Definition
The loss of an airplane's total mechanical energy — the combined kinetic energy from airspeed and potential energy from altitude — to a level where the airplane can no longer maintain controlled flight or recover from a developing sink rate.
Plain English
The airplane has run low on the two things that keep it flying: speed and height. Once both get too low, there isn't enough of either left to fix the situation.
Context Anchor
You will see this term in energy management discussions, especially during approaches, climbs, turns, go-arounds, and any situation where the airplane is getting slower, lower, or both.
Derivation
From Latin deplere, 'to empty out.' Energy depletion is literally the airplane's energy account being drained — speed and altitude spent faster than they're being replaced.
Why Pilots Care
Unchecked energy depletion produces an irreversible sink rate or stall that cannot be corrected in time to avoid ground contact.
Analogy
It is like spending down both your cash and your emergency savings at the same time. If something unexpected happens, there may not be enough left to fix the problem.
Grounding Statement
Picture an airplane on final that is low, slow, and still descending: that is a practical sign that usable energy is being depleted.
Intuition Check
Energy depletion does not mean only that the engine is losing power. In this context, it can also mean the airplane is losing airspeed, altitude, or both, faster than the pilot can safely replace them.
Example Sentence 1
Dragging the airplane in low and slow on a long final set up a classic energy depletion scenario, with no altitude to trade and an engine that couldn't respond fast enough.
Example Sentence 2
Continued energy depletion during the turn to final forced the airplane into a high sink rate that could not be arrested before touchdown.