Definition
The process by which an engine's rotating components accelerate from a lower rotational speed to a higher one in response to a power increase. In turbine engines it refers to the compressor and turbine assemblies (the spools) accelerating to a commanded power setting; in turbocharged piston engines it refers to the turbocharger turbine accelerating until it produces useful boost.
Plain English
The time it takes for an engine's spinning parts to wind up to a higher speed after the pilot adds power.
Context Anchor
Used during engine checks, before-takeoff checks, and power changes when the pilot is watching or listening for the engine to build power smoothly.
Derivation
From 'spool', the cylindrical rotating assembly inside a turbine engine that holds the compressor and turbine on a common shaft. To 'spool up' literally means to bring that spool up to speed.
Why Pilots Care
Adequate spool-up time ensures the aircraft meets required takeoff performance; rushing it can result in insufficient thrust.
Intuition Check
Spool up does not mean the airplane is speeding up. It means the engine itself is increasing speed and power; the airplane may still be stopped.
Example Sentence 1
Before releasing the brakes, the pilot let the engines spool up to takeoff power and confirmed the gauges were stable.
Example Sentence 2
On a short runway, the crew allowed extra seconds after brake release for the engines to spool up fully.