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Aviation History
1961
1961 - 1430.PDF
534 FLIGHT, 5 October 196! Aviation Electronics Oo Autonetics Verdon airborne digital computer de- signed for extreme reliability. It contains 16,000 components in 1.4 cu ft t-h Functional testing under vibration of a VHF transmitter Reliability in Military and Civil Aircraft Electronics By P. B. RAYNER* Verdan's memory unit in which the disc rotates at 6,000 r.p.m. on an air bearing and withstands I5g YEARS ago, the first flight of a new aircraft was an achievementin itself. Today, the first flight is just the beginning of thelong process of proving a complicated and expensive flight system which contains a great deal of electronic equipment.Aircraft are relying more and more on this electronic equipment, both to assist the crew and to control the aeroplane itself. Thefirst electronic aid was communications radio, giving the crew valuable contact with the ground. Failures here were rarelydisastrous and were tolerated without great hardship. Now that autopilots and navigation aids such as radar and Decca havebecome commonplace, and we are in the era of inertial navigation and automatic landing systems, the reliability of electronic equip-ment has become of the utmost importance. An unforeseen failure could, and sometimes does, mean disaster. It is for this reason that the designer of electronic equipmentfinds it necessary, if he is to sell his product successfully, to know how reliable it is and what environment it will stand. Because hisequipment is now playing a vital role in the safety of the aircraft, it is incumbent upon him to build into it the greatest possiblereliability; and this must be built-in to the basic design and main- tained through all the stages to the production of the finished The effect on module serviceability of non-time-dependent failures in groups of individual components r* DEFECTS PER IOO COMPONENTS ^* _^X^1 DEFECT PER IOO ^ COMPONENTS o 4oL article. It is impossible to bolt on reliability as an afterthought. This article describes some of the steps the industry is taking towards more reliable electronic equipment, both in this country and in the US, and indicates some of the future problems in this field. Measurements of Reliability The reliability of equipment is usually defined in terms of themean time to failure deduced by dividing the number of faults into the time over which they have occurred. Because this factoris easy to calculate, too much emphasis tends to be placed on it as an indication of performance in a specific environment. Forinstance, an amplifier having a high, but known, drift rate may show excellent performance in a missile with a flight time of a fewminutes, provided it is calibrated immediately before the flight. A similar unit with a moderate random failure rate, but having alonger mean time to failure, may be quite unsuitable for this missile because of a higher fault rate while the missile is in flight. Nevertheless, a figure such as the mean time to failure can beuseful in trying to assess the performance of a system; and this must be done in order to predict reliability before the system orits component parts are built. For this purpose, an assumption that there are only two general categories of failure, time-dependentand non-time-dependent, allows statistical models to be used to predict an overall reliability. The fact that these assumptions arenot exact naturally results in answers which are only approximate, but encouraging results have been obtained in practice. Non-time-dependent Failures Failures which result in faulty operation, but which developindependently of operating time are considered to be non-time- dependent. Typical of these are reception and installation of faultycomponents; damage to components during handling, testing and installation; misuse or misapplication because of design faults,malfunction of externally connected equipment or incorrect operating procedures; and failures resulting from repeated normaloperating cycles such as switching on and off. The first two types are easier to discover and rectify than the others by good quality- • Elliott Brothers (London) Ltd I DEFECT PER 1,000 COMPONENTS
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