Pilot 1 is doing a run-up and notices an excessive mag drop on one side. He runs it up to nearly full power briefly and the engine smoothes out and the mag drops become equal and normal. He takes off and makes an uneventful flight. When he returns, he tells Pilot 2 of the problem and the solution.
The next day, Pilot 2 is doing a run-up on the same airplane. He notices an excessive mag drop on one side, and recalls Pilot 1s solution. Recalling that full power solved the problem for Pilot 1, he takes the runway and advances the power to full. The plane takes off as usual, but the com radios are filled with ignition noise. At an intermediate destination, a mag check shows equal drops that are on the far end of the allowable range, but within limits. Pilot 2 returns safely and tells Pilots 1 and 3 about the experience.
A mechanic is summoned. After investigation, he determines that the ignition harness needs to be replaced. The part is ordered and installed. Pilot 3 gets the duty of testing the repair.
Pilot 3 notes that the com is free from ignition noise and taxis to the run-up area for a test flight. At run-up, the mags show an equal drop, but the drop is excessive and the engine runs poorly. Recalling Pilot 2s experience, he takes the runway and advances the power to full.
The engine refuses to develop more than 2,000 rpm. Pilot 3 aborts the takeoff and taxis back to maintenance. The new ignition harness is found to have a flaw that allows several of the leads to arc.
Troubleshooting problems in aircraft is complicated when more than one pilot flies the plane. Describing the nature of the problem to other pilots can lead to a distorted impression and risky behavior. If the message must then be relayed to a mechanic, the chances of a misdiagnosis increase even more.
In the true story above, all three pilots believe they acted cautiously and responsibly, especially given the long runway available. The important thing to remember is that not all mechanical failures involve components working fine one minute and not working at all the next. In this case, the ignition harness had hidden corrosion that gradually made performance decline.
It illustrates the necessity to keep careful track of the airplanes performance and note even small changes over time. This will not only help every pilot who flies the airplane make an individual risk assessment, but will help the mechanic pinpoint the problem at whatever point maintenance is done. That will not only save you money, it could save your life.
-Ken Ibold
