The Piper Cheyenne prototype first flew in 1969. Designated the (PA-31T-620), it was essentially a turboprop evolution of the Piper PA-31 Navajo, powered by two Pratt & Whitney Canada PT6A-28 turboprops rated at 620hp each. Certification was granted on 3 May 1972. The original Cheyenne, produced from 1974 to 1977, did not carry a Roman numeral suffix designation. Because of difficulties complying with certification standards for longitudinal stability, the Cheyenne and later Cheyenne II are equipped with a Stability Augmentation System (SAS), consisting of a stick-pusher system (actuated by an angle-of-attack sensor) designed to provide the pilot with the appropriate stick force gradient - and to keep the airplane at its trim airspeed when flying near aft center of gravity loadings, high power settings, low airspeeds and high angles of attack. Without the SAS, the aircraft demonstrated neutral static stability in its flight regime. Later in 1978 it was re-designated as the Cheyenne II (PA-31T-2-620), due to the production of the PA-31T-1-500.