The Blind Landing Experimental Unit, abbreviated BLEU, was a unit of the British government tasked with creating an early autolanding system for military and civilian aircraft from the late 1940s until the mid-1960s.
Background Pilots in the early days of aviation relied on dead reckoning to find out where they were flying, which proved difficult or impossible at night or in bad weather. A 1925 United States Post Office study found that 76% of its forced landings were due to weather, which highlighted the early need for a system to facilitate blind landings. Early suggestions at addressing the problem ranged from using primitive radio signals to placing emergency landing strips periodically near major highways. Experiences from the Second World War called more attention to the problem. Bombers based in Britain would sometimes return to their home bases in the early morning hours to find all of their landing sites completely fogged in, which resulted in the senseless loss of airframes and aircrews. In the post-war period, aviation experts knew that there would be benefits to both military and civilian flyers, as military missions would be made possible in all conditions and airlines could avoid the wasteful and expensive practice of diverting away from fogged in airports.
Several blind-landing systems had been developed in the pre-war era, notably the US Diamond-Dunmore system and the German Lorenz beam concepts. Both relied, to some degree, on the voice radios in the aircraft, which were common on larger aircraft of the era. Diamond-Dunmore appears to have seen no active uses, but the Lorenz system was deployed at major airports in Germany, the UK and other European locations and their related foreign fields. These generally operated in common medium-wave frequencies on the order of 300 to 400 kHz, frequencies that were, by the mid-war era, on the decidedly low-frequency range. The optical resolution of any system is a combination of the wavelength and the size of the antenna system, so using these frequencies resulted in relatively low accuracy. During the war, experiments were carried out with similar systems working at VHF frequencies around 100 MHz and a variety of such systems were used at many military airfields.
Origins of autoland
Before the formation of BLEU, an automatic landing was made at the Telecommunications Flying Unit (TFU) of the TRE at RAF Defford in a Boeing 247D aircraft, DZ203, early in 1945, using the American SCS 51 radio guidance system. Landings were made in complete darkness, with no landing lights and all other lights obscured by the wartime blackout. There was no flare-out; the low approach speed and shallow glide angle meant that the aircraft could be allowed to fly straight onto the ground.R W Howard (1973) Automatic flight controls in fixed-wing aircraft – The first 100 years, The Aeronautical Journal, November 1973, p. 549McDonald J A (1964) Flight International, Letters, p. 727 30 April 1964 SCS 51 was the basis for the Instrument Landing System (ILS), adopted by ICAO in 1948.Sir John Charnley (1989) Presidential Address: Navigation Aids to Aircraft – All-weather Landing, The Journal of Navigation, Vol. 42, No. 2 p. 166 http://journals.cambridge.org/abstract_S0373463300014405
There was an alternative system to the SCS 51 from Col. Moseley and that was a radar-based system wholly devised, developed and trialled by F/O L. C. Barber and his colleagues at Defford.T.F.U. Report 70, February 1945. Report on Controlled Approaches and Landing Trials carried out at Defford, Nov. 1944 – Feb. 1945. [UK National Archives AVIA 7/1599] This system effectively gave range and height data which could be added to autopilot heading info.
The Blind Landing Experimental Unit (BLEU) of the Royal Aircraft Establishment (RAE) was formed at RAF Woodbridge and RAF Martlesham Heath during 1945 and 1946. It was a multi-disciplinary unit, drawing staff from the RAE, Farnborough and the Telecommunications Research Establishment, Malvern (TRE).Michael D. Dobson ed., (2001), Wings over Thurleigh, Chapter 8 p. 123 The terms of reference were that the unit "will operate as a satellite of the RAE and will be responsible for the development on blind approach and landing of RAF, Naval and Civil aircraft".T. W. Prescott (1976) BLEU – The first decade. RAE Tech. Memo, FS 77 p. 21 [UK National Archives AVIA 6/25422] Research during the first few years at BLEU led to the conclusion that a promising approach to blind landing would be a fully automatic system, and produced a definition of the requirements for such a system, later designated Autoland.
The Instrument Landing System (ILS) was introduced in the post-war era based on the SCS 51 concepts. This used two separate radio signals, one for lateral guidance, the "localizer" and the other for vertical guidance, the "glideslope". They both worked on the same basic principle; each of the signals was broadcast on a separate carrier frequency with a fixed relationship so that the glideslope frequency was always a fixed amount away from the localizer.
The signals were split on their way to their respective antennas and amplitude modulated with a low-frequency signal, 90 Hz or 150 Hz. The two signals were then sent out of directional antennas that produced large broadcast patterns aimed slightly left (90) and right (150) of the runway mid-line or above (90) and below (150) the glideslope. The patterns were relatively wide and aimed such that they overlapped in the center, indicating the proper line to fly.
