The Boeing Vertol (model 114) YCH-47A made it's initial hovering flight on 21 September 1961. The all-weather medium lift CH-47A Chinook first entered service in Vietnam about 1966. The CH-47A was powered by either Allied Signal Engines T55-L-5 or T55-L-7 engines. CH-47B was powered by two Allied Signal Engines T55-L-7C engines. Models CH-47A, CH-47B, and CH-47C, all utilized the same airframe, but later models featured upgraded engines.