Checking the mags in an engine-trouble scenario

[photofly]

Interesting comment from Mike Busch, which has relevance to the way we are taught to respond in a power loss scenario.

In the relatively small population of the aircraft that my company manages (hundreds, not thousands), we had a half-dozen failures of magneto distributor gears during a one-year period. When the distributor gear fails, the magneto starts firing random spark plugs at random times and the engine pretty much goes berzerk. The appropriate pilot action in a situation like this is to identify the bad mag by switching to one mag, and if that doesn't solve the problem, switching to the other mag. Once the malfunctioning mag is switched off, the engine runs normally on the other mag.

However, not one of the six affected pilots had the presence of mind to do this. Each treated the berserk engine syndrome as an emergency. The pilots varied in experience from newbies to greybeard CFIs. One pilot experienced the failure at FL230 and had a full half-hour to think about the situation as he descended to an emergency landing, and in all that time it never occurred to him to try switching from BOTH to L or R.

In a situation like this, redundancy helps only if the pilot knows how to use it. In a perfect world, pilots would be trained to deal with such failures and we could run the mags to failure. However, it appears to me that very few pilots are properly trained in this regard.

(my emphasis)

I am in no doubt that everyone who is responsible for ab-initio training and is reading this will be thinking that they already teach their students to check this, but just in case, I thought it was worth posting.

Here's the full article; this quote is from a reply to a comment, at the end.
http://blog.aopa.org/opinionleaders/201 ... ines-fail/

[CpnCrunch]

It does specifically mention that in the 172S POH:

"A sudden engine roughness or misfiring is usually evidence of
magneto problems. Switching from BOTH to either L or R ignition
switch position will identify which magneto is malfunctioning. Select
different power settings and enrichen the mixture to determine if
Icontinued operation on BOTH magnetos is possible. If not, switch to
the good magneto and proceed to the nearest airport for repairs. "

(in the "rough engine operation or loss of power" section).

However, from what I can tell, it doesn't mention this in the actual emergency checklist, only in the amplified procedures.

[Big Pistons Forever]

I had exactly what is described happen to me. Flying along all happy when all of a sudden the engine losses significant power and runs very rough and erratically. I cycle the mags and and as soon as I switch to the left mag the engine picks up nice and smooth. One of the crappy plastic mag gears had failed and advanced the spark on that mag about 40 deg . Needless to say the engine did not like the dueling mags

Anyway after a bit of a fright I had an uneventful flight back to home base and after a mag change the engine was as good as new.

I have related this story to every student I have taught as part of developing an understanding of what a pilot can and can not do if the engine starts misbehaving and the desirability of restoring power before you have to do all the hero pilot shit involved in the forced approached.......

[ahramin]

Exactly photo. Funny I was just talking about practice forced approaches today with a friend of mine who is doing his ppl. He hadn't considered that you would try to deal with the engine issue (ex trike guy, go figure ).

If the aircraft is under control and a safe height, work the problem. Some general rules: As posted sudden rough running try either mag. Surging, fuel flow problem. Progressive power loss, carb ice.

You can try playing with the mixture too but there aren't too many scenarios where this will help. I had one swallow a valve and it ran much better on the remaining 3 leaned out. On that one trying one mag at a time definitely didn't help!

[PilotDAR]

This has happened to me too, and as described, flying home on one mag was no problem. Paying attention to the mixture becomes important, as the EGT will rise on only one mag. The one mag trick is not so certain a cure for those Lycoming 540s whose design incorporates only one mag drive for a dual magneto. I can't imagine how this design met the requirements for certification in terms of redundancy! I have twice seen those single drive gears (one gear drives both mags) with teeth missing.

It is noteworthy that a rough running mag at higher altitudes, which seems okay down lower, is most likely mag lead insulation breaking down, rather than the mag itself failing.

[photofly]

EGT rises because the gas burn is slowed so the gas is hotter as it exits, and is mostly irrelevant; CHT usually goes down.

It's worth spending some time flying on one mag to see what goes on.

[ahramin]

PilotDAR, I'm puzzled as to why you would pay more attention to mixture on one mag. What is the problem with the higher EGT reading and what do you do about it?

[PilotDAR]

EGT rises because the gas burn is slowed so the gas is hotter as it exits

...Is my concern. I once had a stuck exhaust valve (at an extremely inconvenient place during a multi province trip), which I later learned was attributed to hot exhaust gasses not being carried away from the cylinder exhaust port efficiently. I'm uncertain of the exact relationship between that event, and the higher EGT of running on one mag, but for the very few times I have had to run on one mag, I have enrichened as much as practical, just to reduce the possible effect of hotter than normal exhaust gasses, farther downstream than usual. Nothing scientific in my thinking, just an abundance of caution. Only one of the stuck exhaust valves I have had has occurred at a convenient place and time, so I try to minimize the possible contributing factors.

