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The Royal Aircraft Establishment at Bedford

The BLEU Automatic Landing System

The BLEU Automatic Landing System required the following ground and airborne equipment:-

(a) Instrument Landing System (ILS) localiser and glide path transmitters to provide lateral and vertical approach guidance.
(b) Magnetic Leader Cable signals to provide lateral landing guidance.
(c) Radio Altimeter to provide vertical landing guidance.
(d) Gyro Compass for drift removal at touchdown.
(e) Automatic Pilot for controlling the aircraft to the guidance signals.
(f) Automatic Throttle for controlling airspeed.
(g) Flight Instrument Display for presentation of pilot monitoring information.

Aircraft carrying out automatic landings go through a sequence of autopilot/autothrottle flight control phases, each of which is briefly described below. Figure 1 (from a paper by J S Shayler in 1958) shows a simplified schematic diagram of the main elements and phases of the BLEU automatic landing system.

Figure 1 : BLEU Automatic Landing System

The airfield is approached (A-B) with the automatic pilot using barometric height to maintain a constant aircraft altitude (1500 feet for example), and using the ILS localiser signal to capture and then maintain the extended runway centreline (TRACK phase). The automatic throttle accurately controls the approach speed to the value selected by the pilot. When the ILS glide path beam is intercepted (at B), descent is started (GLIDE phase) with the barometric height control being disconnected and the aircraft being controlled to follow the ILS glide path beam (normally defining a 3 degree descent path to the glide path aerial). The ILS localiser signal is still used for lateral guidance.

At a height of approximately 300 feet the aircraft enters the coverage of the Leader Cable signal (at C) and the lateral input to the autopilot is automatically switched from ILS localiser to Leader Cable (LEADER CABLE phase). In elevation the aircraft continues to be controlled to the ILS glide path.

At a height of about 100 feet (D), the ILS glide path signal is disconnected and the aircraft controlled to a mean pitch datum automatically computed while flying down the glide path (ATTITUDE phase). This continues for a few seconds down to a height of about 60 feet (E), when vertical control is transferred to the radio altimeter (FLARE phase), with the rate of descent being gradually reduced to give a smooth touchdown. The throttles are automatically closed at a constant rate down to the engine safe flight idling speed.

At approximately 20 feet (F), the Leader Cable signal is disconnected, the wings levelled and rudder applied to automatically remove any drift due to a crosswind (KICK OFF DRIFT phase). After touchdown the pilot disengages the autopilot and steers the aircraft along the runway (G-H), using either visual information or a steering director driven by a combination of Leader Cable signal and compass heading. The pilot also applies manual braking to bring the aircraft to a safe taxying speed.

 Automatic approach in BAC 1-11 (XX105)

The automatic landing systems in use today are basically the same as that described above, except that the ILS localiser signal has been improved to a level where Leader Cable is no longer required, and at suitably equipped airports, Microwave Landing System (MLS) guidance is used instead of ILS.


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