CA2665963A1 - Onboard aircraft weight and balance system - Google Patents

Onboard aircraft weight and balance system Download PDF

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Publication number
CA2665963A1
CA2665963A1 CA 2665963 CA2665963A CA2665963A1 CA 2665963 A1 CA2665963 A1 CA 2665963A1 CA 2665963 CA2665963 CA 2665963 CA 2665963 A CA2665963 A CA 2665963A CA 2665963 A1 CA2665963 A1 CA 2665963A1
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Prior art keywords
value
aircraft
pressure
landing gear
load
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CA 2665963
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French (fr)
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CA2665963C (en
Inventor
Michael A. Long
Geoffrey E. Gouette
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Boeing Co
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Boeing Co
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01G—WEIGHING
    • G01G19/00—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups
    • G01G19/02—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for weighing wheeled or rolling bodies, e.g. vehicles
    • G01G19/07—Weighing apparatus or methods adapted for special purposes not provided for in the preceding groups for weighing wheeled or rolling bodies, e.g. vehicles for weighing aircraft
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M1/00—Testing static or dynamic balance of machines or structures
    • G01M1/12—Static balancing; Determining position of centre of gravity
    • G01M1/122—Determining position of centre of gravity
    • G01M1/125—Determining position of centre of gravity of aircraft

Abstract

An onboard system for determining the instantaneous weight and balance of an aircraft simply, reliably, accurately, and requiring a minimum amount of calibration includes a memory for storing previously determined friction data of the aircraft's landing gear shock struts, sensors for sensing the pressures in the struts, the vertical loads exerted by the landing gear on the aircraft, and the attitude of the aircraft relative to the horizontal during loading or unloading thereof, and a computer for computing the vertical load in each of the landing gears from the stored calibration friction data and the shock strut pressures, landing gear vertical loads and aircraft attitude sensed during the loading or unloading. The computer then computes the gross weight of the aircraft and the location of its center of gravity (CG) using the computed vertical loads in the landing gears.

Description

ONBOARD AIRCRAFT WEIGHT AND BALANCE SYSTEM
BACKGROUND

This invention relates to aircraft in general, and in particular, to a system carried aboard an aircraft, such as a large commercial jetliner, that is capable of providing the instantaneous weight and balance of the aircraft, i.e., its total weight and the location of its center of gravity (CG), in a quick, reliable and accurate manner.
Weight and balance of an aircraft is one of the most critical factors affecting flight safety. An overweight aircraft, or one having a center of gravity outside allowable limits, is both inefficient and dangerous to aviate. Responsibility for proper weight and balance control begins with engineers and designers who design the aircraft, and extends to Aviation Maintenance Technicians (AMTs) maintaining the aircraft, "loadmasters" responsible for loading fuel, baggage and cargo aboard the aircraft, and ultimately, to the pilot operating the aircraft.
Two elements are vital in weight and balance determinations for an aircraft:
the total weight of the aircraft must be no greater than the maximum gross weight allowable for the particular make and model of the aircraft and the particular types of flight operations that it is to undertake; and the location of the aircraft's center of gravity (CG), or the point at which all of the weight of the aircraft is considered to be concentrated, relative to its center of lift (COL) or mean aerodynamic chord (MAC), which must be maintained within a pre-determined range for the particular operational weight of the aircraft. Weight and center of gravity (CG) determine field lengths, rotation velocities, decision speeds, and horizontal stabilizer settings.
Center of gravity (CG) must be considered relative to the landing gear positions to control loading and ground handling characteristics.
The initial weight and balance of an aircraft is determined while the aircraft is situated on the ground. Accordingly, one practical way of determining the weight and balance of an aircraft is to measure respective loads imposed on the ground by the landing gear of an aircraft and its attitude relative to the horizontal, if any, and then, calculate the weight and center of gravity (CG) location. The best way to determine the landing gear ground loads is to measure them directly, i.e., with a calibrated scale placed under each gear. However, this is an impractical technique for very large aircraft that may be deployed in field locations that lack the requisite weighing facilities. Consequently, weight and balance measurement systems that are carried onboard the aircraft have been developed in an effort to address this problem, but unfortunately, they are complex, expensive, heavy, difficult to calibrate and maintain, and cumbersome to use.
In light of the foregoing, there is a long-felt but as yet unsatisfied need in the aviation industry for an onboard aircraft weight and balance measurement system that is accurate, reliable, inexpensive, lightweight, and easy to calibrate, maintain and use in the field.

