Drift Down

Drift down is a procedure used when a multi-engine aircraft experiences an engine failure at cruise altitude and can no longer maintain that altitude on the remaining engine(s). Rather than making an emergency descent, the aircraft gradually descends at the optimum drift-down speed until it reaches the maximum altitude that can be sustained with the remaining thrust — called the one-engine-inoperative (OEI) ceiling.

Drift-down planning is a critical element of flight planning for ETOPS and oceanic operations, as well as flights over mountainous terrain. Dispatchers must verify that the drift-down profile clears all terrain and obstacles along the route, including a safety margin (typically 2,000 feet over terrain in non-mountainous areas, or specific obstacle clearance altitudes in mountainous regions).

The drift-down profile depends on aircraft weight, atmospheric conditions, and the specific engine-out performance of the aircraft type. Performance data is provided in the Aircraft Flight Manual and is computed by the FMS or flight planning software.