Inflight mag checks?

[Nick - A2A]

Hello all,

Whilst reading this article by AvWeb co-founder Mike Busch as part of the discussion on cockpit engine monitors, I was struck by the mention of inflight mag checks, and how they provide "a far more exacting test of the ignition system than the usual runup mag check".

This was something I hadn't read about before. A quick web search (made more difficult by all those airline inflight magazines ) threw up a few forum topics such as this one and another here. The subject seems to divide opinion: some seem to think that it's downright dangerous and unnecessary; on the other hand there are numerous supporters of the idea of inflight checks of the ignition system. For example, this article on the Columbia Aircraft knowledge base has a procedure for safely conducting inflight mag checks.

John Deakin of the oft-quoted Pelican's Perch articles is also a proponent of an inflight, high-power, lean-of-peak mag check as can be seen here (scroll down about 2/3 of the page to the 'mag check' bit).

The video below shows a lean-of-peak in flight mag check displayed on a JDI EDM 700. If I understand correctly, this is done using the instrument set to 'normalised' mode.

[youtube]http://www.youtube.com/watch?v=qO3didsXCKA[/youtube]

What's the viewpoint of the knowledgable aviators here? Do you perform mag checks in the air?

Thanks,
Nick

[AKar]

Whether to do these checks or not is up to each pilot/operator and I've got no comment on that, it is nonetheless absolutely true that high altitude magneto tests can detect issues that are imperceptible on sea level. The effect from lean mixture is one thing for sure, but at least as importantly thin air up at altitude may provoke sparking within magneto's high voltage sections should there be any abnormal wear or damage much before it would occur at sea level. That may be the only way to get an early warning from impending magneto troubles 'on wing'.

-Esa

[Great Ozzie]

Hi Nick,

I'm sure lots of pilots do it. However, it isn't a good idea, simply because you might switch off the only mag that's working and might, in some types especially ( geared? I don't have any knowledge...), suffer an immediate engine failure. (Magneto Checks Whilst in Flight)

So this guy doesn't "have any knowledge" yet thinks it will cause an engine failure. He does mention flying an R44... I have no idea regarding piston engine rotorcraft if this a bad thing (mag check inflight) so maybe that is a big concern.

Keep it on both unless that is not possible. If you run on one mag, your fuel will burn after leaving the combustion chamber and you destroy your exhaust system, cowling, etc. (Which mag to use?)

Someone needs to explain to achimha that sometimes mags fail inflight and it does not destroy the exhaust system & cowlings.

---------------
At the Columbia wiki for Inflight Mag checks under What can go wrong you will find the following explanation:

For the curious, the reason for #3 is that the exhaust fills up with unburnt fuel (no spark). When you turn the working mag back on, exhaust temp can ignite it and you can get a nasty explosion inside the exhaust which can crack it.

Technically, this is called an "Afterfire" (*not* a Backfire). It can also most certainly happen on the ground - and it can rupture the muffler.

Wrt the (Columbia) procedure, it looks just fine to me (tho I would not run a percent power greater than specified in the POH). I have never performed one (inflight mag check) and truthfully, never heard of it (as far as I can recall) until either a Deakin or Busch (AVweb) article. That said, I do not think I would teach it to a student pilot - certainly not w/o the school's approval. A student has his/her hands full as it is, and I would hate to have a procedure such as this lead to a cracked muffler, or worse, an off-airport landing. It is, however, something I might bring up with an owner, particularly in light of the Columbia wiki. Most certainly heed the "don't panic and by all means do NOT "reverse the last thing you did" (as your instincts will tell you). Be mentally prepared for this before you start the check."

Btw, I've never used the JDI instrument (don't know about the modes - so I'll trust Mike Busch's word on that ).


-Rob

[Nick - A2A]

Thanks as always guys for the expert input. Rob - yes, some of the comment on those 'real pilots' forums suggest that the contributors are pretty narrow-minded at best.