BRIEF SUMMARY
In accordance with one aspect of the invention, there is provided a method for determining a load borne by a landing gear apparatus of an aircraft, the method involves acquiring pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded; acquiring load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded; determining from the pressure signals periods of constant pressure and pressure values associated with respective periods of constant pressure; determining a fluid force value representing fluid force in the strut, the fluid force being the product of the area of a piston in the strut upon which the fluid pressure acts, and a pressure value associated with a period of constant pressure; determining a friction force value associated with the pressure value associated with the period of constant pressure; determining a load change value, wherein the load change value represents a difference between a load measurement at a present time and a load measurement at a beginning of the period of constant pressure; producing a load borne value as a function of the fluid force value, the friction force value and the load change value, the load borne value representing the load borne by the landing gear apparatus; and producing a load borne signal representing the load borne value.
Determining periods of constant pressure may involve obtaining pressure measurement values at successive points in time and replacing the contents of a first buffer containing a previously acquired pressure value with a later acquired pressure value when the later acquired pressure value differs from the previously acquired pressure value by a pre-defined amount, such that the later acquired pressure value represents a pressure associated with a current period of constant pressure.
Determining the fluid force value may involve determining the fluid force value in response to a pressure associated with a current period of constant pressure and determining the friction force value may involve determining the friction force value in response to the pressure value associated with the current period of constant pressure. Determining the load change value may involve determining the load change value at a beginning of the current period of constant pressure and at a current time.
Determining the friction force value may involve at least one of accessing a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded and accessing an unloading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being unloaded.
Producing the load borne value may involve at least one of adding the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and adding the fluid force value and the load change value and subtracting the friction force value when the aircraft is being unloaded.
The method may further involve copying the contents of the first buffer to a second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
Determining the fluid force value may involve determining the fluid force value in response to the pressure value associated with the previous period of constant pressure and determining the friction force value may involve determining the friction force value in response to the pressure value associated with the previous period of constant pressure. Determining the load change value may involve determining the load change value at an end of the previous period of constant pressure and at a current time.
Determining the friction force value may involve at least one of accessing a loading breakout friction lookup table associating pressure with corresponding loading breakout friction values while loading the aircraft and accessing an unloading breakout friction lookup table associating pressure with corresponding loading breakout friction values while unloading the aircraft.
Producing the load borne value may involve at least one of adding the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and adding the fluid force value and the load change value and subtracting the friction force value when the aircraft is being unloaded.
The method may further involve receiving an attitude signal representing an attitude measurement of an angular attitude of the aircraft and associating the attitude measurement with respective points in time, and resolving the fluid force and the friction force into vertical components prior to adding the load change value to produce the load borne value.
Receiving the attitude signal may involve receiving the attitude signal from an inertial navigation system on the aircraft.
In accordance with another aspect of the invention there is provided a method of producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, where the aircraft weight component represents a portion of the weight of the aircraft supported by and associated with the landing gear apparatus. The method involves any of the methods described above and further involves producing a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in the shock strut and producing a landing gear weight component signal representing the landing gear weight component value.
In accordance with another aspect of the invention there is provided a method of calculating a weight of an aircraft. The method involves the method above to produce a landing gear weight component signal for each landing gear apparatus supporting the aircraft and further involves producing an aircraft weight value representing the sum of the values represented by the landing gear weight component values, and producing an aircraft weight signal representing the aircraft weight value for receipt by a display.
In accordance with another aspect of the invention there is provided a method of producing a signal representing a center of gravity of an aircraft. The method involves the method above and further involves calculating a moment value relative to a common datum, for each landing gear apparatus, calculating a net moment value relative to the common datum, from the moments calculated for each landing gear apparatus, calculating a distance value representing a distance from the common datum to the center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into the net moment value, producing a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to the distance value and producing a center of gravity position signal representing the center of gravity position value, for use by a display.
In accordance with another aspect of the invention there is provided a method of causing a tip alarm to be actuated in response to the center of gravity position value, when the center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
In accordance with another aspect of the invention there is provided a computer readable medium encoded with codes for directing a processor to execute any of the methods described above.
In accordance with another aspect of the invention there is provided an apparatus for determining a load borne by a landing gear apparatus of an aircraft.
The apparatus includes provisions for acquiring pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded, provisions for acquiring load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded, provisions for determining from the pressure signals periods of constant pressure and pressure values associated with respective periods of constant pressure, provisions for determining a fluid force value representing fluid force in the strut, the fluid force being the product of the area of a piston in the strut upon which the fluid pressure acts, and a pressure value associated with a period of constant pressure, provisions for determining a friction force value associated with the pressure value associated with the period of constant pressure, provisions for determining a load change value from a difference between a load measurement at a present time and a load measurement at a beginning of the period of constant pressure, provisions for producing a load borne value as a function of the fluid force value, the friction force value and the load change value, the load borne value representing a load borne by the landing gear apparatus, and provisions for producing a load borne signal representing the load borne value.
The determining provisions for determining the periods of constant pressure may include a first buffer and provisions for obtaining pressure measurement values at successive points in time and replacing the contents of the first buffer with a later acquired pressure value when the later acquired pressure value differs from a previously acquired pressure value stored in the first buffer, by a pre-defined amount, such that the later acquired pressure value represents a pressure associated with a current period of constant pressure.
The provisions for determining the fluid force value may include provisions for determining the fluid force value in response to a pressure associated with a current period of constant pressure and the provisions for determining the friction force value may include provisions for determining the friction force value in response to the pressure value associated with the current period of constant pressure and the provisions for determining the load change value may include provisions for determining the load change value at a beginning of the current period of constant pressure and at a current time.
The provisions for determining the friction force value may include at least one of provisions for accessing a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded and provisions for accessing an unloading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being unloaded.
The provisions for producing the load borne value may involve at least one of provisions for adding the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and provisions for adding the fluid force value and the load change value and subtracting the friction force value when the aircraft is being unloaded.
The apparatus may further include a second buffer and provisions for copying the contents of the first buffer to the second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
The provisions for determining the fluid force value may involve provisions for determining the fluid force value in response to the pressure value associated with the previous period of constant pressure and the provisions for determining the friction force value may involve determining the friction force value in response to the pressure value associated with the previous period of constant pressure and the provisions for determining the load change value may involve provisions for determining the load change value at an end of the previous period of constant pressure and at a current time.
The provisions for determining the friction force value may include at least one of provisions for accessing a loading breakout friction lookup table associating pressure with corresponding loading breakout friction values while loading the aircraft and provisions for accessing an unloading breakout friction lookup table associating pressure with corresponding unloading breakout friction values while unloading the aircraft.
The provisions for producing the load borne value may involve at least one of provisions for adding the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and provisions for adding the fluid force value and the load change value and subtracting the friction force value when the aircraft is being unloaded.
The apparatus may further include provisions for receiving an attitude signal representing an attitude measurement of an angular attitude of the aircraft and provisions for associating the attitude measurement with respective points in time, and provisions for resolving the fluid force and the friction force into vertical components prior to adding the load change value to produce the load borne value.
The provisions for receiving the attitude signal may include provisions for receiving the attitude signal from an inertial navigation system on the aircraft.
In accordance with another aspect of the invention, there is provided an apparatus for producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, the aircraft weight component representing a portion of the weight of the aircraft supported by and associated with the landing gear apparatus. The apparatus includes any of the apparatus described above to produce the load borne value and further includes provisions for producing a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in the shock strut and provisions for producing a landing gear weight component signal representing the landing gear weight component value.
In accordance with another aspect of the invention, there is provided an apparatus for calculating a weight of an aircraft, employing the apparatus above to produce a landing gear weight component value for each landing gear apparatus supporting the aircraft and further including provisions for producing an aircraft weight value representing the sum of the values represented by the landing gear weight component signals, and provisions for producing an aircraft weight signal representing the aircraft weight value for receipt by a display.
In accordance with another aspect of the invention, there is provided an apparatus for producing a signal representing a center of gravity of an aircraft, the apparatus comprising the apparatus above and further including provisions for calculating a moment value relative to a common datum, for each landing gear apparatus, provisions for calculating a net moment value relative to the common datum, from the moments calculated for each landing gear apparatus, provisions for calculating a distance value representing a distance from the common datum to the center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into the net moment value, provisions for producing a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to the distance value, and provisions for producing a center of gravity position signal representing the center of gravity position value, for use by a display.
The apparatus may further include a tip alarm and provisions for causing the tip alarm to be actuated in response to the center of gravity position value, when the center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
In accordance with another aspect of the invention, there is provided an apparatus for determining a load borne by a landing gear apparatus of an aircraft.
The apparatus includes an input for receiving pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded, an input for receiving load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded, an output for producing a load borne signal, a processor in communication with the inputs and the output, and a memory encoded with codes for directing the processor to:

determine from the pressure signals periods of constant pressure and pressure values associated with respective the periods of constant pressure;
determine a fluid force value representing fluid force in the strut, the fluid force being the product of the area of a piston in the strut upon which the fluid pressure acts, and a pressure value associated with a period of constant pressure;

determine a friction force value associated with the pressure value associated with the period of constant pressure;

determine a load change value, wherein the load change value represents a difference between a load measurement at a present time and a load measurement at a beginning of the period of constant pressure;

