Plain English Meaning
A spool is one rotating set inside a jet engine: a compressor at the front and a turbine at the back, joined by a shaft so they spin together. Bigger jet engines have two or three of these sets nested inside each other, each spinning at its own speed.
Full Definition
In a turbine engine, a spool is a matched assembly of a compressor and a turbine connected by a single shaft, rotating together as one unit. Multi-spool engines have two or more such assemblies (low-pressure spool, high-pressure spool) mounted concentrically, each free to rotate at its own optimum speed.
Context Anchor
Seen in turbine engine descriptions, engine start procedures, and discussions of how quickly a turbine engine accelerates.
Derivation
From the everyday word 'spool,' meaning a cylindrical object that things wind around. The engine assembly was named for its shape — a shaft with rotating discs along it, resembling a spool of thread.
Why Pilots Care
Turbine engine gauges (N1, N2, sometimes N3) each report the speed of a different spool. Knowing which spool a gauge is showing helps the pilot interpret engine response, set power correctly, and recognise abnormal indications.
Intuition Check
Spool does not mean a roll of thread here. It means a connected set of turbine-engine parts that rotate together.
Example Sentence 1
On a two-spool engine, the N1 gauge shows the speed of the low-pressure spool, while N2 shows the high-pressure spool.
Example Sentence 2
Maintenance replaced seals on the high-pressure spool after detecting unusual vibration at cruise power.