CA1307573C - Optimal flight guidance for aircraft in windshear - Google Patents

Abstract

ABSTRACT

An aircraft guidance system for optimizing the flight path of an aircraft in the presence of a windshear maximizes the time the aircraft remains in the air and the distance traveled regardless of the magnitude of the windshear, in the presence of horizontal or vertical windshear components, while effectively minimizing excitation of the aircraft's phugoid mode. A flight path angle is commanded sufficient to clear any obstacle that may be found in the airport vicinity. For longitudinal or horizontal shears, a slightly positive constant flight path angle which is a function of the magnitude of the vertical wind is added to the slightly positive flight path angle command to produce a modified command that compensates for the decrease in flight path angle relative to the ground caused by the vertical wind. The system inhibits exceeding stick shaker angle of attack by reducing the command signal until the actual angle of attack is equal to or less than the stick shaker angle of attack.

Description

~3~7~73 ' .
7251~-9 OPTIMA1 FLIGHT GUIDANCE FOR AIRCRAFT IN ~INDSH~R
BACK~ROUND OF THE INVENTION
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l. FI~LD OF THE INVENTION
The invention relates ~o aircraft control systems and more particularly to a system for exiting a windshear condition in a manner that maximizes the distance travelled and the time the aircraft remains in the air.

2. D~SCRIPTION OF THE PRIOR ART
Windshear, encountered on takeoff or landing, can pose a serious threat to the safety of aircra~t and has been attributed to several aircra~t accidents, including the recent crash o~ an L-lOll aircraft at Dallas, Texas on August 2, 1985.
Windshear has been de~ined as a rapidly changing horizontal wind or a steady vertical wind whose effect on the alrcra~t is to cause large speed or altitude deviations from normal ~light.
Either as a direct result of loss o~ airspeed and altitude in-duced by the windshear or as a result of maneuver~ by the human pil~t to restore the aircra~t to its normal fligh~ path, wind~
shear can cause the aircraft to ~tall or crash.
Prior art systems have lncluded means ~or detecting and measuriny the magnitude o~ the windshear and for providing guidance to the human pilot or an autopilot which would cause the aircra~t to fly at some fixed speed, usually slightly greater than stall speed. The speed commanded was usually a speed known as stick shaker speed, which is approximately 5%
greater than stall speed, and is the speed where artificial means are - ~L3075~3 coupled to the control colunn or stick to cause a vibration and warn the hunan pilot of i!npendin~ stall. stick shaker spee~3 llas ~enerally been considerc~ to ~e ~he minimuln speed ~or saEe ~ JIit. correspondin~J to stick shaker speed is a stick sllalcer angle oE attack, which is ~enerally considered to be the maximum allowable an~le for attack for safe 1ight of the aircraft.

Since many c~nercial transpork aircraEt~ general aviation 1~ ,, aircraft and military aircraft are equippe~ with a flight director system whereby pitch command signals nay be displayed to the human pilot, the guidance com~and for a win~shear encounter is usually presented as a displ~c~nent of the pitch ca~mand bar. When the h~nan pilot maneuvers the aircraft in such a manner as to r~duce the displac~nent to a null value, he has assured that the aircraEt is at the required pitch angle to satisfy the guidance comnal~. In addition, many aircraft are also equip~ed with an automatic pilot system which can be coupled to manipulate the elevator control sur~ace of an aircraft in order to respond to a predetermined guidance control law, such as one which mlght be used to co~nand the aircraft to the optimum fli~ht path in the event of a windshear encounter.
~ shortc~ning of the prior art is that the canmande~ Eixed speed or ~ngle oE attack may result in causin~ the aircraft to fly at the minitnum sa~e spe~d when the ma~nitu-~e and the ~uration oE the windshear do not in ~act require such a maneuverIn ad~ition, ~ comnand to fly at the maximwm angle oE attack can excite the phu~oid mo~e of oscillation, which is a long, .. .. _ ., _ poor~y damped oeciIlation ~of the aircra~t involving change~ o~ ~peed ~ ltitude with a period that may excee~ two minutes for a large airplane.

