Don't Instictively go Back to Both

[pelmet]

I am talking about a mag check. What do you do after checking a mag individually? Go back to both right away. What happens if the engine dies because you have a bad mag. Instinct makes most of us go back to both as quickly as possible. Then you get a backfire(no need to correct the terminology) which is not good.

Let the engine die. Easy to say but not easy to do. Instinct makes many of us go back to Both. So whenever, I do my mag check, I deliberately, select the desired mag and immediately release my hand from the key. Then, with everything fine, I put my hand back on the key and select back to both. If the engine dies, it dies. If it is running rough, I can think about what I should do next.

What about in the air. One can do a mag check in the air as well as seen in the video below. In fact it is a much better check. I remember flying a Chipmunk once where the mag check on the run-up was just fine. Once in the air, the airframe seemed to be vibrating. Initially one asks if that was the way it always is but after turning to stay near the airport, I did an inflight mag check(toggle switches for that type). One of the mags led to very rough engine operation, much worse than with both mags on. I shut it off and returned for landing. After clearing the runway, I did a another run-up and everything was fine. Then I did a higher power run-up and the roughness was there. Bottom line, normal runup power may not diagnose a mag problem.

So then the question is.....what if the engine dies while doing the mag check in the air? Let the engine die(unless it is unsafe to do so for a few seconds) and seeing as you removed your hand, you won't instinctively go back to both(it helpsd if you were prepared in advance for the engine to die for several aspects). Then mixture to lean, close the throttle, turn on the good mag, mixture rich again, and open the throttle as required.

https://www.flyingtv.com/programs/wta_e ... mp4-106577

A good description is in this video at 10:42

[photofly]

Let the engine die. Easy to say but not easy to do. Instinct makes many of us go back to Both. So whenever, I do my mag check, I deliberately, select the desired mag and immediately release my hand from the key. Then, with everything fine, I put my hand back on the key and select back to both. If the engine dies, it dies. If it is running rough, I can think about what I should do next.

Another way is to mentally prepare for the mag check to shut down the engine as you switch to one mag. Then it’s not a surprise, and you don’t instinctively go back to both. The surprise comes when the engine keeps running!

[digits_]

Let the engine die. Easy to say but not easy to do. Instinct makes many of us go back to Both. So whenever, I do my mag check, I deliberately, select the desired mag and immediately release my hand from the key. Then, with everything fine, I put my hand back on the key and select back to both. If the engine dies, it dies. If it is running rough, I can think about what I should do next.

Another way is to mentally prepare for the mag check to shut down the engine as you switch to one mag. Then it’s not a surprise, and you don’t instinctively go back to both. The surprise comes when the engine keeps running!

Are you saying you want to surprise your students during every mag check when the engine keeps running ?



On a more serious note, interesting subject. What's the harm in going back to both on the ground? The detonation, if it happens, should happen the moment you switch to the broken mag. After that point, why not go back to both so you can taxi back to the ramp?

[ahramin]

On a more serious note, interesting subject. What's the harm in going back to both on the ground? The detonation, if it happens, should happen the moment you switch to the broken mag. After that point, why not go back to both so you can taxi back to the ramp?

As pelmet explained, when you go back to both the ignition of the unburnt fuel in the exhaust pipe can damage the exhaust system. Same problem when doing a live mag check before shutdown but in that case, it's less likely because the RPM is slower and the mixture should be as lean as possible.

A mag check is far more likely to run rough than kill the engine, in which case you probably want to leave it on that mag for a bit anyway to diagnose where the problem is (if you have the instrumentation).

[digits_]

On a more serious note, interesting subject. What's the harm in going back to both on the ground? The detonation, if it happens, should happen the moment you switch to the broken mag. After that point, why not go back to both so you can taxi back to the ramp?

As pelmet explained, when you go back to both the ignition of the unburnt fuel in the exhaust pipe can damage the exhaust system. Same problem when doing a live mag check before shutdown, and the solution is the same:

1. Do the ignition check and the live mag check with as lean a mixture as possible, not full rich.
2. If the engine dies, be quick about getting back to both.
3. If you fail at 2, just let the engine die.

Interesting!

I've only had it happen a couple of times, but I seem to recall the loud bang is caused by switching the mags to off (or L/R and a broken mag) at high power setting. Are you saying that the bang is caused by the switching from OFF back to both?

Or are there multiple bangs?

[photofly]

I've only have it happen a couple of times, but I seem to recall the loud bang is caused by switching the mags to off (or L/R and a broken mag) at high power setting. Are you saying that the bang is caused by the switching from OFF back to both?

The bang definitely comes when you switch back to "both". Most often, in my experience, after a clumsy mag check by a less experienced pilot who overshoots the "L" position to "OFF" then realizes their mistake and fixes it before you have time to intervene by closing the throttle.

EIther way, if you're not expecting silence, it's easy for your hand to jerk back to both after one or the other, and ... BANG!

Rough-running doesn't usually cause a problem, one cylinder not firing I don't believe allows much unburnt fuel/air mixture to build up in the exhaust manifold and muffler. But if all four (or six) stop for half a second or two, then you have a nice stoichiometric mixture filling those tubes....