produce a load borne value as a function of the fluid force value, the friction force value and the load change value, the load borne value representing a load borne by the landing gear apparatus; and control the output to produce the load borne signal in response to the load borne value.
The apparatus may further include a first buffer in communication with the processor and the codes may include codes for directing the processor to obtain pressure measurement values at successive points in time and replace the contents of the first buffer with a later acquired pressure value when the later acquired pressure value differs from a previously acquired pressure value stored in the first buffer, by a pre-defined amount, such that the later acquired pressure value represents a pressure associated with a current period of constant pressure.
The codes may include codes for directing the processor to determine the fluid force value in response to a pressure associated with a current period of constant pressure and the codes may include codes for directing the processor to determine the friction force value in response to the pressure value associated with the current period of constant pressure and the codes may include codes for directing the processor to determine the load change value at a beginning of the current period of constant pressure and at a current time.
The codes for directing the processor to determine the friction force value may include at least one of codes for directing the processor to access a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded, and codes for directing the processor to access an unloading friction lookup table associating pressure with corresponding unloading sliding friction values when the aircraft is being unloaded.
The codes for directing the processor to produce the load borne value may include at least one of codes for directing the processor to add the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and codes for directing the processor to add the fluid force value and the load change value and subtract the friction force value when the aircraft is being unloaded.
The apparatus may further include a second buffer and codes for directing the processor to copy the contents of the first buffer to the second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
The codes for directing the processor to determine the fluid force value may include codes for directing the processor to determine the fluid force value in response to the pressure value associated with the previous period of constant pressure. The codes for directing the processor to determine the friction force value may include codes for directing the processor to determine the friction force value in response to the pressure value associated with the previous period of constant pressure and the codes for directing the processor to determine the load change value may include codes for directing the processor to determine the load change value at an end of the previous period of constant pressure and at a current time.
The codes for directing the processor to determine the friction force value may include at least one of codes for directing the processor to access a loading breakout friction lookup table associating pressure with corresponding loading i breakout friction values while loading the aircraft and codes for directing the processor to access an unloading breakout friction lookup table associating pressure with corresponding unloading breakout friction values while unloading the aircraft.
The codes for directing the processor to produce the load borne value may include at least one of codes for directing the processor to add the fluid force value, the friction force value and the load change value, when the aircraft is being loaded and codes for directing the processor to add the fluid face value and the load change value and subtract the friction force value when the aircraft is being unloaded.
The apparatus may further include codes for directing the processor to receive an attitude signal representing an attitude measurement of an angular attitude of the aircraft and codes for directing the processor to associate the attitude measurements with respective points in time, and codes for directing the processor to resolve the fluid force and the friction force into vertical components prior to adding the load change value to produce the load borne value.
The codes for directing the processor to receive the attitude signal may include codes for directing the processor to receive the attitude signal from an inertial navigation system on the aircraft.
In accordance with another aspect of the invention, there is provided an apparatus for producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, the aircraft weight component representing a portion of the weight of the aircraft supported by and associated with the landing gear apparatus. The apparatus includes any of the apparatus above to produce the load borne value and further includes codes for directing the processor to produce a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in the shock strut and codes for directing the processor to produce a landing gear weight component signal representing the landing gear weight component value.
In accordance with another aspect of the invention, there is provided an apparatus for calculating a weight of an aircraft. The apparatus includes the apparatus described above to produce a landing gear weight component value for each landing gear apparatus supporting the aircraft and further includes codes for directing the processor to produce an aircraft weight value representing the sum of the values represented by the landing gear weight component signals, and codes for directing the processor to cause the output to produce an aircraft weight signal representing the aircraft weight value for receipt by a display.
In accordance with another aspect of the invention, there is provided an apparatus for producing a signal representing a center of gravity of an aircraft. The apparatus includes the apparatus described above and further includes codes for directing the processor to calculate a moment value relative to a common datum, for each landing gear apparatus, codes for directing the processor to calculate a net moment value relative to the common datum, from the moments calculated for each landing gear apparatus, codes for directing the processor to calculate a distance value representing a distance from the common datum to the center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into the net moment value, codes for directing the processor to calculate a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to the distance value, and codes for directing the processor to cause the output to produce a center of gravity position signal representing the center of gravity position value, for use by a display.
The apparatus may further include a tip alarm and codes for directing the processor to actuate the tip alarm when the center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
A better understanding of the above and many other features and advantages of the novel onboard aircraft weight and balance system of the present invention may be obtained from a consideration of the detailed description of some exemplary embodiments thereof below, particularly if such consideration is made in conjunction with the appended drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a left side elevation view of an aircraft incorporating an onboard system for determining the weight and balance of the aircraft in accordance with one embodiment of the present invention;
Figure 2 is a schematic diagram of the onboard system of Figure 1;
Figure 3 is a graphical illustration of load versus shock strut pressures and load sensor readings for the landing gear shock struts and landing gear of the aircraft of Figure 1, showing the effects of breakout and sliding friction in the struts and the pressures sensed therein;
Figure 4 is a block diagram of a processor circuit of the onboard system shown in Figure 2;
Figure 5 is a flowchart illustrating a process executed by the processor shown in Figure 4 for producing a signal representing load borne by one of the shock struts shown in Figures 1 and 2, having regard to breakout friction;
Figure 6 is a flowchart illustrating an alternative process executed by the processor shown in Figure 4 for producing a signal representing load borne by one of the shock struts shown in Figures 1 and 2, having regard to sliding friction in the shock strut;
Figure 7 is a flowchart illustrating a process executed by the processor shown in Figure 4 for producing a landing gear weight component signal representing a component of the weight of the aircraft associated with one of the struts shown in Figures 1 and 2;
Figure 8 is a flowchart illustrating a process executed by the processor shown in Figure 4 for producing a gross weight signal representing a gross weight of the aircraft shown in Figure 1;
Figure 9 is a flowchart illustrating a process executed by the processor shown in Figure 4 for producing a center of gravity signal representing a center of gravity of the aircraft shown in Figure 1; and Figure 10 is a flowchart illustrating a process executed by the processor shown in Figure 4 for producing a tip alarm actuation signal for actuating a tip alarm of the system shown in Figure 2.