While the system worked and was relatively simple to implement using 1940s technology, it was not accurate enough to provide guidance during landing, with accuracy on the order of a few hundred feet. ILS approaches ended at 200 feet over the runway, by which time the pilot had to be able to see the runway visually, or call off their approach. This was not nearly accurate enough for a truly automated system.J. S. Shayler (January 1961) Radio Guidance Elements of the B.L.E.U. Automatic Landing System for Aircraft, Journal Brit. I.R.E., U.D.C 621.396.933.23.
BLEU's first attempt at solving the problem was to place two cables, one mile long, extending along each side of the runway, similar to the Ambrose Channel pilot cable. A detector in the aircraft could see the signal from the cables and align itself very accurately along the midline of the runway. For vertical guidance, a new FM radio altimeter that BLEU developed was capable of resolving height differences to 2 feet at low altitude.J. S. Shayler (1958) The application to civil aviation of an automatic landing system, R.A.E Technical Note No. BL.47 pp 4 & 8. [UK National Archives AVIA 6/17826] The team safely conducted thousands of test landings using this system.
BLEU realized that most airports did not have room to place one-mile cables, so they continued working on a radio-driven solution. In collaboration with Smiths Industries Ltd., BLEU also developed coupling units to derive the commands to the autopilot from the guidance signals, and auto-throttle.
Components of the system were developed separately on several types of aircraft, including the Lancaster, Viking, Devon and the Albemarle. A demonstration of the techniques used was given to military and government representatives in May 1949.B.L.E.U. Operation Order No. 1/49, 12 May 1949 [UK National Archives AIR 29/1295] By 1950 the entire system had been installed on a DH Devon and the first demonstration of Autoland was given on that aircraft on 3 July 1950.Michael D. Dobson ed., (2001) Ibid. p. 126 Over the next 20 years, BLEU in conjunction with UK industry and the UK airworthiness authority, was responsible for almost all of the pioneering work needed to convert the concept of those experimental demonstrations into safe, accurate blind landings by large transport aircraft.Sir John Charnley (1989) Ibid. p. 165 The system in use during the early 2000s is basically the same as that used experimentally in 1950. The following diagram, from J S Shayler's 1958 memorandum, shows how the different components of the system, and guidance signals, were used during the consecutive phases of an automatic landing.
BLEU during the 1950s and 1960s During the early 1950s, as a preliminary to the development of the full Autoland system, automatic approach trials were carried out on Valetta, Meteor and Canberra aircraft. The Canberra, VN799, was acquired in 1953 but was a write-off following a crash landing in August that year due to a double engine failure, fortunately without serious injury to the crew.
At that time, Autoland had lower priority because efforts were concentrated on other projects including rapid landing of aircraft for RAF Fighter Command, visual aids for pilots, runway approach lighting and an approach aid using DME with Barbro. That changed when Operational Requirement 947 (OR947) for automatic landing on the V-Force bomber fleet was issued in 1954.T. W. Prescott (1976) Ibid. p. 19 At that time the V-bomber force was the UK's main contribution to the strategic nuclear power of the westDefence White Paper, 1960, para. 36 [UK National Archives CAB 129/100] and all-weather operation was essential. There was also renewed interest in automatic landing for civil aviation. As the next step in the development, the flare-out and coupling units from the Devon were linked to a Smiths Type D autopilot and installed in Varsity WF417, a much larger aircraft, capable of carrying 38 people rather than 10 in the Devon. The first fully automatic approach and landing was made by WF417 on 11 November 1954 under calm and misty conditions.RAF Martlesham Heath, Operations Record Books, 1951–1955 [UK National Archives AIR 29/2364] A similar system was installed in Canberra WE189 to provide the first application of Autoland to jet-type aircraft.T. W. Prescott (1976) Ibid. p. 41 Automatic approachesJ. W. Birkle (1956) Automatic Approach Tests of a Type D Autopilot in a Canberra T.4 Aircraft at Woodbridge Airfield, R.A.E. Technical Note No: BL.41 [UK National Archives DSIR 23/24675] and automatic landingsRAF Martlesham Heath, Operations Record Books, 1956–1957 [UK National Archives AIR 29/2860] were recorded by WE189 but the development was interrupted in April 1956 when the facilities at Woodbridge, which had the only suitable leader cable installation, ceased to be available to BLEU. Development of auto-flare and automatic kicking-off drift was continued at RAF Wittering, but in September that year WE189, returning from tests at Wittering, crashed due to engine failure on an approach when returning to its base at Martlesham Heath. The pilot, Flt. Lt. Les Coe, and the BLEU scientist in charge of the project, Mr. Joe Birkle, were killed.