In-Flight Mag Checks

[SoonerAviator]

I do the ground check in NRM mode and look for the egt rise on each cylinder. (I don't really even look at the rpm drop.) In the case I'm citing the plug did fire during the ground check on multiple occasions. With the higher demand under power at altitude something in the lead was not adequate to fire the plug. The in flight check showed the cylinder egt drop off when the mag with the bad lead was the one that was on.

Since we have some thread drift going on here anyway, I will say that I think it's a CFI failure that so many pilots are unwilling to do in flight mag checks, and unwilling to lean properly on every flight, and unwilling to use the carb heat long enough to do some good, and unwilling to use all the fuel in an aux tank, and unwilling to at least understand and consider LOP operations, and so much more. All I can say is that because of this so many pilots are not getting full, safe utilization of the planes they are flying. Rant off.

I just don't think the chances of identifying a failed mag or bad plug lead as occurring often enough to warrant the in-flight mag check. It's not that it isn't a useful procedure when something is suspected, but there probably a dozen other checks you could do in-flight that most pilots don't because there's just not a high likelihood of failure. I mean, I suppose you could test the annunciator panel in-flight, or the fuel-cutoff knob, or alternate air source, or glass panel AHRS . . . but why?

 

[Lance F]

. . . but why?

There is the question. And that's what I'm getting at. If the answer is that I will not get any useful information or performance improvement from the test, from the operation, from the adjustment...then fine, don't do it. However if the honest answer is: because I'm afraid of what will happen, or I don't understand what I might gain by doing this, or because my about to go to a regional CFI never explained that to me, well that's another matter.

 

[Kevin Eggert]

And when you have a mag slip its distributor gearing and it starts firing at really bad times and causes massive roughness and power loss, you'll spend valuable time and altitude looking for a runway and neglect the troubleshooting that could save your life.

I am a Canadian aircraft maintenance engineer. I was a flight instructor. I taught Aircraft Systems in a college. It never ceases to amaze me how some pilots can presume to teach stuff they know so little about.

My point is that if you're scared to touch that switch in flight, you're not likely to try it if the engine starts acting up.

We get carb ice accidents because pilots pull the carb heat and the engine runs worse, so they push the knob back in. So the ice continues to build and kills the engine and the airplane crashes for no good reason. We get fouled sparkplugs and terrible fuel mileage because some folks are scared to touch that mixture knob because the engine might quit if they pull it out too far and maybe it won't start again or something. We get people who learned to fly in 172s that are scared to move the fuel selector off of Both. It's silly. IMHO it's too easy to get a PPL, and the accident stats show it.

There's a big difference between being scared to touch a switch and leaving something alone if it is still running well enough to maintain altitude. I had a scenario similar to this actually happen while in instrument training with an instructor. We were experiencing reduced power, but enough to maintain altitude and even climb at about 200fpm. After turning on the carb heat, we found the nearest airport then did these checks over the airport so that if anything was to happen we would be able to glide to a safe landing. We did an in-flight mag check along with a number of other checks which did not clear the problem. We slowly climbed to a safe altitude to make it to our home airport which was close by then took it in for service. Turns out, both sides of the mag were beginning to fail and it was sent in to overhaul. Back to the original point, however, I don't see any upside in doing routine mag checks while in flight. Troubleshooting is one thing, but that is the only exception that makes any sense. Aviate -> Navigate -> Communicate... You seem to be putting "Troubleshoot" somewhere ahead of this I think.

 

[GMascelli]

I did an in flight mag check to troubleshoot a hot EGT issue. Number 4 cylinder would spike 100° over the other three.

The right mag ran rough so it pointed to a potential fouled bottom plug on 4. Once on the ground, and plug pulled, we found a lead snot ball on the fine wire electrode.

 

[N1120A]

Why would this be? How could they get the SureFly certified if there wasn't a smooth transition if either the SureFly or standard mag quit? What would happen with an inflight mag check shouldn't be any different than if either quit for real. Or at least I wouldn't think so.

Read the post after

Yeah, these kind of statements.

I have a SureFly mag and I have no idea why you wouldn't do this.

Do you have toggles or a key?

 

[cowman]

It's not something I would normally do but if something didn't sound/feel right or I had some other reason for an operational check I don't see any reason not to.