~xcitation oE the phugoi~ mode can result in loss of control ar~ a crash of :

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7251g~9 the aircraft eVen a~ter the windshear condition has abated.
Consequently, prior art systems could in fact create dangerous situations wherein the aircraft would crash even in the presence of relatively low magnitude shear.
Another prior art scheme is discussed in Canadian applicatlon serial number 522,848 filed November 13, ~9~6 now Canadian Patent No. 1,270,545, Flight Guidance for Aircraft Windshear, co invented by one of the present inventors and assigned to the assignee of the present invention. In æaid application, a command was generated to reduce the aircraft's true airspeed at a rate proportional to the magnitude of the encountered windshear, rather than to a fixed airspeed. This control law effectively minimized the fllght path angle change in a shear encounter and provided improved guidance commands, but did not adequately take into account the long kerm phugoid mode oscillatlons of the aircraft.
The preæent invention overcomes the limitations of the prlor art by providing a guidance command that effectlvely minimizes excitation o~ the phugoid mode, while commanding a flight path angle at a minlmum elevation sufPiclent to clear any obstacles, such as tall buildings or hills that may be found around airports and compensate for downbursts. The invention maximizes the time the aircraf~ remains in the air and the distance travelled, regardless of the magni~ude of the windshear or whether the winds are horiæontal, vertical, or a combination of the two.
SUUMARY OF THE INVENTION
The present invention provides guidance commands to a human pilot or automatic pilot system for a windshear encounter so as to optimize the resultant flight pakh of the aircraft and maximize time in the air and distance travelled. When operat-ing in the presence of a tail ~3~ 3 . .
72519-g windshear, a fixed flight path angle independent of the magnitude of windshear is commandecl that minimizes excitation of the aircraft's phugoid oscillatory mode while maintaininy adequate clearance of hills and buildings that might be present around the airport. In the presence of a vertical wind component, the effect on the aircraft's flight path angle is computed and added to the fixed flight path angle command. The guidance command is limited in magnitude to preclude commands which would cause the aircraft to exceed the maximum allowable ; 10 angle of attack.
An angle of attack sensor provides a signal which is combined with a sensed pitch angle to der.tve a signal corresponding to the actual flight path angle. A command signal corresponding to a fixed flight path angle at a predetermined elevation is combined with a signal corresponding to the effective change in flight path anyles due to a vertical windshear component and with the actual f].ight path angle to derive a pitch command signal. The derived pitch command - signal may be applied to control the elevator of the aircraf~
by means of the autopilot system or to a flight director instrument for contxol by the human pllot.
In accordance with the present invention there is provided apparatus for controlling the vertical flight path of an aircraft encountering a windshear condition, comprising:
means for pxoviding a signal representative oi an actual flight path angle, means for providing a siynal representative of a predetermined flight path angle, means fox providing a signal representative of a change in vertical ~light path anyle due to a windshear rate component, means ior combining said predetermined flight path angle æignal and said change in fllght path angle signal to provide an algebraic sum thereof, ~3~S7~
..., 7251~9 and means for subtracting said actual fligh-t path angle signal from said combined si~nals to derive a signal representative of an error command signal for correcting said vertical flight path.
BRI~F DESCRIPTION OF THE DRAWINGS
Figure 1 shows computed values of aircraft altitude vs. time for a horizontal windshear condition with angle of attack as a parameter.
Figure 2 is a graph illustrative of prior art schemes and the present invention with respect to flight path as a function of angle of attack for a windshear of fixed maynitude and durakion.
Figure 3 is a graph illustrative of the ~light path commanded by the present invention ~ompared to prior art schemes in the 4a pre.sence of a verticaL wirr~lshear.
Figu.re 4 is a sch~natic block di~ra~ oE the present inventioll .
Dli'.SCRlP'r~(:)N OF TIIE PRI~.FERI~I;'I) FMF3()1)11`11;`N'r ~
~eEore discussing the preEerred ~mbodiments oE thc apparatus of the present inventiorl, a discussion o~ the variou.s mathcnatical r21ationsllips and .E~igllt patll str.~tcYIics will ~ rovidr.~l in ordtr to la facilitate an understandin~3 of the preEerrecJ embodilnents.
e most dan~Jero~ls ty~ o.E wincl!;hcar arc tllr.? t~ii wil~sl~ear alll the clownburst or micro~urst. q`he Eonner is a wilY~I tllat vari~s with time ancl l)low~s in the direc~ion oE aircrat ~otion; the latter ls a constant wind that hlows toward the ~3roul~1. Tlle tail win~shear tends to reduce tllc aircra~'s true airspeecl and thus e~tracts kinetic energy ~r~n the aircrat relative to the air rnass~ ~s the true airsp~ed oE the aircraEt decreases, a stable aircr~Et will at~npt to r~Jain the lost sr~rd hy 2a exchal~iw3 potential ener~y Eor kinetic ener~.~y. The lo.ss oE potential ener~y .; results in loss oE altitude, an~.l i.E the excllan!Je is not ~ corltrolled by the h~nan pilot or autopil.ot, tlle aircraEt may 105e suEEicient altitude to cause a crash. ~ e inllerr.~nt natural ~ner~.3y excllancJe ~lay lY~ red~:ed by ~5 pullin~ u~ the no3e oE the aircraEt, thereby lncreasit~ the an~31e oE attack alld liEt, usinrJ tl~ elevator control sur.Eaces. Ilo~ver, ;ncreas~ dra~
will result in more s~eed loss ar,YJ ie unclleclc~ can cause the aircraEt to stall and cra~ angl.e oE attack ~t ~licl) the aircraEt wi~l stall is a known value that is a function oE tile aircraEt's ~lap position. ~s a result, tlle pilot is constrained in ~is control caL~ability to a ma~imun an~Jle oE
attack which is necessarily less than stall angle.

