A quitting engine...and useful information.

[AdamFrisch]

A year ago I heard an experienced pilot and mechanic talk about the nature of mag failures. How, due to the internal breakdown they end up firing impulses in no particular order or completely randomly, causing detonation, misfires, rough engines and failure. I remember thinking that I didn't know that mags could behave like that - I thought they either ran rough, not at all or good. I stored this information somewhere.

I'm glad I did.

After a two day long cross country to Chicago with the worst weather I've ever encountered (I'll write a trip post on that sometime in the future), tired, on the final leg into Chicago the left engine misfired briefly. It wasn't enough time to try to trouble shoot. Soon after that it came back worse and for much longer, and here I had time to try stuff. I tried the booster pumps, the power, the mixtures, the props etc. Didn't work. I was just about to call the center and declare my intentions of diverting swiftly when I remembered that mag story. I switched mag. Nothing. Then I tried again and it ran like butter! I had missed the detent on the first try.

I know this might be standard procedure for some, but I had never been taught that mags could fail like that, overpowering the good mag. I'd also never been consistently taught to check mags in an emergency. I always thought I had two for redundancy, and when one fails the engine just keeps going on the second. Not so. :uhoh:

With the Iowa flatlands below me and airports every 2 miles it wasn't going to kill me here, but this could have been life saving information any other place - especially since my left engine is the only one with the hydraulic pump on it which means if it fails the gear comes down weather you like it or not. In mountains and at high density altitudes, this could have been a killer.

So if the engines starts to abruptly misfire and run rough, check the mags first thing!:eek::ok:

[foxmoth]

I had never been taught that mags could fail like that,

Poor teaching then - something I certainly cover in Engine failure cause of failure checks, and I would hope most instructors would do the same.:rolleyes:

[AdamFrisch]

Indeed. Nobody's has ever mentioned it - and I've done a Swedish, English and US PPL. Be good to hear from others here if they've had this information as part of their ground school.

[Tankengine]

Normal engine checks.:eek:

[SDB73]

I don't recall ever having been taught this, so thank you!

[stickandrudderman]

Whether one was taught it or not, the post serves a useful purpose in reminding us that with proper thought and action lots of situations can be managed to a successful conclusion. Thanks or a useful post.

[Contacttower]

Normal engine checks.

Well yes; I think most on here have probably come across a mag that makes the engine run rough during a ground run, however as Adam illustrates I don't think I have ever heard the suggestion that during flight it might be advisable to actually just turn one of them off...if I had had a similar situation to Adam's it may or may not have occurred to me to do what he did...whereas now I will forever have this thread in my mind when I have a rough running engine. :cool:

[AdamFrisch]

Yes, that mag did run rough on the ground, but I could always clear it by running up and leaning a bit. It was never as smooth as the other, but that's the case with many aircraft.

I bloody hate mags in general. They're always a bit dodgy and expensive to overhaul. I wish there was some kind of electronic ignition modification available, so I could get rid of them completely.

[172driver]

First off, great to hear that you did get to make that flight :ok:, even if it was perhaps a bit 'interesting'.....

As for the mags - this was indeed covered during my initial training, but I doubt it would have been the first thing to spring to mind. Thanks, Adam, for posting this valuable reminder :D


I bloody hate mags in general

Couldn't agree more. Had my share of trouble with them.

[UV]

especially since my left engine is the only one with the hydraulic pump on it which means if it fails the gear comes down weather you like it or not
That seems strange...what type of aircraft is this?

[AdamFrisch]

It's a 1953 Aero Commander 520. Gear is held up by hyd pressure. It's designed to extend when pressure is lost. There is a hand pump to keep enough pressure to operate the breaks, but it would take a superman to pump the gear up manually.

They did weird things in the 50's...

172Driver - thanks, and nice meeting you!

[POBJOY]

So what was the problem with the mag Adam !!!

I have had a mag switch break down on a single that made the mag run rough,and also the rotor arm fail on a radial that caused spurious sparks going to the wrong cylinder thereby setting up a massive misfire which felt like an actual component breaking.
The safest mag problem is the one when the points fail to operate properly on the inpulse mag and the engine will not start, therefore preventing a flight.
I had an interesting conversation with a 'mag overhaul' man once who told me that mags from 'tow planes' would quite often not get to TBO without extra attention due to the extra(electrical) load on the components.
Once the jets really got going on the North Atlantic the FE thought it was Christmas every day after years of engine changes and shut downs on the piston variety.

[172driver]

172Driver - thanks, and nice meeting you!

Likewise, Adam !

[Tankengine]

I should have said : "normal engine checks when airborne with rough running engine!":ugh:

I suppose it shows the poor standard of some instructors!:=

On Experimental reg aircraft it is common to replace at least one mag with electronic ignition.:ok:

[flyinkiwi]

I have experienced the loss of a mag in a 172 just after takeoff. There were no indications during the run up or power check at the start of the take off roll that there was a problem. Once we got to about 300 feet AGL it let go. I thought the resulting vibration was so bad the prop would shear off so I reduced power and the vibration went away almost immediately. Despite the power reduction we were still climbing so I leveled off at 500 feet AGL and told the Tower I needed priority for the crossing runway. We landed safely 30 seconds later but I was pretty shaken by the whole thing. The main point I want to make is, you can do all the checks in the world, but that won't stop something important failing when you need it most, so be prepared!

[IO540]

It is not really the case that a mag can "overpower" the other mag.

Except in the case of a single shaft dual mag (like the Series 3000 one that I have) there is no connection between the two except the accessory drive gearbox and if that gets shredded then your camshaft will prob99 also develop a mind of its own :)

What can obviously happen is that a fault in one mag, which causes an earlier than normal ignition, can mess up the engine, by igniting the stuff a lot sooner. The fact that the other mag delivers a spark at the right time is of no help.

The dual-mag redundancy assumes that a mag failure will be either (a) a total failure to deliver a spark (probably the most common failure mode) or (b) a late delivery.

Even if you replaced the mags with dual electronic ignition, you still have the same problem because if e.g. the sensor slips on its mounting you can get an early spark with that too. One could devise clever ways to block an obviously early spark, however.