Is there a good technical reason not to other than just discomfort with messing with engine stuff in flight? In my mind I'm doing that every time I touch a throttle or mixture lever anyway so I don't see what the fuss is.

 

[Doug Reid]

I had a magneto coil malfunction in flight...verified with ignition switch...ran engine on distributor alone to a safe landing. The only times I have done an inflight check was when I noticed a rough running engine..

 

[Dan Thomas]

I do the ground check in NRM mode and look for the egt rise on each cylinder. (I don't really even look at the rpm drop.) In the case I'm citing the plug did fire during the ground check on multiple occasions. With the higher demand under power at altitude something in the lead was not adequate to fire the plug. The in flight check showed the cylinder egt drop off when the mag with the bad lead was the one that was on.

The insulation between the center wire and the shielding braaid in a sparkplug lead can break down. Heat, time, vibraton all take their toll. At high power or boost settings, it gets harder for the spark to jump the plug gap in the high cylinder pressures. If that lead insulation is breaking down, the spark current finds an easier path: it sparks through the insulation to the ground lead, and we have a misfire.

One of the 100 hour/annual inspection items on many manufacturers' checklists is a test of those leads. While they're disconnected from the plugs, a high voltage is applied to them and the tester will show if there's a shorting spark. If one lead appears to be shorted, the mechanic moves the prop a little to move the magneto distributor gear electrode finger away from that lead, and the test is redone. If the electrode was lined up the spark will go into the coil and give a false short indication.

 

[Dan Thomas]

There's a big difference between being scared to touch a switch and leaving something alone if it is still running well enough to maintain altitude. I had a scenario similar to this actually happen while in instrument training with an instructor. We were experiencing reduced power, but enough to maintain altitude and even climb at about 200fpm. After turning on the carb heat, we found the nearest airport then did these checks over the airport so that if anything was to happen we would be able to glide to a safe landing. We did an in-flight mag check along with a number of other checks which did not clear the problem. We slowly climbed to a safe altitude to make it to our home airport which was close by then took it in for service. Turns out, both sides of the mag were beginning to fail and it was sent in to overhaul.

Now that right there wasn't smart. It could have failed abruptly at any time. "Both sides of the mag" implies one of those Bendix D-series, and reduced power can come from a slipped magneto mounting or points cam or busted impulse spring causing retarded timing, and one should land as soon as practicable, not as soon as convenient. There have been enough crashes caused by pilots trying to get home instead of getting down soon. If a loose mag slipped further you'd have no power, besides having high EGT that could cause lots of other trouble. It could be an engine starting to seize. There are lots of possibilities, some of them really ugly.

 

[Lance F]

Dan, I used that exact make/model of cable tester on the ground. The lead that was the problem tested OK on the ground with this device. Everything in the ignition system has to work harder at power at altitude. That’s why I consider an inflight mag check much more valid than one done on the ground.

 

[Cervieres]

I think there is "message creep" here. The OP suggested routine in flight mag testing, which is what most of the responses are focused on, versus checking a bad running engine.

If there is an engine problem, my training was 1) establish max glide, 2) look for place to land, and then 3) trouble shoot. These are the priorities which will have a higher chance of saving your life. And with this scenario, Dan's suggestion of doing an in flight mag check sounds good. I was taught to trouble shoot by switching tanks, putting on full mix, turning on carb heat, then checking mags. If it works, great. If not, you're set up for landing someplace.

But should the mags be tested in flight with a good running engine? Is the potential benefit worth the potential problem caused by the test? What is the probability of finding a mag problem that didn't pop up on the ground? My practice is not to do that, as I'm on the side of the juice isn't worth the squeeze, and I might cause more harm than the good I might do.

Well said. The only place cycling the magnetos makes an appearance in my POH is during run up and for rough running engine/loss of power. Freelancing outside of the manufacturer's guidelines is just going to create more potential problems.

 

[Daleandee]

There's a big difference between being scared to touch a switch and leaving something alone if it is still running well enough to maintain altitude. <SNIP> Troubleshooting is one thing, but that is the only exception that makes any sense.

My dad used to ask me a question that may apply here ... "if it ain't broke, why are you fixing it?"

As I noted earlier, I check ignitions before take-off and after landing. No need to check them in flight if it's working correctly.

 

[AA5Bman]

I have done it once, when on normal run up I got a mag check that suggested a fouled plug. I cleared the plug, got a good run up, and then in the flight I did my first and only in-flight LOP mag check to make sure everything was still fine, just prior to going from where there were good landing-out options, to where there were not (mountains).

