FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A student mechanic asks why some reciprocating engines use an impulse coupling on one magneto. What is the primary purpose of this device?
- ATo advance the ignition timing automatically at high altitude
- BTo retard the spark timing and spin the magneto rapidly to produce a hot spark for starting
- CTo reduce magneto RPM during engine shutdown to prevent kickback
- DTo increase magneto output voltage at cruise RPM
Show answer & explanationAnswer & explanation
Correct answer: B. To retard the spark timing and spin the magneto rapidly to produce a hot spark for starting
An impulse coupling temporarily retards the spark and then releases the magneto rotor to spin rapidly, producing a hot, retarded spark needed for easy starting at low cranking speeds.
Why the other options are wrong
- A. Timing advance at altitude is not the function of an impulse coupling.
- C. Impulse couplings act at start-up, not shutdown.
- D. Impulse couplings only function during starting, not cruise.
Impulse Coupling
A spring-loaded mechanism on one magneto that retards spark timing during cranking and then snaps the magneto rotor rapidly for a strong, retarded spark to aid starting.
- Used only on one magneto (usually the left/impulse mag)
- Produces a retarded, hot spark for easy starting
- Releases automatically once engine reaches a set RPM
Memory trick: Impulse = 'I Snap for Start' — winds up, snaps, sparks hot