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Trim Servo Unit

Technical Information

Catalogue No: C1304
Category: Flight Control
Object Type: Actuator
Object Name: Trim Servo Unit
Part No: 16781-1A
Serial No: 805109
Manufacturer: Bendix
Division: Eclipse-Pioneer [of Bendix]
Platform(s): BAC 1-11 , VC10
Year of Manufacture: circa 1962
Dimensions: Width (mm): 75
Height (mm): 75
Depth (mm): 202
Weight (g): 1,471
Location: Rack RAA02 [Main Store]

Servo, Trim
115/115 Volts 400~
28 Volts DC
Stall Torque 75 lb in
No Load Speed 0.75 rpm
Type No. 16781-1A
Wt - lbs
Serial No. 805109
The Bendix Corporation
Eclipse-Pioneer Division
Teterboro, New Jersey
Made in U.S.A.
[maintenance marks]
MA [Marconi Avionics]
Service & Repair Division
EBL. R. 7/87
GAv(R) A.S.&R.D. 132


This functions as a trim device to relieve control pressure and maintain the elevator in the desired position. This is a lower-speed, lower-torque version of the related item.

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.

From the Auto-pilot Computer, amplified commands are passed to the torque motor actuator mounted directly on the powered control unit at the appropriate control surface.

In the elevator channel, the one most directly involved in automatic flare-out, there are four surfaces and four powered control units, and the output of No 1 autopilot is passed to the inner port unit, the output of No 2 autopilot going to the inner starboard unit. Mechanical interconnection ensures that autopilot demands are passed to all four surfaces from any one autopilot and spring links ensure that jamming of one unit cannot immobilize the remainder. Under manual control, mechanical feed-back positions the powered control valve in the normal way and the pilot's input controls are reacted against the artificial feel loads.

When an autopilot is engaged, part of the powered control feed-back linkage is locked and autopilot demands are fed directly into the main valve operating linkage by the torque motor actuator and associated servo valve. The connection is thus direct and the control cable and feel system loads are taken by the powered control and not the autopilot actuator. Control is therefore extremely close and the autopilot can position the surface to ±0.3° with a bandwidth in excess of l0Hz.

With the autopilot engaged, the normal mechanical feed-back of surface movement is replaced by an electrical synchro position feed-back directly to the autopilot servo; and the servo-amplifiers are thus included in the autopilot powered control loop.

The BAC 1-11 AFCS, like that in the VC10, was based on the well proven Bendix PB-20 Autopilot and was designated the Series 2000 AFCS. New features over the PB-20 system include separate pitch and azimuth control computers, a modular Air Data Sensor and a range of units specifically designed for autoflare and autolanding.

Each unit in the BAC 1-11 AFCS is built to a common configuration with circuit modules arranged in stacks either side of the chassis. The stacks are connected by plugs to a mother board and are physically separated into ‘command’ and ‘monitor’ functions to preclude common failures. The computers are entirely solid state and there is a high degree of built-in-test. Self-monitoring techniques and multiple channel redundancy are used to give automatic failure survival in approach and cruise flight.

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