Hawker Siddeley (Avro) 748, XW750 (formerly with Civil registration G-ASJT)
Bedford’s Hawker Siddeley HS748, initially registered G-ASJT, was an early production aircraft designed by Avro in 1958. It was
first delivered to
Smiths Aviation Division on 4 December 1963 and operated from Staverton Airport. Smiths used it for the development
of the Electric Pilot Series 6 (SEP6) and Flight Director which equipped the
Fokker F-28 aircraft and later the HS Nimrod. After
G-ASJT became surplus to requirements at Smiths it was acquired by RAE in 1970. In 1971 it was transferred to Thurleigh for use by
the Blind Landing Experimental Unit and post-1974 with Flight Systems Department. At this time it was added to the military register
as XW750, but still in Smiths' livery, as illustrated opposite. It remained an invaluable research asset at Thurleigh until the
airfield closure in 1994 when it was transferred with the other Bedford research aircraft to Boscombe Down.
XW750, fitted with the Smiths SEP6 Automatic Flight Control System, provided a flexible interface for experimental equipments under test at Thurleigh, for specific research flight trials and for demonstrations. With the addition of the RAE MODAS standard flight recording system, XW750 provided a very capable workhorse; with significant contributions to many research programmes over a period of 35 years. Following a repaint in RAE colours, shown opposite, it was a particularly suitable facility for general avionics research. The SEP6 was modified with suitable changes to allow automatic landings to touchdown from a variety of experimental guidance systems. In addition to the specific research programmes, XW750 supported Industry who made extensive use of the aircraft to develop prototype equipment such as the Monocular Head Up Display (MonoHUD) and the Mil Std 1553B data transmission system. Such was the flexibility of the aircraft, different research programmes could be flown on any day.
The two main programmes to which XW750 contributed were firstly what was known as the UK Economic Cat III programme in the period 1975 - 1980 and secondly the developments which led to the passive terrain-following system and the CO2 Laser Radar (CLARA) between 1983-2003. The passive terrain-following system and CLARA continued further development with Tornado ZA326.
The purpose of this programme was to provide an airfield recovery technique for use in all weather conditions but without the complexity and cost to the operator of a full airworthy automatic landing system. The premise was that if the aircraft was delivered accurately to the airfield by a reliable guidance system, a decision by the pilot at a height of 50ft for landing in runway visual range conditions of 200m was considered realistic. However the regulator, the Civil Aviation Authority, was not receptive to authorising manual landings in Cat III weather conditions so the research focussed on a simpler implementation of the full system. This extension to the SEP6 together with the use of a MonoHUD to provide the pilot with critical information offered an effective application.
Between 1973 and 1974 XW750 first demonstrated airfield approaches at Thurleigh using an early form of Doppler Microwave Landing System (DMLS) guidance to investigate automatic approach performance. At the time the UK needed a demonstration aircraft to present the merits of the DMLS, and XW750 was selected as it had this capability. The first demonstration at an operational airport was made at Gatwick where DMLS had been installed as part of the UK demonstration programme. Demonstrations using DMLS guidance, comparing the performance with the standard Instrument Landing System (ILS) on 31 August and 1 September 1977 were successful. These were the first automatic landings using DMLS at a civil airport and the publicity helped the UK's DMLS profile as a viable operational system. Other demonstrations followed and XW750 is shown on the approach to Berne airport with its restricted runway, surrounded by mountains. Further demonstrations were made at Manchester airport, Mehrabad airport, Iran, and at Montreal airport during the International Civil Aviation Authority (ICAO) meeting in April 1978 to select the future MLS technique. To further support the UK submission for the ICAO competition, automatic segmented approaches which followed natural features or areas of low population were demonstrated. Although the UK system was considered a better technical solution, the US Time Reference Scanning Beam (TRSB) system won the competition by a narrow margin.
