Hot Magneto: What It Is, Why It Happens, and Why It’s Dangerous

“Hot mag” is hangar shorthand for a magneto that will still fire with the switch in OFF. It is one of the few airplane faults that can hurt someone who is not even flying. Here is how it happens, according to the FAA’s maintenance handbooks.

How the ignition switch shuts off a magneto

The ignition switch works backwards from almost every other switch on the airplane. In the off position, it completes a circuit through the switch to ground. Other electrical switches normally open the circuit when they are off (FAA-H-8083-32B, Ch. 4, “Ignition Switches”).

• One terminal of the switch connects to the magneto’s primary circuit, between the coil and the breaker contact points. The other terminal connects to aircraft ground structure (FAA-H-8083-32B, Ch. 4, Fig. 4-15).
• With the switch OFF (closed), primary current is not interrupted when the breaker points open, because there is still a path to ground through the switch. The primary field cannot collapse suddenly, so no high voltage is induced in the secondary coil to fire the plugs. The points are effectively short-circuited, and the magneto is inoperative (FAA-H-8083-32B, Ch. 4).
• With the switch ON (open), the switch has no effect on the primary circuit. The breaker points control the field collapse again, and the magneto fires (FAA-H-8083-32B, Ch. 4, Fig. 4-16).

The wire from the magneto to that switch is the magneto ground lead, more commonly called the P-lead (FAA-H-8083-32B, Ch. 4, “Ignition Switch Check”).

What a broken P-lead causes

A magneto is shut off only when its primary circuit is grounded. If the P-lead is open, the magneto is not shut off even with the ignition switch in the off position (FAA-H-8083-32B, Ch. 4, “Ignition Switch Check”). The General handbook makes the same point: the switches that control the magnetos turn the ignition off by short-circuiting the current. If the switch is faulty, it can be in “off” and still let current flow in the magneto primary circuit, which could allow the engine to start with the switch off (FAA-H-8083-30B, Ch. 1, “Hand Cranking Engines”).

Nothing on the panel tells you. The key says OFF, and the engine is still live.

How to catch it: the ignition switch check

The ignition switch check verifies that all magneto ground leads are electrically grounded (FAA-H-8083-32B, Ch. 4):

• It is usually done at 700 rpm, or the lowest speed possible on engines that will not idle that low.
• Momentarily turn the switch to OFF. The engine should completely quit firing.
• After a drop of 200–300 rpm, return the switch to BOTH as rapidly as possible to avoid afterfire and backfire.
• If the engine does not cease firing in OFF, the P-lead is open and the trouble must be corrected.

Always follow the aircraft manufacturer’s procedures for your specific engine.

Inspection points

When troubleshooting the magneto, AC 43.13-1B says to look for frayed insulation on the leads in the breaker compartment, and to make sure all terminals are secure and wires are properly positioned (AC 43.13-1B, Ch. 8, para. 8-19(e)). It also directs that internal magneto inspection or repair follow the manufacturer’s maintenance and overhaul manuals (AC 43.13-1B, para. 8-19). For harness checks, see AC 43.13-1B, para. 8-18.

Why it’s dangerous: propeller handling

The FAA puts it bluntly. With an open P-lead, turning the propeller can result in personnel injury or death, because the engine can start with someone in the propeller arc (FAA-H-8083-32B, Ch. 4, “Ignition Switch Check”). The General handbook adds these rules (FAA-H-8083-30B, Ch. 1, “Hand Cranking Engines”):

• While touching a propeller, always assume the ignition is on.
• Never allow any part of your body in the way of the propeller, even when the engine is not being cranked.
• When hand-propping, use the callouts “Fuel on, switch off, throttle closed, brakes on” and “contact.” Never turn on the switch and then call “contact.”
• Make sure the ground is firm. Do not lean toward the blade. Push the blade down with your palms; do not curl your fingers over the edge.

Technicians also cannot assume the pilot of a taxiing aircraft can see them, and they must stay within the pilot’s view on the ramp (FAA-H-8083-30B, Ch. 1, “Safety Around Airplanes”).

Hangar takeaway

Treat every prop as live. Do the switch check, and if a mag won’t shut off, the airplane does not fly until it’s fixed. Ignition systems are core material for the Powerplant written and the oral and practical. Our A&P prep course covers them. If your airplane needs a look, see our aircraft maintenance in Kansas.

Frequently Asked Questions