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The phugoid oscillatory mode oE the aircra~t is characterized by flight at an essentially constant an~le of attack ~urin~
tl-e phu~oid oscillations, the exchan~Je oE Icinetic and potential ener~y Oe the aircraEt results in air~peed ~ains and lo3ses accanpanied by altitude losses and gains. Ie the oscillations are of sufficient magnitude, the aircraft may crash durin;3 a cycle of oscillation. Characteristically, the ......
phugoid oscillation can be excited when the pilot att~npts to maintain stick shaker angle of attaclc durin3 a tail win~shear encounter. Figure 1 illustrates tl~e Elight paths of various guidance schemes in a purely tail windshear oE Eixed magnitude, 5 knots per second and inEinite duration.
111is mc~3nitude and duration of shear is i~possible for the aircraEt to safely exit, but does provide a baseline for c~nparing the eEficiencies of various strate3ies. Llne segment l corresponds to the flight path of the aircraft before the win~shear encounter an~ is comnon to all the yuidance strategies. ~t point ~, ~he tail windsl-ear c~n~nences. Line s~3ment 2 2a illustrates the fliyht path of a strategy that co~mands attai~nent an~
maintenance of sticlc shaker an~le oE attack imm~diately at the onset oE
shear. It may be seen that initially that the aircraEt gains a larqe arnount of altitude, but tl1is is Eollow~2d imm~iately by canmencement oE the phu~oid ~5 oscillation which results in the aircraEt ~escen~in~ ~owards the ground and crasl1in;~ at about ela~s~ time oE 26 seconc1s. t~hen phu~oid o~illation has been initiated, the hunan pilot or autanatic pilot is virtually helpless.

Th~ angle oE attaclc cannot ~e incr~ase~ to develop more lift since the 3a aircra~t will stall; conversely pushin~ the aircraft nose down will s~nply result in an increas~1 rate oE descent. Further, it i9 clear that increasing the an~le of attack to sticlc shaker should only be done when absolutely n~cessary to preclude striking ~he gro~md.

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1 Er~n the above, it may be seen that an optimal guidance law ~or use by the hutnan pilot or an aut~natic pilo~ syst~n must provide the best possible utilization oE the aircraEt's available energy to rnaintain flight ~or as long as possible while also minimizing excitation of the phugoid mode oE oscillation.
L.ine segment 3 of Figure 1 illustrates the flight path of a strate3y which att~dnpts to minimize the flicJht path angle change during a 1~ windshear encounter. This strate3y is similar in nature to said application 5~"2 ~
serial number d~,729. The strategy ter~s to prolong the time beore it is necessary to reach stick shaker anc31e of attack and thus delays excitation of the phu~oid ~ode and consequently len~thens the ti~e before i~pact with the ground. It may be seen fr~n ~igure 1 that this strategy provicles a time to impact of 29 seconds, w~ich is clearly su~erior to the previous strate~y of stick shaker angle of attack.
It is well known that a method o~ mini~iziny excitation of the 2~
pl~ugoid mode is either to minimize the altitude cha~3e or speed chanc3e resulti~-3 fran ~he oscillatioll. As minimizing the speed chan~e can result in negative flight path angles relative to the ground, in practice the minimization can only be done by minimizin~.3 the altitude change. Ideally, ~ this could be done by flyin.~3 the aircraEt at zero flight path angle. ~n practice, howeve~, when a win~lshear is encountered at very low altitudes, such as in takeo~ or lan~in3, flyin~ zero flight path angle (i.e., flying 3~ at constant altitude ) could result in the aircraEt striking tall buildings or hills near the airport. ~lence it is desirable to have a slight positive fli~ht path an~le to avoid obstacles on the ground. Federal Aviation Adninistration re3ulations state that the minimum allowable Elight path .