But in general, I think I feel like this:

Not terribly fond of messing with my powerplant in flight more than operationally necessary. Just my opinion.

 

[Dan Thomas]

"if it ain't broke, why are you fixing it?"

There's another area where that philosophy is popular. Cheap or incompetent annual or 100-hour inspections that don't adequately inspect the airplane, which is why we constantly read, on this site, stories of serious malfunctions and component failures in flight. It wasn't broke, so they didn't inspect it. They didn't inspect it and didn't see that it was worn to a thread and about to fail. It wasn't broke, so they didn't fix it. And so the pilot flirted with an accident or worse.

 

[WannFly]

"if it ain't broke, why are you fixing it?"

doesnt always work in aviation... thats why we take proactive steps to identify if something is off.

 

[steingar]

My thinking when trying to identify a problem is always "what am I going to do when I find the problem?". If I find a problem with the ignition system that is only revealed in the middle of a cruise flight, what am I going to do about it? What is my plan of action if I DO find a problem. My guess is if I found a problem in cruise my plan would be the same as if I didn't, since the mill was working right in the first place. So why make the check? What does it gain you?

And yes, there is the possibility of turning your one working engine into a non working engine, which seems like a truly bad idea in the middle of wherever you are. You do you, I'll do me.

 

[WDD]

doesnt always work in aviation... thats why we take proactive steps to identify if something is off.

If it isn’t broken don’t fix it response was intended to apply to a normally running engine in flight. No one is advocating not checking at all (we do mag checks before every flight) or skipping annual inspections, etc.

There is a reasonable time and place for everything. At some reasonable point you take risk and go fly.

If something isn’t normal then yes you investigate. At the right time and place

 

[Bell206]

There's another area where that philosophy is popular. Cheap or incompetent annual or 100-hour inspections that don't adequately inspect the airplane, which is why we constantly read, on this site, stories of serious malfunctions and component failures in flight. It wasn't broke, so they didn't inspect it. They didn't inspect it and didn't see that it was worn to a thread and about to fail. It wasn't broke, so they didn't fix it. And so the pilot flirted with an accident or worse.

+1. This sums up the root cause of most maintenance issues whether flying or not. As to mag checks in flight I've never been a fan of them as there's no procedure I'm aware of that quantifies what you are actually checking/verifying when performing one in flight. On the normal ground mag checks there are procedures with correction trees if there is a problem. If the ground check doesn't pick up the problem then usually additional troublshooting does find the issue without an inflight mag check. But that is my experience. Unfortunately, no one seems to properly troubleshoot anymore and instead invent new ways of looking for something that may not be there or simply buy/replace enough new parts until the problem goes away.

 

[Craig Hammond]

At altitude with less dense air, you are more likely to find an issue early. Less dense air has less electrical resistance which allows the ignition spark to jump than on the ground where dense are has more resistance. That is the stress that the article talks about. You can't get that kind of stress on the ground.

 

[bflynn]

Yeah, that's gonna be a no from me. I know the odds are small, but I'm not going to create an emergency when there wasn't one.

 

[Tom-D]

Let's talk about parts wearing out. Mag switches are two friction surfaces that do wear out.
Is your last "CLICK" the last click?

 

[Bell206]

After reading Mike Busch’s article

That is the stress that the article talks about.

Curious. Outside the MB article, are there any other references out there addressing in-flight mag checks and what they indicate?

 

[Daleandee]

If it isn’t broken don’t fix it response was intended to apply to a normally running engine in flight. No one is advocating not checking at all (we do mag checks before every flight) or skipping annual inspections, etc.

Thank you! I trust no one would suggest that I meant to advocate waiting until something has broken to fix it. We were talking mags checks in flight.

Again ... I check ignitions before take off and after landing. Unless I have an ignition concern in flight I'm leaving well enough alone.

It might cause a ruckus to suggest that if a breaker trips in flight to land and find out why instead of resetting it ...

 

[nrpetersen]

Hmmmmm.... Originally I said OK for mag checks in the air over flat country, but I'd never do it with anything except my own aircraft. Maybe because looking for maintenance issues? I haven't rented for 53 years.

 

[Magman]

A routine RunUp procedure for Radial Engines was the “Power/Mag Check.”

