What is the minimum allowable idle RPM during an engine run up?

The minimum allowable idle RPM during an engine run-up is critical for ensuring that the engine is operating within safe parameters. An idle RPM that is too low may not provide adequate oil pressure for lubrication, could lead to engine roughness, or may not allow for proper functioning of the engine’s ignition and fuel systems.

Typically, the idle RPM specified in most aircraft engine manuals for single-engine aircraft ranges between 600 and 750 RPM. A figure of 650 RPM represents a balance that ensures the engine is lightly loaded but still functioning effectively. This RPM allows the pilot to verify that the engine is running smoothly before taking off while avoiding dropping below the threshold where operational issues might arise.

Moreover, running the engine at idle RPM within the manufacturer’s specifications is essential for conducting proper checks during run-up procedures, which include magneto checks, carburetor heat checks, and ensuring that the propeller operates correctly. Any deviation from the recommended idle RPM may signal an underlying issue with the engine, thus making 650 RPM a safe and recommended minimum during the engine run-up.