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an~le for multi-en~3ine passen~er-carrying jet aircraEt is 1.55 degrees. ~Ihis represents a positive fliyht path angle Eor obstacle clearance which also will maxi~ize phu~oid damping. Ilence, the present invention ca~mands a flight path angle of 1.55 degrees in the event oE a horizontal windshear encounter and assures obstacle clearance capability ~hile preventing excitation of the phuyoid mode by delayin~ attairment oE stick anyle of la attack as long as possible. ~
Line segment 4 illustrates the flight path prod~ced by the present invention where a constant flight path angle of 1.55 degrees is comnanded upon an encounter with a purely longit~dinal or horizontal windshear. This strategy provides the greatest time to impact of the three strategies previously discussed, resultiny in 34 seconds to impact. Further, it ~ay be noted that the rate at which the ground is struck, represented by the slope of the flight path anyle lines just prior to ~ro~nd impact, is the smallest for constant ~ , thus giving the aircraEt and occu2ants a better chance of surviving the crash. ~nle present invention accomplishes this by c~nandin~ a slightly positiVe path angle and thereby maximizin~ the time beEore the aircraEt must achieve sticls shaker an~le of attack to r~nain aloft.
Referrin~ now to Fiyure 2, the encountered win~shear is at a fixed mc~3nitude of 5 knots per second and fixed duration of 25 seconds. Line sec~nent 10 represents the flight path of the aircraEt before the windshear encowlter at point ~ he win~shear ~ ins at point ~ and continues ~or 25 ~
seconds thereafter. Line segment 11 represent.s the flight path for a strategy of attaining and maint~ininy stick shaker angle of attack. Line segnent 12 represents the strateyy of minimizing the flight path angle loss. Line seyment 13 represcnts the fli~ht path produced by the present - . ~3~S~

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1 invention ~ere it is attempted to maintain a positive flight path angle with respect to the ground of 1.55 degrees.~ It ~ay be seen that-the present invention produces the only flight path oE those consider~3 capable of exitir~ the wirdshear without a crash.
The effect o~ a downw~rd flowing vertical wind must also be considered. The eEfect is to reduce the aircraft's flight path angle relative to the ground. A ne3ative Elight path ar~le will result in descent Oe the aircraft and, if uncorrected, contact with the ground. The flight path angle due to a purely vertical wind may be calculated accordin~ to the following well known approximate equation:

9L ~ L =lh~L ~

G = flight path angle relative to the ground in radians h AIR = the aircraEt's rate of clirnb relative to the airmass in feet feet per secon3 h WIND = the vertical velocity oE the wind in feet per second, do~lward bein~ n~ative v = the aircraft' 5 sp~ed in feet per second Fron the above equation it may be seen that the yreater the maynitude of the vertical wind, the greater the inEluence on the aircraft's flight path anqle relative to the grounc3. ~lence, a further function of an optimal aut~natic guidance law would be to account for the 3~ reduction in fli~ht path angle relative to the ground due to a vertical windshear. A strategy which attempts to correct for the change in flight path angle requires increasir~ the liEt o~ tne aircraEt by increasing the angle of attack through the aircraft's elevator control. If the vertic~l _9_ , ~ . .

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wind is of suEEicient ma~nItude and duration, the an~le of attack wiLl be 1 continually increas~d until the maximum allowable limit, stick shaker an~le of attack, is obtained. As discussed previously, this may result in phugoid mode osclllatiol~s. Conse~uently, it is desirable as it was in the case of purely horizontal or longitudinal windshears to maximize the time before stick shaker angle of attack is reache~. In the present invention this is accomplished by camputin~ the net change in Elight path angle due to the vertical wind and alterin~ the constant fli~ht path anqle command oE 1.55 1~ deyree accordingly.
~s an exa;nple, assu~e an aircraEt ~lying at 150 knots and cli~bin~ at l~ feet per second in a downward vertical wind of 25 feet per second. The flight path an~le relative to the air m~ss may be ccnputed by the followin~ well known appro~imate equation:
.~" , . o A = h ATR (2) ... v Where: ~ A = the aircraEt's flight path angle relative to the air 23 mass in radians.

= the aircraEt's vertical velocity relative to the air mass in ~eet per second.

v - the aircraEt's tru~a airspe~d in Eeet per second.

