Definition
A turboprop engine in which the propeller, the reduction gearbox, the compressor, and the turbine are all mechanically connected to a single common shaft. Because everything spins together, propeller speed is controlled by varying the propeller blade pitch rather than by changing engine RPM, and the engine must be running at high RPM whenever the propeller is producing thrust.
Plain English
A turboprop where the propeller and the engine are locked together on one shaft, so they always turn at related speeds. To change thrust you change the angle of the propeller blades, not the engine speed.
Context Anchor
Seen in turbine engine descriptions, aircraft systems training, and comparisons between different turboprop engine designs.
Derivation
Fixed means held in place or permanently connected. Shaft is the rotating rod that transmits power. The name describes the mechanical layout: the shaft connecting the turbine to the propeller is fixed -- it is one continuous drive train rather than two independent ones.
Why Pilots Care
Power management is simpler because throttle movement directly changes both engine and propeller speed, but it limits flexibility in feathering and ground operations compared with free-turbine designs.
Analogy
It is like a fan belt connection in a simple machine: when the driving part turns, the connected part is tied to that motion. The parts may have gearing between them, but they are still mechanically linked.
Intuition Check
Do not read “fixed shaft” as “a shaft that does not move.” In this term, “fixed” means mechanically connected, and the shaft still rotates.
Example Sentence 1
In a fixed shaft turboprop engine, the pilot manages thrust by adjusting propeller blade pitch while the engine continues running at its governed RPM.
Example Sentence 2
During the preflight inspection the mechanic confirmed the aircraft was equipped with a fixed shaft turboprop engine rather than a free-turbine model.