What is the danger of working around reciprocating engines with an ungrounded magneto?

• A. The spark can occur at any time even if the propeller is stationary, starting the engine.
• B. You must never disconnect ignition leads.
• C. The impulse coupling snaps and sends a spark to the cylinder; stay clear of the propeller and disconnect ignition leads if possible.
• D. The ignition system cannot produce a spark when ungrounded.

Answer: (C)

The danger comes from the high-voltage spark being able to occur at any moment when a magneto is ungrounded. Without a ground reference, the ignition system can fire even with the propeller stationary, or at times you don’t expect, which could ignite fuel vapors or injure you if you’re near the ignition leads or spark plugs. The impulse coupling part of some magnetos is a starting feature that can deliver a strong spark to the cylinder, so staying clear of the propeller and disconnecting ignition leads when working around the engine reduces the chance of an accidental spark reaching you or the fuel system.