Definition
The loss of an airplane's available mechanical energy — the combined total of kinetic energy (from speed) and potential energy (from altitude) — to a level so low that the airplane can no longer recover from a high sink rate or deceleration before contacting the ground. Once mechanical energy is depleted, neither pitch nor power can restore controlled flight in time.
Plain English
The airplane has run out of the speed and height it needs to recover. Without enough of either, there is nothing left to trade for lift or climb performance, and a crash becomes unavoidable.
Context Anchor
Seen in discussions of preventing a low-speed, high-sink-rate condition during approach, landing, or other low-altitude flight.
Derivation
Mechanical energy in physics means the energy an object has because of its motion (kinetic) plus its position (potential). 'Depletion' comes from the Latin deplere, 'to empty out.' Together the phrase describes an airplane whose energy reserves have been emptied out to the point of no return.
Why Pilots Care
Uncorrected depletion removes the energy margin needed for flare, go-around, or stall recovery, often resulting in ground contact.
Analogy
Think of airspeed and altitude like money in two accounts. You can spend one to help the other, but if both accounts get too low at the same time, you have very little left to work with.
Grounding Statement
If you have no altitude to trade for speed and no speed to trade for lift, you have nothing left to fly with.
Intuition Check
Mechanical energy depletion does not mean a mechanical failure or an electrical problem. Here, it means the airplane is losing the usable energy stored in its speed and altitude.
Example Sentence 1
The instructor explained that pulling back hard at low altitude with reduced power is a fast path to mechanical energy depletion.
Example Sentence 2
A late turn to final can produce mechanical energy depletion if descent rate is not adjusted to match the reduced airspeed.