Head Up Displays (HUD) had shown their merit in military aircraft but little interest in this equipment had been shown by civil aviation up to 1979. In an attempt to break into this potential market Marconi Avionics (MAv) developed the MonoHUD primarily for the retrofit civil market where space constraints introduced installation difficulties. As part of the Economic Cat III programme, the MonoHUD was fitted to XW750 to investigate its value for manual and automatic landings in low visibility. XW750 first demonstrated the MonoHUD in fog approaches and landings in runway visual ranges down to 200m at RAF St Mawgan, Cornwall, in conditions where landings would have otherwise been impossible without it. This was followed in May 1980 by demonstrations at Manchester and Thurleigh in RVRs down to 200m and at Birmingham Airport in 100m RVR. The MonoHUD installation in XW750 is illustrated on the left hand side of the cockpit; the SEP6 control panel is at the bottom of centre console of the left hand photo. The photo on the right shows its operation by Sq Ldr Dennis Stangroom. When not required the MonoHUD could be hinged out of the pilot's field of view. With the introduction of flat panel electronic displays to replace dials in civil aircraft, as pioneered by the BAC1-11 XX105 aircraft programme, the principle of the MonoHUD was eclipsed.
By January 1991 the major programme using XW750 was work on a covert technique for low-level navigation and the detection of cables
between pylons. XW750 trialled the early development of a map-referenced navigation system combined with Global Positioning
System (GPS) navigation and the development of a multi-mode CO2 Laser Radar (CLARA).
The map-referenced navigation system was designed to allow safe low-level flight without the need for forward-looking radar to define the terrain in front of the aircraft. This method was made possible by the developing technique of digital mapping, the ability to store large quantities of data, accessibly, large quantities of data, linked with GPS and other conventional navigation aids. The aircraft installation, including the MonoHUD, provided a demonstration of the principal benefits of a highly integrated system. It involved the specification and procurement of data bases, extensive algorithm design and requirements capture for future systems.
To underpin the fidelity of the data bases and the overall system integrity, the development of a covert sensor to detect obstacles not included in the data base was introduced. This sensor began development with the LOCUS system, a CO2 laser radar developed by GEC to meet a US Navy requirement for obstacle detection. With the basics established with LOCUS, CLARA became the product definition. An early development of CLARA is illustrated in the nose cone of XW750, the aircraft also having high visibility propellers (picture right).
CLARA was specific to the detection of cables and the development was done in collaboration with France, Dassault Electronique, with GEC as the prime UK contractor. Two sets of equipment were produced, one for each nation. Development flying in XW750 continued into the late 1990s, successfully demonstrating the world’s first multi-mode laser radar. CLARA demonstrated a highly effective method for obstacle detection including cables in front of the aircraft's flight path. These systems were selected by the RAF for Tornado GR4 equipment update but because of cost it was not implemented.
In the early 1970s, to alleviate noise problems around airports, XW750 was used to investigate steep 6 degree approaches, two-stage flare manoeuvres to reduce passenger discomfort and provide more precise landing. The steep approaches required the development of experimental visual aids to provide pilot guidance close to the ground. These aids included the first trials of what became the highly successful Precision Approach Path Indicator (PAPI) now a standard feature at most airports in the world.
In support of the accident investigation of the Dan Air HS748 (G-KEBF) crash during take-off from Sumburgh, Orkney Islands in July 1979 with the loss of 17 lives, XW750 performed flight tests to establish the cause of the accident. The investigation centred on the elevator gust lock mechanism which had become engaged during pre-flight checks and which had contributed to loss of control.
On 30 September 1982 a Mil Std 1553B digital data transmission system was flown for the first time in the world in XW750. The equipment was designed and manufactured by Smiths Industries. This equipment replaced heavy and bulky cable looms and this data-bus standard was widely used until it was surpassed by higher transmission data rates.
XW750 was a highly effective flying laboratory for all its programmes, continuing the legacy of its first owner at Smiths Industries.
XW750 eventually became surplus to research requirements and was sold at auction. It did not suffer the ignominy of being scrapped and went to Robin Hood Airport in an attractive new livery, bearing the registration number N748D.
It was later transferred to Southend Airport where the fuselage was repainted, shown opposite, and where it has remained in storage since 2008, understood to be the oldest airworthy HS748.
This page last updated - 07/06/2023
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