Definition
A turbine engine compressor design with two independent rotating assemblies (spools) arranged on concentric shafts, each driven by its own turbine stage and free to rotate at different speeds. Air flows axially (parallel to the engine's centerline) through both compressor sections in sequence: a low-pressure compressor (N1) followed by a high-pressure compressor (N2). Each spool self-adjusts its rotational speed to match airflow demands, improving efficiency, compression ratio, and engine response across a wide operating range.
Plain English
A turbine engine compressor built in two separate rotating sections that spin at different speeds. Air passes straight through both sections, getting squeezed harder at each stage before reaching the combustion chamber.
Context Anchor
Seen in turbine-engine instrument discussions, especially when explaining what an N1 indicator is measuring.
Derivation
"Spool" refers to a shaft-and-rotor assembly that spins as a unit, named for its resemblance to a thread spool. "Axial-flow" means the air moves along the axis (centerline) of the engine, as opposed to a centrifugal compressor where air is flung outward. "Dual" simply indicates two such spools working in series.
Why Pilots Care
Independent spool speeds let the engine maintain good compression and fuel efficiency from idle to high power and across wide altitude ranges.
Analogy
Think of two fans in line inside the same tube. Air moves through both fans in the same direction, but each fan can spin at its own speed.
Intuition Check
Do not read spool as a loose coil of thread here. In this engine context, a spool is a rotating compressor-and-shaft group that has its own speed.
Example Sentence 1
Most modern turbofan engines use a dual-spool axial-flow compressor, which is why the cockpit shows both N1 and N2 RPM indications.
Example Sentence 2
During climb the low-pressure spool of the dual-spool axial-flow compressor automatically adjusted its rpm to keep the engine operating on the optimum compressor map.