← All explained questions · Supplemental · II — Preflight Procedures
When checking magnetos during the engine run-up, you observe an RPM drop of 200 RPM on the LEFT magneto and 50 RPM on the RIGHT magneto. The maximum allowable drop in the POH is 175 RPM with no more than 50 RPM differential between magnetos. The correct action is to
Choices
depart — the right magneto is fine.
Departing with a fouled/weak magneto risks engine roughness or stoppage in flight.
✓ abort the run-up.correct
both the absolute drop on left magneto AND the 150 RPM differential exceed POH limits; investigate or report for maintenance. Two limits are violated: (1) 200 > 175 RPM drop is excessive on the left mag, suggesting fouled plug or timing issue; (2) 150 RPM differential is far above the 50 RPM allowed, indicating one magneto/ignition system is significantly weaker. Either is grounds to abort. Try leaning + burn-off to clear plug fouling first, then re-check; if still out of limits, write up for maintenance.
lean the mixture and re-test.
Lean burn-off is a reasonable first step but doesn't justify departure if drop persists.
shut down the engine and walk back to the FBO.
Shutdown is appropriate only if there's no chance to clear the issue; standard sequence is troubleshoot first.
Why
both the absolute drop on left magneto AND the 150 RPM differential exceed POH limits; investigate or report for maintenance. Two limits are violated: (1) 200 > 175 RPM drop is excessive on the left mag, suggesting fouled plug or timing issue; (2) 150 RPM differential is far above the 50 RPM allowed, indicating one magneto/ignition system is significantly weaker. Either is grounds to abort. Try leaning + burn-off to clear plug fouling first, then re-check; if still out of limits, write up for maintenance.
FAA source: FAA-H-8083-3C, AFH Ch. 2, before-takeoff check; aircraft POH/AFM magneto limitsbrowse the reference library →
Original study question written for this course — representative of FAA knowledge-test topics, not an actual current FAA exam question.