Measuring and testing – Navigation – Take-off and landing monitors
Reexamination Certificate
2001-04-12
2003-05-20
Oen, William (Department: 2855)
Measuring and testing
Navigation
Take-off and landing monitors
Reexamination Certificate
active
06564628
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to back-up instruments which deliver a minimum of three critical items of information to the pilots of an aircraft, in the event of a failure of the main instruments, these items being: the speed of the aircraft relative to the air or conventional speed deduced from a measurement of dynamic pressure, the altitude of the aircraft deduced from a measurement of static pressure and the attitude of the aircraft obtained on the basis of information originating from inertial sensors.
The conventional speed is the image of the dynamic pressure on which the lift depends. In particular, it enables the pilot of an aeroplane to decide the instant of take-off and to estimate his safety margin with respect to the risk of stalling or of abrupt loss of lift.
The altitude is used differently depending on the phases of flight. When cruising, the aeroplane must comply with the air corridor allocated to it and, to do so, must maintain a given flight level defined in terms of a standard altitude or pressure altitude. The standard altitude or pressure altitude is a theoretical altitude deduced from the measurement of the static pressure and translated into a theoretical height after making a number of assumptions regarding the physical properties of the atmosphere in which the aircraft is flying. It is very different from the actual altitude relative to the ground or to sea level. The disparities may reach several thousand feet but the indication provided in terms of relative value for one and the same place is very accurate and leads to a very safe definition of the air corridors since all the aircraft use the same model of the atmosphere.
The flight levels or corridors allocated to the aircraft by the air traffic controllers have a height which takes account of the accuracy of measurement of the standard altitude or pressure altitude. For example, above the level
290
corresponding to 29,000 feet, an air corridor has a height of 1000 feet whereas the accuracy of measurement of the standard altitude or pressure altitude is better than 100 feet. The risk of collision is therefore almost zero.
In the event of inadequate visibility, the pilot must be furnished with a vertical reference, given by an attitude indicator, in order to fly the aircraft in complete safety and avoid placing the aircraft into an attitude which is prohibited by the construction specifications of the machine.
These three basic items of information, regarded as critical in the safety sense, are provided to the pilot and to the copilot of an aircraft in a redundant form so that the probability of total loss or of erroneous information is almost zero. For aircraft which are required to transport passengers, it is the rule that the on-board instruments providing this basic information are triplicated. On board there are, almost routinely, three independent sets of instruments: a first for the pilot, a second for the copilot and a third as back-up. The pilot and the copilot are each furnished with their own set of instruments together with so-called main displays based on cathode-ray tubes or liquid crystals whilst the back-up set of instruments is arranged on a central console separating the pilot's seat from the copilot's seat.
The back-up instruments need not be quite as accurate as the main instruments intended for the pilot and for the copilot since, normally, they merely serve as reference for testing the proper operation of the main instruments. But they must however exhibit very high operational reliability.
Until very recently, the reliability of the back-up instruments was based on purely mechanical or pneumatic embodiments not requiring recourse to the on-board electrical supply network or to the mount on the outside of the fuselage of measurement probes other than pressure probes, with no moving parts, connected up by pneumatic ducting. Conversely, in order to improve the accuracy of the indications provided, the embodiments of the instruments of the main systems are calling upon an ever larger portion of electronics involving the use of the on-board electrical supply network and upon various measurement probes, including aerodynamic incidence probes, which are mounted on the outside of the fuselage of the aircraft and enclose electromechanical sensors requiring electrical cable wiring. The reliability of the main instrument systems is affected by the reliability of the on-board electrical network and of the non-purely static measurement probes used on the outside of the fuselage. It is also affected by a degree of sensitivity to the radioelectric disruptions of the environment of the aircraft such as lightning strikes due to the presence of the electrical cable wiring connecting the instruments to certain measurement probes placed outside the fuselage.
In view of these considerations, the search for the greatest possible safety of operation has hitherto led to the use as back-up instruments of: a pneumatic altimeter, a mechanical anemometer or “air speed indicator” and an attitude indicator or gyroscopic “artificial horizon”.
The current technological advances have now made it possible to attain a level of safety in respect of the electrical supply network of an aircraft such that it can be presumed that an electrical power source will always be available on board for a minimum of instruments and equipment regardless of the failure conditions encountered. This is why the introduction of electronics into the back-up instruments has recently been planned so as to improve their accuracy and reduce their maintenance costs, without thereby affecting their safety of operation. In particular, it is envisaged to replace the three purely mechanical and pneumatic conventional back-up instruments with fully electronic combined instruments which provide on a liquid crystal screen the three critical items of information with regard to the conduct of a flight, namely: the pressure altitude, the conventional speed and the attitude of the aircraft. However, in order to ensure a high level of safety, it still remains necessary to maintain complete independence between the system of back-up instruments and the systems of main instruments for the pilot and the copilot and also to avoid the system of back-up instruments having recourse to measurement probes mounted on the outside of the fuselage of the aircraft which are not purely static or require connection by electrical wiring. This leads to the system of back-up instruments keeping its own measurement portions and its customary measurement probes. Thus, the system of back-up instruments will still possess its own inertial means for determining the attitude of the aircraft relative to the vertical and its own measurement portions linked to pneumatic measurement probes for determining the conventional speed and the pressure altitude.
The inertial means of the system of back-up instruments consist, for example, of a gyroscopic top slaved to the apparent vertical by an erector device which maintains a vertical reference accurate to within a few tenths of a degree, it being possible to synthesize this gyroscopic top by means of inertial sensors of angular velocity and of accelerometers or inclinometers.
The conventional speed measurement portion of the system of back-up instruments deduces, in a standard manner, the conventional speed from the dynamic pressure which is the difference between the total pressure and the static pressure, by implementing St Venant's law or Rayleigh's supersonic equation.
The portion for measuring the pressure altitude of the system of back-up instruments deduces, in a standard manner, the pressure altitude from the static pressure, by applying the benchmark relations arising from the Laplace equations for the standard atmosphere.
The measurements of total pressure and static pressure on behalf of the system of back-up instruments are made independently of the main systems of instruments for the pilot and the copilot, the total pressure being measured with the aid of a specific pres
Chevalier Jean-Rene
Escobar Edouard
Leblond Henri
Oen William
Thomson-CSF Sextant
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