From my layman's perspective, it does seem quite understandable that many pilots would instinctively be uncomfortable with shutting-off a mag in flight. Obviously no-one in their right mind would do this, say, at night over the Rockies. However, it does seem like a useful exercise in a controlled situation. I can see why it's not PPL material, and I guess it's something that really up to the individual to decide as Esa says.

However, I was quite struck by the lack of literature/guidance on this subject. As I mentioned though, maybe there are other articles buried amongst all the inflight mag(azine) links that my searches turned up!

Anyway, here's another Youtube vid which shows an inflight LOP mag check identifying a problematic cylinder in a twin.

[youtube]http://www.youtube.com/watch?v=mm5n04as2kA[/youtube]

Cheers,
Nick

[pjc747]

Well in flight, especially with a single, one lacks the luxury of being stationary while doing these. Everybody can make mistakes and slip, and especially with one key for magnetos, one could accidentally switch the engine off. This seems like an excessive and unnecessary practice; if you are flying over an area in a single where your ignition system heightens your concern, either you should: a) not be flying there b) your ignition system is unreliable or c) land, and check things out first. Magnetos are checked before flight to ensure the system is functioning properly, otherwise, flight cannot be undertaken (legally). If a mag fails en-route, this solves any questions you may have about the necessity of having redundant systems. The likelihood is you won't lose one, and even more unlikely to lose two. Shutting off systems while they are in use is opening the door to messing things up, the mag could easily fail when you turn it back on. No need to remove half your cams during a mountain ascent to make sure they're all working. You inspected them before, and the likelihood of them failing is low, and you always have backups.

[DHenriques_]

Well in flight, especially with a single, one lacks the luxury of being stationary while doing these. Everybody can make mistakes and slip, and especially with one key for magnetos, one could accidentally switch the engine off. This seems like an excessive and unnecessary practice; if you are flying over an area in a single where your ignition system heightens your concern, either you should: a) not be flying there b) your ignition system is unreliable or c) land, and check things out first. Magnetos are checked before flight to ensure the system is functioning properly, otherwise, flight cannot be undertaken (legally). If a mag fails en-route, this solves any questions you may have about the necessity of having redundant systems. The likelihood is you won't lose one, and even more unlikely to lose two. Shutting off systems while they are in use is opening the door to messing things up, the mag could easily fail when you turn it back on. No need to remove half your cams during a mountain ascent to make sure they're all working. You inspected them before, and the likelihood of them failing is low, and you always have backups.

My respect for John Deakin aside, I tend to strongly agree with this. Checking mags on the ground gives the pilot data that both mags are functioning and with an idle off check that they are grounded. The nuances of going any deeper such as an inflight check might be left for the venue of a maintanence flight check for specific reasons.
In my entire career as a pilot I never once saw the need to do an in-flight mag check.
Dudley Henriques

[Nick - A2A]

Thank you gents for the comments.

Everybody can make mistakes and slip, and especially with one key for magnetos, one could accidentally switch the engine off.

My understanding was that even if one did suffer from 'finger trouble' and accidentally switched the mags to 'off', this wouldn't be too much of a problem provided you followed the correct procedure: i.e. lean the mixture, mags to both, and enrich as necessary. Granted, this might not hold true for something like a microlight where the inertia might not keep the prop windmilling for very long. However, on the subject of 'finger trouble' one comment I read warned that you definitely don't want to be dropping the key on the floor if attempting this!

Shutting off systems while they are in use is opening the door to messing things up, the mag could easily fail when you turn it back on.

This bit I don't follow. Why would the mag fail more easily under these circumstances?

The nuances of going any deeper such as an inflight check might be left for the venue of a maintanence flight check for specific reasons. In my entire career as a pilot I never once saw the need to do an in-flight mag check.

I think your point about a planned maintenance flight is well made Dudley. Certainly the impression I got is that this isn't something to be done casually. The material I read advised to run through the procedure and "be mentally prepared" as Rob reiterated, whether on a maintenance flight or if conditions under cruise flight are favourable.

