Drift down occurs when a multi-engine aircraft experiences engine failure during its flight. As a result of an engine failure occurring during the final stage of a climb or on cruise flight, as result the aircraft cannot maintain its level with the thrust created by its remaining engine(s) and has to descend.
Drift down refers to the set of procedures implemented when such a problem occurs.
In the event of an engine failure, the maximum continuous thrust is calculated for the remaining engine(s). And the speed is reduced to optimum drift down speed. Then the plane begins to descend with a slow vario. In summary, this is “drift down”.
So what happens if drift down is not applied when necessary? If the aircraft is under autopilot control, the system tries to keep the level, then starts to lift the nose of the aircraft because the thrust power is not enough, the speed starts to decrease, and at the end of this cycle the speed of the aircraft may drop to the critical stall speed.
There may be differences between the drift down procedures of each aircraft type and operator. For example it may not be necessary to disconnect the autothrottle (AT) connection on some aircraft types. Checking out and following the specific procedures of the aircraft must be priority.
As a result, drift down is the descend of plane under engine problem conditions by losing minimum altitude in the minimum glide angle without losing speed.
In case of such a situation, if the plane is above the One Engine Inoperative (OEI) service ceiling, the plane should descend. However, the conditions in which the aircraft is currently in must be evaluated. Situations such as terrain condition, air traffic situation and ATC coordination are effective in determining the nature of this descent.
Depending on the obstacle conditions, the descend process may be delayed or gradually planned. Or, if there is no constraint, optimum descent can be made smoothly. In all this process, it is necessary to be in contact with ATC in charge and follow the instructions.