DETAILED DESCRIPTION

Referring to Figure 1, a portion of an aircraft is shown generally at 100. The aircraft includes a fuselage 98, wings, one of which is shown at 96, and a plurality of landing gear 103. The fuselage 98 has a longitudinal axis 106 that may be disposed at an angle (8) relative to a horizontal reference plane 108. The angle 8 is a measure of the pitch attitude (P) of the aircraft.
The landing gear 103 may comprise a nose gear 94 and first and second spaced apart main gear, only one of which is shown at 92 in Figure 1.
Generally, each landing gear 103 includes a shock strut 110 comprising a fluid filled cylinder 90 containing a piston 88. A connecting rod 86 has a first end 84 connected to the piston 88 and a second end 82 connected to a wheel assembly 80. The shock struts 110 are connected to the fuselage 98 and are generally foldable into the fuselage from a deployed position. The shock struts 110 are shown in the deployed position in Figure 1.
When the shock struts 110 are in the deployed position, they extend from the fuselage 98 at respective angles. For example, the nose gear 94 extends at a first angle (Anlg) from a line 109 which is perpendicular to the longitudinal axis 106 and the main gear 92 extends at a second angle (Amlg) from the line 109. As will be seen later, the vertically upward forces provided on the landing gear 103 by the ground 101 will be resolved along the shock struts 110 as part of the determination of the weight of the aircraft 100.
Referring to Figure 2 a system for determining weight and balance of the aircraft shown in Figure 1 is shown generally at 300. The system 300 includes a plurality of pressure sensors 128 on each of the shock struts 110 that produce signals representing fluid pressures in the cylinders 90, a plurality of load sensors 130 that produce signals representing loads on the respective landing gear 103, an onboard weight and balance system (OBWBS) control unit 134, and a display unit 136. The aircraft 100 will normally have an inertial navigation system (INS) 140 and in the embodiment shown, the INS is in communication with the OBWBS control unit 134 to provide at least pitch attitude signals representing pitch attitude (P) of the aircraft relative to the horizontal reference plane (108 shown in Figure 1).
The aircraft (100) may also include a flight deck display 142 in communication with the OBWBS control unit 134 to display information such as weight and center of gravity location information produced by the OBWBS control unit. The aircraft (100) may also include an airplane tip alarm 144 in communication with the OBWBS control unit 134 to allow the OBWBS control unit to cause the alarm to be actuated when the weight and/or center of gravity determined by the OBWBS control unit 134 meet certain criteria.
The pressure sensors 128 measure hydraulic fluid pressure, for example, in the shock strut cylinders 90. The load sensors 130 measure strain acting on the landing gear 103 or on the structure of the aircraft (100). The load sensors 130 are conventional and are typically incorporated in large aircraft to provide signals to an onboard avionics system which automatically determines whether the landing gear 103 of the aircraft is in contact with the ground 101, for example a runway or tarmac.
These load sensors 130 are not sufficiently accurate for measuring absolute load values in the landing gear 103, but they are capable of measuring relative values quite accurately and this feature is put to good use in this embodiment. As illustrated in Figure 3 at 129, each of the load sensors 130 exhibits a single line load versus strain (e.g. an output voltage reading) in response to the loading or unloading of the aircraft (100), as represented by the sloped line 132 therein.
Referring back to Figure 2, in this embodiment, the signals produced by the load sensors 130 are considered to represent force (in Newtons, or pounds, for example) measured in the shock struts 110. The load sensors 130 sense only the vertical component of the loads acting on the aircraft 100 by the landing gear 103.
Referring to Figure 4 the OBWBS control unit 134 is shown in greater detail and includes a processor 150, an input port 152, an output port 154, and variable memory 156 for storing values represented by the pitch attitude signal, the gear load sensor signals and the cylinder pressure signals and for scratchpad storage of values used during calculations. The OBWBS control unit 134 further includes a program memory 158 for storing a block of codes 159 for directing the processor 150 to determine load borne by each individual shock strut (110), a block 161 for determining a weight component of the aircraft (100), associated with the landing gear, a block 163 for determining a gross weight of the aircraft, and a block 165 for determining a center of gravity (CG) of the aircraft. The program memory 158 may also include a block of codes 167 for directing the processor 150 to produce a signal to actuate the airplane tip alarm 144.
For ease of understanding, it will be assumed that the input port 152 samples the cylinder pressure signals, the gear load signals and the pitch attitude signal and stores values representing instantaneous amplitudes of these signals in buffers readable by the processor 150. Similarly, it will be assumed that the processor 150 is operable to write to one or more buffers (not shown) in the output port 154 to cause the output port to produce signals for use by the display unit 136, the flight deck display 142 and/or the airplane tip alarm 144. The output port 154 may produce signals in accordance with a network protocol, for example, where the display unit, flight deck display 142 and/or airplane tip alarm 144 have network interfaces, or alternatively, the output port may directly control the display unit 136, flight deck display, 142, and/or airplane tip alarm 144.
The blocks of codes 159, 161, 163, 165, 167 stored in the program memory 158 may be loaded into the program memory from a separate computer readable medium such as a flash card, a CD-ROM, or from an interface that receives signals from a network, for example.
Effectively, the block of codes 159 directs the processor 150 to calculate loading on the landing gear (103) with due regard to mechanical characteristics of the shock strut (110). For example, the actual load-pressure response of a large aircraft shock strut to the application of monotonically increasing or decreasing i loadings is not a single, smooth, linear response curve, but rather, a series of "stair-step" responses, such as the two "loading" and "unloading" response curves respectively illustrated in Figure 3 by the dashed lines 112 and 114, which result from the operation of frictional forces.
As illustrated in the graph of Figure 3, as the shock strut (110) is either loaded or unloaded, the fluid pressure within the shock strut tends to remain constant over a relatively substantial interval, because no relative movement occurs between the two opposite ends of the shock strut until the static friction force in the shock strut that resists the movement, referred to herein as the "breakout friction," is overcome, at which point, the two ends of the shock strut either move suddenly toward (during loading) or apart from each other (during unloading), resulting in a sudden increase or decrease, respectively, in the pressure sensed in the shock strut. That is, the static friction in the shock strut (110) acts to resist compression of the shock strut during loading and extension thereof during unloading. Accordingly, to obtain an accurate determination of the load acting through the shock strut (110), and hence, through the landing gear (103), it is essential to take into account the effects of the friction forces acting in the shock strut.
As illustrated graphically in Figure 3, shock strut (110) friction is of two types, viz., a static, or "breakout" friction, which must be overcome for shock strut movement to occur, and a dynamic, or "sliding" friction component, which continues to resist movement. If no friction forces were acting in the shock strut (110), the load versus pressure relationship for both loading and unloading of the shock strut would be represented by the single, smooth phantom line 120 in Figure 3, and if only dynamic or sliding friction were acting, the load versus pressure relationship would be represented by a respective one of the two solid lines 116 (for loading) and 118 (for unloading). Typically, as load, due to passengers, luggage, cargo, fuel, food, equipment and/or supplies, for example, is applied to the shock strut (110), when the aircraft (100) is first being loaded for a flight operation, there is initially no movement of the shock strut, and therefore, no change in the pressure measured within it.
However, once the load overcomes the static breakout friction (Fcbo) acting in the shock strut (110), the shock strut compresses, thereby raising the pressure in the shock strut to a new value. As illustrated in Figure 3, as the pressure in the shock strut at one of these breakout pressures begins to rise, a point is reached at which the force resulting from the pressure and the sliding friction equals the force exerted by the load and the shock strut stops compressing. This creates a "stair step"
pattern in pressure vs. loading, which occurs repeatedly during the loading of the aircraft (100), resulting in the stair-step load versus pressure plot 112 of Figure 3.
The reverse of the foregoing pattern is exhibited during the unloading of the aircraft, as represented by the dashed line 114 of Figure 3.
There are two ways to determine the load on the landing gear 103. The first way is to predetermine breakout friction values (Fcbo) 121 and measured fluid pressure values for various loads and store in a lookup table the breakout friction values (Fcbo) and measured fluid pressure values (Pcbo) for a range of loading.
Then, given a fluid pressure value (Pcbo), a corresponding breakout friction value (Fcbo) 121 can be determined. As can be seen from Figure 3, the sum of a force 123 associated with a previous breakout pressure, the breakout friction value (Fcbo) 121 and a difference 125 in load measured by the load sensors gives the current landing gear load, when loading the aircraft (100) Referring back to Figure 2, lookup table data associating breakout friction values (Fcbo) with breakout pressures (Pcbo), is preferably generated early in the initial rollout of the aircraft (100), i.e., after a few test flight cycles and prior to delivery to the customer. Each landing gear 103 of the aircraft (100) is placed on a respective scale whereby actual loads acting on the landing gear are measured.
As the aircraft (100) is loaded and unloaded with dummy weights, e.g. sandbags or fuel, the actual loads acting on the landing gear 103, as measured by the scales, and the pressure in the landing gear, as measured by the pressure sensors 128, are monitored. During loading or unloading, when the piston 88 of the landing gear begins to move at the breakout pressure (Pcbo), the friction force (Fcbo) associated with the breakout pressure may be found by subtracting the fluid force value at the breakout pressure (Pcbo*A) prior to the movement of the piston 88 from the actual load acting on the landing gear 103 as measured by the scales. A loading breakout friction lookup table is produced by loading the aircraft (100) with dummy weights and an unloading breakout friction lookup table is produced by unloading the dummy weights from the aircraft. The loading breakout friction lookup table is subsequently used by the OBWBS control unit 134 during loading of the aircraft (100) and the unloading breakout friction lookup table is subsequently used by the OBWBS
control unit during unloading of the aircraft.
New loading and unloading breakout friction lookup tables can be created at any time during the life of the aircraft (100) and may be necessary to create for a given shock strut (110), in the event that the mechanical characteristics of the shock strut changes due to maintenance or replacement, for example.
Referring back to Figure 3, the second way to determine load on the landing gear (103) is to predetermine sliding friction values (Fcs) 127 and measured fluid pressure values (Pcs) for various loads and store in a lookup table the sliding friction values (Fcs) and measured fluid pressure values (Pcs) for a range of loading.
Then, given a fluid pressure value (Pcs), a corresponding sliding friction value (Fcs) 127 can be determined. As can be seen from Figure 3, the sum of a force 129 associated with a current breakout pressure (Pcs), the sliding friction value (Fcs) 127 and a difference in load measured by the load sensors gives the current landing gear load.
Referring back to Figure 2, as before, lookup table data associating sliding friction force values (Fcs) with breakout pressures (Pcs), is preferably generated early in the initial rollout of the aircraft (100), i.e., after a few test flight cycles and prior to delivery to the customer. Each landing gear 103 of the aircraft (100) is placed on a respective scale whereby actual loads acting on the landing gear are measured. As the aircraft (100) is loaded and unloaded with dummy weights, e.g.
sandbags or fuel, the actual loads acting on the landing gear 103 as measured by the scales and the pressure in the landing gear as measured by the pressure sensors 128 are monitored. During loading or unloading, after the piston 88 of the landing gear 103 moves at the breakout pressure (Pcbo), it will stop moving at the breakout pressure (Pcs). The sliding friction force value (Fcs) associated with the breakout pressure (Pcs) may be found by subtracting the fluid force value at the breakout pressure (Pcs*A) after the movement of the piston 88 from the actual load acting on the landing gear 103 as measured by the scales. A loading sliding friction lookup table is produced while the aircraft (100) is being loaded with dummy weights and an unloading sliding friction lookup table is produced while the dummy weights are being unloaded from the aircraft. The loading sliding friction lookup table is subsequently used by the OBWBS 134 during loading of the aircraft (100) and the unloading sliding friction lookup table is subsequently used by the OBWBS
during unloading of the aircraft.
New loading and unloading sliding friction lookup tables can be created at any time during the life of the aircraft (100) and may be necessary to create for a given shock strut 110, in the event that the mechanical characteristics of the shock strut change due to maintenance or replacement, for example.
The present description describes both ways of determining landing gear loading.
Referring to Figures 4 and 5, the block of codes 159 that direct the processor 150 to determine the load borne by a single landing gear according to the first way, i.e. using breakout friction values, direct the processor to perform the operations indicated at 160 in Figure 5.
The blocks of codes 159 for determining the load borne by a landing gear (103) include a first block 162 that directs the processor 150 to acquire or receive pressure signals representing pressure measurements of fluid in the shock strut (110) of the landing gear apparatus and to acquire or receive load signals from the corresponding load sensor, at successive points in time while the aircraft (100) is being loaded or unloaded. This is achieved by causing the processor 150 to communicate with the input port 152 at successive points in time, such as every second, for example to produce a time stamp and to associate with the timestamp a pressure value representing a pressure measured at the cylinder and to associate with the time stamp a load value representing a load measured at the shock strut (110). The time stamps and their associated pressure and load measurement values are stored in the variable memory 156. It will be appreciated that the same is done for the signals from each landing gear (103) so that each time stamp is associated with a complete set of pressure and load signals for each landing gear.
i In other words the pressure and load signals associated with all of the landing gear (103) on the aircraft (100) are sampled substantially simultaneously.
Block 164 directs the processor 150 to determine from the pressure measurements associated with the shock strut (110) of interest, periods of relatively constant pressure. In this embodiment, to do this, block 164 directs the processor 150 to maintain a first buffer in the scratchpad memory and to store in the first buffer successively acquired pressure values. More particularly, block 164 directs the processor 150 to replace the contents of the first buffer containing a previously acquired pressure value with a later acquired pressure value when the later acquired pressure value differs from the previously acquired pressure value by a pre-defined amount, such as a few pounds per square inch. Thus, the later acquired pressure value represents a pressure associated with a current period of constant pressure.
Essentially, this amounts to recording the pressure in the shock strut (110), waiting for a pressure change to occur for a period of time and if no pressure change occurs in this period of time, select a last recorded pressure as the constant pressure. This constant pressure may be referred to as a breakout pressure (Pcbo) because it represents the pressure after the last movement of the shock strut (110) during loading or unloading. Of course, a tolerance of a few pounds per square inch above and below a previously acquired pressure value should be allowed, before taking a newly acquired pressure as a new breakout pressure.
In this embodiment, block 164 also directs the processor 150 to maintain a second buffer in conjunction with the first buffer, whereby block 164 directs the processor to copy the contents of the first buffer to the second buffer prior to replacing the contents of the first buffer so that the second buffer holds a pressure value associated with a previous period of constant pressure. Thus, in effect, the first and second buffers are controlled by the processor 150 to act as a first-in first-out (FIFO) buffer for successively acquired breakout pressure values.
Next, the processor 150 is directed to calculate the load borne by the landing gear for a point in time. To do this, block 166 directs the processor 150 to determine a breakout pressure value (Pcbo) to be used in calculations and to determine a load measurement associated with the point in time. The pressure measurement is given by the contents of the second buffer, i.e. the previously acquired breakout pressure (Pcbo(t-1)). The load measurement is the load measurement value associated with a time stamp nearest the point in time.
Block 170 then directs the processor 150 to use the previously acquired breakout pressure value (Pcbo(t-1)) to determine a fluid force value representing a fluid force provided by the fluid in the shock strut (110). The program memory is pre-loaded with an area value representing cross sectional area (A) of the piston (88) in the shock strut (110) so block 170 directs the processor 150 to calculate the fluid force value from the relation:
F = (Pcbo(t-1)) * A

