Definition
A turbine engine in which the power-producing turbine is mechanically independent of the turbine that drives the compressor. The two turbines are not connected by a shaft; they share only the gas path. Hot gases from the combustion section first drive the compressor turbine (which keeps the engine running), then pass through a separate power turbine that drives the output shaft connected to a propeller, rotor, or gearbox.
Plain English
An engine with two turbines that spin independently. One turbine keeps the engine itself running. The other is spun by the leftover hot gases and is the one that actually delivers power to the propeller or rotor. Because they are not linked by a shaft, the output side is 'free' to turn at its own speed.
Context Anchor
Seen in turbine engine maintenance, especially with turboprop and helicopter powerplants where shaft arrangement and engine indications matter.
Derivation
Free' here means mechanically uncoupled — the power turbine is free of any shaft connection to the compressor turbine. It spins purely from the gas flow passing through it.
Why Pilots Care
This design lets the propeller run at its best speed for efficiency while the engine core adjusts power independently, reducing wear and improving control.
Intuition Check
“Free” does not mean the turbine is loose or uncontrolled. It means the power turbine is mechanically independent from the turbine that keeps the compressor turning.
Example Sentence 1
The PT6 is a free-turbine engine, which is why the propeller stays still during the initial part of the start sequence.
Example Sentence 2
During cruise, the free-turbine engine allowed the pilot to set propeller RPM separately from core engine speed.