Definition
The time required for a turbine engine to accelerate from a lower power setting to a higher one, as the rotating compressor and turbine assemblies build rotational speed and produce increased thrust.
Plain English
The lag between pushing the throttle forward in a jet or turboprop engine and the engine actually delivering the higher power you asked for.
Context Anchor
Used in turbine-powered airplane operations, especially during approaches, go-arounds, and power changes from idle or low power.
Derivation
From 'spool,' the rotating shaft assembly inside a turbine engine that carries the compressor and turbine wheels. When the engine accelerates, the spool literally spins up to a higher RPM. The term describes the physical process: the spool is winding up to speed.
Why Pilots Care
The lag in thrust response affects how quickly an aircraft can climb or stop during critical maneuvers.
Analogy
Like a ceiling fan that takes several seconds to reach full speed after you switch it to high.
Grounding Statement
If the pilot advances the power lever from low power, the engine may need a moment to speed up before the airplane gets the requested power.
Intuition Check
Spool-up does not mean the engine is already producing the selected power. It means the engine is in the process of speeding up toward that power.
Example Sentence 1
On approach in the jet, the captain kept a little extra power in to reduce spool-up time in case of a go-around.
Example Sentence 2
On go-around the spool-up delay required the pilot to hold the runway heading longer than expected.