Where, F: is the fluid force provided by fluid in the shock strut at the previously acquired breakout pressure;
(Pcbo(t-1)): is the previously acquired breakout pressure; and A: is the pre-loaded cross sectional area value representing the area of the shock strut piston.

Next, block 172 directs the processor 150 to determine a breakout friction force value (Fcbo(t-1)) from the loading breakout friction lookup table or the unloading breakout friction table, depending on whether the aircraft (100) is being loaded or unloaded. The selected friction force value Fcbo is the one associated with the previously acquired breakout pressure (Pcbo(t-1)), i.e. the contents of the second buffer.
Block 173 then directs the processor 150 to produce a force entity value Fe which is the sum of the fluid force value F and the friction force value Fcbo according to the relation:
Fe = F +/- Fcbo(t-1) I

If the aircraft (100) is being loaded, the term Fcbo(t-1) is added to the fluid force value (F) and if the aircraft is being unloaded, the term Fcbo(t-1) is subtracted from the fluid force value (F).
Next, block 174 directs the processor 150 to determine a load change value (Al) representing a difference in a load measurement associated with the point in time (Lf) and a load measurement (Lcbo(t-1)) associated with a point in time at which the corresponding breakout pressure occurred. The load change value (Al) is positive when loading and negative when unloading. The load change value (AI) (125 in Figure 3) is calculated according to the relation:
(AI) = (Lf) - (Lcbo(t-1)) Block 176 then directs the processor 150 to add the load change value (Al) to the force entity (Fe) to produce a value (Fib) representing the load borne by the landing gear (103) and then to store the value as a load borne value representing the load borne by the landing gear. The load borne value (Fib) is produced according to the relation:
Fib = Fe +(Al) Block 178 directs the processor 150 to cause the output port 154 to produce a load borne signal representing the load borne by the landing gear (103) in response to the load borne value. To do this, the processor 150 communicates the load borne value to the output port 154 to cause this value to be communicated to the display unit 136, the flight deck display 142 and/or the airplane trip alarm 144. The load borne value (Fib) can be used to indicate to the pilot or loadmaster the load borne by the associated landing gear (103).
The above actions are useful for determining the force on a vertically oriented shock strut (110). Where the shock strut (110) is not vertically oriented, such as specified by an aircraft designer during design of the aircraft, the processor 150 is directed to block 180 after executing block 173. Block 180 directs the processor 150 to modify the force entity (Fe) by the cosine of an angle entity (R), which, in this embodiment is the angle of the shock strut (110) to the vertical. The force entity is modified according to the relation:

Fern = Fe /cos (R) The angle R is equal to the angle of the nose gear (94) or the main gear (92), as appropriate from the vertical. Where the aircraft (100) has a pitch attitude angle P relative to the horizontal, the pitch attitude angle P is added to the designed strut angle where the pitch attitude increases the angle of the shock strut (110) to the vertical and is subtracted from the strut angle where the pitch attitude decreases the angle. This is accomplished by causing the processor 150 to receive pitch attitude signals from the inertial navigation system (140) and to store values representing these signals in the variable memory 156. The processor 150 then adds or subtracts the current pitch attitude P value to or from the designed strut angle (tnlg or Amlg) to produce the angle R to be used in the calculation of the modified force entity described above. In other words, R is calculated as follows:

R = P + Onlg or Omlg, as appropriate It should be noted that since the load sensors (130) sense only the vertical component of the loads acting on the aircraft (100) by the landing gear (103), it is not necessary to resolve these loads vertically by their multiplication with 1/cos P.
To determine the load borne by any individual shock strut (110), the above process is followed with the indicated modifications depending on pitch attitude and designed strut angle, to produce the load borne value (Flb).
Referring back to Figure 3, the second way of producing the load borne value (Flb) involves the use of the sliding friction loading and unloading lookup tables which relate sliding friction values (Fcs) 127 and measured fluid pressure values (Pcs) for a range of loading. Then, given a fluid pressure value (Pcs), a corresponding sliding friction value (Fcs) 127 can be determined. Thus in an implementation involving this second way of producing the load borne values (Flb) and/or signals, blocks 172 to 176 of Figure 5 are replaced with blocks 200-204 shown in Figure 6.
Referring to Figure 6, block 200 directs the processor 150 to use the current breakout pressure (Pcs), stored in the first buffer, to determine a fluid force value representing a fluid force provided by the fluid in the shock strut (110). As before, the program memory 158 is pre-loaded with an area value (A) representing cross sectional area of the piston (88) in the shock strut (110) so block 200 directs the processor 150 to calculate the fluid force value from the relation:

F=Pcs*A
Where, F: is the fluid force provided by fluid in the shock strut at the current breakout pressure;
Pcs: is the current breakout pressure; and A: is the pre-loaded cross sectional area value representing the area of the shock strut piston.

