FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
During an engine runup, a CFI has a student switch the magneto selector from BOTH to LEFT only. The RPM drops slightly and runs smoothly. The student then switches to RIGHT only, and the engine stops firing completely. What should the instructor conclude?
- AThe impulse coupling on the right magneto has engaged, cutting spark timing
- BThis is normal because the right magneto always fires fewer cylinders than the left
- CThe right magneto has failed or its ground (P-lead) wire is grounded, killing the engine
- DThe engine stopping confirms both magnetos are functioning normally and properly grounded
Show answer & explanationAnswer & explanation
Correct answer: C. The right magneto has failed or its ground (P-lead) wire is grounded, killing the engine
If the engine dies completely when selecting a single magneto position, it indicates that magneto (or its ignition circuit) is not producing spark — either failed internally or its P-lead is shorted/grounded, effectively killing all ignition since only one magneto is now supposed to be firing. A normal single-magneto check should show an RPM drop but the engine continues running smoothly.
Why the other options are wrong
- A. Impulse couplings function during starting, not during runup checks.
- B. Both magnetos should fire all cylinders in a normally operating dual ignition system.
- D. An engine stopping on a mag check indicates a problem, not normal grounding.
Dual Magneto Ignition Check
Selecting each magneto individually during runup verifies both ignition systems function; a normal drop in RPM (not stoppage) confirms proper operation.
- BOTH position uses both magnetos for redundancy and smoother combustion
- Selecting L or R alone should cause a small RPM drop but engine keeps running
- Complete engine stoppage on one position indicates a magneto or ignition circuit failure
Memory trick: Drop is fine, dead is denied