The Boeing Chinook is a large, tandem rotor helicopter operated by the Royal Air Force (RAF). A series of variants based on the United States Army's Boeing CH-47 Chinook, the RAF Chinook fleet is the largest outside the United States. RAF Chinooks have seen extensive service in the Falklands War, the Balkans, Northern Ireland, Iraq, and Afghanistan. The Chinook, normally based at RAF Odiham in England, provides heavy-lift support and transport across all branches of the British armed forces. The RAF has a total of sixty Chinooks in active inventory as of 2015. In 2018, the UK issued a request to the United States to purchase sixteen additional aircraft. The Chinook is expected to remain in RAF service until the 2040s. Design and development Chinook HC1 In March 1967, the United Kingdom placed an order for fifteen Boeing Vertol CH-47B Chinook to replace the Royal Air Force's Bristol Belvedere HC.1 fleet. In British service the new aircraft was to be designated as the Chinook HC Mk 1 (also formatted as HC.1 or HC1) standing for Helicopter, Cargo Mark 1. However in November of that year, the order was cancelled in a review of defence spending. UK Chinook procurement ambitions were revived in 1978 with an announced requirement for a new heavy-lift helicopter to replace the Westland Wessex. Thirty Chinooks were ordered at a price of US$200 million. These helicopters, comparable to the CH-47C with Lycoming T55-L-11E engines, were again designated Chinook HC1, and entered service in December 1980. Eight more HC1 were delivered from 1984 to 1986, with the CH-47D's Lycoming T55-L-712 turbo-shaft engines. The replacement of the HC1s metal rotor blades with aluminium and glass fibre composite rotor blades saw these aircraft designated Chinook HC1B. All surviving aircraft were later returned to Boeing and updated to the Chinook HC2 standard for further service within the RAF. Chinook HC2 The US Army's next generation Chinook, the CH-47D, entered service in 1982. Improvements from the CH-47C included upgraded engines, composite rotor blades, a redesigned cockpit to reduce pilot workload, redundant and improved electrical systems, an advanced flight control system (FCS) and improved avionics. The RAF returned their original HC1 fleet to Boeing for upgrading to CH-47D standard, the first of which returned to the UK in 1993. Three additional HC2 Chinooks were ordered with delivery beginning in 1995. Another six were ordered in 1995 under the Chinook HC2A designation; the main