FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A technician is about to remove a spark plug from a radial engine cylinder for inspection. The ignition switch is confirmed OFF. What additional precaution should be taken before touching the propeller or plug leads?
- ASet the mixture to idle cutoff only, since this disables the magnetos
- BDisconnect the battery and treat the P-leads as if they were still capable of firing the magneto
- CAssume the engine is safe since the switch is off and proceed without further checks
- DRemove only the top spark plugs since bottom plugs cannot fire with the switch off
Show answer & explanationAnswer & explanation
Correct answer: B. Disconnect the battery and treat the P-leads as if they were still capable of firing the magneto
A broken or open P-lead can leave a magneto 'hot' even with the switch in the OFF position, so the propeller and ignition system must always be treated as capable of firing an engine, and P-leads should be verified grounded before working near the ignition system.
Why the other options are wrong
- A. Mixture position has no effect on magneto grounding.
- C. Trusting the switch alone is a known cause of prop-strike injuries.
- D. Cylinder location does not affect whether a magneto can fire.
P-lead Safety
The primary (P) lead grounds the magneto when the ignition switch is OFF; if it is broken or disconnected, the magneto remains 'hot' even with the switch off.
- P-lead ground failure is a common cause of prop strikes
- Always treat propellers as if the ignition is live
- Ground checks are part of 100-hr/annual inspections
Memory trick: 'P' for Protect — a broken P-lead means the mag is still alive