Fighter Interception Unit The Fighter Interception Unit ( FIU ) was a special interceptor aircraft unit of the Royal Air Force ( RAF ) during the Second World War . It was part of Air Defence of Great Britain . The Fighter Interception Unit was initially set up to evaluate technological advances such as airborne interception ( A . I . ) equipment ( an on - board interception radar ) and other operational innovations , to counter increasing night raid s 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 Blenheims equipped with the latest A . I . 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 . However on the night of the 22 / 23 July 1940 they achieved the first airborne radar intercepted 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 ( A . I . operator ) , patrolled the Sussex coast at . They were directed to a possible intercept by the controller at Poling Chain Home radar station who reported an incoming raid . Sgt . Leyland reported a response on the A . I . 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 . The crew was later rescued from the sea . 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 . Between 1940 and June 1944 , some 21 victories were claimed by the FIU . A FIU detachment was at RAF Newchurch to complement the already formed 150th Wing with the Hawker Tempest V , where it tested the AN / APS 13 range - determining radar for night use . This special flight of Tempest V fighters was formed to counter the 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 claimed 52 V - 1s to become the RAF ' s top scorer against the flying bombs . 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 ) - comprising the ' Bomber Support Development Unit ' ( BSDU ) , the FIDS , and the ' Fighter Experimental Flight ' ( FEF ) , the latter specialising in day ' Ranger ' operations with 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 in an effort 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 . 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 Holland and flying out over the North Sea at a height of less than 300 feet . Once the Heinkels neared the East Anglain 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 initial trials , low level night patrols off northern Holland were carried out by the Wellington with several Mosquito night fighters . In order 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 .