FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A technician turns the ignition switch to OFF before pulling the propeller through by hand, but the engine continues to fire on one magneto. What is the most likely cause of this hazardous condition?
- AAn open (broken) primary ground (P-lead) wire between the switch and the magneto
- BA weak or open magneto condenser
- CThe E-gap has been set to too great an advance angle
- DExcessive breaker point gap on the affected magneto
Show answer & explanationAnswer & explanation
Correct answer: A. An open (broken) primary ground (P-lead) wire between the switch and the magneto
If the P-lead (primary ground wire) that grounds the magneto's primary circuit when the switch is OFF is broken or disconnected, the magneto remains ungrounded and 'hot,' allowing it to fire even with the switch in the OFF position. This is why the propeller must always be treated as if the ignition is live.
Why the other options are wrong
- B. A bad condenser causes arcing/weak spark, not a hot mag on switch-off.
- C. E-gap affects timing of spark, not the grounding circuit.
- D. Point gap affects timing/output, not the ability of the switch to ground the magneto.
Hot Magneto (Open P-Lead)
A magneto that continues to fire with the ignition switch OFF because its primary grounding (P-lead) circuit is broken.
- Switch OFF normally grounds the primary circuit through the P-lead, stopping magneto firing.
- A broken P-lead leaves the magneto ungrounded and capable of firing at any time.
- Always treat a propeller as if the ignition is live, regardless of switch position.
Memory trick: No ground, no OFF — a broken wire keeps the mag alive.