[photofly]

The problem with this is that the temperature away from the cylinder at a random place in the exhaust stream doesn't correlate with the temperature at the valve or the cylinder head in general. Which is why we measure and limit CHT, and interpret EGT only in a relative way using the peak value as a measure of mixture rather than caring about the absolute temperature.

The person doing the attributing in your case was either an engine genius, or making false correlations. Hard to say which.

[cgzro]

The person doing the attributing in your case was either an engine genius, or making false correlations. Hard to say which.

Oooh thats at great line. I can use that at work almost every day

[PilotDAR]

The person doing the attributing in your case was either an engine genius, or making false correlations. Hard to say which.

The person doing the attributing in this case was Bill Brogden, retired Senior Powerplant Engineeer from Continental, so I unreservedly accepted his advice.

During Continental engine detonation testing I conducted decades back, one of the things I learned was that the flame front movement has a lot to do with how heat is distributed around (hopefully uniformly) the combustion chamber. Changing the flame front movement (including running on one mag, or detonation) may cause undesirable redistribution of heat. The engine can tolerate it for short periods of time, but longer periods can create harm. I find fuel to be a very cheap coolant, and use it if I think it will prolong engine health/life.

[photofly]

On the one hand running on one mag is an "unusual" scenario that will cease at the latest at the planned destination, and (unlike detonation) doesn't cause near instant damage; on the other hand since it's only temporary there's no reason not to run rich of peak or very rich, if one is uncomfortable with the EGT temperatures.

Do any of the engine operating manuals have anything to say?

[pelmet]

I had a discussion with another poster on this exact subject a while back. His actual suggestion was to do nothing as you might make things worse. Seems like his methods are wider spread than I thought.

You can see some of the discussion here...

http://www.avcanada.ca/forums2/viewtopi ... 50#p899887

The final statement from said poster was a smart-ass statement. Is it possible that Mike Busch's quote below is because pilots really are getting bad advice, whether on the internet or elsewhere.

"The appropriate pilot action in a situation like this is to identify the bad mag by switching to one mag, and if that doesn't solve the problem, switching to the other mag. Once the malfunctioning mag is switched off, the engine runs normally on the other mag.

However, not one of the six affected pilots had the presence of mind to do this. Each treated the berserk engine syndrome as an emergency. The pilots varied in experience from newbies to greybeard CFIs. One pilot experienced the failure at FL230 and had a full half-hour to think about the situation as he descended to an emergency landing, and in all that time it never occurred to him to try switching from BOTH to L or R.

In a situation like this, redundancy helps only if the pilot knows how to use it. In a perfect world, pilots would be trained to deal with such failures and we could run the mags to failure. However, it appears to me that very few pilots are properly trained in this regard.

[photofly]

There is a difference between a little tough running (like when you lose one plug) and a mag failure that causes mistimed ignition on all four or six cylinders and a massive reduction in available power.

I can understand why pilots are reticent to risk sacrificing their remaining magneto juju in the first case (although I don't agree) but in the second case appropriate action to avoid a forced landing in a single shouldn't be overlooked. It would be an error to do so.

I also believe that appropriate training involves having pilots switch mags in flight when all is well, along with appropriate discussion of what to do if one mag turns out to be dead at that time and how to bring the engine back to power without risking the exhaust.

[Big Pistons Forever]

I have to say the idea of enriching the mixture when running on one mag never occurred to me, so I found Pilot DAR's information quite interesting. That is what I like most about Avcanada, those little nuggets of information that get you thinking

[photofly]

Quick test today, during cruise:

switching to one mag resulted in, on one cylinder picked at random, a 100F rise in EGT (1412 to 1509), a slight drop in CHT (312 to 309) and a six knot drop in airspeed (120 to 114).

[crazyaviator]

The problem with this is that the temperature away from the cylinder at a random place in the exhaust stream doesn't correlate with the temperature at the valve or the cylinder head in general. Which is why we measure and limit CHT, and interpret EGT only in a relative way using the peak value as a measure of mixture rather than caring about the absolute temperature.

I disagree. The idea of an internal combustion engine is to have INTERNAL combustion finished by the time the exhaust valve opens, otherwise, the flame front is heating the valve stem and burning the lubricating oil onto the stem ( like coking) and expanding the stem, causing sticking valves

People shy away from lean or beyond lean operation because of exhaust wear/sticking and other maladies. Without gamis and tuning etc, there may be 1 cylinder out of 4-6 that is self destructing

[photofly]

I'm not sure that slower combustion and a rise in EGT means you can draw any conclusion about where the flame front is or isn't, exactly.

