A flight engineer (FE), also sometimes called an air engineer, is the member of an aircraft's flight crew who monitors and operates its complex aircraft systems. In the early era of aviation, the position was sometimes referred to as the "air mechanic". Flight engineers can still be found on some larger fixed-wing airplanes, and rotary wing helicopters. A similar crew position exists on some spacecraft. For U.S. civilian aircraft that require a flight engineer as part of the crew, the FE must possess an FAA Flight Engineer certificate with reciprocating, turboprop, or turbojet ratings appropriate to the aircraft. Whereas the four-engine Douglas DC-4 did not require an FE, the FAA Type Certificates of subsequent four engine reciprocating engine airplanes (DC-6, DC-7, Constellation, Boeing 307 and 377) and early three- and four-engine jets (Boeing 707, 727, early 747, DC-10) required FEs. Later three- and four-engine jets (MD-11, B-747-400 and later) were designed with sufficient automation to eliminate the FE position.