« Previous Next »

Pitch Amp and Computer

Technical Information

Catalogue No: C1499
Category: Flight Control
Object Type: Signal/Data Processor
Object Name: Pitch Amp and Computer
Part No: 10940-A-1
Serial No: 100/71
Manufacturer: Elliott
Division: Flight Controls [FCD]
Platform(s): VC10
Year of Manufacture: circa 1971
Dimensions: Width (mm): 190
Height (mm): 190
Depth (mm): 50
Weight (g): 10,340
Location: Rack RAA01 [Main Store]

Pitch Amp & Computer
Type No. 10940-A-1

Ser.No 100/71


This box is called a Pitch Amplifier and is visibly different from the Longitudinal Amplifier. This is possibly a later version of the Lateral Amplifier and now used more modern DC Amplifiers. It was part of the flight director or auto- flare systems. This is the same unit as collection entry 1309 but with different part No.

From the early days of the company it had been hoped to enter the civil aircraft flight control field, in order to reduce dependence on military projects. The late 1950s was a time of significant change in the automatic flight control field. Elliott made a major contribution to this evolution by the design and development of actuation systems which integrated the electronic control input with the hydraulics of the main flying control power actuator.

The opportunity to take this step came in the late 1950’s with the planning of the Vickers 'VC 10' for which Elliott Brothers secured an order to provide a complete automatic flight control system. This led to considerable shared responsibility with the airframe designs of the Vickers VC 10, where the main control surfaces were split into several separate units. From the outset, the 'VC 10' system was planned to make provision for fully automatic landing of the aircraft. For certification ever to be possible an extremely high standard of reliability was essential, and even in the case of failure of the equipment it was a requirement that the aircraft must not be subjected to violent manoeuvres. After a detailed study of possible alternatives, the solution chosen was to duplicate the whole of the major system, one half to be operative while the other was to be 'standing by', with a changeover mechanism of the utmost reliability to permit instant switching from one to another. By 1960 the basic development was substantially complete and the requirements for automatic landing were being explored in detail with full 'autoland' capability available from January 1963. Successful development of the 'VC 10' system resulted in the opportunity to supply broadly similar equipment for the British Aircraft Corporation 'BAC 111', which has been produced in substantial numbers. The automatic flight control system of the Standard and Super VC10 was designed to be capable of development to full blind landing. To meet this requirement the system had to be capable of failure survival and this includes associated services such as power supplies and flying controls. The method of autopilot failure survival chosen was to provide two monitored systems which are fail soft, i.e. there is negligible aircraft disturbance after a failure. Only one autopilot is used to fly the aircraft, and the two systems, including power supplies, are completely independent. Each autopilot has a comparison monitor which detects faults and, in flight, will disconnect the system if these faults are likely to lead to dangerous conditions. For autoflare the system provides for automatic changeover to the second monitored autopilot system, in the event of fault in the first. Under these conditions the second autopilot is primed and ready to take over. If for any reason the monitoring system fails to prevent an autopilot runaway, the control movement is limited to a safe amount by the yielding of a torque-limiting spring. Many of the needed components were already present in the autopilot fit on the Standard VC10s, to achieve the autoland capacity the system on the Super received some additional items. The system, supplied by Elliott Brothers (London) Ltd, was based largely on components of the well-proved Bendix PB-20 autopilot, made under licence by Elliott, and interchangeable with American built components as installed in Boeing 707s. However, the system as a whole i.e., the dual autopilot concept was novel, and designed entirely by Elliott.           

A comprehensive description of the VC10 systems will be found at this VC10 website.

The VC10 had two autopilots which were normally both energized as soon as the aircraft power supply was switched on, but only one autopilot at a time was used during cruising flight. Only during the automatic flare-out, and later during automatic landing, was the automatic changeover facility available. In this case monitoring was in full operation on both autopilots and the second autopilot was electromechanically slaved to the one which had control authority. The separation of the two channels was maintained right through to the individual inputs to the powered control units and there were individual circuits from the pilot's control panels and individual power supplies to both main and comparison channels. Monitoring was applied so that each important element throughout the chain was covered in such a way that disengagement would automatically follow a discrepancy between any significant components in the system. The main computer of each autopilot contains all the adders and shapers required to produce the control demands related to the inputs being used. The circuitry is built up in plug-in card modules as in the original Bendix design base. About 25 different electronic and electro-mechanical modules are involved and a special automatic test installation was designed to isolate faulty cards in boxes.

There are two computers divided into longitudinal (or Pitch) and lateral (or Azimuth) amplifier and computer channels. All longitudinal aerodynamic parameters, throttle control, pitch autostabilization, height, i.a.s., Mach number and glide-slope functions are in the longitudinal channel and the lateral channel accounts for yaw damper, azimuth, aileron and rudder, aileron damper and the dynamic vertical sensor functions.

Errors revealed by differences in signals between main and monitor units are passed to the error disconnect system which then disengages the autopilot by releasing a lock integrated with the powered control unit.
From this Lateral Amplifier and Computer signals go to this Amplifier and Computer from which amplified commands are passed to the torque motor actuator mounted directly on the powered control unit at the appropriate control surface.

Each main computer was housed in two 1 ATR cases.

The basic requirement for an automatic landing is that the equipment must survive a single failure and continue to operate. Fundamentally, this can be achieved by triplication of all equipment. But in providing and justifying redundant equipment in civil passenger aircraft, consideration must be given not only to overall safety, reliability and performance, but also to weight, installation difficulties, overall cost, maintenance problems and many other factors. Unnecessary redundancy must therefore be avoided.

It is essential that effective autopilot disconnection should occur in the event of a failure and that the pilot should be warned of the failure and the control runs automatically freed. The disconnection and warning unit can only be electrical and must be made truly fail-safe. In practice, failure of the system to disconnect following an autopilot failure will occur only if both the autopilot and the disconnection device fail. The likelihood of this is remote as it involves a product of small probabilities in the landing phase. The acceptance of an electrically actuated disconnect device permits further simplifications of the duplicate channel, with an increase in system reliability and a saving in weight.

The operation can be checked in a different way by comparing the demand of the second autopilot with the effective demand of the first which is obtained by suitably processing the actual control output with the approximate inverse transfer function of the servo motor control loop. This concept is called a "monitored-duplicate" system and is the design used by Elliotts on the VC10. The comparison concept is used throughout the Autopilot and the Flight Director system with the various flight parameters derived in a stand-alone units. Because the duplicate sensors are used for comparison and not for actual control, they can be considerably simplified and therefore made more reliable and lighter than those used in the autopilot; and the inherent differences make them less liable to fail from a common environmental cause.

Longitudinal and Lateral Computers have equivalent Comparison computers, the Vertical Gyro has a simple comparison unit and the Air Data Computer core elements are separated for this purpose. Not all the functional boxes are compared in this way; in some cases such as the Polar Path Compass the units are duplicated and are compared electro-mechanically but there is not a Comparison unit.

Click to enlarge Click to enlarge