WO1995021766A1 - Detector for indicating ice formation on the wing of an aircraft - Google Patents

Abstract

A detector indicating ice formation on the wing of an aircraft or any planear surface, based on a thread-like or a tape-like transducer, through which a supersonic signal is transmitted at one end. The attenuation of the signal having passed through the thread is measured with a receiver at the opposite end while the thread is simultaneously being heated such that ice that may surround it melts again, the attenuation thus resuming its initial level.

Description

Detector for indicating ice formation on the wing of an aircraft

In modern aircrafts the wing profile is crucial for the flight. If this profile is altered for some reason, the flying characteristics and consequently the stalling characteristics of the wing are significantly deteriorated.

The wing profile hardly changes in normal conditions, except when ice is formed on the wing surface for various reasons. In fact, ice formation on the wings of aircrafts has become a noticeable hazard in air traffic, since it has been noted that the ice layers may grow a thickness of up to one inch, whereby the flying characteristics of the wing are substantially weakened. Ice formation on the wing has proved to be the very reason for a number of recent passenger flight accidents. Ice formation involves a second drawback: in many jet planes the motors and their air intakes are located at the rear end of the fuselage, implying that, as the wing is bent at takeoff, ice is detached and absorbed directly into the air intakes of the motor, causing the turbine wings of the motor to break.

Ice formation may occur in various ways: during the flight weather conditions may be such that ice starts forming on the wings; also weather conditions during taxiing may generate ice on the surface of the wing; however, the most unexpected situation arises, when an aircraft having flown at high altitudes at a low temperature (e.g. -50C) accumulates a thick ice layer on its wings after landing at the airport. This is due to the fact that the fuel has been deeply cooled in its tank during the flight. The design of fuel tanks allow the fuel to get into contact with the upper surface of the wing, the upper surface of the wing being extremely cooled and accumulating ice on the surface, although the air temperature at the airport would be above zero. This ice formation process has been difficult to verify and has caused surprises in air traffic. Aircraft manufacturers have of course taken measures to eliminate the risks caused by ice formation. The most common method is spraying the wings with glycol liquid, which melts snow and ice that may have accumulated on the wing. Another method consists in checking the humidity and the temperature of outside air, enabling to anticipate conditions in which ice formation occurs, and to conduct combustion heat from the motors to the front edge of the wing to melt the ice. A special ice detector is further provided at the front end of the fuselage, an ice formation alarm going off if ice is formed on the surface of the detector. In this situation, precautions can be taken to prevent ice formation. Nevertheless, this detector does not indicate whether ice has accumulated precisely on the surface of the wing.

Finnair in particular has implemented a very simple ice detection in their aircraft: a special kind of strips are fixed on the surface of the wing, the strips fluttering in the air current if the wing surface is bare and no ice is present. Furthermore, Finnair has used a kind of clearly visible flange that can be observed from the ground. If the flange and the scale attached to it cannot be clearly seen, this implies that there is ice on the wing surface.

As to prior art, we note that the most common solution is a detector attached to the front end of the fuselage, the detector consisting of a short, vibrating stick which vibrates continuously at its natural resonance frequency. If ice is formed on the vibrator stick, the ice and its high viscosity will change the vibration frequency and attenuate the vibration amplitude. A slightly different detector based on supersonics, placed at any point on the wing, has been recently launched. This detector consists of a metal disc mounted on the surface of the wing plane, and a supersonic crystal below the disc causes the metal disc to vibrate. If water is present on the disc, the viscosity of water is low enough not to attenuate the vibration. Conversely, if the water on the disc freezes, the viscosity is altered abruptly, and the vibration amplitude decreases noticeably, the decrease being detectable at the supersonic crystal under the disc. This detector has the advantage of being mountable at the point of the fuel tank, whereby it detects the ice formation at the very point where this would be hazardous. The drawback of this detector is difficult mounting, since it must be mounted inside the fuel tank, which is a dangerous and expensive solution because of explosion risks, among others. In addition, this detector detects ice formation only at a given, spot-like point, which is not sufficient for total security.

FI patent specification 61249 describes a supersonic detector enabling to detect the presence of "black ice" on a bituminous road by transmitting a supersonic pulse along a thin thread, the pulse being subsequently reflected at the end of the thread. If the thread is covered with ice, no echo pulse will be obtained at the end of the thread. This detector has the inconvenience that the thread being only partly covered with ice, the freezing point may cause an extra reflection, which reduces the operating reliablity of the detector. Moreover, the detection electronics required for the detector is relatively complicated. It is known that aviation electronics must be as reliable and straightforward as possible.

The object of the present invention is to provide a device, which detects by means of supersonics whether ice has been formed on the wing of an aircraft or for instance around the air intake of the motor. The device must have as simple a design and as reliable an operation as possible to comply with security device requirements in aviation electronics.

