Inflight mag checks?

[Oracle427]

I recently read an NTSB accident investigation report where an aircraft suffered a magneto failure and subsequent total engine failure resulting in a forced landing. I'm not sure that I'll be able to find this report again.

I'm not suggesting that the same events would happen here due to an in-flight mag check. What I am trying to say is that these machines are very complex and one may not understand the entire chain of consequences for each action they take.

The airplane had a pinion gear break on one of the magnetos causing the ignition timing to change dramatically for that magneto. The engine began to run very rough and the pilot decided not to change mags as per the rough running engine checklist. I can't remember what the pilot did or didn't do, but I recall that they made a choice to not change mags.

The change in timing from the broken pinion damaged the pistons and even damaged the remaining magneto. The engine was completed disabled after several minutes of operation. It was determined that if the pilot had followed the checklist and switched to the remaining good mag, the engine would likely have ran smoothly at reduced power and suffered no further damage.

Reports like that make me cautious about doing things differently without careful consideration and discussion about a specific engine/type, etc.

[AKar]

As this discussion is alive again, I might as well share a few words.

First, this is solely consisting of my opinions, and is in no way intended to discuss pilot technique. What I write, as always, is written solely in interest of airplane and systems knowledge in technical sense, and no suggestions are made towards any particular way of operating an airplane - outside the educational environment of a simulator, of course!

That said,

The airplane had a pinion gear break on one of the magnetos causing the ignition timing to change dramatically for that magneto.

A mis-timed ignition from any reason is one of the most potentially harmful issues the engine can be subjected to, aside any catastrophic mechanical failure. In particular, any failure that causes the timing to advance can have serious consequences, and needs to be immediately dealt with. That's also why the timing should be checked as often as is practicable during maintenance checks. Most emergency checklists, I'd figure, cover the items needed, but I'm not immediately that convinced every pilot/operator, even if possessing a wealth of information will quickly recognize the situation as such that requires immediate corrective action. Also, the situation described above actually makes a case that underlines the very need to be comfortable with the basics of an engine operation instead of seeing them overwhelmingly complex machines - which they are not! A very interesting post!

I understand from the above that the pilot chose not to disable the failed magneto. It would be interesting to know what was driving him into that decision, as it would have been immediately logical and correct action to take - which I believe many experienced pilots would have taken automatically by the time the list was first brought up in this one. I wouldn't want to learn it was 'shyness' towards taking the action that led into that. As it is often difficult to think rationally when things go haywire, I think the training should give enough technical confidence for - not only pilots but every one involved in safety of flight - to take the necessary actions promptly and positively, with a presence of mind to understand the expected results and to realize if the actual ones are not those.

In that sense, a few points:

• 1) Under any circumstances, the engine must operate on a single magneto. Should conditions require one to switch to only one mag, there is no reason to hesitate, because
  • 1.1) if one selects a completely faulty magneto, it will cause the engine to flame out. That is immediately recoverable by re-selecting the working magneto.
    1.2) if one selects the faulty magneto that causes the bad running, it already determines the engine performance, if it has failed into dangerous way. Selecting the good magneto will improve the situation.
  2) A regular ground check done by the POH/AFM is not a complete, thorough test of the ignition system, and therefore, it will not give reliable evidence that the ignition system is in fully working order. An ignition problem may develop, especially at high altitude (due to reduced dielectric strength of the air as its density decreases), even if the 'drop test' was performed successfully. Basically, the drop test identifies things such as a dead magneto (or a poor plug or harness if it is bad enough), actually giving some confidence on how well these things work on one mag! It will not identify most, if any, of more subtle issues. At that low RPM and rich mixture, even rather bad timing on another one could go unnoticed. In that sense, it is important to understand that a 'passed' drop test is not a reason for total confidence.
  
  3) In my personal opinion, a high power, lean mixture ground run drop test should be performed sparingly, but regularly, to every airplane. I would not recommend the pilots to do that at their own discretion, however, because it is hard to the engine and requires some attention to the engine temperatures, and enough competence and instrumentation to analyze the results on the fly. But, again, in my opinion, the maintenance crews and pilots participating on the maintenance run-ups should be educated on that.
  
  4) Magneto checks in flight are more demanding to the ignition as a system, and therefore make much better diagnostics of it. They also do not pose any realistic, immediate danger, however, whether they should or should not be performed by individual pilots is not something I'm willing, or would have any competence to comment. But, I think, once again, in my opinion that it would be beneficial, if practicable, that those are to be performed regularly by crew competent to handle such tests and to provide any meaningful information of the results. Perhaps before the maintenance event and then during the acceptance test flight.

-Esa