Lockheed C-5 Galaxy Operations Limits Practice Test

Driftdown is the prescribed speed you follow when one or more engines fail, to ensure you can descend safely while keeping control and staying within performance limits with the remaining engines. For a C-5 with two or three engines out, the driftdown setting is chosen to balance enough airflow for control with the reduced thrust available. In this context, the Mach number for two- or three-engine driftdown is 0.60. This speed provides the necessary energy margin and airspeed to maintain controllability and a safe descent path with the engine(s) inoperative. Slower speeds would reduce airflow too much and risk stall or insufficient control, while faster speeds would demand more thrust than the remaining engines can reliably provide to sustain safe flight. The 0.60 value is the setting that best fits the expected performance envelope for these engine-out conditions.