Definition
A condition in which the airplane has less total mechanical energy — the combination of altitude (potential energy) and airspeed (kinetic energy) — than is required for the current or upcoming phase of flight. The shortfall must be corrected by adding energy with thrust, by trading one form of energy for another, or by revising the flight plan to match available energy.
Plain English
The airplane is too low, too slow, or both, for what the pilot is trying to do next. There isn't enough height or speed in the bank to fly the planned profile without doing something to fix it.
Context Anchor
Used when viewing the airplane as an energy system, especially during climbs, approaches, turns, slow flight, and recovery from low-speed or low-altitude situations.
Derivation
From Latin deficere, meaning 'to fall short' or 'to be lacking.' The word energy comes from Greek energeia, 'activity' or 'capacity to do work.' Together they describe a shortage of the capacity needed to fly the airplane through the next part of its flight.
Why Pilots Care
An uncorrected energy deficit can produce an unstabilized approach, high sink rate, or stall.
Analogy
Like coasting toward a hill on a bicycle and realizing you lack enough speed or height to reach the top without pedaling harder.
Grounding Statement
Picture turning final, low and slow, with the runway still well ahead — you don't have enough altitude to glide there and not enough speed to stretch it. That gap between what you have and what you need is the energy deficit.
Intuition Check
An energy deficit does not mean the airplane has no energy at all. It means it has less energy than it needs for the current task.
Example Sentence 1
Recognizing the energy deficit early on final, the pilot added power and adjusted pitch before the situation became critical.
Example Sentence 2
Proper speed and altitude planning prevents an energy deficit during the landing flare.