FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesEasy
A mechanic is troubleshooting a reciprocating engine that is difficult to start when hand-propped in cold weather. The magneto is equipped with an impulse coupling. What is the primary function of the impulse coupling during engine start?
- AIt grounds the magneto primary circuit to prevent accidental firing during shutdown
- BIt regulates magneto output voltage during normal high-speed operation
- CIt advances ignition timing to improve fuel efficiency at cruise RPM
- DIt retards ignition timing and snaps the magneto rotor through the magnetic field rapidly to produce a hot spark at low cranking speed
Show answer & explanationAnswer & explanation
Correct answer: D. It retards ignition timing and snaps the magneto rotor through the magnetic field rapidly to produce a hot spark at low cranking speed
An impulse coupling mechanically retards the spark and stores energy in a spring, then releases the magneto rotor with a sudden snap so it spins fast enough to generate a hot spark even at slow hand-propping or starter cranking speeds.
Why the other options are wrong
- A. Grounding for shutdown is the function of the P-lead/ignition switch, not the impulse coupling.
- B. Magneto output voltage regulation is not a function of the impulse coupling.
- C. Impulse couplings function only at low starting speeds, not cruise, and they retard rather than advance timing.
Impulse Coupling
A spring-loaded mechanism between the magneto drive and rotor that retards timing and snaps the rotor rapidly during starting to produce a hot spark at low cranking speed.
- Only engages during starting/low RPM
- Automatically disengages once engine speed increases
- Retards spark to prevent kickback during hand-propping
Memory trick: Snap, retard, spark hard — that's the impulse coupling's job at start.