Definition
A fixed-shaft turboprop engine originally designed by the Garrett AiResearch company, in which the propeller, reduction gearbox, compressor, and turbine are all mechanically connected on a single shaft that rotates as one unit. Engine power is controlled primarily by varying fuel flow, while propeller blade angle is adjusted to maintain a constant engine RPM.
Plain English
A specific type of small turboprop engine where the propeller and the engine's spinning parts are all locked together on one shaft. It is a well-known example of a fixed-shaft turboprop, commonly used in smaller turboprop airplanes.
Context Anchor
Seen in discussions comparing fixed-shaft turboprop engines with free-turbine turboprop engines.
Derivation
Named after Garrett AiResearch, the U.S. company that designed the engine. The 'T' stands for turbine, 'P' for propeller, and 'E' for engine, with '331' as the model number. The Garrett line is now produced under Honeywell, but pilots still refer to the engine by its original Garrett name.
Why Pilots Care
Because everything is on one shaft, the propeller is always being driven directly by the engine's core. This affects how the engine is started, how power is managed, and how the airplane responds to power changes. Pilots transitioning to a TPE331 need to learn its specific start procedure and power-handling feel, which differs noticeably from free-turbine engines like the PT6.
Grounding Statement
When the engine turns, the propeller system is mechanically tied into that rotation through gearing, so the engine and propeller are more directly connected than in a free-turbine design.
Intuition Check
Do not read Garrett TPE331 as an airplane model. It is an engine model, and in this context it is used as an example of a fixed-shaft turboprop engine.
Example Sentence 1
The Garrett TPE331 in our Metroliner spools up quickly on takeoff because the propeller and engine are linked on the same shaft.
Example Sentence 2
Maintenance on the Garrett TPE331 includes checking the reduction gearbox for proper oil pressure.