A mechanic disconnects a magneto primary (P-lead) from the ignition switch to perform maintenance, but does not ground the lead to the engine or airframe. What hazard does this create?
- AThe magneto could remain in a hot condition and fire if the propeller is moved
- BThe oil pressure will drop due to loss of grounding
- CThe battery will discharge through the open circuit
- DThe engine will backfire immediately upon shutdown
Show answer & explanationAnswer & explanation
Correct answer: A. The magneto could remain in a hot condition and fire if the propeller is moved
An ungrounded P-lead removes the path that normally grounds out the magneto when the switch is OFF, leaving the magneto "hot" and capable of producing spark if the propeller is moved by hand, creating a serious personal injury hazard. This is why an open or broken P-lead is treated as an airworthiness and safety hazard requiring the magneto to be grounded through another means before any propeller handling.
Why the other options are wrong
- B. Oil pressure is unrelated to magneto grounding or the P-lead circuit.
- C. P-leads are part of the ignition system, not the battery circuit, so this has no effect on battery discharge.
- D. Backfiring relates to induction system fuel/air issues, not P-lead grounding.
Magneto P-Lead Grounding
The primary lead (P-lead) grounds the magneto when the ignition switch is OFF; if disconnected or broken without an alternate ground, the magneto remains "hot" and can fire if the propeller is turned.
- Switch OFF grounds the magneto through the P-lead
- An open P-lead = magneto stays live/hot
- Always treat propeller as if the ignition is on when P-lead is disconnected
Memory trick: No ground on the P-lead means the prop can bite—treat it as LIVE.