If the helicopter, after an engine failure, cannot maintain its altitude with the remaining engine(s) at Maximum Continuous Power (MCP) setting above the One-Engine-Inoperative service ceiling, then a "Drift Down" procedure is commenced.

The altitude at which a driftdown may commence varries. It may be a high or a lower cruising altitude, depending each time on the margin between its power required and power available, which is determined by its mass and the ambient conditions:

1. MCP is set and the published speed for drift down is maintained (normally V_Y) to give the minimum descent gradient, which permits obstacle clearance. There is not a regulated minimum descent angle for the drift down procedure.
2. As altitude decreases, power available increases gradually, while power required decreases.
3. The flight path will be mostly vertical in the early stages, because the remaining engine(s) will have little power, but as the air density improves, it will become more horizontal. During the drift down the obstacle clearance must be at least 2000 ft.
4. As the power deficit reduces, the rate of descent and descent gradient also reduce, until power available equals power required, when the helicopter can maintain level flight, with 1000 ft above the highest point of the crossing. This will be slightly higher than the planned level-off altitude, because that requires 50 ft/min rate of climb to be possible.

If the mass of the helicopter at the beginning of the drift down indicates that the helicopter will stabilise below the Minimum Safe Altitude or it is expected that it will be incapable of meeting the fore mentioned obstacle clearance requirements, then fuel jettisoning (dumping) must be planned.

NOTE:
Drift down is permitted in Performance Class 1 and 2 helicopters.