Throttle is advanced to a given Manifold Pressure ( generally same as

Altimeter Setting ) and Mags are checked and RPM noted.

Think of Manifold Pressure as “ Power In” and RPM as “ Power Out”.

Note that if both plugs are dead in one jug you will not have a Mag Drop

but the RPM will be low.

Inflight concerns could be addressed with the “ Ignition Analyzer” where

you could observe the firing of any of the 128 Spark Plugs in the Engines.

As in post 49 you can see some plugs misfire at Max Power.

Not a good thing as you may have Detonation.


Of course this is the technology of 75 years ago.

 

[Tom-D]

A routine RunUp procedure for Radial Engines was the “Power/Mag Check.”

Throttle is advanced to a given Manifold Pressure ( generally same as

Altimeter Setting ) and Mags are checked and RPM noted.

Think of Manifold Pressure as “ Power In” and RPM as “ Power Out”.

Note that if both plugs are dead in one jug you will not have a Mag Drop

but the RPM will be low.

Inflight concerns could be addressed with the “ Ignition Analyzer” where

you could observe the firing of any of the 128 Spark Plugs in the Engines.

As in post 49 you can see some plugs misfire at Max Power.

Not a good thing as you may have Detonation.


Of course this is the technology of 75 years ago.

we just did "sweeps" the "O" scope would tell us right away.

 

[Ghery]

The insulation between the center wire and the shielding braaid in a sparkplug lead can break down. Heat, time, vibraton all take their toll. At high power or boost settings, it gets harder for the spark to jump the plug gap in the high cylinder pressures. If that lead insulation is breaking down, the spark current finds an easier path: it sparks through the insulation to the ground lead, and we have a misfire.

One of the 100 hour/annual inspection items on many manufacturers' checklists is a test of those leads. While they're disconnected from the plugs, a high voltage is applied to them and the tester will show if there's a shorting spark. If one lead appears to be shorted, the mechanic moves the prop a little to move the magneto distributor gear electrode finger away from that lead, and the test is redone. If the electrode was lined up the spark will go into the coil and give a false short indication.

View attachment 92483

Question - what is that "high voltage" that appears on the test leads? We tested insulation resistance on the dam I worked on in college in the early 1970s using a 1000 V Megger.

My dad used to ask me a question that may apply here ... "if it ain't broke, why are you fixing it?"

As I noted earlier, I check ignitions before take-off and after landing. No need to check them in flight if it's working correctly.

doesnt always work in aviation... thats why we take proactive steps to identify if something is off.

That may explain the quote on a t shirt of mine. "Engineer's motto. If it isn't broken, take it apart and fix it!"

 

[Magman]

we just did "sweeps" the "O" scope would tell us right away.

Type aircraft?

 

[Tom-D]

Type aircraft?

WV-2

 

[Dan Thomas]

Question - what is that "high voltage" that appears on the test leads? We tested insulation resistance on the dam I worked on in college in the early 1970s using a 1000 V Megger.

It would be, judging by the spark gap in the tester window, around 20,000 volts. The idea is that if the spark appears between the electrodes behind that window, the lead isn't leaking. If it doesn't appear, the lead is leaking internally or the spark is jumping the gap in the distributor cap to the coil secondary. Shift the prop and try again.

It takes 30Kv to jump a one-centimeter gap.

 

[Ghery]

It would be, judging by the spark gap in the tester window, around 20,000 volts. The idea is that if the spark appears between the electrodes behind that window, the lead isn't leaking. If it doesn't appear, the lead is leaking internally or the spark is jumping the gap in the distributor cap to the coil secondary. Shift the prop and try again.

It takes 30Kv to jump a one-centimeter gap.

Makes sense. We were testing wires and motors that would see a maximum of 480 VAC, so the lower voltage would work. Thanks for the clarification.

 

[Rgbeard]

I flew from Phoenix to St. Louis today. Much of it was what you might deem as inhospitable terrain.

I performed zero in-flight mag checks.

 

[jesse]

I have personally discovered failing ignition systems on airplanes many times with in flight mag checks. Everything from small 65 hp engines up to 300 some horsepower turbocharged six cylinders. These issues were not visible on the ground check. Even after the flight, not visible on the ground check...and if ignored, failure soon follows.