In the exa~ple given, a spe~ of 150 knots is equivalent to 253.35 eeet p~r second. T~ereforc, ~ A = 0.03~ radians or 2.26 de~rees.
From equation (l), the flight path angle relative to the ground is equal to - ~.059 radians or -3.39 degrees. From an initial altitude above the ground oE 100 Eeet, the aircraEt would strike the ground in approximately 6.6 seconds at a flight path an~le of -3.39 degrees.

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1 It may be seen by takinc3 the diE~e~rence o~ equations (1) and ~2) that the net chan~e in flic3ht ~ath angle between air mass and ground references is ~iven by:
S ~ n (3) ' v Where: ~ ~ = the difference between air mass and groun~ flight angles in radians.
I; WIN~ = the ma3nitude of the vertical wind in ~eet per second, .~
v = the velocity of the aircraft in feet per secol~.
Fr~n equation (3), the computed difference Eor the example above would be -0.0987 radians or -5.65 degrees.
~5 Thus, consideration of the longitu~inal or horizontal windshear and the,vertical windshear conponents requires adding 5.65 degrees to the constant value of 1.55 de~rees for a new c~manded flight path angle 2~ of 7.21 degrees. Flyin~ this modified flic3llt path'angle relative to the air mass would then assure flyiny 1.55 degrees relative to the ground while also providi~-3 the maximun time available beEore the aircra~t achievcs stick ' shaker angle of attack and therefore minimize the excitation oE the phugoid moc3e osci11ations.
~ Referrin~3 now to Figure 3. flight paths of an aircraft usinc~ various ~light patll strategies are canpar~l when f'~yin~3 throu~3h a vertical windshear o~ 50 feet per second and 25 seconds duration. Line 3~ sec~nent 14 represents the fligl~t path of the aircraft before encountering the vertical wind. The windshear condition begins at point A and persists - for a ~uration of 25 secol~1s thereafter. Line s~nent 15 represents the ~light path of the ,; 3S

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, ~3~Ys~3 1 aircraft ~ile rnaintaining .stick shaker angle o~ at~ack. Line s~ment 16 ; represents the flight path while minimizin~ flight path angle loss. Line s~3ment 17 represents the flight path prod~ ed by Elyinc) a constant flight path of 1.55 degrees relative to the air. Line segnent 18 is a representation of the flight path produced by the present invention, wherein the comnanded flight path o~ 1.55 degrees is au~ented by the change in fli~ht p~th ~n~l~ ca~s~3 by the vertical wind. Fi~ure 3 cl~arly shoh.r th~
l~ superiority oE the present invention over the other con~idered flight"path strategies.
~igure 4 is a block diagraln o~ the present invention ~hich provides an opti~un guidance signal for exiti~3 a win3shear. ~or clarity and understanding of -the present invention it will be explained by using a generally analo~ format/ it bein~ understo~3 that the same analo~3 format may also represent'the pro~ra~ming of a programable di~3ital camputer wherein the various analo3 inputs are converte~ to digita'l siynals for di~ital processing and the various digital outputs are converted to analog signals for driving either a flight director in~:licator or aut~matic pilot syste~.
. In operation windshear detection device 20 supplies a signal hw on lead 21 that is proportional to the maynitude anc3 ha~ the - same siyn as the time rate of chan~e of vertical wind measured in units of the ~3ravitational constant or g's. ~he windshear detection device may be that as described in U.S. patent 4,593,285 filed May 6, 1983 and issued June 3~ 3, 1986 to the present assi~3nee. The detector disclos~3 therein is capable of supplying a signal proportional to the rate of change of vertical wind and a logic signal indicating when a pre~etermin~3 threshold denotin~3 a serious or minor windshear condition has been detected~

