Definition
A power setting that is lower than required to maintain a desired flight condition, such as airspeed, altitude, rate of climb, or rate of descent. In instrument flying, underpowering produces a deviation from the target performance — most commonly a sink below the desired altitude or a slower-than-planned airspeed.
Plain English
Using less engine power than the situation needs, so the airplane slows down, sinks, or fails to climb at the rate you wanted.
Context Anchor
Seen when discussing instrument flying power settings, especially when adjusting power to hold a desired speed, altitude, climb, or descent.
Derivation
Built from 'under-' (less than, below) and 'power' (engine output). The everyday sense — 'not enough power for the job' — carries directly into the aviation meaning.
Why Pilots Care
Failure to recognize and correct underpower can result in loss of airspeed or altitude, especially dangerous in instrument conditions.
Intuition Check
Underpower does not mean the engine is broken or too small. In this context, it means the selected power is too low for the flight condition you want.
Example Sentence 1
She noticed the altimeter creeping down and realised she was underpowered for level flight, so she added a small amount of throttle.
Example Sentence 2
With a full load and a tailwind component the pilot elected to delay takeoff rather than risk being underpower on departure.