Mag Failures

[markejackson02]

I have had three mag/engine failures now on 52M.  The first occurred last Sept.  Right mag failed.  Took the mags off and had them overhauled.  Flew okay for a couple months.  Then I took off last May and at 3000' the left right failed.  Replaced the plugs, cleaned the mags, one flight was okay.  Next flight the left mag failed.  Off came the mags, back to the shop, overhauled.  Tonight buttoned everything up.  Engine started okay, run-up was okay but at 500' full power the left mag failed again.  It remained dead at low power.

 

We have replaced the mags, the harness, and the plugs.  Any ideas on what kind of thing could cause this failure mode?

 

Thx

 

Mark Jackson

M20C N9752M KSGR

[Jerry 5TJ]

How about the magneto switch on the panel?  Any chance it is intermittently grounding a magneto even when it is selected to "both?" 

 

[carusoam]

I am with Jerry on this one.  Magnetos are pretty well independent of everything.

Mechanically they are driven by the engine.  Electrically their output is to the spark plugs (top on one side, bottom on the other).

The only thing left is the "P-Leads".  These are the wires that ground out the system when you want to keep the magnetos from firing.  Check your wiring diagram,  the P-leads are run through the complex and expensive ignition switch that may be 40 years old.

Check to see if these wires are not grounding out somewhere in between.

Broken P-leads (no ground) lead to the ignition system being always on.  This is why we always treat the prop with care.  If a P-lead is not grounded, than the engine can fire by moving the prop.

Jose can probably give the best advice on this topic.

Best regards,

-a-

[Piloto]

Quote: jerry-N5911Q

How about the magneto switch on the panel?  Any chance it is intermittently grounding a magneto even when it is selected to "both?" 

 

[markejackson02]

I will check with a meter this weekend.  It is a total failure, not one plug missing.  Silence at 800' at night <> good.

[DaV8or]

I agree with Jose, check the wiring too. A old C-150 we had in the flying club I was in was lost because of mag wiring. Lost both mags at once and during the forced landing, nosed over in the mud. People fine, plane total. The wires had chaffed where they passed through the firewall and eventually grouned there. This had a really hard life as a trainer pretty much it's entire life with all the maticulous care and maintenance that goes into a 40 year old trainer.

[markejackson02]

Appears the p-lead is shorting out.  Is the plane ferriable with the p-lead removed (I know about the hot prop)?  It is the left p-lead so it should be okay to start without the p-lead.  About a 20 minute flight to were repairs can be made. 

[conom06d]

good catch guys. I dont know about ferrying it if its died at 800' in the past. Thats about 1 min flight time for a 20 min flight.... but thats just me

[DaV8or]

Quote: conom06d

good catch guys. I dont know about ferrying it if its died at 800' in the past. Thats about 1 min flight time for a 20 min flight.... but thats just me

[Barry]

At this point the plane is "unairworthy" and to move it he has to get a ferry permit. The FAA will decide.

[DaV8or]

Quote: Barry

At this point the plane is "unairworthy" and to move it he has to get a ferry permit. The FAA will decide.

[Piloto]

To make things more confusing I remember from my old days that there are magnetos that ground the P-lead terminal when the P-lead plug is removed to avoid having a hot mag. This is not the case for mags with regular P-wire terminals attached with a nut.  

José

[conom06d]

oh i see. Well if faa deems ok i say go for it

[markejackson02]

Turns out that there is a spring inside the mag that is compressed when you insert the p-lead into it's socket.  If there isn't a p-lead, it is supposed to ground the mag as a safety measure.

Also turns out that to start, you don't "need" but should have the shower of sparks available, which requires the p-lead to be connected.

A short term cure is a wooden chopstick which prevents the p-lead from flexing and shorting during the ferry flight.  Should keep everyone happy and my engine running.

Learning lots about the ignition system.

[carusoam]

Does your chopstick come with a roll of electrical tape?

Since your research is in high gear....

Are you able to purchase a replacement P-lead? 

My thought.... so you have the right one on hand, that you can hand to your mechanic in a timely fashion?  This way you won't be surprised by the mechanic needing to source one and it may take a day or two....

Best regards,

-a-

[markejackson02]

Yes, had to order from Aircraft Spruce.