The drift-down procedure requires a minimum descent angle after an engine failure at cruising altitude.
When determining the obstacle clearance during drift-down, fuel dumping may be taken into account.
The drift-down procedure requires a minimum obstacle clearance of 35 ft.
An engine failure at high cruising altitude will always result in a drift-down, because it is not permitted to fly the same altitude with one engine inoperative as with all engines operating.
In the case of an engine failure, the remaining thrust may 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 level off altitude is determined by the actual air density and also by the actual aircraft mass. A higher aircraft mass requires more thrust to balance the drag and consequently a lower level off altitude. It can happen that the level off altitude is lower than altitude to clear the obstacles with a safety margin. Jettison should be considered to lower the aircraft weight, increasing the level-off altitude. Also, jettison may be required to lower aircraft mass below the aircraft maximum landing mass.
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