Also, I suppose that safety concerns aside, there's little benefit in performing an inflight mag check unless the aircraft is equipped with a probe-per-cylinder graphical engine monitor.

Cheers,
Nick

[AKar]

Magnetos are checked before flight to ensure the system is functioning properly, otherwise, flight cannot be undertaken (legally).

Though folks that occasionally do these tests argue that their very reason is to check for conditions not found in ground checks.

If a mag fails en-route, this solves any questions you may have about the necessity of having redundant systems.

This rises an interesting point regarding use of word 'redundant'. It depends on who you ask, but I tend to say that many aircraft systems that are said to be redundant in reality are not, light aircraft ignition system being a good example of such. This is because both individual systems are required for proper, full-capability working; using two magnetos each firing a separate set of plugs gives us tolerance against failure, but it is not redundant system. To call it redundant, I'd have two of those ignition systems in there configured so that either one could provide full all-plugs ignition capability. But that's somewhat off topic.

My understanding was that even if one did suffer from 'finger trouble' and accidentally switched the mags to 'off', this wouldn't be too much of a problem provided you followed the correct procedure: i.e. lean the mixture, mags to both, and enrich as necessary. Granted, this might not hold true for something like a microlight where the inertia might not keep the prop windmilling for very long.

In just about any piston-engine airplane, if one has an in-flight flameout, the single often-forgotten tip that can save one's day is to reduce the throttle to the range that corresponds to the speed at which the engine windmills. Having the throttle wide open at lowish windmilling speed could easily prevent the resulting mixture from sustaining combustion, and the engine from starting. Though in airplanes having constant speed props, an in-flight shutdown at cruise speeds may not end up with any resultant rpm drop at all after conditions stabilize.

Also, I suppose that safety concerns aside, there's little benefit in performing an inflight mag check unless the aircraft is equipped with a probe-per-cylinder graphical engine monitor.

I really don't think the engine analyzer would add much of anything to in-flight mag check it wouldn't add to the ground checks as well. If you've got a bad mag, switching to it in flight would result in roughness same way it does on the ground, except in earlier stages as things like internal sparking may not occur at sea level when the damage is only minor. The roughness may be even worse as the prop helps to drive the engine regardless of the power loss.

-Esa

[Nick - A2A]

n just about any piston-engine airplane, if one has an in-flight flameout, the single often-forgotten tip that can save one's day is to reduce the throttle to the range that corresponds to the speed at which the engine windmills. Having the throttle wide open at lowish windmilling speed could easily prevent the resulting mixture from sustaining combustion, and the engine from starting.

Thanks Esa, throttle setting is something I definitely hadn't considered. This is perhaps something we can get away with ignoring in the sim for now - much like the danger of an 'afterfire' I guess.

I really don't think the engine analyzer would add much of anything to in-flight mag check it wouldn't add to the ground checks as well.

Yes, afraid I phrased my comment rather badly. What I meant is that with that probe-per-cylinder engine monitor, we could identify if there's a problem with a specific spark plug for example, as opposed to the actual mag itself. (Just as is shown with that second, 'Tubreaux Aviation' video I embedded above.)

In other words, with the graphical engine monitor, a pilot seems to have a much better tool for checking the ignition system as a whole, particular with the data logging feature. Hopefully I made the point a bit better in this post on the thread in the Skylane forum.

I also wanted to add a link to this Lycoming Service Instruction I happened across. It describes a more demanding ground check of the ignition system than the standard basic magneto drop-off check.

Thanks,
Nick

[DC3]

Just my opinion, but in flight, I am reluctant to "test" anything that appears to be working. This (for me) comes under the "if it ain't broken, don't fix it" rule. If the flight is a maintenance flight designed to test these things with provisions like a nearby airport to land at and other support options then I think being a test pilot is fine. But to do this on a normal cross country flight, well I would have to wonder about the mind set of the pilot that does this. Again, please note, this is just my opinion.

