We're scheduled for an early morning takeoff. At first all goes well, with a recorded start time of 05:52 a.m. (that's translated from Zulu time to local time) on my operational flight plan. The engines warm quickly and I taxi to the runup area near the threshold where I go through the systems checks. Propellers work, electrical works, deicing works, gauges work. I select the left engine left magneto off to ensure that the right magneto works independently, but it doesn't. The left engine rpm drops precipitously. I quickly flick the magneto back on again, but it's too late, the left engine dies. Damn.
I restart the engine and run it up hot, leaning the mixture for a high cylinder head temperature. The right magneto is connected to one spark plug in each cylinder so if the right plugs are fouled, turning off the left mag could leave some cylinders without a working spark plug. I don't really believe this is the case, but it's the sort of thing the maintenance manager will ask me to do when I call, so I want to have it done before they ask. By running the engine up hot it can burn off the carbon deposits. It can also damage the cylinders, so I don't do it for long. The procedure has no effect today. The engine still will not run on the right magneto alone.
It's six o' clock on a Sunday morning. Things like this always happen on the weekend. I advise the customer that we may be able to get the airplane fixed this morning, but no guarantees. I advise company immediately (text message to PRM says something like "bad mag. no flight. looking for local help") and then I wait until seven to call the 24-hour contact number for the local FBO. Sure it's "twenty-four hours" but it's still a real person who has to pick up that cellphone, and I know he's probably sleeping. He isn't sure if he can get maintenance for me on a Sunday, but says he'll give them another hour to sleep and then call them. The good news is that this company operates the same type, and magnetos are parts one expects to fail now and again--that's why I have two per engine--so chances are good they have the part as well as the expertise.
Meanwhile my PRM is a little disdainful of my diagnosis. The reply is "What do you mean, 'bad mag'? How much of a drop is there?" Properly my snag should read, "Left engine will not run with left magneto selected off." I knew that. I don't actually know it's a bad magneto. It could be a lot of things. I was lazy with the text message. Company wants me to disconnect the P-lead to check if the magneto is bad or just accidentally grounded. The way these things work is that when they are switched off, they are connected to ground, to avoid accidental injury. If the grounding wire rubs on something so the insulation wears off, it can be grounded all the time and the magneto will behave as though it is off, regardless of the switch position. With the mechanic on the phone, I pull off the engine cowling and find the right magneto. It has two wires coming from it, not the one that was described. I feel like I'm in one of those movies where the hero has to decide which wire to cut in order to disarm the bomb. I carefully describe what I'm looking at. This is why pilots have to know the parts of their airplanes. The mechanic identifies the extra wire as part of an optional starter system. I remove a nut to disconnect the wire he specifies, folding it out of the way and securing the ends so they won't contact anything. I put the cowling back on and restart that engine. If the problem was that wire, then the engine should continue to run now whether or not the left magneto magneto switch is on. In fact, now it should continue to run with both magneto switches selected off. It doesn't. It behaves exactly as before I disconnected the lead.
My PRM is forced closer to my abbreviated diagnosis of "bad mag." The FBO has got a hold of the local mechanic, and he's fairly sure he does have the part, but he is fixing his roof, and has hired a crew, so can't work on my airplane today. He promises first thing tomorrow morning. I tape a sign to the mag switch that the p-lead is disconnected, even though it's just me. You never know.
I have to tell the customer we're down for the day.