Two Pratt & Whitney F100-100 turbofan engines first flew in an F-15 'Eagle' in 1972 with a thrust of 23,930 lbs. Due to the advanced nature of engine and aircraft, numerous problems were encountered in its early days of service including high wear, stalling and "hard" afterburner starts. These "hard" starts could be caused by failure of the afterburner to start or by extinguishing after start, in either case the large jets of jet fuel were lit by the engine exhaust resulting in high pressure waves causing the engine to stall. Early problems were solved in the F100-PW-220, and the engine is still in the USAF fleet to this day.