Next, block 202 directs the processor 150 to determine from the sliding friction loading or unloading look up table stored in the program memory, a sliding friction force value, Fcs (127), associated with the current breakout pressure Pcs.
Block 204 then directs the processor 150 to produce a force entity value Fe which is the sum of the fluid force value F and the sliding friction force value Fcs according to the relation:
Fe = F + Fcs If it is desired to use the process during unloading, the friction force value Fcs is instead subtracted from the fluid force value F.
The force entity value (Fe) may be modified to produce a modified value (Fem), in response to the angle of the shock strut (110) to the vertical using the procedure described in connection with blocks 180 and 182 in Figure 5.
Referring back to Figure 6, next, block 206 directs the processor 150 to determine a load change value (Al) representing a difference in a load measurement associated with the point in time (Lf) and a load measurement (Lpcs) associated with a point in time at which the current breakout pressure (Pcs) first occurred.
Again, the load change value (Al) is positive when loading and negative when unloading.
The load change value (Al) is calculated according to the relation:

Al = (Lf) - (Lpcs) Block 208 then directs the processor 150 to add (when loading) or subtract (when unloading) the load change value (Al) to the force entity to produce a value (Fib) representing the load borne by the landing gear (103) and then to store the value as the load borne value (Fib) representing the load borne by the landing gear.
The load borne value (Fib) is produced according to the relation:
Fib = Fern + (Al) Landing gear Weight component While the load borne signal produced at block 178 of Figure 5 can be provided to the display unit 136, the flight deck display 142 and/or the airplane tip alarm 144, it need not be provided directly and it may be more useful to provide a landing gear weight component signal to these devices. To do this, referring to Figure 7, the landing gear weight calculation block of codes 161 is executed after block 176 in Figure 5 or after block 208 in Figure 6 and includes a first block 184 that directs the processor 150 to produce a landing gear weight component value representing the sum of the load borne value (Fib) and an unsprung weight value representing the weight of components of the landing gear (103) that are not supported by fluid pressure in the shock strut (110). This unsprung weight value may be pre-defined, by weighing the landing gear (103) before it is attached to the aircraft (100) or by summing the weights of the individual components of each landing gear. The unsprung weight value for each landing gear (103) may be pre-stored in the program memory, for example. Block 185 then directs the processor 150 to store the calculated landing gear weight component values write it to the output port 154 to produce a landing gear weight signal that can be provided to the display unit 136, the flight deck display 142 and/or the airplane tip alarm 144.

Gross Aircraft Weight Referring to Figures 4 and 8, the gross ai6rcraft weight is determined under the control of block 163. It is assumed that the process described through Figure 7 has been executed for each landing gear (103) such that respective landing gear weight component values for each landing gear have been produced and stored in the scratch pad memory for use by the processor 150 when it executes block 163.
Block 163 includes a first block 186 that directs the processor 150 to sum the landing gear weight component values calculated for each shock strut (110) to produce a gross weight value representing the gross weight of the aircraft (100).
Block 188 then directs the processor 150 to communicate the gross weight value to the output port 154 to produce a gross weight signal and to communicate this signal to the display unit, 136, the flight deck display 142 and/or the tip alarm 144.

Center of Gravity Referring to Figures 4 and 9, block 165 directs the processor 150 to calculate a center of gravity value for the aircraft (100). The program memory 158 is preloaded with distance values representing distances of respective landing gear (103) from a pre-determined position on the aircraft (100). Using these distance values, and after the process described in connection with Figure 8 has been completed, block 190 directs the processor 150 to calculate a moment relative to a common datum for each landing gear (103). Block 192 then directs the processor 150 to calculate a net moment value relative to the common datum, from the moments calculated for each landing gear apparatus. Block 194 then directs the processor 150 to calculate a distance value representing a distance from the common datum to the center of gravity by dividing the aircraft weight value produced by executing the codes shown in Figure 8 into the net moment value produced at block 192. Block 196 then directs the processor 150 to produce a center of gravity (CG) position value signal representing the position of the center of gravity of the aircraft (100) in a coordinate system understandable by the loadmaster, in response to the distance value. Block 198 then directs the processor 150 to communicate the center of gravity (CG) position value signal to the output port 154 and the output port produces a center of gravity (CG) signal that is provided to the display unit 136, the flight deck display 142, or the airplane tip alarm 144 to cause a representation of the center of gravity (CG) value signal to be displayed or to cause the tip alarm to be actuated. The representation of the center of gravity (CG) value signal may include a graphic representation or a text representation, for example.
Referring to Figures 4 and 10, codes 167 direct the processor 150 as to whether or not to actuate the tip alarm 144, to do this block 201 directs the processor to determine whether the center of gravity (CG) value signal meets a certain criterion, such as if the center of gravity signal indicates a center of gravity within a range determined by the aircraft designer to be an unsafe operating range.
If this criterion is met, the processor 150 is directed to a further block of codes 203 that directs the processor to communicate with the output port 154 to cause an actuation signal to be sent to the tip alarm 144.
The aircraft "Tip Alarm" can be used to warn the loadmaster that the airplane center of gravity (CG) is nearing the main gear fore/aft location. If the center of gravity (CG) moves aft of the aft-most main gear 92, the aircraft (100) can tip back onto its tail, causing extensive aircraft and cargo damage, as well as injury to personnel in the vicinity. In order to prevent this type of accident from happening, the processes shown in Figures 5-10 may be continuously run to continually compute the location of the center of gravity (CG). When the center of gravity (CG) approaches a tipping limit for the aircraft, the tipping alarm actuation signal is produced to cause the tipping alarm to provide an aural, visual, or other warning.
The tipping alarm actuation signal may also be provided to cargo handling systems to prevent such systems from moving or transferring any more cargo aft in the aircraft. Alternatively, the tip alarm may have its own processor operable to actuate the tip alarm in response to any load borne signal, any weight component signal, any gross weight signal or any center of gravity (CG) signal.
Additionally, it may be noted that, by observing the sequence of breakout pressures, i.e. increasing or decreasing during a loading or an unloading operation, the system can also determine automatically whether the aircraft (100) is being loaded or unloaded.
The present invention monitors shock strut pressures and calculates shock strut friction. Load sensor readings are then used to further increase the accuracy of the landing gear loads. Accordingly, the system of the present invention does not require the complexity, hardware and weight of an additional system to pump hydraulic fluid in and out of the shock struts (110), and precludes the added risk of leakage of the shock strut fluid from the pump-type system components.

Claims (46)

1. A method for determining a load borne by a landing gear apparatus of an aircraft, the method comprising:

acquiring pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;

acquiring load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;

determining from said pressure signals periods of constant pressure and pressure values associated with respective said periods of constant pressure;

determining a fluid force value representing fluid force in said strut, said fluid force being the product of the area of a piston in said strut upon which said fluid pressure acts, and a pressure value associated with a period of constant pressure;

determining a friction force value associated with said pressure value associated with said period of constant pressure;

determining a load change value, wherein the load change value represents a difference between a load measurement at a present time and a load measurement at a beginning of said period of constant pressure;

producing a load borne value as a function of said fluid force value, said friction force value and said load change value, said load borne value representing a load borne by the landing gear apparatus; and producing a load borne signal representing said load borne value.
2. The method of claim 1 wherein determining said periods of constant pressure comprises obtaining pressure measurement values at successive points in time and replacing the contents of a first buffer containing a previously acquired pressure value with a later acquired pressure value when said later acquired pressure value differs from said previously acquired pressure value by a pre-defined amount, such that said later acquired pressure value represents a pressure associated with a current period of constant pressure.
3. The method of claim 2:

wherein determining said fluid force value comprises determining said fluid force value in response to a pressure associated with a current period of constant pressure;

wherein determining said friction force value comprises determining said friction force value in response to said pressure value associated with said current period of constant pressure; and wherein determining said load change value comprises determining said load change value at a beginning of said current period of constant pressure and at a current time.
4. The method of claim 3 wherein determining said friction force value comprises at least one of:

accessing a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded; and accessing an unloading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being unloaded.
5. The method of claim 4 wherein producing said load borne value comprises at least one of:

adding said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and adding said fluid force value and said load change value and subtracting said friction force value when said aircraft is being unloaded.
6. The method of claim 2 further comprising copying the contents of the first buffer to a second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
7. The method of claim 6:

