In 1959, Sikorsky Aircraft began testing of a proof-of-concept heavy lift helicopter called the S-60. Initial tests proved successful and the U.S. Navy and Army showed interest in the concept. Sikorsky immediately began development of a more powerful version of the helicopter powered by two turbo-shaft engines. Designated the S-64 by Sikorsky and the H-54A by the U.S. Army, it made its first flight in May 1962. The U.S. Army named the helicopter Tarhe, but most people call them Skycranes. The CH-54 left military service in the early 1990s