Mag Failure

M

Mtns2Skies

Final Approach
Joined
Jul 12, 2008
Messages
5,631
Display Name
Display name:
Mtns2Skies
What would cause a mag to just fail? I went to pick up my plane from the avionics shop all the way across the state, go to do runup and bam L mag has nothing. Engine goes quiet. Runs okay on the right, but a lot of backfiring on startup.

They tried messing with the P-lead to no avail.

Vac pump was removed which is directly adjacent to the mag, also some wiring, but not with the mags.

Thoughts?
 
Big shot in the dark...they inadvertently swapped the sensor lead from the vac pump with the p lead of your left mag?
 
The only time I've had a magneto fail like that the coil had failed open. Slick magneto.
 
As @Ryanb said, when it rains it pours.

I forgot to specify Slick impulse coupled mags.

Could there be a gear issue where the vac pump was an intermediate gear to the accessory drive? And now the mag isn't even making contact so it's not spinning?
 
Mtns2Skies said:
Vac pump was removed which is directly adjacent to the mag, also some wiring, but not with the mags.
Click to expand...
Did they take one mag off to get at those stupidly inaccessible nuts on the vacuum pump flange? They might have reinstalled the mag badly out of time. Sometimes they misinterpret the lights on the mag timer, setting it so the light goes off instead of on at the timing mark (old-style buzzer mag timer; newer electronic ones work the other way, adding to the confusion). A mistake like that makes the mag fire really late.

Or, worse, they left the mag timing pin in it and turned the prop and munched the plastic distributor gears.
 
Dan Thomas said:
Did they take one mag off to get at those stupidly inaccessible nuts on the vacuum pump flange? They might have reinstalled the mag badly out of time. Sometimes they misinterpret the lights on the mag timer, setting it so the light goes off instead of on at the timing mark (old-style buzzer mag timer; newer electronic ones work the other way, adding to the confusion). A mistake like that makes the mag fire really late.

Or, worse, they left the mag timing pin in it and turned the prop and munched the plastic distributor gears.
Click to expand...
No, but there wasn't clearance between the vac pump and the firewall so they lowered the engine a bit with a hoist to be able to get it out.
 
Okay the current theory is that the rubber mag coupling fell into the engine on removal of the vac pump. And as such the mag isn't spinning on the accessory gear.

If this is the case, how critical is it to remove the soft rubber from the sump if it fell in? I'm hearing conflicting theories on this.

@Bell206 @Dan Thomas
 
Impulsed mag? Are they both impulsed? If so do they snap together when turning prop?

Bendix or Slicks? Remove the plug in the top of both bendix mags. Set the painted mark in the distributor gear to middle of the window on the undisturbed mag and compare it to the other.
 
Our trusty A&Ps will advise expertly shortly, I’m sure. But I think you know the answer.

Anything loose in the crankcase is either going to bounce around hard in there within the oil tornado, or get sucked up against the oil intake/screen. Neither happenstance is acceptable to my level of risk tolerance when aviating, but the trade off is a maintenance induced error (you already had one, so there’s your empirical data, at least with this crew).

Sorry man. This crap is expensive.
 
bnt83 said:
Impulsed mag? Are they both impulsed? If so do they snap together when turning prop?

Bendix or Slicks? Remove the plug in the top of both bendix mags. Set the painted mark in the distributor gear to middle of the window on the undisturbed mag and compare it to the other.
Click to expand...
Slicks, impulsed. I'm not sure if they both are.
 
Mtns2Skies said:
Slicks, impulsed. I'm not sure if they both are.
Click to expand...

Normally only one magneto, usually the LH one, has an impulse coupling for starting purposes.

As for the rubber coupling, how on earth does taking off the vacuum pump disturb the physical magneto drive coupling? I'm not familiar with Continental engines, having only owned Lycomings.
 
Mtns2Skies said:
kay the current theory is that the rubber mag coupling fell into the engine on removal of the vac pump.
Click to expand...
Whose theory? Perhaps I'm missing something in your explanation but I find it hard for the rubber mag drive to "fall" into the engine if the mag was not removed? Something don't sound right. But if as you say there is a rubber mag drive in your sump then yes it needs to be cleaned out.
 
Bell206 said:
Whose theory? Perhaps I'm missing something in your explanation but I find it hard for the rubber mag drive to "fall" into the engine if the mag was not removed? Something don't sound right. But if as you say there is a rubber mag drive in your sump then yes it needs to be cleaned out.
Click to expand...

I'm having difficulty imaging how the rubber pieces in the magneto drive could "fall into the engine" even when the magneto is being removed?
 
Just the theory from the avionics techs. Will learn more on Monday. Nothing has been opened up and looked at yet.
 
GRG55 said:
I'm having difficulty imaging how the rubber pieces in the magneto drive could "fall into the engine" even when the magneto is being removed?
Click to expand...
Same here. Don't recall that ever being a problem but then again stranger things have happened.
 
Mtns2Skies said:
Just the theory from the avionics techs. Will learn more on Monday. Nothing has been opened up and looked at yet.
Click to expand...
Best bet is a simple wire issue grounding the mag. But for future reference the Australian CASA put out an Airwothiness Bulletin that consolidated most mag failure modes along with a list of mag SBs as reference. It's one of the best T/S-ing docs out there for mag problems.
 
