Engine Failure and Drift Down

In the case of an engine failure, the remaining thrust is not sufficient to balance the drag force and the cruise speed cannot be maintained. The only solution is to descend to a lower flight altitude, where the remaining engine can provide enough thrust to balance the drag and allow level flight. As the airplane descends into the lower atmosphere where density is greater, the remaining engine can develop more thrust which will equal the drag force, this is the GROSS level-off altitude, but would give no performance margin. So the DRIFT DOWN PROCEDURE is continued to a lower altitude, the NET level-off altitude.

The main purpose of the drift down procedure is to bring the aircraft to an altitude where the aircraft can generate enough trust with the remaining to balance the drag. The level-off altitude is determined by the actual air density.