Definition
A turbine engine shaft arrangement in which the compressor and turbine sections are divided into two independent rotating assemblies (spools) mounted on concentric shafts, each free to rotate at its own optimum speed. Typically a low-pressure spool (low-pressure compressor driven by the low-pressure turbine) rotates inside a hollow high-pressure spool (high-pressure compressor driven by the high-pressure turbine), with the two shafts mechanically independent of each other.
Plain English
Inside the engine there are two separate rotating units, one inside the other, that can spin at different speeds. This lets each one run at the speed that works best for it, rather than forcing everything to spin together.
Context Anchor
Seen in jet engine basics when learning how turbine engines are built and why different engine sections may have different speed indications.
Derivation
Spool' in turbine engineering refers to a complete rotating assembly of compressor and turbine sections joined by a shaft -- named for its spool-like shape. 'Split' indicates the assembly is divided into two independent units rather than one solid piece. Knowing this helps because it explains why pilots see N1 and N2 gauges -- each gauge reads one spool.
Why Pilots Care
The design improves engine efficiency and response across a wide range of thrust settings and altitudes by letting each spool run at its optimal RPM.
Analogy
Think of two bicycle wheels mounted on separate axles. They are part of the same bicycle, but one wheel is not forced to turn at the exact same speed as the other.
Intuition Check
“Split” does not mean the shaft is cracked or broken. Here it means the engine’s rotating sections are intentionally separated so they can turn independently.
Example Sentence 1
The split-spool shaft design allows the high-pressure section to spin faster than the low-pressure section, improving overall engine efficiency.
Example Sentence 2
During a high-altitude cruise, the low-pressure spool on the split-spool shaft slows while the high-pressure spool maintains its RPM for best fuel economy.