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Aviation History
1960
1960 - 2237.PDF
FLIGHT, 7 October 1960 AVIATION ELECTRONICS . . . complement of "black boxes" associated with the VC10 autopilotand the equipment by which each of two separate autopilots is individually monitored. The separation of the two channels ismaintained right through to the individual inputs to the powered control units and there are individual circuits from the pilot'scontrol panels and individual power supplies to both main and comparison channels. Monitoring is applied so that each import-ant element throughout the chain is covered in such a way that disengagement will automatically follow a discrepancy betweenany significant components in the system. The two autopilots are normally energized as soon as the aircraftpower supply is switched on, but only one autopilot at a time is used during cruising flight. Only during the automatic flare-out—and later during automatic landing—is the automatic change- over facility available. In this case monitoring is in full operationon both autopilots and the second autopilot is electromechanically slaved to the one which has control authority. Almost all of themechanical actions normally required for changing from one to the other are already performed. Should the automatic change-over be required, only two mechanical actions need be performed, and these are automatic and mechanically simple. It should benoted that the monitored duplicate arrangement requires the automatic changeover to operate on average only for half thetotal number of faults, because half of them will occur in the stand-by system. The exact weight of the whole system has not been revealed.Elliott state only that the equipment weighs less than two average human pilots and that a comparison channel weighs only one-quarter as much as its related main channel. Both duplication and monitoring features are considered to be required for cruisingas well as automatic flare-out and it is almost impossible to assess the weight penalty of the automatic flare-out facility by itself. It is worth pointing out at the outset that the flight directorinstruments are integrated with the automatic control loop to only a limited extent—mainly in using the same compass and ver-tical gyro references and supplying pilot's heading demand infor- mation to the autopilot. The remaining flight instruments areentirely separate. They have their own dynamic and static pres- sure capsules and servos and their own pitot heads. To this extent,the failure of equipment not directly related with autopilot control cannot affect the automatics. Protection has been provided against stray mechanical faults.For example, guards will prevent a wiring lead which has broken away from its terminal from making contact with any other ter-minals. Where a single output drives two synchros or potentio- meters, electrical isolation is rigorously maintained. All circuitryin the interlocks is isolated and a unique supply of 115V d.c. is derived in each autopilot power supply and distributed to theisolated high-impedance engage circuits so that the possibility of stray pick-ups is exceedingly remote. Moreover, the monitoringconditions will be broken by a wrong voltage in the comparison selection system and all circuits are so designed that failure ofwiring or connectors in zero signal lines is also detected. In many cases mechanical computation with electrical pick-offs is em-ployed in order to maintain component reliability as high as possible. If a fault is detected, special disconnect circuitry is activated toisolate part or all of the autopilot. If the error disconnection system fails, it means that there must have been two concurrentfaults, an event which has never yet occurred even in the less sophisticated system of the Boeing 707. Micro-switches andtorque-limiting devices at the powered control units provide addi- tional safeguards to cover even this eventuality.Both autopilots are controlled from a panel on the centre console between the pilots, the basic selections being either for yaw-damping alone or for full autopilot control. As soon as an auto- pilot is selected, the engage circuitry analyses the complete systemand, if a fault is present, the engage switch will return to the "off" position as soon as it is released. Both yaw damper systems can-not normally be switched on together and a special yet simple pro- cedure is followed before engaging both autopilots together inpreparation for the automatic flare-out manoeuvre. Each control The 25 different plug-in card modules used in the VC10 autopilot The pilot's controller with autothrottle speed selecter above it Simulator testing of the "bread board" VC10 system with a much modified Link trainer
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