FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsEasy
A pilot is conducting an engine run-up prior to takeoff. When the magnetos are switched from 'BOTH' to 'LEFT' only, a slight drop in RPM is observed. What is the most likely cause of this RPM drop?
- AAn electrical short in the left magneto's wiring harness.
- BNormal operation, indicating both magnetos are functioning.
- CInsufficient fuel pressure to the cylinders.
- DA fouled spark plug in the right magneto's circuit.
Show answer & explanationAnswer & explanation
Correct answer: B. Normal operation, indicating both magnetos are functioning.
When switching from 'BOTH' to a single magneto, the engine operates on only one ignition system. This typically results in a slight but noticeable drop in RPM, as the single magneto provides less efficient ignition than both working together. This is a normal and expected indication of proper magneto function during a run-up.
Why the other options are wrong
- A. An electrical short in a magneto's wiring would likely cause that magneto to fail completely, resulting in a much larger RPM drop or engine stoppage when selected.
- C. Insufficient fuel pressure would affect all cylinders and likely cause engine roughness or power loss regardless of magneto selection.
- D. A fouled spark plug would likely cause a more significant, rougher drop in RPM, and would be isolated to one cylinder, affecting the magneto it's connected to.
Magneto Check
A pre-takeoff check to ensure both ignition systems (magnetos) are functioning correctly by momentarily operating the engine on each magneto individually.
- Performed during engine run-up.
- Expect a slight RPM drop when switching from 'BOTH' to 'LEFT' or 'RIGHT'.
- Excessive RPM drop or no drop indicates a problem.
Memory trick: Magneto Checks Reveal Power Differences.