[ahramin]

On a more serious note, interesting subject. What's the harm in going back to both on the ground? The detonation, if it happens, should happen the moment you switch to the broken mag. After that point, why not go back to both so you can taxi back to the ramp?

As pelmet explained, when you go back to both the ignition of the unburnt fuel in the exhaust pipe can damage the exhaust system. Same problem when doing a live mag check before shutdown, and the solution is the same:

1. Do the ignition check and the live mag check with as lean a mixture as possible, not full rich.
2. If the engine dies, be quick about getting back to both.
3. If you fail at 2, just let the engine die.

Interesting!

I've only had it happen a couple of times, but I seem to recall the loud bang is caused by switching the mags to off (or L/R and a broken mag) at high power setting. Are you saying that the bang is caused by the switching from OFF back to both?

Or are there multiple bangs?

You quoted me too soon ! I've already fixed it upon reflection. Yes, the loud bang is when you go back to both. When you turn the mags off, the fuel air mixture no longer ignites and is pushed out into the exhaust system. When you go back to both, the fuel air mixture is very hot and can still be combusting as it goes into the exhaust system, igniting the unburnt mixture. Bang. Why did you switch the mags off at a high power setting?

[digits_]

Thank you all for the clarification.

Why did you switch the mags off at a high power setting?

It wasn't intentional, it was during the run up that I accidentally went to OFF instead of L/R. I could have sworn the bang happend before I went back to both, but it's been quite a few years, so my recollection of the event might be faulty.

[ahramin]

I suppose the unburnt fuel air mixture could have been ignited by something hot in the exhaust system while the mags were off?

Maybe a better title for the topic would be only select mags off with low RPMs and lean mixture .

[photofly]

Thank you all for the clarification.

Why did you switch the mags off at a high power setting?

It wasn't intentional, it was during the run up that I accidentally went to OFF instead of L/R. I could have sworn the bang happend before I went back to both, but it's been quite a few years, so my recollection of the event might be faulty.

I would put money on you switching back to L from OFF to cause the bang. Very easily done.

[pelmet]

As pelmet explained, when you go back to both the ignition of the unburnt fuel in the exhaust pipe can damage the exhaust system. Same problem when doing a live mag check before shutdown, and the solution is the same:

1. Do the ignition check and the live mag check with as lean a mixture as possible, not full rich.
2. If the engine dies, be quick about getting back to both.
3. If you fail at 2, just let the engine die.

Interesting!

I've only had it happen a couple of times, but I seem to recall the loud bang is caused by switching the mags to off (or L/R and a broken mag) at high power setting. Are you saying that the bang is caused by the switching from OFF back to both?

Or are there multiple bangs?

You quoted me too soon ! I've already fixed it upon reflection. Yes, the loud bang is when you go back to both. When you turn the mags off, the fuel air mixture no longer ignites and is pushed out into the exhaust system. When you go back to both, the fuel air mixture is very hot and can still be combusting as it goes into the exhaust system, igniting the unburnt mixture. Bang. Why did you switch the mags off at a high power setting?

Most often, in my experience, after a clumsy mag check by a less experienced pilot who overshoots the "L" position to "OFF" then realizes their mistake and fixes it before you have time to intervene by closing the throttle.

EIther way, if you're not expecting silence, it's easy for your hand to jerk back to both after one or the other, and ... BANG!

Which is why it is best to..........

......quickly remove your hand from the key/switch when reaching the detent. Because instinct is to go from off to an operating mag. With your hand removed, hopefully(never a guarantee in this world) you won't do the lightning quick improper selection.

Or if you are an instructor....teach the student to do this.

[CpnCrunch]

I would put money on you switching back to L from OFF to cause the bang. Very easily done.

Don't you mean R? The sequence is OFF, R, L, Both.

I did this myself last week. I think I must have been taxiing on the left magneto by mistake, as I'd been testing both magnetos before taxiing (after a magneto repair). During the run-up the engine started dying as I moved the igintion to what I thought was R. I instinctively put it back to R almost immediately, and there was a loud bang. Lesson learned.

[trey kule]

I was alway taught to do a dead mag check at low RPMs after start.
But the same applies

[HHI]

A long time ago while doing a mag check during a run-up the engine started to quit while selected to a single mag. I knew better but instinctively switched back to both. There was a very loud bang. I taxied back to the ramp and the faulty mag was replaced.

That bang turned out to be very expensive. Two flights later (while away from home base) the exhaust riser on one of the cylinders had fractured. Loud engine noise during taxi out.

Taxied back and got a hotel room for the night.

That cost the owner (me) the price of a new exhaust system.

My checklist now includes a “dead mag” check at as low an idle as possible prior to taxi to the run-up area. If a mag is failed and I do not let the engine die but do switch back to both, there will not be a loud bang - with potential damage to the exhaust system. The odds of a mag failure after engine start and prior to the runup are (hopefully) very slim and if it did happen I hope to not switch back to both out of habit/instinct.