The test engine, which was named the Allison 501-M78,[17] had a thrust rating of 9,000 lbf (40 kN).[18] It was first operated in flight on March 28, 1987.[19] The extensive test program, which cost about $56 million,[20] racked up 73 flights and over 133 hours of flight time before finishing on March 25, 1988.[21] In 1989, however, the testbed aircraft returned to the air from April 3 through April 14 to measure ground noise levels during flight.[22][23] The engine was removed after that, and the aircraft was converted to a space shuttle training aircraft later that year.[24]