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Lead 21 supplies the signal hW to conventional integrator 22 whose function is to provide an output hW on lead 23 that is proportional to the time integral of the input siynal appearing on lead 21. qhe signal h~
represents the vertical velocity of the win~ in g-seconds. Win~shear detection device 2~ provides a logic signal via lead 33, junction 34 and inverter 35 which is a lo~ical 1 on lead 71 whenever a windshear has not been detected and a logical 0 ~henever a windshear has been detected. A
logical 0 on lead 71 enables the integrator 22. A lo~ical 1 applied to integrator 22 results in a 0 output, that is the integrator is reset.
Consequently, the signal appearinc~J on lead 23 is proportional to the magnitude o~ the vertical wind and has units of g-seconds whenever a windshear exce~din~ the predetennined threshold of detection occurs;
otherwise the signal appearing at lead 23 will be at null.
Lead 23 is coupled to conventional gain el~nent 24. Gain 2~ 24 multiplies the signal on lead 23 by a value of 19.05. Multiplication by 19.05 converts the units of the sk~nal on lead 23 from g secon~s into knots.
The output of gain 24 appears on lead 25 and is supplied to limiter 26.
Limiter 26 is used to preclude driEt and dc o~fset o~
_ integrator 22 Erom presenting a runaway condition in the presence of a windshear. Limiter 26 acts to limit the maynitude of the signal appearing on lead 25 betw~en predeter~ined values, as for example, +100 knots and -100 knots. Thus, a signal exceeding 100 knots is limited to a value oE l00 knots 3~
and a signal less than -100 knots is set equal to -10~ knots. The resultant limited signal is outputted on lead 27. Values of the signal on lead 25 falling between the predetennined limit values appear unchanged on lead 27.
Lead 27 supplies a conventional divider 28 with a value to ~l3~

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1 be used as the nu~erator tN) oE the quotient. ~rhe den~ninator ~D) i5 supplied by lead 39 and is a si~nal prop~rtional to the true airspeed of the aircraft at the time that t~ wi~3shear is initially encountere~.
The signal VT on lead 39 is derived as follo~s.
Conventional air data canputer 6B provides a si-;nal VT proportional to the true airspeed of the aircraft, measured in knots, to conventional latch 38 via lead 37. Latch 3~ operates in such a fashion as to store the value present on lead 37 when a logical high signal, denotin~ the presence of a windshear, appears on lead 36. ~he signal on lead 36 is supplied by windshear detection device 20 via lead 33 and junction 34. Thus~ whenever a win~shear is detect~d, having either horizon~al or vertical components, latch 38 stores the value of true air speed at the time and supplies the value as a continuous output on lead 39 to divider 28. IE the signal on lead 36 is a logical null, indicating the absence of windshear, the instantaneous airspeed signal appears on lead 39 unaltered. Lead 35 also provides a corresponding logic signal on lead 7~ to varlable gain 55, whose function will be described.
The purpose oE storing and utilizing the true airspeed existing at the time of the initial win~shear encounter is to p~ovide a - constant reference for divider 2B. ~llus, in the event oE both a tail windshear and a vertical wind, the continuous reduction in air speed due to the tail windshear could result in a flight path angle correction greater 3~ than required and could drive the aircraEt to stick shaker angle of attack sooner than necessary to obtain the desired flight path correction. By latching and utilizing the air speed at the time o~ wi~lshear detection, this anomaly is overcane and the optimum flight path angle increment is mputed.

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1 rlllle ou~put oL dlvidfr 28 appear.s on lead 29 and i.9 a signal hW/VT ~ ~
representin;~ the quotifnt oE thc ma-3nitude of the vertical wind ~ivid~d by the latched air speed. ThiS signal is the tenn ~ of equation (3) in units : of radians. T~e si~nal on lea~ 29 is applied to conventional gain 30 which multiplies the signal on lead 29 by a value of 57.3 to convert radians to df~grees. lhus the ou~put o~ gain 30, app~aring on lead 31, represents the 1~ increnental flight path angle due to a vertical win!lshear measured in degrees.
A signal having a predetennined value correspondi~ to 1.55 degrees, but which may he any other suitable constant angular value, is impresse~ on lead 40 and applied to convention~l summir~s junction 32. ~l`he - increnental ~light path angle signal on lead 31 is also applie~ to summing .. . .
j~ction 32 where the signals are algebraically added to provide an output signal 41. Tl~e signal on lead 4l therefore represents the desir~d fli~ht 2e path angle of 1.55 degrees for horizontal or longitudinal shears au~mented by the incr~nen~al ~ t pa~h anf~sle canput~d fran ts~e ma~nitude Oe the vertical wind.
Since it is desired to cf~mnand a cllarlf~e in ~he ac~ual ~ ~light path an~le to correct Eor the windshear, the actuaL ~ligllt path an~le m~lst Eir-;t i~e detemlin~l. Conventiollal vertical gyroscop~ 69 ~uenishes a signal proportional to the pitch attitude oE the aircraEt, measured in 3~J deyrees, on lead 42. A si~nal proportional to the actual arr~le of attack oE
the aircraft, measured in degrees, is supplied by sensor 7~ on lead 45 to - junction 99, lead 48, and lead 50. Tl~e si~nals on lead 42 and 48 are applied to summing junction 43 which operates in such a manner as to pro-~uce th~