wherein determining said fluid force value comprises determining said fluid force value in response to the pressure value associated with said previous period of constant pressure;

wherein determining said friction force value comprises determining said friction force value in response to said pressure value associated with said previous period of constant pressure; and wherein determining said load change value comprises determining said load change value at an end of said previous period of constant pressure and at a current time.
8. The method of claim 7 wherein determining said friction force value comprises at least one of:

accessing a loading breakout friction lookup table associating pressure with corresponding loading breakout friction values while loading the aircraft; and accessing an unloading breakout friction lookup table associating pressure with corresponding loading breakout friction values while unloading the aircraft.
9. The method of claim 8 wherein producing said load borne value comprises at least one of:

adding said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and adding said fluid force value and said load change value and subtracting said friction force value when said aircraft is being unloaded.
10. The method of any one of claims 1 to 9 further comprising receiving an attitude signal representing an attitude measurement of an angular attitude of the aircraft and associating said attitude measurement with respective said points in time, and resolving said fluid force and said friction force into vertical components prior to adding said load change value to produce said load borne value.
11. The method of claim 10, wherein receiving said attitude signal comprises receiving said attitude signal from an inertial navigation system on the aircraft.
12. A method of producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, the aircraft weight component representing a portion of the weight of the aircraft supported by and associated with the landing gear apparatus, the method comprising the method of any one of claims 1 to 11 to produce said load borne value and further comprising producing a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in said shock strut and producing a landing gear weight component signal representing said landing gear weight component value.
13. A method of calculating a weight of an aircraft, employing the method of claim 12 to produce a landing gear weight component signal for each landing gear apparatus supporting the aircraft and further comprising producing an aircraft weight value representing the sum of the values represented by said landing gear weight component values, and producing an aircraft weight signal representing said aircraft weight value for receipt by a display.
14. A method of producing a signal representing a center of gravity of an aircraft, the method comprising the method of claim 13 and further comprising:

calculating a moment value relative to a common datum, for each landing gear apparatus;

calculating a net moment value relative to said common datum, from the moments calculated for each landing gear apparatus;

calculating a distance value representing a distance from said common datum to said center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into said net moment value;
producing a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to said distance value; and producing a center of gravity position signal representing said center of gravity position value, for use by a display.
15. The method of claim 14 further comprising causing a tip alarm to be actuated in response to said center of gravity position value, when said center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
16. A computer readable medium encoded with codes for directing a processor to execute the method of any one of claims 1 to 15.
17. An apparatus for determining a load borne by a landing gear apparatus of an aircraft, the apparatus comprising:

means for acquiring pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;

means for acquiring load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;

means for determining from said pressure signals periods of constant pressure and pressure values associated with respective said periods of constant pressure;

means for determining a fluid force value representing fluid force in said strut, said fluid force being the product of the area of a piston in said strut upon which said fluid pressure acts, and a pressure value associated with a period of constant pressure;

means for determining from a look up table, a friction force value associated with said pressure value associated with said period of constant pressure;

means for determining a load change value from a difference between a load measurement at a present time and a load measurement at a beginning of said period of constant pressure;

means for producing a load borne value as a function of said fluid force value, said friction force value and said load change value, said load borne value representing a load borne by the landing gear apparatus;
and means for producing a load borne signal representing said load borne value.
18. The apparatus of claim 17 wherein said determining means for determining said periods of constant pressure comprises a first buffer and means for obtaining pressure measurement values at successive points in time and replacing the contents of said first buffer with a later acquired pressure value when said later acquired pressure value differs from a previously acquired pressure value stored in the first buffer, by a pre-defined amount, such that said later acquired pressure value represents a pressure associated with a current period of constant pressure.
19. The apparatus of claim 18:

wherein said means for determining said fluid force value comprises means for determining said fluid force value in response to a pressure associated with a current period of constant pressure;

wherein said means for determining said friction force value comprises means for determining said friction force value in response to said pressure value associated with said current period of constant pressure; and wherein said means for determining said load change value comprises means for determining said load change value at a beginning of said current period of constant pressure and at a current time.
20. The apparatus of claim 19 wherein said means for determining said friction force value comprises at least one of:

means for accessing a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded; and means for accessing an unloading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being unloaded.
21. The apparatus of claim 20 wherein said means for producing said load borne value comprises at least one of:

means for adding said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and mean for adding said fluid force value and said load change value and for subtracting said friction force value when said aircraft is being unloaded.
22. The apparatus of claim 18 further comprising a second buffer and means for copying the contents of the first buffer to said second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
23. The apparatus of claim 22:

wherein said determining means for determining said fluid force value comprises means for determining said fluid force value in response to the pressure value associated with said previous period of constant pressure;

wherein said determining means for determining said friction force value comprises determining said friction force value in response to said pressure value associated with said previous period of constant pressure; and wherein said determining means for determining said load change value comprises means for determining said load change value at an end of said previous period of constant pressure and at a current time.
24. The apparatus of claim 23 wherein said determining means for determining said friction force value comprises at least one of:

means for accessing a loading breakout friction lookup table associating pressure with corresponding loading breakout friction values while loading the aircraft; and means for accessing an unloading breakout friction lookup table associating pressure with corresponding unloading breakout friction values while unloading the aircraft.
25. The apparatus of claim 24 wherein said means for producing said load borne value comprises at least one of:

means for adding said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and means for adding said fluid force value and said load change value from said fluid force value and for subtracting said friction force value when said aircraft is being unloaded.
26. The apparatus of any one of claims 17 to 25 further comprising means for receiving an attitude signal representing an attitude measurement of an angular attitude of the aircraft and means for associating said attitude measurements with respective said points in time, and means for resolving said fluid force and said friction force into vertical components prior to adding said load change value to produce said load borne value.
27. The apparatus of claim 26, wherein said means for receiving said attitude signal comprises means for receiving said attitude signal from an inertial navigation system on the aircraft.
28. An apparatus for producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, the aircraft weight component representing a portion of the weight of the aircraft supported by and associated with the landing gear apparatus, the apparatus comprising the apparatus of any one of claims 17 to 27 to produce said load borne value and further comprising means for producing a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in said shock strut and means for producing a landing gear weight component signal representing said landing gear weight component value.
29. An apparatus for calculating a weight of an aircraft, employing the apparatus of claim 28 to produce a landing gear weight component value for each landing gear apparatus supporting the aircraft and further comprising means for producing an aircraft weight value representing the sum of the values represented by said landing gear weight component signals, and means for producing an aircraft weight signal representing said aircraft weight value for receipt by a display.
30. An apparatus for producing a signal representing a center of gravity of an aircraft, the apparatus comprising the apparatus of claim 29 and further comprising:

means for calculating a moment value relative to a common datum, for each landing gear apparatus;

means for calculating a net moment value relative to said common datum, from the moments calculated for each landing gear apparatus;
means for calculating a distance value representing a distance from said common datum to said center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into said net moment value;

means for producing a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to said distance value; and means for producing a center of gravity position signal representing said center of gravity position value, for use by a display.
31. The apparatus of claim 30 further comprising a tip alarm and means for causing said tip alarm to be actuated in response to said center of gravity position value, when said center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
32. An apparatus for determining a load borne by a landing gear apparatus of an aircraft, the apparatus comprising:

an input for receiving pressure signals representing pressure measurements of fluid in a shock strut of the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;
an input for receiving load signals representing load measurements of load on the landing gear apparatus at successive points in time while the aircraft is being loaded or unloaded;

an output for producing a load borne signal;

a processor in communication with said inputs and said output; and a memory encoded with codes for directing the processor to:

determine from said pressure signals periods of constant pressure and pressure values associated with respective said periods of constant pressure;

determine a fluid force value representing fluid force in said strut, said fluid force being the product of the area of a piston in said strut upon which said fluid pressure acts, and a pressure value associated with a period of constant pressure;

determine a friction force value associated with said pressure value associated with said period of constant pressure;
determine a load change value, wherein the load change value represents a difference between a load measurement at a present time and a load measurement at a beginning of said period of constant pressure;

