Royal Air Force and replacement by the F-35, the RAF Lossiemouth-based 15 (Reserve) Squadron (the Tornado Operational Conversion Unit) was disbanded in March 2017. Lightning: On 23 November 2015 it was announced that a total of 138 F-35Bs will be ordered. 617 (The Dambusters) Squadron officially reformed on 18 April 2018 as an RAF Lightning Squadron. The first four aircraft arrived at RAF Marham on 7 June 2018. The Sentry AEW1, based at RAF Waddington, provides airborne early warning to detect incoming enemy aircraft and to co-ordinate the aerial battlefield. The Sentinel R1 (formally known as ASTOR – Airborne STand-Off Radar) provides a
No. 421 (Reconnaissance) Flight RAF were issued to 421 Flight and the Hurricanes went to other squadrons. No. 421 (Reconnaissance) Flight RAF No. 421 (Reconnaissance) Flight was a specialist RAF fighter flight created on 21 September 1940 to patrol the Channel and provide early warning of the types of incoming Luftwaffe raids from occupied France. It was later expanded to full squadron strength and renumbered as No. 91 Squadron on 11 January 1941. Its role led to its pilots being nicknamed "Jim Crows". In late September 1940, Air Chief Marshal Hugh Dowding was faced with two problems. The first was a change in Luftwaffe tactics.