Helicopter flight controls blades collectively ("i.e.," all at the same time) and independent of their position. Therefore, if a collective input is made, all the blades change equally, and as a result, the helicopter increases or decreases its total lift derived from the rotor. In level flight this would cause a climb or descent, while with the helicopter pitched forward an increase in total lift would produce an acceleration together with a given amount of ascent. The collective pitch control in a Boeing CH-47 Chinook is called a "thrust control", but serves the same purpose, except that it controls two rotor systems, applying
Boeing Chinook (UK variants) such as the AgustaWestland Merlin HC.3 of the Royal Navy's Commando Helicopter Force and the RAF's Westland Puma HC.2, based at RNAS Yeovilton and RAF Benson. The RAF has a total of 60 active Chinooks in inventory as of 2015. In 2018 the UK issued a request to the United States to purchase 16 additional aircraft. The Chinook is expected to remain in RAF service until the 2040s. In March 1967 an order was placed for fifteen Chinook HC Mk1s, standing for "Helicopter, Cargo Mark 1", for the Royal Air Force to replace the Bristol Belvedere. This original MK1 variant