Definition
A gas turbine engine that uses three concentric, independently rotating shafts (spools), each connecting a separate compressor stage to its own turbine stage. The three spools — typically a low-pressure (LP), intermediate-pressure (IP), and high-pressure (HP) spool — rotate at different speeds, allowing each compressor and turbine pair to operate near its own optimum efficiency.
Plain English
A jet engine built around three separate spinning shafts stacked inside one another. Each shaft turns at its own best speed, which makes the engine more efficient than designs that use only one or two shafts.
Context Anchor
Seen in turbine engine design, aircraft systems descriptions, and engine performance discussions for some large turbofan engines.
Derivation
Spool here refers to the rotating shaft-and-disk assembly inside the engine — named for its resemblance to a thread spool, with compressor and turbine blades mounted on a central shaft. Triple simply means three of them, nested concentrically.
Why Pilots Care
The extra spool allows each compressor stage to run closer to its optimum speed, improving fuel efficiency, thrust response, and altitude performance on long-range aircraft.
Analogy
Think of three nested spinning assemblies inside one engine, each free to turn at its own best speed rather than all being forced to spin together.
Intuition Check
A triple-spool engine is not three engines. It is one engine with three separate rotating sections inside it.
Example Sentence 1
The Rolls-Royce Trent is a triple-spool engine, so the cockpit displays N1, N2, and N3 readings rather than just N1 and N2.
Example Sentence 2
Maintenance manuals for the triple-spool engine require separate vibration checks on each of the three shafts during borescope inspections.