would calculate the best flap angle for various flight conditions given the current gross weight. Together these modifications offered a reduction in approach speed of 10 kn. For the YC-15, four engines were used, versions of the Pratt & Whitney JT8D-17 widely used on the Boeing 727 and Douglas DC-9. The YC-15 borrowed components from other McDonnell Douglas aircraft, with its nose gear coming from the Douglas DC-8 and the nose section and cockpit being derived from the Douglas DC-10. Parts borrowed from other aircraft included the Universal Aerial Refueling Receptacle Slipway Installation (UARRSI), taken from a Fairchild A-10, anti-tipover stabilizer struts from the Lockheed C-141 Starlifter, pumps taken from the McDonnell Douglas F-15 Eagle, Lockheed C-5 Galaxy, DC-9 and C-141 and actuators taken from the C-5 Galaxy and DC-10. In addition, the environmental cooling system was composed of components taken from the DC-9, C-141 and Boeing KC-135. The YC-15 interior cargo hold was large, with dimensions 47 x 11.8 x 11.4 feet. The YC-15 was the first aircraft to offer pilots a heads up display. It was especially designed to help with short field landings, showing the pilot the horizon, flight path scale, airspeed indexer and touchdown point. Operational history Two YC-15s were built, one with a wingspan of (#72-1876) and one of (#72-1875). Both were long and powered by four Pratt & Whitney JT8D-17 engines, each with of thrust. The first flight was on 26 August 1975. The second prototype followed in December. They were tested for some time at McDonnell Douglas as the Boeing entry was not ready until almost a year later. In November 1976, both designs were transferred to Edwards Air Force Base for head-to-head testing, including lifting heavy loads like tanks and artillery from dirt airfields at Graham Ranch, off the end of Runway 22. In Phase II of the flight test program, a "refanned" Pratt & Whitney JT8D-209 was tested in No. 1 nacelle of 72–1876, and a CFM International CFM56 was tested in the No. 1 nacelle of 72–1875. In addition, a new wing with increased chord and span was flown on 72–1875. The YC-15 met specifications under most, but not all, conditions. It was a very good STOL plane for its size. At a gross weight of 149,300lbs, the YC-15 flew short landing approaches at only 87 kn at a 6 degree glide slope yielding a sink rate of 15.4 feet