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Autotrim Computer

Technical Information

Catalogue No: C0589
Category: Flight Control
Object Type: Signal/Data Processor
Object Name: Autotrim Computer
Part No: 48-006-02-EX1
Serial No: 005
Manufacturer: Marconi-Elliott Avionic Systems Ltd
Division: Combat Aircraft Controls [CACD]
Platform(s): Sea Harrier
Year of Manufacture: 1989
Dimensions: Width (mm): 127
Height (mm): 197
Depth (mm): 385
Weight (g): 5,280
Location: Rack RAA01 [Main Store]

Marconi-Elliott Avionics
Auto-Pilot Computer
Part 48-006-02-EX1
Ser 005
Code K0656
Part No. 48-006-02-EX1
Serial No. 005
Non Standard
Authority [signature] -/6/89
HA 887
Autotrim Demo Model


This unit is an experimental Autotrim unit (EX1) fitted to an FRS1 Sea Harrier for demonstration.

In 1988 the UK Ministry of Defence awarded British Aerospace a £170 million contract to      upgrade the Royal Navy's Sea Harriers with new radars, avionics, and weapons. The contract   covered modification of all RN aircraft to FRS.2 standard, and work started in October 1990.  The RN had 42 Sea Harrier FRS.ls out of a total of 57 delivered and an additional contract, to build 16 new FRS.2s was negotiated. The Automatic Flight Control System supplied by GEC Avionics includes:

·         Autostabiliser which provides three axis stability augmentation during take-off, transition and hover

·           Autopilot to provide height, heading and attitude hold.

An Autotrim function was incorporated during the aircraft mid-life update to further enhance the AFCS.



The basic function of the autopilot is to control the flight of the aircraft and maintain it on a pre-determined path in space without any action being required by the pilot. (Once the pilot has selected the appropriate control mode(s) of the autopilot.) The autopilot can thus relieve the pilot from the fatigue and tedium of having to maintain continuous control of the aircraft’s flight path on a long duration flight. The pilot is thus free to concentrate on other tasks and the management of the mission.

A well-designed autopilot system which is properly integrated with the aircraft flight control system can achieve a faster response and maintain a more precise flight path than the pilot. Even more important, the autopilot response is always consistent whereas a pilot’s response can be affected by fatigue and work load and stress. The autopilot is thus able to provide a very precise control of the aircraft’s flight path for such applications as fully automatic landing in very poor, or even zero visibility conditions. In the case of a military strike aircraft, the autopilot in conjunction with a T/F guidance system can provide an all-weather automatic terrain following capability. This enables the aircraft to fly at high speed (around 600 knots) at very low altitude (200 ft or less) automatically following the terrain profile to stay below the radar horizon of enemy radars. Maximum advantage of terrain screening can be taken to minimise the risk of detection and alerting the enemy’s defences.

The basic loop through which the autopilot controls the aircraft’s flight path is by means of an inner and outer loop. The autopilot exercises a guidance function in the outer loop and generates commands to the inner flight control loop. These commands are generally attitude commands which operate the aircraft’s control surfaces through a closed loop control system so that the aircraft rotates about the pitch and roll axes until the measured pitch and bank angles are equal to the commanded angles. The changes in the aircraft’s pitch and bank angles then cause the aircraft flight path to change through the flight path kinematics.

Autopilots in modern complex aircraft are three-axis and generally divide a flight into taxi, take off, climb, cruise (level flight), descent, approach, and landing phases. Some Autopilots can automate all of these flight phases except taxi and take off. An autopilot-controlled landing on a runway and controlling the aircraft on rollout (i.e. keeping it on the centre of the runway) is known as a CAT IIIb landing or Autoland and is available on many major airports' runways today, especially at airports subject to adverse weather phenomena such as fog. Landing, rollout, and taxi control to the aircraft parking position is known as CAT IIIc.
An autopilot is often an integral component of a Flight Management System
An autopilot takes the aircraft's position and attitude from an inertial guidance system and then controls a Flight Control System to guide the aircraft. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the airspeed, and move fuel to different tanks to balance the aircraft in an optimal attitude in the air.


There may be a need for improved damping and stability about all three axes. This can be achieved by an auto-stabilisation system, or, as it is sometimes referred to, a stability augmentation system.

Yaw auto-stabilisation systems are required in most jet aircraft to suppress the lightly damped short period yawing motion and the accompanying oscillatory roll motion due to yaw/roll cross coupling known as Dutch roll motion which can occur over parts of the flight envelope. In the case of military aircraft, the yaw damper system may be essential to give a steady weapon aiming platform as the pilot is generally unable to control the short period yawing motion and can in fact get out of phase and make the situation worse.

A yaw damper system is an essential system in most civil jet aircraft as the undamped short period motion could cause considerable passenger discomfort.

A yaw damper system may be insufficient with some aircraft with large wing sweepback to suppress the effects of the yaw/roll cross coupling and a roll damper (or roll auto-stabilisation) system may also be necessary. The possible low damping of the short period pitch response  can also require the installation of a pitch damper (or pitch auto-stabilisation) system. Hence, three axis auto-stabilisation systems are installed in most high-performance military jet aircraft and very many civil jet aircraft.

Background for Elliott Bros in Autopilots

In the 1950s Smiths Industries  was the traditional supplier of autopilots for transport and bomber aircraft but the RAE [Farnborough | with the UK government wanted an alternative supplier to tackle the new supersonic combat aircraft field. Elliott Brothers (London) Ltd. (at Borehamwood), with its long history of making aircraft instruments, and another company, Louis Newmark Ltd., were asked to go into competition for the honour, by bidding for the two systems required for a 3-axis autostabiliser and the MK13 autopilot. During 1954 Elliott was chosen for both contracts and Newmark was directed into the helicopter controls business. Initial projects involved the development of analogue autopilots for unmanned experimental aircraft and an analogue 3-axis autostabilisation system for a Mach 2 fighter under the official requirement F23/49, which was to become the English Electric Lightning. By 1960 this activity had expanded to cover the autopilots for both the Lightning and Buccaneer transonic strike aircraft as well as complementary development of analogue air data systems for these aircraft.

Most importantly work had begun, also under Ron Howard’s leadership, of a dual channel fail-operative autopilot/automatic landing system for the Vickers VC'10 long-range jet airliner. In 1960 Elliott’s expertise in advanced flight control systems in the UK was recognised by the award of the triplex automatic terrain following autopilot and autostabliser contract for the Mach 2-plus TSR 2 strike aircraft. These two systems, along with the earlier Lightning system, can be considered as the foundation stones of much of what was to follow in Flight Control system developments at Elliott’s.

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