On 22 March 2010 the British Ministry of Defence announced that it had reached agreement with the US Government to purchase three RC-135W Rivet Joint aircraft to replace the Nimrod R1, which was subsequently retired in June 2011. The aircraft, to be styled as 'Airseeker', were scheduled to be delivered by 2017 at a total cost of around £650 million, including provision of ground infrastructure, training of personnel and ground supporting systems. In 2013, the UK government confirmed that crews from the RAF's 51 Squadron had been training and operating alongside their USAF colleagues since 2011, having achieved in excess of 32,000 flying hours and 1,800 sorties as part of the 55th Wing at Offutt AFB. The RAF received the first RC-135W in September 2013, which was deployed from July 2014 to support coalition action against combat Islamic State of Iraq and the Levant militants in Iraq. The second aircraft was delivered seven months ahead of schedule in September 2015, with over sixty improvements incorporated ranging from upgrades to the aircraft's mission systems to engine improvements providing increased fuel efficiency and durability. In due course, the first Airseeker will receive the same upgrades. The aircraft will be air-to-air refuelled in service by USAF tankers based in Europe, as the UK does not operate boom-equipped refueling aircraft and has no plans to adapt drogue-equipped aircraft.