Mtns2Skies said:
Just the theory from the avionics techs. Will learn more on Monday. Nothing has been opened up and looked at yet.
Click to expand...

Ahhh. Avionics techs troubleshooting magneto problems. :)
Let's hope it's something quite simple, perhaps in the wiring as @Bell206 speculates, and you're back in the air soon.
 
I flew my airplane from TX to here. Just fine, no problems. A day or two later, I want to go fly. It won't start. Impulse coupling broke spring.
Oh well time for the 500 hour anyway. Slick mags. So, Yes, mags can, and will, just up and quit with no warning.
 
Bell206 said:
But for future reference the Australian CASA put out an Airwothiness Bulletin that consolidated most mag failure modes along with a list of mag SBs as reference. It's one of the best T/S-ing docs out there for mag problems.
Click to expand...
For those interested here is the CASA AB I mentioned above:
https://www.casa.gov.au/files/005pdf-15
 
take it to the a holes who f it up and have them hire a a&p to come and repair there f up .stop payment on check and or credit card.and keep your log books in your possession . it might be something simple like over torquing the p lead on a slick mag .when they say12 in lbs on the nut they mean it. have seen many trashed condenser because of a holes
 
hotprops said:
take it to the a holes who f it up and have them hire a a&p to come and repair there f up .stop payment on check and or credit card.and keep your log books in your possession . it might be something simple like over torquing the p lead on a slick mag .when they say12 in lbs on the nut they mean it. have seen many trashed condenser because of a holes
Click to expand...

What is that saying? You can catch more flies with honey than vinegar. :)
 
What was it in the avionics shop for? Inspect the wiring behind the panel at the switch.
 
Any marks on it? Having this weird feeling I should mention maybe someone dropped something heavy on it. Like a vacuum pump. LOL.
 
Mtns2Skies said:
Planes fixed. I'll get it back tomorrow. The mag shaft was 180* out of phase so they put it back and re-timed it. No parts needed, no parts broken.
Click to expand...
Fantastic! Glad to hear.
 
Mtns2Skies said:
Planes fixed. I'll get it back tomorrow. The mag shaft was 180* out of phase so they put it back and re-timed it. No parts needed, no parts broken.
Click to expand...

Great to hear that. But how did it get installed 180 deg out if they didn't remove either of the magnetos?

Or, as @Dan Thomas mentioned, did they have to take out one mag to get at the vacuum pump fastners?
 
Apparently the mag and the vac pump shared a shaft of sorts that went onto the accessory gear. They were exactly opposite of eachother on the engine, the mag on the fore part of the gear, the vac pump on the after part.. They pulled the shared shaft out with the vac pump and reinstalled it out of phase.

I'm not sure if that makes, sense, I haven't seen it but it's apparently more complex than I realized.
 
that doesn't make a lot of sense. what engine are we talking about? i really need to lookup the gear train to visualize how they got it 180 degrees out.
 
Must be a 470 or 520. The mags are on the front of the accessory case, above the crankcase. The vac pump is on the back. The guy took the whole vac pump drive out instead of just the vac pump. No reason to do that unless the drive was leaking.

upload_2020-9-26_17-16-35.png

#20 are the mag drive rubber couplings. They go into #19, which goes into #17, the drive gear. #12 is the accessory drive assembly they took off the engine. They should have take off just the vacuum pump, which fits onto the four #14 studs. The #1 plate is usesd to cover the drive if the vac pump is not installed. #5 is a gasket.

Now, I hope they got the #5 and #16 gaskets right. There have been service bulletins on certain gaskets that failed in flight. There's an oil feed port, under pressure, that passes oil though small holes in the gaskets to the wet-type vacuum pumps. The dry pumps have no oil inlet in their flanges and just blank the port off. Some gaskets would blow out under pressure and pump oil overboard.

upload_2020-9-26_17-30-53.jpeg

See the right mag and vacuum pump facing each other?
 
Dan Thomas said:
Must be a 470 or 520. The mags are on the front of the accessory case, above the crankcase. The vac pump is on the back. The guy took the whole vac pump drive out instead of just the vac pump. No reason to do that unless the drive was leaking.

View attachment 90383

#20 are the mag drive rubber couplings. They go into #19, which goes into #17, the drive gear. #12 is the accessory drive assembly they took off the engine. They should have take off just the vacuum pump, which fits onto the four #14 studs. The #1 plate is usesd to cover the drive if the vac pump is not installed. #5 is a gasket.

Now, I hope they got the #5 and #16 gaskets right. There have been service bulletins on certain gaskets that failed in flight. There's an oil feed port, under pressure, that passes oil though small holes in the gaskets to the wet-type vacuum pumps. The dry pumps have no oil inlet in their flanges and just blank the port off. Some gaskets would blow out under pressure and pump oil overboard.

View attachment 90384

See the right mag and vacuum pump facing each other?
Click to expand...
Yep I've got an O-470. For me it was a wet pump that opposed the LH mag not the RH, but same idea. The A&P that did it was not my normal one, just the one that my avionics shop uses which is 80nm from home. I'll have my A&P go over their work shortly.

Sure is nice to have all the vac pump plumbing and junk out of the engine though!
 
You must log in or register to reply here.
Back
Top