produce a load borne value as a function of said fluid force value, said friction force value and said load change value, said load borne value representing a load borne by the landing gear apparatus; and control the output to produce said load borne signal in response to said load borne value.
33. The apparatus of claim 32 further including a first buffer in communication with the processor and wherein said codes include codes for directing the processor to obtain pressure measurement values at successive points in time and replace the contents of said first buffer with a later acquired pressure value when said later acquired pressure value differs from a previously acquired pressure value stored in the first buffer, by a pre-defined amount, such that said later acquired pressure value represents a pressure associated with a current period of constant pressure.
34. The apparatus of claim 33:

wherein said codes include codes for directing the processor to determine said fluid force value in response to a pressure associated with a current period of constant pressure;

wherein said codes include codes for directing the processor to determine said friction force value in response to said pressure value associated with said current period of constant pressure; and wherein said codes include codes for directing said processor to determine said load change value at a beginning of said current period of constant pressure and at a current time.
35. The apparatus of claim 34 wherein said codes for directing the processor to determine said friction force value comprise at least one of:

codes for directing the processor to access a loading sliding friction lookup table associating pressure with corresponding sliding friction values when the aircraft is being loaded; and codes for directing the processor to access an unloading friction lookup table associating pressure with corresponding unloading sliding friction values when the aircraft is being unloaded.
36. The apparatus of claim 35 wherein said codes for directing the processor to produce said load borne value comprise at least one of:

codes for directing the processor to add said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and codes for directing the processor to add said fluid force value and said load change value and to subtract said friction force value when said aircraft is being unloaded.
37. The apparatus of claim 33 further comprising a second buffer and wherein said codes include codes for directing the processor to copy the contents of the first buffer to said second buffer prior to replacing the contents of the first buffer, such that the second buffer holds a pressure value associated with a previous period of constant pressure.
38. The apparatus of claim 37:

wherein said codes for directing the processor to determine said fluid force value comprises codes for directing the processor to determine said fluid force value in response to the pressure value associated with said previous period of constant pressure;

wherein said codes for directing the processor to determine said friction force value comprises codes for directing the processor to determine said friction force value in response to said pressure value associated with said previous period of constant pressure; and wherein said codes for directing the processor to determine said load change value comprises codes for directing the processor to determine said load change value at an end of said previous period of constant pressure and at a current time.
39. The apparatus of claim 38 wherein said codes for directing the processor to determine said friction force value comprise at least one of:

codes for directing the processor to access a loading breakout friction lookup table associating pressure with corresponding loading breakout friction values while loading the aircraft; and codes for directing the processor to access an unloading breakout friction lookup table associating pressure with corresponding unloading breakout friction values while unloading the aircraft.
40. The apparatus of claim 39 wherein said codes for directing the processor to produce said load borne value comprise at least one of:

codes for directing the processor to add said fluid force value, said friction force value and said load change value, when said aircraft is being loaded; and codes for directing the processor to add said fluid force value and said load change value and to subtract said friction force value when said aircraft is being unloaded.
41. The apparatus of any one of claims 32 to 40 further comprising codes for directing the processor to receive an attitude signal representing an attitude measurement of an angular attitude of the aircraft and codes for directing the processor to associate said attitude measurements with respective said points in time, and codes for directing the processor to resolve said fluid force and said friction force into vertical components prior to adding said load change value to produce said load borne value.
42. The apparatus of claim 41, wherein said codes for directing the processor to receive said attitude signal comprises codes for directing the processor to receive said attitude signal from an inertial navigation system on the aircraft.
43. An apparatus for producing a landing gear weight component signal representing an aircraft weight component associated with a landing gear apparatus of an aircraft, the aircraft weight component representing a portion of the weight of the aircraft supported by and associated with the landing gear apparatus, the apparatus comprising the apparatus of any one of claims 32 to 42 to produce said load borne value and further comprising codes for directing the processor to produce a landing gear weight component value from a sum of the load borne value, and an unsprung weight value representing the weight of components of the landing gear apparatus that are not supported by fluid pressure in said shock strut and codes for directing the processor to produce a landing gear weight component signal representing said landing gear weight component value.
44. An apparatus for calculating a weight of an aircraft, employing the apparatus of claim 43 to produce a landing gear weight component value for each landing gear apparatus supporting the aircraft and further comprising codes for directing the processor to produce an aircraft weight value representing the sum of the values represented by said landing gear weight component signals, and codes for directing the processor to cause the output to produce an aircraft weight signal representing said aircraft weight value for receipt by a display.
45. An apparatus for producing a signal representing a center of gravity of an aircraft, the apparatus comprising the apparatus of claim 44 and further comprising:

codes for directing the processor to calculate a moment value relative to a common datum, for each landing gear apparatus;

codes for directing the processor to calculate a net moment value relative to said common datum, from the moments calculated for each landing gear apparatus;

codes for directing the processor to calculate a distance value representing a distance from said common datum to said center of gravity by dividing the aircraft weight value represented by the aircraft weight signal into said net moment value;

codes for directing the processor to calculate a center of gravity position value representing the position of the center of gravity of the aircraft relative to the common datum, in response to said distance value; and codes for directing the processor to cause the output to produce a center of gravity position signal representing said center of gravity position value, for use by a display.
46. The apparatus of claim 45 further comprising a tip alarm and wherein said codes include codes for directing the processor to actuate said tip alarm when said center of gravity position value represents that the center of gravity of the aircraft meets a criterion.
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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012123757A3 (en) * 2011-03-17 2012-11-15 Messier-Dowty Limited METHOD AND SYSTEM FOR DETERMINING FRICTION COEFFICIENT μ FOR AN AIRCRAFT LANDING EVENT
CN107719695A (en) * 2017-09-08 2018-02-23 中国飞行试验研究院 The Flight Test Method on large-scale amphibious aircraft water surface pitching examination border
US9914532B2 (en) 2015-03-19 2018-03-13 Safran Landing Systems Uk Ltd Shock absorber
CN111323168A (en) * 2020-04-16 2020-06-23 一汽红塔云南汽车制造有限公司 Method for measuring and calculating height of mass center of automobile
CN115183983A (en) * 2022-09-13 2022-10-14 中国航空工业集团公司沈阳空气动力研究所 Control surface hinge moment balance verification loading device
EP4245667A1 (en) 2022-03-16 2023-09-20 Fundación Tekniker Method, system and computer program product for obtaining the position of the centre of gravity of an aircraft
EP4253930A1 (en) 2022-03-28 2023-10-04 Airbus Operations Limited Determining a load condition of an aircraft

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012123757A3 (en) * 2011-03-17 2012-11-15 Messier-Dowty Limited METHOD AND SYSTEM FOR DETERMINING FRICTION COEFFICIENT μ FOR AN AIRCRAFT LANDING EVENT
US9798838B2 (en) 2011-03-17 2017-10-24 Safran Landing Systems Uk Ltd Method and system for determining friction coefficient μ for an aircraft landing event
US9914532B2 (en) 2015-03-19 2018-03-13 Safran Landing Systems Uk Ltd Shock absorber
CN107719695A (en) * 2017-09-08 2018-02-23 中国飞行试验研究院 The Flight Test Method on large-scale amphibious aircraft water surface pitching examination border
CN107719695B (en) * 2017-09-08 2023-10-20 中国飞行试验研究院 Test flight method for water surface pitching test boundary of large amphibious aircraft
CN111323168A (en) * 2020-04-16 2020-06-23 一汽红塔云南汽车制造有限公司 Method for measuring and calculating height of mass center of automobile
EP4245667A1 (en) 2022-03-16 2023-09-20 Fundación Tekniker Method, system and computer program product for obtaining the position of the centre of gravity of an aircraft
EP4253930A1 (en) 2022-03-28 2023-10-04 Airbus Operations Limited Determining a load condition of an aircraft
CN115183983A (en) * 2022-09-13 2022-10-14 中国航空工业集团公司沈阳空气动力研究所 Control surface hinge moment balance verification loading device
CN115183983B (en) * 2022-09-13 2022-11-15 中国航空工业集团公司沈阳空气动力研究所 Control surface hinge moment balance verification loading device

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