One Time Mag Issue Question

DutchessFlier

DutchessFlier

Line Up and Wait
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Hudson Valley NY
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DutchessFlier
O-360-A3A AirPlains 180 Conversion
Slick Mags New in 2005 now with 300hrs

Flew last week:
Cold AM start/runup/mag check fine-2 hour flight-shutdown for about 2 hrs.

Restart to leave:
Statup fine (3 blades) Running a bit rough: Decided to do a Mag Check on the ramp at about 1000RPMs: Rt Mag fine-75RPM drop/ On Lt Mag No Spark at all??? / Key Back to Both to not stall the engine.

Repeated the above a few times with the same result.

Avionics Off / Master Off then On (I know it has no relationship to the Mags) cycled through carb heat, etc. etc.

Repeated the Mag Check: Now everything is totally fine! 75RPM drop on each mag, smooth running at 600/1200/2200 RPMs on each mag and on both mags.

Taxi over to an open part of the ramp: Shutdown for 5 mins and restarted. Ran bout 10 runups and mag checks various RPMs-no problems. Wiggled the key around in the ignition switch in each position to see if the switch itself was an issue.

Decided to fly home: VFR day, plenty of airports enroute if we had to land. One more runup and mag check at the hold line. Uneventful flight, and an in-flight mag check was also fine.

Next day, out to the plane on the home ramp. Started up and another 15 or so mag checks, etc. No issues at all....:dunno:

The ignition switch was serviced to meet the AD that was out in 1993. The mags also fall under an SB from Unison SB3-08 regarding carbon brush wear, but no evidence of that was found at last annual in December.

Looked for evidence of chafing and any loose connections on the leads, none found.

Any thoughts?
 
Temperature? Humidity? Moisture in the mag can lead to shorts that disapate with heat.

You might consider having your mechanic pop the cap off the back of the mag to look for carbon tracking just to be sure that it was truly a one time event. (also recomend having the P-leads checked to be sure they aren't crossed and it was really the left mag)
 
A mag that was temporarily totally dead? I would suspect a short in the P-lead somewhere, perhaps caused by the vibration of cranking, and the restart might have moved the wire enough to unshort it. The whole length of the lead needs to be checked, with special attention paid to the connections at the mag and at the switch. I've seen loose terminals at the switch, too, and they can short on ground terminals.

Dan
 
Dan Thomas said:
A mag that was temporarily totally dead? I would suspect a short in the P-lead somewhere, perhaps caused by the vibration of cranking, and the restart might have moved the wire enough to unshort it. The whole length of the lead needs to be checked, with special attention paid to the connections at the mag and at the switch. I've seen loose terminals at the switch, too, and they can short on ground terminals.

Dan
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Any loose connection or open in the "P" lead will create a hot mag.
 
Tom-D said:
Any loose connection or open in the "P" lead will create a hot mag.
Click to expand...


The loose connection I refer to is the P-lead terminal screw that can come loose and let the crimp terminal rotate and touch the ground lug on the switch, or any other lug for that matter. Any such short will kill the mag. The mag isn't hot because the P-lead is still attached to the terminal and is shorted when the switch is shut off.

Dan
 
Dan Thomas said:
The loose connection I refer to is the P-lead terminal screw that can come loose and let the crimp terminal rotate and touch the ground lug on the switch, or any other lug for that matter. Any such short will kill the mag. The mag isn't hot because the P-lead is still attached to the terminal and is shorted when the switch is shut off.

Dan
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Hugely remote chance of that happening… like its a good theory, but impossible to happen in real life.

In the case the op is talking about it is the switch its self, where the key tumbler portion gets worn enough that wiggling the key shorts the mag to off position. Very well known problem with the old Bendix and Gerties switches
 
Tom-D said:
I'm wondering why the weird stuff always happens to you?
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I imagine you've had a lot of singular snags, too, that no one else has ever encountered. Must have, with so many years doing maintenance.

Dan
 
Dan Thomas said:
I imagine you've had a lot of singular snags, too, that no one else has ever encountered. Must have, with so many years doing maintenance.

Dan
Click to expand...
I must lead a sheltered life, I've always had the benefit of QA in the Navy, but out here in the real world, i see more maintenance induced failures than usage failures.

Every time I install a new engine I build a set of new "P" leads and rework the Ignition switch, plus a new controls, knowing if it is new it should feel new too.

