A magneto check on an aircraft includes checking the entire ignition system. Performing a magneto check during the before-takeoff checklist is a task required for spark-ignition piston engines. The preflight magneto check verifies each magneto's operation to see that all plugs are firing evenly and regularly on each magneto, and it tests each magneto for ignition fault. By grounding one magneto and observing RPM drop, the pilot confirms that the magneto ground is intact and that the entire ignition system is functioning properly.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a magneto check on an aircraft?
A magneto check on an aircraft involves checking the magnetos, which is a check of the entire ignition system, and during pre-flight, it verifies each magneto's operation to see that all plugs are firing evenly and regularly on each magneto. The check is performed during the before-takeoff checklist at a specified rpm setting - approximately 1,800 RPM for fixed-pitch propeller aircraft - and is accomplished by grounding one magneto, then the other, while the engine runs. In flight, a tougher test is carried out at normal cruise power and an aggressively lean mixture, preferably lean-of-peak, while watching the engine monitor in normalize mode. By sensing each cylinder's EGT and CHT, the pilot can identify the cylinder exhibiting a lower-than-normal EGT or the one that drops out completely on one mag, directing the mechanic to the specific spark plug to clean or change. The usual pre-flight mag check will only detect gross defects in the ignition system, whereas the in-flight mag check is a more demanding and discriminating way to test the ignition system. After the check, the switch is returned to BOTH, guaranteeing the engine runs smoothly on each mag and that magneto grounding wires are still connected.
What is the function of the P-lead in an aircraft magneto?
The P-lead is a wire that runs from the ungrounded end of the magneto coil's primary winding to the cockpit ignition switch. Its sole purpose is to disable the magneto by grounding the primary coil: when the switch is OFF, the P-lead shorts the primary to ground so the magneto cannot fire. In the L position the P-lead grounds the right magneto and in the R position it grounds the left magneto. In BOTH the leads are opened and both magnetos operate. A broken P-lead leaves the magneto hot: the ground path is lost, the primary is never shorted, and the engine can keep firing even with the switch OFF. Because the wire is normally a 16-gauge shielded conductor, any break in the center conductor or its connection to the switch creates this dangerous condition. Shielding is vital - an unshielded P-lead acts as an antenna that radiates ignition pulses and interferes with aircraft radios. So the shield is grounded to the magneto case and, according to installation manuals, at both the magneto and switch ends.
How to test an aircraft magneto?
To test an aircraft magneto, first turn the key one click to select the left magneto. This grounds the right magneto so only the left ignites. Switch to the right mag in the same way to isolate the opposite side. While the magneto is on the bench, measure resistance across the breaker points to confirm continuity. For a quick compression feel, place a thumb over the spark-plug hole of cylinder one and turn the prop slowly, a firm puff indicates the selected magneto is firing.
When I was learning solo ignition verification, the instructor demonstrated the operation and emphasized the importance of the procedure. I moved the spark coil toggle from BOTH to RIGHT setting, then progressed the accelerator to accomplish the particularized RPM. I saw a standard RPM fall and the motor persisted to operate smoothly. The RPM decrease was within ranges provided in the aircraft's manual. I reverted the toggle to BOTH, then proceeded to change to the LEFT side. I reiterated the process for the port area, and both drops were within acceptance. Examining the two drops was vital. Both the drops were the same, indicating no substantial discrepancy that would signify a possible question with one ignition system.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Why does the RPM drop during a magneto check?
The RPM drops when the magneto switch is turned to the L or R position because the ignition system is now firing only half the spark plugs. This drop shows that the engine will still keep the aircraft flying on one magneto. A normal drop of 50-70 RPM at 3,400 RPM or 80-100 RPM at 4,000 RPM occurs. A drop of 100 RPM or less on both magnetos is nominal, while a differential between the two magnetos does not exceed 50 RPM and neither side exceeds 150 RPM or the maximum-allowable 175 RPM.
What does a magneto drop check indicate in an aircraft? The point of the RPM drop is to validate the tests. Too great a drop indicates several problems: a bad magneto, fouled plug, bad ignition wire, or improper timing. Excessive RPM drop when the engine runs smooth but all EGTs rise indicates retarded ignition timing. An absence of RPM drop indicates that the magneto is not grounded - the grounding wire is broken, the P-lead is open, or the timing is set in advance of the setting specified. If the engine runs rough on one magneto and is accompanied by a larger RPM drop, it indicates a fouled spark plug. Rough running on one magneto indicates a bad magneto with 80% certainty. If doubt remains, RPM checks at higher engine speed will usually confirm whether a deficiency exists.
What is a hot magneto?
A hot magneto is one whose primary circuit is not grounded. Although the cockpit ignition switch is OFF, the magneto itself is still live. Because a magneto is a self-contained electrical generator, any rotation of its permanent-magnet rotor will produce a 20,000-30,000 volt pulse. If the propeller is moved even slightly, that pulse reaches the spark plugs, and the engine fires.
The hazard is twofold. First, the unexpected start jerks the propeller through several degrees of travel fast enough to injure hands or feet. Second, if the impulse-coupled magneto is hot, its retarded spark produces violent backward rotation, while the non-impulse unit, with its advanced spark, produces forward rotation. Either direction is strong enough to damage the engine and dangerous to personnel.
Any indication like MAGNETO #2 HOT on a checklist is therefore a no-go item: the flight must be postponed until the grounding fault in the P-lead is found and the magneto is safely returned to a cold state.
