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Aviation History
1964
1964 - 0472.PDF
filCHT International, 20 February 1964 FLIGHT SYSTEMS SURVEY Strictly for the Record . . . (11) Installation In the rear of the aircraft and, whenever practicable,in the tailcone. (12) Protection The construction of the flight recorder must be suchthat the record medium is afforded protection against conditions likely to be experienced in a crash. The recorded data must becapable of intelligible analysis to within the overall specification requirements after the protected record medium has been subjectedto each of the following tests: (a) impact shock of lOOg applied at the attachment points (b) a static load of one ton in any direction (c)exposure to flames at 800° over 50 per cent of the surface area for a period of 15min, the recorder being allowed to cool naturally aftersuch exposure. (13) Water resistance The record must be capable of intelligibleanalysis after the record medium [sic] has been immersed for 36hr in sea water.(14) Resistance to other fluids The record must be capable of intelligible analysis after the record medium [sic] has been immersed for a periodof 5min in petrol, kerosine and all hydraulic and fire extinguisher fluids likely to be used. (15) Termination of the record Provision must be made against self-erasure of the record after a crash. (16) Operation (a) Operation of the flight recorder must be auto-matically initiated and terminated by essential pre-flight and post- flight actions, (b) There must be a visual indication to the crew ofthe aircraft that the flight recorder is receiving sufficient power to function correctly. (17) Record presentation The recorded data must be readily capableof presentation in graphical form, each parameter measured against a common time-base. (18) Accuracy and ranges The accuracy tolerance is to be measuredbetween the final graphical data presentation after read-out and the aircraft system from which the data originates. Although this specification involves the monitoring of six channels, or parameters, the rate of sampling demanded means that 24 sampled per second must be taken. It is therefore possible to term the recorder a 24-channel system. The point of this observa- tion is that, in defining the capacity of a recorder it is the rate of sampling per second which counts. In practice all channels are not necessarily sampled at the same rate and, whereas g, for example, is sampled every one-fifth second, other factors may be sampled much less frequently. Conversely, a so-called 200-channel recorder may take 200 samples per second of far fewer than 200 channels; and it may take 200 samples every 6sec for lOOhr, or 200 samples every 0.6sec for lOhr. A clear definition helps in assessing claims. The Technical Aspects Despite the intentions of the MoA speci- fication, the problems of getting recorders into service are more than adequately tangled; and the technical side is positively at boiling point. Royston have for years been the champions and pioneers in FM tape techniques and have carried out a highly praiseworthy campaign for acceptance of the whole maintenance recording philosophy. Their practical, airborne experience is vir- tually unmatched anywhere in the world and their equipment has been widely accepted. But they are now being hotly pursued, tech- nically and commercially. The table on this page shows the com- panies known to be active in the field. There are numerous thorny problems. On the face of it, a tape or wire transport deck is absolutely straightforward. After all, the shop windows are full of perfectly effective tape and wire recorders. But one after another, manufacturers new to the airborne recording field are running into unforeseen problems. Signal conditioning equipment, from which the tape or wire deck must take its signals, can also be difficult. One major problem, already mentioned, has been spurious inputs caused by pick-up in long cable runs, and "wow" or flutter in recorders. Another has been the encumbrance and weight of transducers and associated cabling, although the 240-channeI Royston Midas installation studied for a Comet in- volved no more than four additional transducers. The recording equipment itself must show a rather better reliability than the sys- tems it is designed to monitor. Finally, the MoA raised a significant problem by stating in its specification that the recording medium, te distinct from the case enclosing it, must survive contact with fluids such as fire extinguishant, Skydrol, fuel, and sea water. They allow that, if the medium itself will not stand these, then the en- closure can be proofed against them, but this raises problems of its °wn. Wire retains its magnetic record up to the curing point at600 C but taped signals are lost at from 110°C to 120°C, so thatm ore heat protection is needed. Each manufacturer is only too ready to point out the disadvan- 287 Manufacturer Royston Instruments LtdRedifon Ltd (Flight Simulator Division) S. Davall & Sons LtdPlessey Group Sperry Gyroscope Co Ltd M.L. Aviation Ltd Penny & GilesColnbrook Instrument Develop- ment Ltd S.E. Laboratories Ltd British Aircraft Corporation(Filton) Ltd Elliott Brothers (London) LtdSmiths Aviation Division E.M.I. Electronics LtdHawker Siddeley Dynamics Ltd Epsylon Industries Ltd Activity Midas systemMilitary crash recorder to OR.8003Wire recording unit Electronic signal conditionerfor above Digital data acquisition systemEjection equipment for recordersWire recorder deck Tape decks to RAE design for Redifon recordersTransducers Reported system designReported system design Reported system designActive in field Reported system designApplying tape experience tages of rival systems, and only practical proof will distinguish propaganda from reliable fact. Tape has a greater capacity than wire, they say. One foot of tape will hold as much information as one mile of wire, but others say that, for a given capacity, tape occupies at least ten times the volume of wire. Wire will not accept broad-band signals. There are manifold mechanical problems with wire. Everyone can point a technical finger at something. Never- theless, the proponents of wire are showing considerable confidence —though the FAA, for example, now specify plastic-based tape. A relatively new factor is that current problems in recording elements using either tape or wire can apparently be overcome by digitizing the signals to be recorded. With traditional analogue methods, precise voltages and frequencies have to be derived from the transducers, registered on the tape or wire and extracted from it during play-back, each stage of which presents ample opportunity for inaccuracy or outright loss or distortion of the signal. With digital techniques, all quantitative information can be reduced to binary code and is registered on the tape or wire simply in the form of a sequence of negative or positive magnetic charges. Error checking processes become possible and deterioration of the signal does not degrade its accuracy. Digital systems tend to be heavier for a given recording capacity, but the increased weight may repre- sent an insignificant proportion of the total. Sperry Gyroscope Co Ltd are in the final development stage with a digital data acquisition system, which will be capable of matching any aircraft system to any recorder. It incorporates solid-state modules to pulse-modulate the transducer outputs and convert them into codes proportional to transducer shaft position. Time- This could be caused by pilot error or equipment failure. A recorder might well indicate which
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