Flight Control Panel Fascia

By the early 1950’s all automatic flight control systems were designed to reduce to the absolute minimum the number of moving parts. Suitable transistors were not readily available until the mid-1950s and valves were not suitable for the military jet environment. So for a short period the Magnetic Amplifier came into prominence. These had been used during WWII largely by German designers of automatic flight controls. Post war higher permeability magnetic materials and new germanium and silicon diodes became available and gave a significant improvement in the reliability of the ‘Mag Amp’. The Elliott Mk 13 and subsequent automatic flight control systems installed in the English Electric Lightning are representative of such technology. These have magnetic "operational amplifiers" in which the majority of gearing adjustments in the computers are effected in the amplifier feedback loops. These also employed the newly available silicon diodes and ultimately the Lightning system was designed so as to be able to withstand a temperature environment limited only by the dissipation capability of the silicon junctions.

The Autostabiliser/Autopilot for the Lightning provided largely automatic control in any of a variety of flight modes. The system drove fast response electro-hydraulic actuators to give three-axis stabilisation. The details of the system were classified; but the ‘FLIGHT’ report of July 1961 gave some idea of the capability:

"A feature of the control panel of one of them was the use of mechanical interlocks between mode selectors to 'save panel space. A climb setting covers optimum climb performance under autopilot control and the inclusion of "track" and "glide" switches indicates automatic or near-automatic landing.

"Automatic throttle control is included. The main hand control is designed for use with the right hand outside the field of vision. Airpass has a controller for the left hand. It was stated that the autopilot hand controller could govern either extent or rate of turn, according to the mode in use. The barometric height lock is monitored, especially at low altitudes, by a radio altimeter to avoid excessive pitch demands.

"Another logical assumption is that the Elliott autopilot is also linked to the radar fire-control, to fly the Lightning on the correct trajectory to effect the most economical interception. All signals from the system are passed as demands to the autostabiliser actuators inserted in each control circuit. All four autostabiliser actuators (there are two in the aileron circuit, one in each wing) are Hobson electrically signalled, rotary hydraulic motors, with a linear output connected to the appropriate control system in such a manner that it moves the surface but not the cockpit control.

"Artificial feel is provided about all three axes. In the aileron circuit a simple torsion bar is inserted between the control column and the (normally fixed) aileron-trim drive, to provide feel directly proportional to stick deflection. In the rudder and tailplane systems, any control movement is resisted by a separate feel unit, in which deflection from neutral pushes a piston against hydraulic pressure governed by the feel simulator according to q (dynamic head) pressure, which varies with airspeed and altitude. These hydraulic feel units may be cancelled by a cockpit switch, and are automatically disengaged by a landing-gear DOWN selection. Further centring forces are provided by coil springs in each feel unit, which remain operative in the event of loss of hydraulic pressure or pitot/static differential, and a non-linear spring unit in the rudder circuit applies additional centring force and feel to the pedals.

"Trimming is effected from cockpit switches, the rudder having a double switch on the port console and the other surfaces a four-way thumb switch on the control column. Each switch controls an electric actuator with a linear output which displaces the complete control run; the aileron trimmer is attached to the control-column torsion bar and the other units are linked to the autostabiliser/feel assemblies in the rear fuselage.”

This was one of the early products from the Aviation Division established at Borehamwood in 1953/54 and eventually transferred to Rochester.

Autopilot

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.

Autostabiliser

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.