FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
A CFI is explaining the purpose of the impulse coupling found on some aircraft magnetos. What function does it serve during engine starting?
- AIt momentarily retards spark timing and mechanically snaps the magneto to produce a stronger spark at low cranking speed
- BIt advances the ignition timing to provide a hotter spark at high RPM
- CIt automatically switches between the left and right magnetos during runup
- DIt reduces magneto output voltage to prevent damage to the spark plugs during starting
Show answer & explanationAnswer & explanation
Correct answer: A. It momentarily retards spark timing and mechanically snaps the magneto to produce a stronger spark at low cranking speed
The impulse coupling retards ignition timing and stores energy in a spring, then releases it to spin the magneto rotor rapidly, producing a strong, hot spark even at the low RPM of hand or starter cranking. This ensures reliable starting despite the low rotational speed of the magneto during engine start.
Why the other options are wrong
- B. Impulse couplings retard timing during starting, not advance it, and are not used at high RPM.
- C. Magneto selection is done manually via the ignition switch, not by the impulse coupling.
- D. The impulse coupling increases spark energy at low RPM rather than reducing voltage.
Impulse Coupling
A spring-loaded mechanism on one or both magnetos that retards timing and delivers a rapid, high-energy spark during engine starting.
- Produces a strong spark at low cranking RPM
- Retards timing to prevent kickback during start
- Often makes an audible clicking sound during hand propping
Memory trick: 'Wind it up, snap it loose—hot spark for a slow start'