British Airborne Early Warning Aircraft (repost)

[Bernard Woolley]

British Airborne Early Warning Aircraft


Fixed Wing.


Fairey Gannet AEW.3
Procured to replace the Doulgas Skyraider AEW.1, the Gannet used the same AN/APS-20 (in fact the actual same radar sets). The AEW.3 operated from Royal Navy carriers until the decommissioning of HMS Ark Royal. Serious consideration was given to bringing them back into service during the Falklands War, but without a suitable carrier this was not possible. The Sea King AEW.2 (q.v.) would eventually fill the gap.
In 1984 five Gannets survived in museums. Three of these aircraft were made airworthy and refitted with spare radar sets. They operated from RNAS Yeovilton, providing radar coverage over the Western Approaches until 1987, when a lack of spare parts grounded them.


Avro Shackleton AEW.2
Very much intended as a ‘stop-gap’ aircraft, procured while the Nimrod AEW.3 (q.v) was developed. Problems with the latter meant that the cramped, loud Shackleton remained in service far longer than planned. The AEW.2 was fitted with the radar and electronic systems from the Gannet AEW 3 (which, as noted had come from the Skyraider). By 1984 the Shackleton and its systems were beyond obsolete, but they were all the RAF had and the crews of the aircraft performed miracles with aircraft and electronics that were over forty years old. Once more modern AEW aircraft entered service, the Shackleton was not retired, but the survivors relieved the Gannet in providing radar coverage over the Western Approaches and Irish Sea. The AN/APS-20 radar might well have been obsolete, but it was still capable of picking up periscopes and snorkels. The AEW.3 could also still carry Lindhome Gear, which meant it was useful for Search and Rescue Work. The last Shackletons were retired from RAF service in 1991.


BAe (Hawker Siddeley) Nimrod AEW.3
A somewhat notorious aircraft, possibly unfairly. The Nimrod was fitted with a Forward and Aft Scanner System (FASS) that used Frequency Modulated Interrupted Continuous Wave radars. In theory at least, the FASS and FMICW radars had an advantage over the American AN/APY-1 radar used on the E-3A. The scanners were not blanked out to the same degree as the saucer on the Sentry and moreover, the FMICW radar had the potential to provide better overwater performance and had a superior capacity to track rotary winged targets. In practise, however, the overall system proved to be troublesome. The main computer could not cope with all the inputs and got confused by ground targets, such as cars, and routinely overloaded. GEC-Avionics did solve the worst of the problems and when six Nimrods were put into service in 1986, the aircraft did actually meet the requirements that the RAF and MoD had laid down for the project. However, the RAF was never satisfied with the AEW.3 (it had always really wanted the E-3A) and the computer system was always working right on the edge of its full performance. The Nimrod airframe was also too small to install any more computer power. Stories of the cooling system was flawed seem to have been a red herring. [NOTE 1]


BAe (BAC) VC10 Guardian AEW.1
With the Nimrod proving to be an unreliable platform, although the overall performance of the radars and associated systems was good when it did work, the RAF sought an AEW platform that had greater growth potential and reliability. Fortunately, British Aerospace had discovered the VC10 jigs gathering dust in one of their more remote facilities and had put a version of the aircraft known as the ‘modular VC10’ into production to meet a number of requirements. The original work on that variant had included an AEW version which essentially used the same systems as the Nimrod AEW.3. One advantage that the Guardian AEW.1 had over the Nimrod was that it was a larger aircraft, which meant that there was space to install a more powerful computer, which eliminate the issue of the Nimrod suffering overloads at critical moments. However, very soon after entering service a serious problem was discovered with vibration affecting the rear scanner. Since the Guardian was powered by a pair of rear mounted RB.211 engines, there was nothing that could be done about this, other than trying to modify the radar scanner and software to be better able to cope with the vibration. Like the Nimrod, the Guardian would frequently fly with GEC-Avionics technicians onboard to fix problems caused by vibration and it was not unknown for aircraft to operate with only the forward radar scanner operational.
With the improved computing power, the RAF did experiment with pointing the radars towards the ground and telling the computer to ignore targets moving above a certain speed, rather than vice-versa. It was found that the Guardian was very capable of tracking vehicles, such as tanks and three Guardians were deployed to Canada with tracking enemy ground vehicles as their primary mission.
Post-war the RAF did explore fitting the AN/APY-1 radar system to the Guardian and the US DoD did offer the system at a very affordable price. However, the Guardians had been used very hard during the war and had used around half of their airframe life. BAe also reported that significant strengthening of the aircraft’s spine would be needed, so that plan was abandoned. [NOTE 2]


