How It Works: A pilot’s guide to magneto P-leads

During a recent gathering, the discussion involved magneto P-leads, the small wires that start at the magnetos and seem to disappear into the firewall. A local aviator suggested that if he had to make an emergency departure in his trusty flying machine without his ignition key, he would simply cut, or disconnect, both P-leads. He claimed this procedure would render his magnetos hot and it would be a simple matter of starting the engine by hand.

A qualified technician provided some additional considerations, but his comments were met with “deer in the headlights” stares. It appeared the average pilot may be missing a rudimentary understanding of magneto P-leads.

A student pilot learns that magnetos are driven by the engine and operate independently from the electrical system (see “How It Works: Magneto”). A spinning magnet induces a large current and small voltage in the primary winding of a coil. A secondary winding then develops a small current and a large voltage which is then routed to the spark plugs. Even if the aircraft generator and battery experience failures, there should always be dependable ignition. But, a way to shut off each of the magnetos is required. That’s where the P-leads come in.

The “P” in P-lead comes from the primary winding in the magneto’s coil. To deactivate the magneto, the primary winding is grounded. An ignition switch opens and closes the P-lead circuits to a suitable ground. The internal workings of an Off/R/L/Both ignition switch contain the required connections. When selected to OFF, both left and right P-leads are grounded and both magnetos are off. In the LT position, the right P-lead is grounded. The RT position grounds the left. In Both, both P-lead circuits are open, and both magnetos are on. Without P-leads, both magnetos would be hot at all times.

Some ignition switches have a Start position. If one of the magnetos has an impulse coupling or an induction vibrator, typically the left, the engine is started on that magneto. The Start position is identical to having the switch selected to the LT position and includes powering the starter. Some magnetos have two P-leads with two independent sets of points, known as retard and advance. The Start position utilizes the retard points. After starting, with the switch in Both, the advance points become active. A starting induction vibrator is used to create a continuous spark for the retard points, and requires its own electrical power source.

If possible, the P-leads should be visually checked for fraying, chafing, or other damage during preflight. Damage along their length, or a short to aircraft structure, can render the magneto erratic or uncontrollable. Also, running functions of the P-leads should occasionally be checked. Prior to engine shutdown, at idle RPM, position the ignition switch to OFF. When the engine RPM is clearly decreasing, return the switch to Both. But, if the engine continues to run, at least one of the magnetos is still on. This is a dangerous situation wherein the engine could start by simply moving the propeller, and should be addressed by a maintenance technician.

Now, it’s back to our intrepid aviator who is planning his great escape by cutting or disconnecting his P-leads. To be successful, he would need easy access to the back of both magnetos, which is not often possible depending on cowling design. There would have to be at least one impulse coupling. Also, some magnetos have a large nut which disconnects the P-lead, and some connectors of this type have a safety feature that renders the magneto inoperative if unfastened.

If both magnetos are somehow rendered hot, there is the distinct possibility that during manual engine rotation an impulse coupling magneto, with its retarded spark, could induce a sudden engine start. The other hot magneto, with the advanced spark, could create violent backward rotation. Engine damage and personal injury are likely.

Any attempt to start an aircraft engine by bypassing safety features, including the ignition switch and P-leads, could be a very expensive and extremely dangerous procedure, and should never be attempted. The best advice is to learn all you can about your particular aircraft and operate it in strict accordance with the manufacturer’s instructions. Oh, and keep a spare ignition key in a handy location.