What is the maximum allowable drop during a magneto check?

During a magneto check, the purpose is to evaluate the performance of the aircraft's ignition system and ensure that the magnetos are functioning correctly. The maximum allowable drop is indicative of how well the ignition system can maintain engine operation when the electric circuit is disrupted by checking each magneto individually.

The maximum allowable drop of 50 RPM between both magnetos means that during a magneto check, if the RPM drops by more than 50 RPM from the baseline while checking either magneto, it could indicate a problem with the ignition system. This limit is used as a guideline to assess whether the magnetos are functioning within acceptable parameters. If the drop exceeds this threshold, it may suggest issues such as faulty connections, worn components, or other mechanical problems that could affect the engine's reliability during flight.

The other options like no drop allowed or specific RPM limits per magneto are either too stringent or not standard practice for ensuring proper magneto function, thus reinforcing that 50 RPM between both magnetos is the reasonable and acceptable standard.