When performing a magneto check, what RPM drop indicates a potential issue?

In the context of performing a magneto check during a pre-flight or run-up procedure, an RPM drop of 175 indicates a potential issue because it suggests that the engine is not operating properly on one of the magnetos. Typically, manufacturers often specify acceptable limits for RPM drop during a magneto check, which generally range from 50 to 200 RPM for a dual ignition system like that found on the PA-28A. A drop below these acceptable levels could indicate a malfunction in the magneto system, such as a faulty spark plug or an issue with the magneto itself.

In practical terms, while slight variations exist among different aircraft models, the 175 RPM drop is a clear signal to the pilot that further investigation is needed. It is essential to address any indications of an excessive RPM drop promptly to ensure safe operation of the aircraft. An RPM drop within the acceptable range, generally seen above 200, does not typically raise significant concerns regarding the integrity of the ignition system.