Definition
A condition in which the engine power currently being produced is less than the power required to maintain level flight at the current airspeed and configuration, causing the aircraft to lose altitude or decelerate unless corrected.
Plain English
The engine isn't making enough power to hold the aircraft steady at its current speed, so without more throttle the airplane will sink or slow down.
Context Anchor
Seen in instrument flying discussions of speed stability, especially when comparing power required with the power setting being used.
Derivation
From Latin deficere, meaning 'to fall short.' A power deficiency is simply the amount by which available power falls short of what the airplane needs right now.
Why Pilots Care
It produces gradual speed loss that, if uncorrected, can lead to a stall or loss of control in instrument conditions.
Grounding Statement
At a fixed power setting, if the airplane’s current condition requires more power than that setting provides, there is a power deficiency.
Intuition Check
Do not read “power deficiency” as an engine failure or a weak engine. Here it means a shortage between the power needed and the power currently being supplied.
Example Sentence 1
As the airspeed bled off below the target on final, the pilot recognised the power deficiency and smoothly added throttle to recover.
Example Sentence 2
The pilot noticed the power deficiency during level flight and lowered the nose slightly while advancing power to regain the target speed.