1. Field of the Invention
The invention pertains to a method for verifying the consistency of values given by an angle of attack probe of an aircraft and to a device implementing this method.
2. Discussion of Prior Art
The control of any aircraft makes it necessary to ascertain its relative speed with respect to the air, that is to say to the relative wind. This speed is determined with the aid of sensors of static pressure Ps, of the total pressure Pt, of the angle of attack α and of the angle of sideslip β. The angle of attack α of an aircraft is commonly defined as being the angle of the air speed vector with respect to a horizontal plane of the aircraft. Likewise, the angle of sideslip β of an aircraft is defined as being the angle of the air speed vector with respect to the vertical plane of the aircraft.
The expression vertical plane of the aircraft, perpendicular to the horizontal plane of the latter, is understood to mean the symmetry plane of the aircraft which is parallel to the fin of the tail empennage of the aircraft. The value of α is traditionally measured by angle of attack probes which dispatch an electrical signal as a function of time, indicative of the locally measured angle of attack. These probes are of vane type, also termed wind vanes, and are generally mounted on the nose of the craft so as to measure the angle of the air speed vector with respect to the horizontal plane of the aircraft. They comprise a movable part which is flush with the skin of the aircraft.
More precisely, FIG. 1 represents such a type of vane probe. The movable vane G overhangs the base S which is implanted in the fuselage. It is understood that the vane G is a sensitive part of the probe since it is subjected to the outside environment. Indeed, such a probe is situated at sensitive locations of the aeroplane and may be damaged by ground maintenance operations, in particular by the use of ladders or cleaning lances, particular meteorological conditions or bird strikes in flight. It may therefore become disabled, distort or break. It is then said to have seized up.
The principle of measuring the angle of attack (AOA) is as follows: the movable part G orients itself along the direction of the local air stream, parallel to the relative wind of the craft. Electronics implanted in the base S measures the angle that the movable part G makes with a reference angle, and converts the difference into an angle of attack measurement signal, which will be denoted Sα subsequently in the description.
For measurement redundancy purposes, modern craft possess several channels for measuring the angle of attack. Each measurement channel comprises a distinct angle of attack probe. In general three probes are implanted on the nose of the craft where the air stream is the least disturbed by the lift-producing elements of the craft. The craft thus comprises three measurement channels, generally each implanted in an ADR (“Air Data Reference”) module of an ADIRU (“Air Data Inertial Reference Unit”) unit of the craft. A problem arises in the case of a fault with one or more of the probes. Indeed, it is difficult to detect a failure due to the deformation of the vane G.
It is known from patent EP 1 354 212 B1 to verify the consistency between three measurement probes by applying the triplex principle. According to this principle, it is possible to log a defect when one of the probes gives a very different result from the other two. It is then deduced therefrom that this probe is defective. The triplex principle is illustrated in FIGS. 2a and 2b. In FIG. 2a, probes 2 and 3 indicate an angle of attack value of 3° for probe 2 and of 3.1° for probe 3. Probe 1 gives for its part a value of 8° very different from the others. The measurements of probe 1 are therefore rejected.
In other cases, the application of the triplex principle may possibly turn out to be insufficient. Indeed, when two probes give erroneous but mutually consistent readings, the correct measurement is rejected. This case is illustrated in FIG. 2b. Probe 1 gives a value of 8° and probe 3 gives a value of 8.1°. These readings are erroneous but mutually consistent. Probe 2 gives a value of 3°, this in fact being the correct value of angle of attack, but the application of the triplex principle implies that it is the measurements of this probe 2 which are rejected.
The measurements carried out by an angle of attack probe can be used by various electronic systems fitted to an aircraft, such as for example the stall warning device or the system of electric flight controls. It is then understood that it is necessary to know whether the readings given by the angle of attack probes are correct, so as to indicate to the computer whether or not it can take these readings into account.
A first solution consists in having a greater number of probes than that necessary for the measurements or else in installing several systems of additional monitoring probes as is described in patent EP 1 354 212. These solutions lead, however, to problems with integrating these systems into aeroplanes.
A second solution consists in estimating the value of the angle of attack and in comparing the estimated value with the measured value so as to detect possible errors which may arise subsequent to the application of the triplex principle alone. This solution is effective and is generally employed on craft of recent design. To further improve this solution, provision may be made to supplement it with means for verifying the consistency of values given by the angle of attack probes.
One of the objectives of the invention is to provide a system for verifying the consistency of the angle of attack probes which does not require the installation of additional probes.