Definition
The time delay between advancing the throttle on a turbine engine and the engine actually producing the commanded thrust. During this interval, the engine's rotating components (compressor and turbine) accelerate from a lower rotational speed up to the higher speed needed to generate the requested power.
Plain English
The lag between pushing the throttle forward and the jet engine actually delivering more thrust. The engine's spinning parts have to speed up first, and that takes a few seconds.
Context Anchor
You will encounter this term when operating turbine-powered airplanes, especially during takeoff, go-around, approach, and low-speed maneuvering where quick power changes may be needed.
Derivation
From 'spool,' the term for the rotating shaft assembly inside a turbine engine that carries the compressor and turbine blades. To 'spool up' is to bring that assembly up to a higher rotational speed. The time it takes to do that is the spool-up time.
Why Pilots Care
Pilots must anticipate this delay to avoid descending below safe altitudes during sudden power demands like missed approaches.
Intuition Check
Do not assume moving the power control forward means the power is available instantly. In a turbine engine, the command happens first; the engine’s actual power increase follows after a short delay.
Example Sentence 1
Because of the engine's spool-up time, the pilot kept the power setting slightly higher on final approach to ensure thrust would be available immediately if a go-around became necessary.
Example Sentence 2
Knowing the spool-up time of the engines allowed the crew to plan a safe missed approach without losing altitude.