FAA Powerplant Written Practice Test

What indicates normal operation when switching the ignition from both to either left or right during an engine ground check?

1. Increase in RPM

2. No change in RPM

3. Slight drop in RPM

4. Complete engine shutoff

The correct answer is: Slight drop in RPM

When switching the ignition from both to either the left or right during an engine ground check, a slight drop in RPM is expected if the engine is operating normally. This is because switching to a single ignition system typically results in reduced spark energy and might not ignite the fuel-air mixture as effectively as when both magnetos are operational. The slight drop indicates that while one magneto is functioning adequately, the remaining spark sources may not be providing full ignition stability. If the ignition system is in good condition, switching to one magneto should only result in this minimal decrease in RPM instead of any significant or complete changes in engine speed. In contrast, if there is no change in RPM, it could suggest that there is a malfunction in either the selected magneto or the ignition system overall. An increase in RPM would be abnormal, signaling potential issues with one of the magnetos or possibly an incorrect linkage in the ignition system. Complete engine shutoff is indicative of a more severe malfunction, indicating a failure of both ignition systems or a serious issue with ignition timing or fuel delivery.