Definition
A gas turbine engine built with two independent rotating assemblies, called spools, that turn on the same centerline but at different speeds. A low-pressure spool connects the low-pressure compressor to the low-pressure turbine, and a separate high-pressure spool connects the high-pressure compressor to the high-pressure turbine. The two spools are mechanically independent, sharing no shaft connection, allowing each to run at its own optimum speed.
Plain English
A jet engine with two sets of compressors and turbines that spin separately at different speeds, instead of everything being locked together on one shaft. This lets each part of the engine run at the speed where it works best.
Context Anchor
Seen in turbine engine descriptions, maintenance manuals, engine start procedures, and cockpit engine speed indications on many turbine-powered aircraft.
Derivation
Spool here refers to a rotating assembly resembling a thread spool — a shaft with compressor and turbine wheels mounted on it. Dual simply means two. The word came into engine use because each compressor-turbine assembly visually resembles a spool when viewed in cross-section.
Why Pilots Care
This design improves compressor efficiency across a wide range of power settings and allows quicker engine response.
Analogy
Think of two fans mounted on separate shafts, one inside the other. They are part of the same machine, but each can spin at its own speed.
Intuition Check
Do not read “spool” as a storage reel. In this term, a spool is a rotating engine group: compressor, turbine, and connecting shaft.
Example Sentence 1
Most modern turbofan engines use a dual-spool gas turbine engine design to improve efficiency across a wide range of operating conditions.
Example Sentence 2
Technicians balance each spool separately when performing maintenance on a dual-spool gas turbine engine.