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 endurance requirement led to retaining the normal engines in production.
Xa que os avións de proba non requerían ter as 14 horas de aguante demandadas para os exemplares de produción, os EC-137 mantiveron os motores comerciais Pratt & Whitney JT3D, e unha posterior redución destes requisitos permitiron manter estes motores nas unidades de produción.