I'm one of the poor folks that must do it right the first time, because I can't afford to do it over.
 
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Tom-D said:
I must lead a sheltered life, I've always had the benefit of QA in the Navy, but out here in the real world, i see more maintenance induced failures than usage failures.

Every time I install a new engine I build a set of new "P" leads and rework the Ignition switch, plus a new controls, knowing if it is new it should feel new too.

I'm one of the poor folks that must do it right the first time, because I can't afford to do it over.
Click to expand...

The ignition switches we have are mostly the ACS make that have the 1000-hour inspection AD against them so they're opened often enough. We do replace all the engine controls at engine change time, after having had several failures of old controls.

Dan
 
Dan Thomas said:
The ignition switches we have are mostly the ACS make that have the 1000-hour inspection AD against them so they're opened often enough. We do replace all the engine controls at engine change time, after having had several failures of old controls.

Dan
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ACS is gerties, why not change them out to new style bendix. ?
 
I H A T E one time events...
They leave you flying along with your shoulders hunched, just waiting for it to become a two time event...

denny-o
 
Dr. O said:
I H A T E one time events...
They leave you flying along with your shoulders hunched, just waiting for it to become a two time event...

denny-o
Click to expand...
Its hard to fix some thing if it ain't broke, but We could fix some thing until it is.
 
Tom-D said:
ACS is gerties, why not change them out to new style bendix. ?
Click to expand...

Canadian law requires us to use parts listed in the aircraft's IPC. Unfortunately, those switches are listed for the older 172s and other Cessna singles.

Dan
 
Dan Thomas said:
Canadian law requires us to use parts listed in the aircraft's IPC. Unfortunately, those switches are listed for the older 172s and other Cessna singles.

Dan
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Canada doesn't have means of legally upgrading aircraft ? ???
 
Tom-D said:
Canada doesn't have means of legally upgrading aircraft ? ???
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Yup, but it can be a hassle. We can use PMA'd parts as long as the PMA documentation lists the aircraft make and model. We can use most STC'd stuff. Anything else and it's a Major Modification Report, and there had better be a good reason for it. I don't think it's quite as convenient as your 337 process.

Canadian aviation law is based on the British model and is much more stringent than the American model. The Australians also base theirs on the British and they're probably the toughest in the world. To get a mechanic's license here takes four years no matter what, and there are two categories of maintenance licenses, one of structures, and one of avionics. There used to be 17 classifications of maintenance alone. I wrote ten exams for my license. We can't do NDI and can't weld structures without further ratings, and they take a long time to get. Makes for considerable delays sometimes.

Dan
 
Dan Thomas said:
Yup, but it can be a hassle. We can use PMA'd parts as long as the PMA documentation lists the aircraft make and model. We can use most STC'd stuff. Anything else and it's a Major Modification Report, and there had better be a good reason for it. I don't think it's quite as convenient as your 337 process.

Canadian aviation law is based on the British model and is much more stringent than the American model. The Australians also base theirs on the British and they're probably the toughest in the world. To get a mechanic's license here takes four years no matter what, and there are two categories of maintenance licenses, one of structures, and one of avionics. There used to be 17 classifications of maintenance alone. I wrote ten exams for my license. We can't do NDI and can't weld structures without further ratings, and they take a long time to get. Makes for considerable delays sometimes.

Dan
Click to expand...

When I graduated trade school in 1960, there were 12 parts to the airframe 6 for the power plant and one for the general. The practical took 3 days, we were required to disassemble a Hamilton standard hydraulic prop, re-index it and reassemble, time the cam on a 975, patch fabric, make a radio wire harness, flush patch aluminum skin. splice a wooden spar, splice a tube, and and splice a cable by long splice method, and some other things I've forgot.

I've had several dealings with transport canada thru the VanCouver offices, and they always were easy to deal with and willing to help. I do know your rules are different, but I also know you can up grade any aircraft in Canada easier than down here.
 
Tom-D said:
Would you like to have your airworthiness certificate re-issued every year, by their AME?
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Only Transport Canada, or a Minister's Delegate For Airworthiness issues Certificates of Airworthiness, and they remain in force as long as the aircraft is in compliance with the regs and the manufacturer's type design. The C of A isn't reissued every year. We have airplanes here with certs 40 years old.

Dan
 
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