What condition indicates a weak-engine when operating with a fixed-pitch propeller?

Lower than normal static RPM indicates a weak engine when operating with a fixed-pitch propeller for several reasons. In a reciprocating engine, the static RPM is a measure of the engine's ability to produce power while the aircraft is stationary. If the engine is producing lower RPM than expected, it suggests that the engine may not be generating sufficient power for optimal performance.

This condition can be caused by various factors, including fuel mixture issues, mechanical problems, or inadequate engine efficiency. A fixed-pitch propeller does not adjust its blade angle to optimize performance at different speeds or power settings, so any deficiency in engine power directly manifests as lower RPM readings.

In contrast, higher than normal static RPM could indicate excessive engine power, while fluctuating RPM at full throttle may point to inconsistent engine performance rather than outright weakness. Excessive vibration during operation indicates potential mechanical issues but does not directly correlate with RPM output as a measure of engine weakness. Overall, lower than normal static RPM serves as a clear indicator of a lack of engine power in this context.