On the other hand I read that the mixture was set on big radials at night by watching the colour of the flames from the exhaust, so perhaps you're right. But on the other other hand nothing in those references suggested a worry of those same flames coking the exhaust valves.

I guess what I'm saying is this: my EGT is 1100F in the climb, and 1400F at cruise. Unlike CHT, I don't have a maximum EGT in mind. Why should an EGT of 1500F on one mag, concern me more than 1400F in cruise on two mags? Are we suggesting that 1400 is ok, and 1500 isn't? If so, on what evidence? Or is it just hearsay/wives' tales? Genuine question. And if I am recommended to make a mixture adjustment for single-mag running, on what should I base it? How will I know when I've got it right?


(I'm aware of Lean of Peak operating, but what does "beyond lean" mean btw?)

[PilotDAR]

Are we suggesting that 1400 is ok, and 1500 isn't?

Well, I am, personally, and informally - meaning that I do not base my opinion upon authoritative data, but rather my understanding some of what I have experienced in this realm.

We agree that 1400F is a "nice" leaned EGT to see in two mag cruise flight, on a normally aspirated engine, which is running properly. I opine that the actual temperature for the flame in the combustion chamber is considerably hotter than that. The difference being the cooling of the flame as it begins to exit the exhaust port. A part of this [heating then] cooling occurs as a flame originates (at the spark plugs), expands outward like a balloon constrained by the combustion chamber, and then contracts (in the temperature sense), as the fuel is burned, and combustion completed. But the key is that the flame front of the normal combustion balloons out, then "contracts" like a balloon deflating, rather than bulging non uniformly.

The combustion chamber was not designed for normal operation on one mag, so the flame originating at only one spark plug, will advance and retreat differently than it would with two spark plugs. We know that the power being produced on only one mag/plug is less than on two. So one would expect that there would be less total heat of combustion - all the fuel may not be being burned in the intended place in the combustion chamber.

I cannot say what temperature the exhaust port "zone", down stream of the of the exhaust valve, is designed to withstand, though with 3500 hours on a Continental, and a Lycoming I own, 1400F EGT seems to have never created a problem for me. Of course, 1400F EGT is a very relative temperature, variable based upon probe location (which is rarely the same for all the cylinders of the engine). We're not actually measuring the precise temperature of the exhaust gasses as they are exhausted, they've cooled a bit already.

So if I see 1500F EGT, I know that the local exhaust area is experiencing hotter than normal temperatures. Is it designed for this? I don't know. I do know that Bill, the retired Continental Engineer, told me that failure to get the exhaust heat away from the exhaust port area of the cylinder could result in the valve coking up and sticking - and this has happened to me. I opine that the hotter EGT indication on one mag is a result of some combination of non uniform flame front, so hotter gases have the opportunity to exit before cooling as much as desired, and fuel which did not burn because of abnormal ignition continuing to burn much later than intended.

I do know that nothing in the design requirements for piston engines requires that the manufacturer demonstrate any prolonged operation nor durability while running on only one mag. It only says that engines shall have two independent ignition systems, not how well it runs on only one, nor for how long before you damage it!

So, with all of that as my basis for thinking, and the fact that I have actually made a Continental 520 denonate during testing (which included leaning it), and measured in real time, I like to err to the conservative, and enrichen mixture as much as I can while running on only one mag - just to be on the safe side. As said, if you're running on only one mag, you're probably no longer planning planning to fly to the full capacity of the aircraft's endurance anyway!

[photofly]

I don't hold anyone out as having all the answers, but here's some comment from Mike Busch, again:

"By contrast, high EGTs do not indicate that the engine is under excessive stress. They simply indicate that a lot of energy from the fuel is being wasted out the exhaust pipe rather than being extracted in the form of mechanical energy sent to the propeller. High EGTs do not represent a threat to engine longevity. The engine is simply not capable of producing EGTs that are high enough to harm anything. Therefore, attempting to limit EGTs in an attempt to be kind to the engine is simply wrongheaded.

Limiting CHTs is essential to ensure cylinder longevity.
Limiting EGTs accomplishes nothing useful.
One caveat: Turbocharged engines usually have a turbine inlet temperature (TIT) red line that should be observed, particularly when flying at Flight Level altitudes. The purpose of the TIT limit is to protect the fast-spinning turbine wheel from blade stretch. The TIT limit is usually either 1650°F or 1750°F, depending on the model of turbocharger in-stalled."

https://www.savvyanalysis.com/articles/ ... ht-and-egt

[PilotDAR]

I also consider with great credibility what Mike Busch says, though I would expect that what he is saying (unless qualified otherwise) is in reference to an engine running normally. Running on one mag is abnormal, and therefore associated conditions might be beyond the scope of what Mike is addressing (of course that's for him to say, I'm not attempting to speak on his behalf).

That said, I welcome any wisdom on this topic....