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Blind Landing Experimental Unit (BLEU)
The Blind Landing Experimental Unit
(BLEU) of the Royal Aircraft Establishment (RAE) was formed in 1945, initially at RAF
Woodbridge but moving to RAF Martlesham Heath in early 1946. It was a multi-disciplinary
unit, drawing staff from RAE Farnborough and the Telecommunications Research Establishment,
Malvern (TRE), tasked with the development of blind approach and landing of RAF, Naval and
Civil aircraft.
The system developed by BLEU used
radio guidance signals from an early Instrument Landing System (ILS), which defined the
extended centre line of the runway and a 3 degree approach path to the runway. A magnetic
leader cable system was used for azimuth guidance during the final stages of the approach,
with a radio altimeter being developed for height guidance during the flare-out, and an
automatic throttle system to control aircraft speed.
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Components of the system were developed separately on several types of
aircraft and by 1950 the entire automatic landing system had been installed and
demonstrated on a Devon aircraft. In 1953 the development extended to a Canberra jet
aircraft, although at that time, automatic landing had a low priority, with effort
concentrated on other projects including rapid landing of aircraft for RAF Fighter
Command. This changed when an Operational Requirement for automatic landing for the
V-Force bomber fleet was issued in 1954.
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| Devon WF984 at Martlesham Heath |
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The complete
system was then installed on a larger (Varsity) aircraft, with the
first fully automatic approach and landing in the Varsity being made
on 11 November 1954. Development of the full system continued at Martlesham Heath until
early 1957 when BLEU moved to Thurleigh (RAE Bedford). |
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Varsity WF417 autolanding |
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Autoland development continued
using three Varsity aircraft and a third Canberra which replaced the two lost through
engine failures. The system was demonstrated to many organizations and individuals,
including the Duke of Edinburgh in 1959.
By October 1958, BLEU aircraft had completed over 2,000 fully automatic landings. |
Canberra WJ992 autolanding at Thurleigh |
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The project to provide Autoland for the V-bomber force ran in parallel with the Varsity and
Canberra work, with a Vulcan aircraft being fitted during 1959. Development and flight testing
of Autoland progressed successfully, with the Vulcan system being accepted for military service
in 1961.
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Vulcan XA899 on approach at Thurleigh |
The safety level required for
Military Autoland was specified as a failure rate no greater than 1 in 120,000 landings.
A single channel system (one of each of the component parts) was adequate to meet this
failure rate, and therefore a single component failure would require the pilot to take
manual control and either land the aircraft visually, or carry out a missed approach.
An intensive program was carried out to establish the ability of pilots to recognize and
successfully recover from system failures. In addition to flight trials in simulated fog
conditions, a BLEU simulator developed specially for research into low visibility approach
and landing was also used.
Considerable research was carried out into ways of providing the pilot with augmented
visual information on a cockpit screen (Televiewer), and providing monitoring information
overlaid on the outside view (Head Up Display).
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Another area of research focused on the visual aids needed to enable the pilot to monitor the
aircraft situation during the final stages of an approach, including during the flare-out.
In addition sufficient runway visual cues were needed to allow the pilot to take-off safely
and to keep the aircraft straight after touchdown while bringing the aircraft to a stop.
These visual aids were also vital for pilot detection of any system failures. The lighting
pattern developed and flight tested by BLEU, in conditions down to zero cloud base and 50 feet
forward visibility, was eventually adopted as the International Civil Aviation Organisation
(ICAO) standard for low visibility operation, and is now installed at major airports worldwide.
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Approach and Runway Lighting |
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Civil Autoland in BLEU
The development of a Military Autoland capability had been closely monitored by civil airlines. Thick radiation fogs during the winter months were common in Europe, and were made worse by smoke close to major cities (e.g. London “smog”). The delays and diversions caused by these conditions were very expensive for the airlines, particularly for BEA with its home base at London
Heathrow.
During the late
1950s and early 1960s increased cooperation between BLEU, the UK Air
Registration Board, the aviation industry and airlines, led to the Air Registration Board defining the safety requirement for Autoland as no more than one fatal accident in 10 million landings (10 times safer than being achieved by pilots landing manually).
To meet such a stringent safety requirement, the system had to be able to tolerate failures during Autoland. This could be achieved using a “triplex” system (three independent channels with one being disconnected if it disagreed with the other two), or a duplex monitored system with failures being detected in a similar way.
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It was recognised that leader cable would be impractical to install
at civil airports and considerable effort went into improving the ILS localizer
throughout the 1950s. By the early 1960s radically new aerial designs for the ILS
transmitters improved ILS to such an extent that leader cable was no longer
required. BLEU played a leading role in the ILS Feasibility Study
undertaken to prove that ILS could meet the stringent accuracy,
integrity and reliability requirements for civil autoland.
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ILS Localiser aerial on Runway 27 |
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In 1961 the
US Federal Aviation Authority sent a Douglas DC-7 to RAE Bedford for the BLEU system
to be installed and flight tested. After that, and further tests on return to
Atlantic City, the FAA strongly supported the UK fully automatic solution to the
all-weather landing problem. |
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DC-7 N464 autolanding at Thurleigh |
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The BLEU fleet was up-dated to
be more representative of civil airliners. A DH Comet 3B (XP915) was acquired in 1961, and a
DH Comet 2E (XV144) fitted with a full “triplex” system, in 1966.
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Comet XV144 autolanding in fog |
In 1963 BLEU was awarded the Cumberbatch Trophy for its outstanding contribution to air
safety.
Methods were developed for
measuring Runway Visual Range (RVR) and Slant Visual Range (SVR), and trials undertaken to
determine pilot monitoring and take-over capabilities in all visibility conditions.
This work was an important input into the definition of a set of Categories for all
weather operations, specifying the Minimum Decision Height and Minimum RVR required for
each Category, adopted by ICAO in 1965.
In 1972 the Comet aircraft were
replaced by a BAC 1-11 XX105 fitted with two channels of the Trident triplex system.
The last of the Varsity aircraft (WF417), which had been the “workhorse” for most of
the BLEU trials, was replaced by a HS 748 XW750.
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The introduction
of Autoland in BEA’s Trident fleet required a huge effort by BEA, Hawker Siddeley
Aviation, Smiths Industries and BLEU. Certification progressed from Category 1 in
1965, through Category 2, 3(a), 3(b) and finally, Category 3(c) in 1979. During
this time some 40,000 automatic landings were analysed.
A triplex system was also developed by Smiths and BLEU for the RAF’s Belfast freighter.
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Trident Two G-AVFA at Thurleigh, March 1968 |
BLEU were world leaders in the development of automatic landing, and the
systems provided for low visibility operation in today’s civil and
military aircraft are essentially the same as developed by BLEU.
In 1974 BLEU was
renamed Operational Systems Division, part of Flight Systems Department.
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