[Nick - A2A]

Just my opinion, but in flight, I am reluctant to "test" anything that appears to be working. This (for me) comes under the "if it ain't broken, don't fix it" rule. If the flight is a maintenance flight designed to test these things with provisions like a nearby airport to land at and other support options then I think being a test pilot is fine. But to do this on a normal cross country flight, well I would have to wonder about the mind set of the pilot that does this. Again, please note, this is just my opinion.

Thanks DC3 - opinions are exactly what I was interested in hearing. This was a procedure I hadn't even heard about until a few days ago and the fact that it comes recommended by both Mike Busch and John Deakin—both pretty eminent and well-respected aviators—sparked my interest. From what I've read, neither are afraid of iconoclastic or controversial advice and this is certainly no exception.

The impression I've got is that it's not really possible to 'break' the ignition system doing these tests. Rather that the risk lies in responding improperly to a mag which is already defective. Having a suitable field within gliding distance is a good point, and this was mentioned in the Columbia Aircraft knowledge base procedure that I linked in the first post.

Cheers,
Nick

[William Hughes]

Most piston singles have pistons that are over five inches across. The dual ignition is there to get more of the fuel/air mixture to burn in the time allowed during the cycle. Generally the only time you need ALL the power is during takeoff. During normal flight you might lose half your system and not notice depending on the power settings used. This has happened to me, anyway. Felt quite stupid when the system utterly failed a mag check during run-up a few days later.

You can also lean the engine out on the ground to get a better indication of the system. But one of the reasons you do it with a full rich mixture and a high power setting is to ensure that it works during take-off, when you have a full rich mixture and a high power setting.

I myself would not choose to fiddle with the magnetos in flight "just to see if they are working ok". Perhaps in a particular setting, for a particular reason, in very controlled circumstances, it might make sense, as indicated in the earlier posts.

If the engine is working leave it alone.

[DHenriques_]

Just my opinion, but in flight, I am reluctant to "test" anything that appears to be working. This (for me) comes under the "if it ain't broken, don't fix it" rule. If the flight is a maintenance flight designed to test these things with provisions like a nearby airport to land at and other support options then I think being a test pilot is fine. But to do this on a normal cross country flight, well I would have to wonder about the mind set of the pilot that does this. Again, please note, this is just my opinion.

Thanks DC3 - opinions are exactly what I was interested in hearing. This was a procedure I hadn't even heard about until a few days ago and the fact that it comes recommended by both Mike Busch and John Deakin—both pretty eminent and well-respected aviators—sparked my interest. From what I've read, neither are afraid of iconoclastic or controversial advice and this is certainly no exception.

The impression I've got is that it's not really possible to 'break' the ignition system doing these tests. Rather that the risk lies in responding improperly to a mag which is already defective. Having a suitable field within gliding distance is a good point, and this was mentioned in the Columbia Aircraft knowledge base procedure that I linked in the first post.

Cheers,
Nick

My problem with this isn't the ignition system. It's the off chance that accidentally turning the mags OFF while at cruise power then ON again as fuel is still being eaten by the cylinders just might cause one HELL of a rod shock as they fire again.
This sounds to me like being one of those things that if done absolutety correctly and without mistake might yield some useful data BUT if done wrong could cause the engine problems it might otherwise have avoided.
I'm sure an AI or even a good A&P could do a check like this in flight and do it right, but the average ham handed pilot isn't really all that sharp with what's happening inside the engine.
My bottom line is that I'd leave the in-flight mag checks to the experts and don't ask for trouble
Dudley Henriques

[William Hughes]

Restarting a wind-milled engine does involve leaning it right out, and then slowly enriching it until it starts working again, and I presume that would be to prevent that rod shock as well as find a proper mixture that will allow operation at altitude. Hadn't though of the shock loading before but that makes a lot of sense. There is also the potential for shock cooling and such.

[LeighP]

As a retired Airforce Flight Engineer on piston (radial) aircraft, I can say that switching your mags while in normal operations was prohibited....this was an action reserved for when you had a non-normal situation in flight and thought that trying each mag may isolate the problem. Other than that, I wouldn't do it.