Definition
A controlled descent following engine failure in a multi-engine airplane, in which the airplane is flown at the speed that produces the smallest rate of descent on the remaining engine(s) until it stabilizes at the highest altitude it can maintain in the new configuration. The descent is gradual rather than abrupt because the airplane still has thrust, just not enough to hold its original altitude.
Plain English
After losing an engine, the airplane can no longer hold its cruise altitude, so the pilot lets it sink slowly at the best speed until it levels off at a lower altitude that the working engine(s) can sustain.
Context Anchor
Seen in engine-failure discussions, especially when deciding what the airplane can do after losing power at altitude.
Derivation
The phrase is plain English: 'drift' suggests a slow, unhurried movement, and 'down' is the direction. Together they describe a descent that is controlled and gradual, not a dive or an emergency plunge.
Why Pilots Care
It is the safest way to lose altitude so the airplane can reach a height where single-engine performance is adequate for continued flight or diversion.
Grounding Statement
Picture an airplane high above the ground after a power loss: it can still fly, but it must gradually descend because it no longer has enough power to stay level at that height.
Intuition Check
Do not read “drift down” as an uncontrolled sink or wandering descent. It is a deliberate, controlled descent when the airplane cannot maintain its current altitude.
Example Sentence 1
After the right engine failed over the ridge, the captain set max continuous power on the left engine and began the drift down to the single-engine service ceiling.
Example Sentence 2
The drift down continued until the aircraft leveled off at 24,000 feet where single-engine cruise was possible.