Definition
The shortfall between the engine power currently being produced and the higher power required to maintain level flight at a given airspeed on the back side of the power curve, where induced drag rises sharply as airspeed decreases.
Plain English
The gap between how much power the engine is putting out and how much it actually needs to keep the airplane flying level at a slow speed. When the airplane is flying slowly enough that drag is climbing fast, the engine may not be producing enough power to hold altitude, and that missing amount is the power deficit.
Context Anchor
Seen in slow-speed flight and speed stability discussions, especially where slowing down can require more power instead of less.
Derivation
Deficit comes from the Latin deficere, meaning 'to fall short' or 'to be lacking.' In aviation, it describes power that is falling short of what level flight requires at that airspeed.
Why Pilots Care
Recognizing a power deficit prevents the pilot from continuing to pull back on the yoke while already at full power, which leads to a stall.
Grounding Statement
Picture the airplane flying very slowly with full power in, yet still settling downward because it is asking for more power than the engine and propeller can deliver.
Intuition Check
A power deficit is not just “low power” or a weak engine. It means the airplane’s power demand at that condition is higher than the power available.
Example Sentence 1
Flying behind the power curve on a short final, the student noticed a growing power deficit and added throttle to stop the sink rate.
Example Sentence 2
The instructor demonstrated that adding power could not correct the power deficit until the nose was lowered to regain airspeed.