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.
Plain English
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.
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.