Definition

Driftdown is a maximum thrust/minimum rate descent necessitated by an engine failure in a multi-engine aircraft in the latter stages of climb or during cruise when an aircraft cannot maintain its current altitude and terrain clearance or other factors are critical.

Description

The optimum cruising altitude for an aircraft with all engines operating normally is primarily dependent upon aircraft mass and the temperature deviation from ISA. In almost all cases, an aircraft’s optimum cruising altitude will exceed the One Engine Inoperative (OEI) service ceiling. An engine failure occurring above OEI service ceiling will therefore necessitate a descent and in most cases a Drift Down procedure will be followed.