Definition
An axial-flow compressor in a turbine engine built in two independent rotating sections, called spools, that turn on the same centerline but at different speeds. Each spool has its own set of compressor stages and is driven by its own turbine through a separate concentric shaft. The forward (low-pressure) spool turns more slowly and feeds compressed air into the rear (high-pressure) spool, which turns faster and delivers air to the combustion section.
Plain English
A jet engine compressor made of two separate rotating groups stacked one behind the other. They share the same centerline but spin at different speeds, each driven by its own turbine. This lets each section run at the speed that works best for it.
Context Anchor
Seen in turbine engine descriptions, engine start procedures, and engine speed indications for engines that show separate rotating-section speeds.
Derivation
‘Spool’ here refers to the rotating assembly of a compressor and its driving turbine connected by a shaft — the shape resembles a thread spool with a wheel at each end. ‘Split’ means the compressor is divided into two such assemblies running independently. Knowing this makes terms like ‘N1’ (low spool speed) and ‘N2’ (high spool speed) easier to follow.
Why Pilots Care
Allows better efficiency across a wide range of engine speeds and improves acceleration response in jet engines.
Analogy
Think of two fans on separate shafts inside the same tube. They both help move and squeeze the air, but each one can spin at the speed that works best for its job.
Grounding Statement
When power changes, the two compressor sections do not have to speed up or slow down as one single unit.
Intuition Check
Do not read “spool” as a loose reel of material here. In this engine context, a spool is one rotating shaft assembly, and a split spool compressor has more than one of those assemblies.
Example Sentence 1
The split spool compressor allowed the low-pressure section to spin slowly during start while the high-pressure section came up to speed independently.
Example Sentence 2
During climb the flight crew monitors N1 and N2 to confirm both spools of the split spool compressor are accelerating normally.