BAe.847 Sentinel AEW.1
With the Guardian AEW.1 only having a limited life left, unless an expensive rebuild programme was undertaken, the RAF began to look at alternatives. The obvious replacement was a version of the Sentry, indeed Boeing had set aside a designation, E-3D, for a British version of the aircraft. However, the 707 airframe was now out of production and the only option available was to use second-hand airliner airframes, which the RAF was not very keen on. Fortunately, BAe, which had recently become a full partner in Airbus (having previously been a sub-contractor), through events which will be explored elsewhere, offered a new AEW aircraft based on the A300C4, which was also offered as a Multi-Role Tanker Transport (again, something that will be explored elsewhere). During the war GEC-Marconi had been working on a second generation FASS with a new phased array radar and improved computer system. Computer technology had moved on in leaps and bounds during the Third World War and the old issue of the computer being overwhelmed by too much input was very much a thing of the past. The computers fitted to the Sentinel were, in fact, far smaller than those fitted to the Nimrod and Guardian yet had many times the computing power. BAe would also fit the same FASS and Mission Systems Avionics to a version of the Airbus A310-200, which it designated the BAe.844. That aircraft was sold to Australia, Canada and (in modified form) France. [NOTE 3]
Like the Guardian, the Sentinel could also be used to track ground targets, however the RAF chose to procure a dedicated variant of the aircraft, the R.1. That variant was fitted with a ventral canoe that contained a Raytheon dual-mode synthetic-aperture radar / moving target indication (SAR/MTI) radar known as Sentinel Dual Mode Radar Sensor (DMRS) and a hump on the spine aft of the cockpit for satellite communications and a datalink.


Airbus Military Voyager AEW.4
While the Sentinel was an excellent aircraft, the RAF chose to take advantage of improvements in airborne radar technology that took place shortly after the turn of the century. Namely the phased array radar and Mission Systems Avionics being developed for the E-7B ‘Wedgetail’, an upgrade of the RAAF’s BAe.844 fleet (designated the E-7A). The Australian programme happened to coincide with the RAF procuring the A330 Voyager tanker to replace the last of its VC10s. Rather than choose to upgrade the Sentinel with the system, which would have involved a lot of structural work, the RAF chose to integrate the ‘Wedgetail’ radar and MSA on the Voyager airframe. The airframe of the Voyager would give a great deal of space for the battle management and control staff to work with and the AEW.4 was even fitted with a pair of wing mounted refuelling pods, which meant it could, if necessary, refuel any escorting fighters.
The Sentinel aircraft that the Voyagers would replace still had more than half of their airframe life left in them. Some of them were converted to tanker/transports, while others were converted to Sentinel R.2 ELINT/SIGINT standard. [NOTE 4]


Rotary Winged.


Westland Sea King AEW.
Rapidly developed during the Falklands War, the Sea King AEW.2 filled a gap that had existed since the retiral of the Gannet AEW.3. Based on the airframe of the HAS.2, the AEW.2 was fitted with a Searchwater radar fitted in an inflatable bag, that could be lowered below the aircraft when it was in flight. Not as capable as fixed wing aircraft like the E-2C Hawkeye, the AEW.2 proved to be very effective, operating from all classes of Royal Navy carriers, MAC ships and occasionally from land bases. During the war, production switched to the AEW.5, which was based on the HAS.5 airframe. Existing Mk.2 helicopters were brought up to that standard. Post-war a major upgrade of the helicopter, the ASaC.7, was developed. Surviving Mk.5 aircraft were brought up to that standard as well as new aircraft built. The Mk.7 was a fully digital aircraft, using the upgraded Searchwater 2000AEW radar and new MSA. During operations over Iraq and Afghanistan, it was discovered that the new radar was capable of tracking ground targets and Moving Target Indicator software was added to the Mk.7. That last iteration of the Sea King AEW would continue to fly from vessels of the Royal Navy until 2018.

Westland Augusta Merlin HM.2 ‘Crowsnest’
When it came to replacing the Sea King ASaC.7, several options were studied, including the E-2D Hawkeye and a version of the MV-22B Osprey. However, the most cost-effective solution was to use a variant of the Merin helicopter. The AEW variant, to be procured under Project Crowsnest, was to have been originally designated the ASaC.5. However, the Crowsnest system, which has an inflatable ‘bag’ like the Sea King, can be installed on any Merlin Mk.2. Which means that there is no longer a dedicated fleet of AEW helicopters. A ‘Crowsnest’ Merlin going into deep maintenance may emerge as a standard Mk.2, or vice-versa. It was even possible to rapidly reconfigure Merlins aboard a Queen Elizabeth class carrier, should the air group need to be reconfigured.
Merlin ‘Crowsnest’ is likely to be the last in the line of rotary winged AEW aircraft in Royal Navy service. A project for an unmanned AEW aircraft capable of operating from the QE class was in its early stages in 2022. However, it is likely to be at least a decade before any prototype flies.



*

1) See Gibson, Chris, ‘The Air Staff and AEW – Royal Air Force Airborne Early Warning Projects’ (Blue Envoy Press 2013), p.37.

2) AEW and other VC10 variants are explored in Gibson, Chris, ‘Vickers VC10 – AEW, Pofflers and Other Unbuilt Variants’ (Blue Envoy Press, 2009).

3) See Gibson, ‘The Air Staff and AEW’ etc, ps.34-35.

4) British ELINT/SIGINT aircraft will be explored in a future fact-file.