 As the test-beds did not need the same 14-hour endurance demanded of the production aircraft, the EC-137s retained the Pratt & Whitney JT3D commercial engines, and a later reduction in the endurance requirement led to retaining the normal engines in production.The first EC-137 made its maiden flight on 9 February 1972, with the fly-off between the two radars taking place from March to July of that year.Favorable test results led to the selection of Westinghouse's radar for the production aircraft.