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1 al~ebraic difEerence thereof representative of flig ~ path angle, on lead 44. Lead 4~ supplies the actual flight angle of the aircraft to swnmin~
junction ~5. ~s previously describ~, sumniny junction 32 provides a 6ignaL
on lead 41 representing the desired or co~nanded Elight path an~le for the aircraft in a wir~shear encounter ~o junction 45. ~unction 45 provides an output on lead 54 that is the algebraic difference between the desired fliyhk path angle and the actual flight path angle and is thus a s}gnal 1~1 representative o an error co~mand for correcting the vertical flight path due to windshear.
Su~nation device 51 is supplied with a signal representative o the actual angle of attack oE the aircraEt, measured in degrees, via lead 46, junction 49 and lead 50. Flap position sensor 71 supplies a siynal proportional to an3ular position of the aircraEt ~laps on lead 72 to stick shaker angle oE attack canputer 73. CGmputer 73 provides pr~determined values of stick shaker anqles of attack corresponding to each flap position, l~his signal appears on lead 52 whicll is applied to swmming junction 51. Junction 51 produces an algebraic diEference between the stick shaker ar~le o~ at~.ack on lead 52 and khe actual angle o~ aktack on ]ead 5~.
Tl~e differenca, ~ C~, is applied on leacl 53 to variable gain 55. Thus, the si~nal on lead 53 is a c~np~rison of the actual an~le oE aktack with stick shaker angle o~ attack. ~len the two signals are identical the signal on lead 53 will be null. If the two signals dif~er by 5 de~rees, the signal on 3~ lead 53 will corre~pon~ to 5 degrees.

hhen the value on lead 53 is ne~atlve, indicating the ~ actual angle of attack exceeds tlie stick shaker angle of attack, the output appearin~ on lead 56 will be identical to the signal on lead 53. Since ~his ~ .

1 si~nal has a ne3ative sign it results in an ultimate conmand to decrea~e the angle oE attack back to the stick shaker value r,~hen the di~ference signal on line 53 is le.s.s th~n a ~r~etermine~ val~e, ~or examplc 3 d~Jrees, the output on lead 56 will be the value of the signal on lead 54 multiplie~ by a c~nputed EactorO l~le valu~ of the computed factor is depen~3ent on the value of the signal appearing on lead 53, the difference betw~en stick shaker ; angle oE attack and actual anc~le of attack. When the dircference betw~en the a two sic~nals exceeds the predetennined value, the gain ~actor is a constant, for exanple 1Ø For values less than the predetermined value, the multiplier is a direct function of the magnitude of the le~l signal appearing on lead 53. ~or example if the signal on lead 53 is 2~, the lS multiplier value would be 0.67; if the signal on lead 53 is 1, the value of the multiplier ~ould be ~.33; and i~ the signal on lead 53 is of null value, the multiplier will be zero.
hrough the above dc-.~scribed action, ~ain 55 serves the 2~
function of precludin~ guidance commands that would exceed the stick shaker angle of attack of khe aircraEt. At .~ct~al angles oE atkack less than the sticls shaker angle o~ attack, for exanple 3 or greater, the signal on lead S~, which represents khe co~nanded ~light path direckicn error signal, would ~ be outpu~ on lead 56 unchan~d. As the angle o~ at~ack approaches stick shaker angle of attack, the multiplication factor is reduced and hence the signal on lead 56 would be less than that on lead 54. When the ackual angle 3a of aktack is equal ko the stick shaker angle of attack, the signal on lead 56 is a null resulting in a ~ero command and hence no change in angle of attack ~ould be c~nanded.
T~e signal on lead 56 is ultimately coupled at junction 74 ; -17-~3~ 3 1 to produce an angle of attaclc command to the autopilot system or the flight director system. Lead 56 supplies gain blo,ck 5~ via junction 74 and lead 57.
Gain 58 multiplies the si~nal on lead 57 by an appropriate constant gain KAp that is deten~ined by the characteristics of the autopilot system 62. The output of gain 58 appears on lead 61 and is coupled to a conventional autopilot syst~n and elevator servo 6~. The servo output is coupled to the elevator 75 in a conventional manner. Elevator control surface will either l~ cause an increase or decrease in angle oE attack and pitch ang]e which is sensed by a Eeedback control system, until the si~nal on lead 61 is a null.
When the signal on lead 61 is null~ the guidance control has been satisfied and the aircraft is on the correct flight path.