The present invention is mainly characterized by the feature that the device comprises as a measuring transducer a thin thread-like or tape-like acoustic waveguide having a supersonic transmitter at one end and a supersonic receiver at the other end, and that the device comprises electronic devices for measuring the intensity and the attenuation of a supersonic pulse having passed through the transducer thread in the case of ice formation, and that the electric resistance of the thread is measured simultaneously with the measurement of the attenuation of the ultrasound, and that the thread can be optionally heated electrically to melt surrounding ice, whereby the ultrasound intensity resumes its initial level.

Thus the ice formation detector according to the invention is based on the feature that a mechanical supersonic signal is transmitted along a thin thread or strip at one end, and the intensity of the ultrasonic signal having passed through the thread is measured at the other end. If the thread is covered with a water layer, the ultrasound will not be attenuated, however if the water freezes, the ultrasound cannot propagate in the thread, but will be abruptly attenuated. If the thread is covered with sludge, the ultrasound will be somewhat attenuated to a kind of intermediate level, at which detection of sludge is also possible. There is a great viscosity difference between ice and water, and thus the intensity of the ultrasound having passed through the thread will also be highly different. The thread being used may have a diameter of 1 to 2 mm and it can be replaced by a tape-like waveguide. The thread may be made of say steel, nickel or any other material having a similar low acoustic attenuation, agnetostrictive nickel being however the most appropriate, since it is not corrosive, it has a relatively high electric resistance (for electric heating) and an ultrasound can easily be generated in it by means of a magnetostrictive transducer (or piezoelectric transducer) . It is known per se that an ultrasound can be generated in a thin thread by magnetostriction by winding a coil around the thread, and a supersonic pulse being generated also in the thread by conducting an electric pulse to the coil. A supersonic pulse can be received with a similar coil and transformed into an electric signal. An essential feature of this invention is that it is not based on the "pulse-echo" principle, but on the supersonic signal having passed through the thread and on the detection of its attenuation. This constitutes a basic difference over the invention mentioned above. If no ice occurs on the thread the signal having passed through will be unattenuated and the situation is under control. Only when the signal having passed through is abruptly attenuated, it is probable that ice has been formed on the thread. The transducer thread itself is mounted on the wing as described below. Another essential advantage is that the receiver does not have to be synchronized with the transmission pulse, which simplifies the electronics considerably, and the receiver may be switched on all the time. The reason for the disappearance of the supersonic pulse having passed through may also be that the thread is broken. However, breakage of the thread is easily detected by measuring the resistance of the thread from one end to the other at the point of the detectors. If the resistance is unaltered, the attenuation is not due to breakage of the thread. When the thread is being mounted on the wing of an aircraft, the mounting is carried out on an insulated support in order to avoid short-circuiting of the thread resistance by the metal wing. A supplementary checking is done by heating the thread with electric current via resistance measuring contacts so that ice around the thread melts again. Now the supersonic signal, which has already been attenuated once, should return, since the thread is only surrounded by water. By these means the necessary safety is provided for the ice indicator according to the invention.

Figure 1 shows the device according to the invention as a block diagramme.

Figure 2 is a schematic view of the placement of the transducer on the wing of an aircraft.

Figure 3 is a schematic view of the placement of the transducer at the mouth of an air intake of the motor.

Figure 4 is a schematic view of some optional cross-sections of the transducer thread as well as a removable spindle-type transducer.

In terms of figure 1 supersonic pulses at an appropriate frequency, preferably in the range of 150 to 250 kHz, are generated in the transducer thread 1 by an electronically pulsated signal generator 2. This supersonic signal is generated in the supersonic transducer 3 at the end of the thread, which has been attached to the transducer thread such that the ultrasound is transferred to the transducer thread in a suitable manner. The transducer 3 may be for instance a piezoceramic transducer having an appropriate opening into which the thread is threaded, or else the transducer may be a magnetostrictive transducer known as such, consisting of a coil wound around the thread. An identical transducer 4 is provided at the other end of the transducer thread 1 to receive a transmitted supersonic signal and to transform it into an electronic signal which is detectable with an electronic amplifier 5, which amplifies the received signal to such an intensity that it can be treated by means known per se in electronics, for instance by taking the received signal to a treshold detector 6, which gives an alarm with the alarm device 7 if the level of the supersonic signal having passed trhough the transducer thread has been too much reduced. The device according to the invention comprises further as an essential part measuring conductors 8 attached to each end of the transducer thread, by means of which the resistance of the thread and its variations can be continuously measured with an electronic meter 9. The same conductors may optionally be used for heating the transducer thread with the heater 10 so as to melt the ice possibly surrounding the thread 1, which provides an additional confirmation of potential ice formation. Thus the operation of the device according to the invention is such that in a normal situation, when the transducer thread is not surrounded by ice, or when it is surrounded by water or perhaps a liquid anti-freeze agent, such as glycol, the ultrasound transmitted along the transducer thread is not attenuated and the receiver 5 detects an intense supersonic signal and the threshold detector 6 connected to the receiver 5 does not give any alarm. If the trandsucer thread is covered with ice the ultrasound is abruptly attenuated and the receiver 5 and the threshold detector 6 do not detect any signal, whereby the pilot's cockpit receives a freezing alarm. Breakage of the thread is detected by continuously measuring the resistance of the thread, which increases strongly when the thread is broken, as is known. When a freezing alarm goes off, vital extra confirmation is obtained by heating the thread electrically until the surrounding ice melts. At this moment the freezing alarm should stop.

