Major fuselage changes were made, including an internal weapons bay, an extended nose for radar, a new tail with electronic warfare (ESM) sensors mounted in a bulky fairing, and a MAD (magnetic anomaly detector) boom. After the first flight in May 1967, the RAF ordered 46 Nimrod MR1s. The first example (XV230) entered service in October 1969. A total of five squadrons using the type were established; four were permanently based in the UK and a fifth was initially based in Malta. R1 Three Nimrod aircraft were adapted for the signals intelligence role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974. The R1 was visually distinguished from the MR2 by the lack of a MAD boom. It was fitted with an array of rotating dish aerials in the aircraft's bomb bay, with further dish aerials in the tailcone and at the front of the wing-mounted fuel tanks. It had a flight crew of four (two pilots, a flight engineer and one navigator) and up to 25 crew operating the SIGINT equipment. Only since the end of the Cold War has the role of the aircraft been officially acknowledged; they were once described as "radar calibration aircraft". The R1s have not suffered the same rate of fatigue and corrosion as the MR2s. One R1 was lost in a flying accident since the type's introduction; this occurred in May 1995 during a flight test after major servicing, at RAF Kinloss. To replace this aircraft an MR2 was selected for conversion to R1 standard, and entered service in December 1996. The Nimrod R1 was based initially at RAF Wyton, Cambridgeshire, and later at RAF Waddington, Lincolnshire, and flown by 51 Sqn. The two remaining Nimrod R1s were originally planned to be retired at the end of March 2011, but operational requirements forced the RAF to deploy one to RAF Akrotiri, Cyprus on 16 March in support of Operation Ellamy. The last flight of the type was on 28 June 2011 from RAF Waddington, in the presence of the Chief of the Air Staff, ACM Sir Stephen Dalton. XV 249, the former MR2, is now on display at the RAF Museum Cosford, West Midlands. The R1 was replaced by three Boeing RC-135W Rivet Joint aircraft, acquired under the Airseeker project; the first aircraft was delivered in late 2013. MR2 Starting in 1975, 35 aircraft were upgraded to MR2 standard,