The risk is that the switch comes apart in your hand while you’re operating it. Unlikely, but possible. Weighing risks, I’d say the benefit of early failure detection at least equalizes if not exceeds that risk.

Unlikely you’re going to suddenly discover you were only running on one mag and have the engine quit. If you were literally only running on one mag you’d already know it. If you didn’t...maybe time to find another hobby...

What you’re looking for is differences between the two mags. If the engine runs about the same on either mag relative to each other, your ignition system is probably pretty healthy. If one mag is pretty smooth, and the other stumbles, then that’s something you’ll want to get looked into.

I would much rather discover I have a mag problem over a maintenance friendly airport on a beautiful VFR day, then discover it a few weeks later when it ****s out in IMC on a dark and stormy night over less than friendly terrain.

One time I discovered the problem on a Mooney in cruise, knowing it had the “two mags in one box” design, I got myself quickly to the ground. The engine completely quit when I taxied clear of the runway and refused to start again. Had to be towed to a hangar to get the mags overhauled.

One final comment...the problem may not be the mag. You’re testing each spark plug and each wire of the harness individually. The problem could be one or more components. Towards the end of mag life, usually the harness is in just as bad of shape...

 

[Scrabo]

I DEFINITELY wouldn't do this in a plane with a SureFly and a keyed ignition. Anyone who has one will know why.

We just installed one during the annual but have not had an opportunity to fly behind it yet, I did see Surefly FAQ comment on their website, I assume that’s what you are referring to.

 

[RyanB]

All good discussion. So a question... Can in-flight mag checks create a problem in the ignition system by stressing it in anyway?

 

[Jeff767]

Let's talk about parts wearing out. Mag switches are two friction surfaces that do wear out.
Is your last "CLICK" the last click?

The mag switches ground the mag to stop its operation. If there is a switch failure the mag will be hot and operate. In fact the best reason to do a ground mag check is to verify the switch or P lead to the mag is not broken and the mag hot all the time. If you want to know if both mags are truly functioning correctly a airborne check is the only way to determine that. Done at top of decent there is no safety issue. Even if you somehow managed to kill the mag you still have another one. I can’t however figure out how a mag check can kill the mag.

 

[Tom-D]

The mag switches ground the mag to stop its operation. If there is a switch failure the mag will be hot and operate. In fact the best reason to do a ground mag check is to verify the switch or P lead to the mag is not broken and the mag hot all the time. If you want to know if both mags are truly functioning correctly a airborne check is the only way to determine that. Done at top of decent there is no safety issue. Even if you somehow managed to kill the mag you still have another one. I can’t however figure out how a mag check can kill the mag.

Easy,, depends upon which switch..the little brass contacts become displaced when they are worn short enough.
another thing that happens the Wires that are the "P" become worn and short out.

 

[j1b3h0]

According to the guys from Ada Oklahoma, who teach LOP engine operation, checking your mags inflight at cruise power and LOP is a much better test of your engine’s ignition system than in the runup area at 1700rpm. I check them inflight, and if they check good, I don’t check them again on the ground on the next flight. Question to those who stress over “tempting fate” for lack of a better term: are you as squeamish when switching fuel tanks?

 

[RyanB]

Question to those who stress over “tempting fate” for lack of a better term: are you as squeamish when switching fuel tanks?

I don’t stress over tempting fate, but switching fuel tanks does heighten my attention every time I do it, especially if it’s over mountains.

 

[Dan Thomas]

. Question to those who stress over “tempting fate” for lack of a better term: are you as squeamish when switching fuel tanks?

Some owners (and renters) should be squeamish about switching tanks in some airplanes. I have found some fuel valves so hard to turn that there's a risk of something breaking when the pilot forces it. Had the shaft of a fuel shutoff in a Citabria fail that way. Found one C150 valve lever with its indexing flat torn out from that. Plug-type valves use Fuel-Lube (now EZ-Turn) fuel-proof grease, which gradually gets extruded out over the years, leaving the plug in metal-to-metal contact with the valve bore so it galls and scores and wants to seize.

I have found some Cessna fuel selector shaft U-joints badly worn so that there's a lot of slop apparent in the selector lever, making me wonder just how often the little rivets that hold it together fail and leave the pilot with no control over the fuel. There isn't much to that assembly, and the U-joints are skinny little bits of steel riveted to the shaft and the U-joint "cross."

Maintenance. It's all about maintenance and inspections.