Updated: Jun 30, 2019
Not long ago I took off out of a small 1900ft strip in a light twin, shortly after take-off the right engine started to surge. After a quick check to make sure the engine wasn't failing, I climbed to a safe altitude and started to troubleshoot the issue. What I found was the right hand magneto on the right hand engine had partially failed causing the engine's timing to change. Once I turned off the faulty magneto the engine's operation returned to normal and I continued to my destination which was only 10nm away and had a much larger runway than the airport I was leaving. I had checked the mags as per the pre-flight checklist (run-up) and they tested ok. The experience got me thinking about how magnetos work, how we test them, and what we're looking for when we test them.
As pilots of small aircraft, run-ups are a critical part of our pre-flight safety checks. We need to know with a reasonable level of confidence the aircraft's engine is performing as designed and it's going to continue to do so for the duration of our flight. One important check that is required in the pilot operating handbook of all the aircraft that I know of is a magneto check so what are we looking for when we check the magnetos?
Types of checks
There are three types of magneto checks, a dead mag check, a mag drop check, and a live mag check.
1. Dead Mag check
The dead mag check is usually completed during the after-start checks by turning one mag off and ensuring the engine will continue to run on one mag, then turning them both on and turn off the other, ensuring the engine will run on that magneto as well. Refer to the procedure in your aircraft's operating handbook for the procedure specific to your airplane. If you turn the switch to either mag and the engine quits, it's time to shutdown and find some help, one of the mags isn't working properly.
2. Mag drop check
The drop check is completed during the run-up, in most small piston airplanes we set the power to approximately 1700 RPM then turn off one mag and look at the drop in RPM. For most small piston engines we're looking to see if the RPM drops more than 125 RPM, if not we switch both mags on again and then switch off the other mag. Again, we're looking to see if the RPM drops more than 125 RPM or the difference in the drops of the two mags is more than 50 RPM.
If the drop is more than 125 or the engine runs rough on one mag there's a chance the problem is just a fouled spark plug but there's a chance it's something more. A fouled plug can be cured by increasing the power to 2000 RPM and leaning the mixture to best power for 30 seconds or so. If this doesn't clear the problem you should consult your mechanic, it could be a more serious problem with a cylinder or timing.
3. Live mag check
The live mag check is done at the end of the flight just prior to engine shutdown. During this check we quickly shut-off all the mags, listen for the engine to die and then turn them back on. If the engine does not quit with the mags off there is likely a P-lead which is the wire that grounds out the mag.
If a fault found during any one of these checks can't be cleared, the flight should be delayed until the problem can be found and fixed. If the engine stops during a dead mag check or there is a significant drop during a mag drop check, there could be a serious problem with the aircraft's ignition system and it should be checked. As with any defect, it's both good practice and a regulatory requirement that you record it in the aircraft's journey log and have someone appropriately qualified rectify the defect and release the aircraft back into service.
More information about magnetos and magneto related problems is available for our members.