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{{Use British English|date=August 2015}}
The '''Fighter Interception Development Unit RAF''' was a special [[interceptor aircraft]] unit of the [[Royal Air Force]] (RAF) during the [[World War II|Second World War]]. It was part of [[Air Defence of Great Britain]] and was previously the '''Fighter Interception Unit''' ('''FIU''').

== Origins ==
The Fighter Interception Unit was initially set up to evaluate technological advances such as [[aircraft interception radar|aircraft interception (AI) radar]] and other operational innovations, to counter increasing [[night bombing|night raids]] by the ''[[Luftwaffe]]''. The unit was formed at [[RAF Tangmere]] in April 1940, under the command of [[Squadron Leader]] George Philip (Peter) Chamberlain, with a strength of 5 [[Bristol Blenheim|Blenheims]] equipped with the latest AI Mk III radars. Operations initially consisted of daytime practice interceptions and operational night defence flights. The [[night fighter]] Blenheims were directed several times to possible targets, in the early days of [[ground-controlled interception]] (GCI) to acquire and then maintain a radar contact and finally to intercept target proved a very difficult task.

== Initial combat success ==
On the night of the 22/23 July 1940, they achieved the first aircraft-interception radar kill in history. A Blenheim Mk IF flown by Flying Officer G. Ashfield, with a crew of Pilot Officer G.E. Morris (Observer) and Sergeant R.H. Leyland (AI radar operator), patrolled the Sussex coast at {{convert|10000|ft|m|abbr=on}}. They were directed to a possible intercept by the controller at [[Poling, West Sussex|Poling]] [[Chain Home]] radar station who reported an incoming raid. Sgt. Leyland reported a response on the AI at a range of 8,000 feet and presently P/O Morris made a visual sighting of a [[Dornier Do 17]] to port and below the Blenheim. Ashfield closed the distance to 400 feet and then opened fire. Strikes were observed on fuselage and engines, the Dornier lurched to starboard and fell away, 5 miles south of [[Bognor Regis]]. The aircraft, a Dornier 17Z of ''2 Staffel'', ''[[Kampfgeschwader 3]]'', crashed into the sea and the crew was later rescued. The Blenheim was so close to the Dornier during the attack that the cockpit perspex was covered in oil, resulting in Ashfield losing control and the Blenheim flipping onto its back. He was able to regain control of the Blenheim at an altitude of 700 feet and landed at Tangmere just after midnight.

The unit was later also equipped with the [[Hawker Hurricane]] and were the first unit to receive the new [[Bristol Beaufighter]] (on 12 August 1940), still stationed at RAF Tangmere.<ref>[http://www.battle-of-britain.com/BoB2/Squadrons/raf_fighter_command_squadrons.htm Battle Of Britain RAF Squadron Numbers]</ref> Between 1940 and June 1944, some 21 victories were claimed by the FIU.

A FIU detachment was at [[RAF Newchurch]] to complement No. 150 Wing RAF with the [[Hawker Tempest]] V, where it tested [[Monica tail warning radar|Monica radar]] (known officially by the RAF as ARI 5664 and by the US military, as AN/APS 13), a range-determining, tail warning system, for night use.<ref>[https://web.archive.org/web/20071110040356/http://www.hawkertempest.se/storymonica.htm The Hawker Tempest Page]</ref> This special flight of Tempest V fighters was formed to counter the [[V-1 flying bomb|V-1 "Flying bombs"]] which had begun falling on south-east England. The flight operated mainly by night and claimed 86 ½ V-1s destroyed before being absorbed into [[No. 501 Squadron RAF]]. The FIU's Squadron Leader [[Joseph Berry (RAF officer)|Joseph Berry]] claimed 52 V-1s to become the RAF's top scorer against the flying bombs.

== Transition to Fighter Interception Development Squadron ==
On 23 August 1944, the FIU became the '''Fighter Interception Development Squadron''' (FIDS). By the latter war years the unit had become an element of the Night Fighter Development Wing (NFDW), which also included the Bomber Support Development Unit (BSDU) and the Fighter Experimental Flight (FEF), which specialised in "Ranger" (daylight [[Intruder (air combat)|intruder]]) operations with [[de Havilland Mosquito|Mosquitoes]]. Two [[Westland Welkin]] s served with the Fighter Interception Unit from May to November 1944, where they were used to evaluate suitability for high-altitude fighter operations. A two-seat night fighter version – the Welkin NF Mk II – was also evaluated but only two were produced was not ordered into production. During the closing months of the war the BSDU claimed four victories, the FIDS two victories and the FEF eight victories plus a large number of aircraft destroyed on the ground.

In late 1944, the Fighter Interception Development Squadron (of the Night Fighter Development Wing) carried out operational trials at RAF Ford (and later Manston) under the code name Operation Vapour to counter Heinkel He 111 H-22 aircraft of III/[[KG 53]] air launching V-1 flying bombs. A radar-equipped [[Vickers Wellington]] was modified for use by the Fighter Interception Unit as one of the first [[Airborne Early Warning and Control]] (AEW&C) aircraft.<ref>{{cite book |title=Britain's Greatest Aircraft |last=Jackson |first=Robert |year=2007 |publisher=Pen & Sword Books Ltd. |location=Barnsley |isbn=978-1-84415-383-1 |pages=217 }}</ref> It operated at an altitude of 4,000 feet over the North Sea to control Mosquito night fighters intercepting the [[Heinkel He 111]]s flying from Dutch airbases and carrying out airborne launches of the V-1 flying bomb. The modus operandi typically involved the Heinkels leaving bases in the Netherlands and flying out over the North Sea at a height of less than 300 feet. Once the Heinkels neared the East Anglian coast they would increase speed and release their flying bombs before turning for home at low level. To assist in detecting the Heinkels the Fighter Interception Development Squadron borrowed a Coastal Command Wellington equipped with a modified ASV Mk VI radar set and PPI to act as Airborne Early Warning and Control. After trials, low level night patrols off the north of the Netherlands were carried out by the Wellington with several Mosquito night fighters. For the night fighters to locate and keep station with the Wellington, the aircraft was fitted with a special homing beacon. Despite encouraging results, the Luftwaffe stopped air launches by mid January 1945, and the operational trials ended.

==Bases==
* [[RAF Tangmere]] 18 April 1940 to 20 August 1940
* [[RAF Shoreham]] 20 August 1940 to 1 February 1941
* [[RAF Ford]] 1 February 1941 to 3 April 1944
* [[RAF Wittering]] 3 April 1944 to 23 August 1944 <ref>[http://www.rafweb.org/Stations/Stations-W.htm Stations-W]</ref>

==Notes and references==
{{reflist}}

[[Category:Fighter aircraft units and formations of the Royal Air Force]]
[[Category:Royal Air Force night fighter units]]
[[Category:RAF squadrons involved in the Battle of Britain]]
[[Category:Military units and formations established in 1940]]

Fighter Interception Development Unit RAF - Revision history

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