Figure 2 shows the mounting of the transducer thread on the wing 11 of an aircraft above the fuel tank 12. The thread can be bent through holes in the wing so that the transducers and their electric contacts will be located below the surface of the wing. It is obvious that the thread should be mounted in the direction of the air current so as to create a minimum of turbulence. Figure 3 illustrates an optional mounting of the transducer thread in the air intake 13 of a jet motor. Figure 4 illustrates the various optional cross-sections of the transducer thread 1. A round transducer thread may have a diameter of 1 to 2 mm, which does not disturb the aerodyamics of the wing. A transducer which actually is a waveguide may alos have the shape of a strap or a semi-circle. It is easier to attach such transducers to the wing surface than round ones. An insulating layer, e.g. epoxy paint, should be applied to the wing surface below the transducer thread, allowing to measure the resistance without problems. The device according to the invention can also be used to detect various intermediate freezing stages, such as the presence of sludge or the presence of thick anti-freeze glycol, by checking the amplitude of the ultrasound passing through in various intermediate situations. The device according to the invention enables to measure ice formation on a very large area, e.g. over the entire fuel tank, and if necessary, several transducer threads can be mounted along the wing.

Finally we note once more that in the pulse-echo device according to FI patent specification 61249, partial freezing of the thread or thread holders pressing the thread to the support surface may produce a false echo, and thus the reliablity of this device is doubtful. In the device according to the present invention echo signals are not detected, but a supersonic signal having passed through the thread, which is not sensitive to false echoes. Further this device is not sensitive to the length of the thread since the standing waves of ultrasound are not being used, but specifically pulse-like propagating waves. The supersonic transducer 3 at the end of the transducer thread may also consist of a kind of a spindle tightened and loosened with a nut 14, in the way a bit is attached to the spindle in drilling machines. The actual piezocrystal 15 is located at one end of the spindle, from where the ultrasound can proceed via the spindle jaws to the transducer thread 1 as in figure 4. In this way the supersonic transducer is easily detached and attached anew.

Claims

Claims

1. A device using supersonics intended to indicate ice formation on the wing of an aircraft, a height stabilizer, the air intake of a motor or any similar surface, characterized in that the device consists of a thread-like or tape-like waveguide (1) placed on the wing or a similar surface, through which the transducer (3) transmits a supersonic signal to the receiver (4) at the other end of the thread, which enables to draw conclusions about ice formation on the wing by observing the attenuation of the supersonic signal having advanced in the thread by means of an amplifier (5), the resistance of the transducer thread being simultaneously measured and the transducer thread being electrically heated with conductors (8) connected to its ends such that potential ice around the transducer (1) melts, whereby the attenuation of the signal detected by the receiver (4) resumes its initial level before the ice formation.

2. A device using supersonics according to claim 1 intended to indicate ice formation on the wing of an aircraft or a similar surface, characterized in that after the electronic amplifier (5) an electronic threshold detector (6) is mounted, which gives an alarm when the signal received from the amplifier sinks below a given limit, and if necessary, the threshold detector (6) switches on the electronic heater (10) automatically, whereby potential ice around the transducer thread melts, and the attenuation of the supersonic signal and the alarm signal should resume their initial level before the ice formation.

3. Device according to claims 1 and 2 intended to indicate ice formation on the wing of an aircraft or a similar surface, characterized in that the metal transducer thread (1) has a circular, semi-circular or tape-like cross-section, and in that the transducer is insulated from the metal support by means of an appropriate insulating paint or a similar insulation.

4. A device according to claims 1 and 2 intended to indicate ice formation on the wing of an aircraft, characterized in that the supersonic transducer situated at the end of the transducer thread is removable and consists of spindle jaws clamped with a nut (14) and conducting the ultrasound generated by the piezocrystal (15) at the end of the spindle to the transducer thread.