In a similar fashion a comnand signal is supplied to the flight director syst~n 66. The output of variahle gain 55 appears on lead 56 and is coupled at junction 74 to lead 59. Lead 59 supplies a conventional ~in 6~. Conventional gain 60 multiplies the signal on lead 59 by an appropriate factor I~FD for energizing the flight director system 66, The output oE ~ain 6~ ~pp~ars on lead 65 and is coupled through conventional electronic or mechanical means to a pitch comnand har 67. The hunan pilot observes the position oE the pitch comman~ bar and chan~es the an~le of attack and pitch an~le oE th~ aircra~t in the direction indicated by the c~nmand bar mov~nent ~mtil the signal on lead 65 is null, so that the pitch command bar 67 i3 returned to its null position, whereupon the guidance 3~ control has been satisEied and the aircraEt is on the correct flight path~
It will be a~preciated fr~ the fore~oing discussion that in a windshear encounter a fixed predetermined Elight path angle of 1.55 is commanded in the event of a lon~itudinal or horizontal win~shear. The ~l 3~u~

l co~mandes3 angle is altered as a functi~n of the rnagnitude o a verticalwindshear to prod~ce a resultant I.55 Elight path angle-relative to the ~round. ~n error comnand produced by subtractin~ the actual flight path angle fro~ the commanded flight path angle is modified as a function of the difference between stick shaker angle of attack and actual anr~le of attack.
EXceedin~ stick shai~er angle of attack will result in co,nmandin~ a reduction in angle oE attack. The combination of the above functions serves to produce 1~ a guldance c~mmand that results in an optimal flight path angle ~or windshear encounters.
While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are w~rds of description rather than limitation and that changes may be made within the purview oE Lhe appended claims without departin~ from the true scope and spirit of the invention in its broader aspects.

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Claims (8)

1. Apparatus for controlling the vertical flight path of an aircraft encountering a windshear condition, comprising:
means for providing a signal representative of an actual flight path angle, means for providing a signal representative of a predetermined flight path angle, means for providing a signal representative of a change in vertical flight path angle due to a windshear rate component, means for combining said predetermined flight path angle signal and said change in flight path angle signal to provide an algebraic sun thereof, and means for subtracting said actual flight path angle signal from said combined signals to derive a signal representative of an error command signal for correcting said vertical flight path.

2. The apparatus as set forth in claim 1, further comprising:
means responsive to said error command signal for controlling the pitch of said aircraft in accordance therewith.

3. The apparatus as set forth in claim 2, wherein said signal representative of a vertical windshear component comprises:
means for providing a signal representative of airspeed of the aircraft with respect to an air mass, means for providing a signal corresponding to the vertical wind velocity due to windshear, and means responsive to said vertical wind velocity signal and said airspeed signal for providing a signal corresponding to the quotient thereof.

4. The apparatus as set forth in claim 3, further comprising:
means for detecting the presence of said windshear condition exceeding a predetermined threshold and for providing a signal corresponding thereto, and means responsive to said airspeed signal and said windshear detection signal to provide a predetermined airspeed signal corresponding to the true airspeed upon activation of said detection signal.

5. The apparatus as set forth in claim 4, further comprising:
means for providing a signal representative of stick shaker angle of attack, means for providing a signal representative of actual angle of attack, means for combining said signal representative of stick shaker angle of attack and said signal representative of actual angle of attack to provide a resultant signal corresponding to any difference there -between, and limiter means responsive to said error command signal and to said difference signal for providing a pitch command signal bounded by predetermined limits such that said limited pitch command signal has a zero value when an actual angle of of attack is at least equal to said stick shaker angle of attack; said limited pitch command signal results in a pitch down command when said stick shaker angle of attack is exceeded by an actual angle of attack; and said Ditch command signal results in a pitch up command corresponding to said error command signal when said stick shaker angle of attack exceeds said actual angle of attack, said pitch up command hounded by said predetermined limits when said difference signal exceeds a predetermined value.

6. The apparatus as set forth in claim 5, wherein said means for controlling the pitch of said aircraft comprises:
means for applying said limited command signal to displace the pitch command bar of a flight director instrument proportional to the magnitude and sense of said signal.

7. The apparatus as set forth in claim 5, wherein said means for controlling the pitch of said aircraft comprises:
an automatic flight command system coupled to an elevator of said aircraft and responsive to said limited command signal, thereby to produce movement of said elevator proportional to the magnitude and sense of said signal.

8. The apparatus as set forth in claim 7, wherein said predetermined flight path angle is 1.55°.