First Mag Failure

[ARFlyer]

Long story short I had my first mag failure tonight while working on my 90 Day Currency. I was shooting a practice ILS and noticed a slight RPM drop with momentary roughness. It cleared up and I didn’t think twice about it until I started the engine after picking up some fuel.

When I tried to start the engine ran like it was coughing up a lung before stabilizing with a slight roughness. Some inner voice told me to check the mags and I found when I switched to L the engine would die. If I giggled the key slightly it would kick back running but would only last for a few seconds. So I decide against my wish to fly the short distance home and shut down.

I’m wondering if it’s the P Lead or if the switch is bad. We’ve been having a sometimes somewhat violent nose shimmy issue that I wonder if it’s shaken stuff loose. We haven’t been able to narrow it down in the last few weeks and it’s very intermittent.

 

[Tom-D]

start with.
Disconnect both "P" lead wires and see if it will run..

 

[Dan Thomas]

As Tom says, see if it runs ok with the P-leads disconnected, although you can't check the mags that way.

Jiggling the key slightly should not make a difference like that. It suggests a defective switch. If it's an ACS switch it could be one that never had the AD done on it: https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument

The contacts get burned in them if the AD-mandated diode isn't installed on the starter contactor.

 

[flyingron]

The roughness is more likely to be plugs than the mag itself. The jiggling of the key is that a lot of mag switches are junk. I had one you could do mag tests all day long (including turning to OFF) but once you took the key out at least one side was ungrounded.

 

[Tom-D]

IF it runs with the "P" lead disconnected secure with the mixture, and replace the ignition switch
and "P" leads.

 

[bbchien]

How many hours since IRAN for that mag- I'm lucky if I get 500 hrs....(but mine are pressurized).

 

[Let'sgoflying!]

Aren't you glad you did not do that mag test at cruise power, in flight?
Or maybe it would not have harmed your exhaust system, who knows?

 

[Clip4]

Sounds to me like you need an AP for both the mag and the violent nose shimmy issue.

 

[Tom-D]

As Tom says, see if it runs ok with the P-leads disconnected, although you can't check the mags that way.

Will they run or not, is what we are checking.

 

[Dan Thomas]

The roughness is more likely to be plugs than the mag itself. The jiggling of the key is that a lot of mag switches are junk. I had one you could do mag tests all day long (including turning to OFF) but once you took the key out at least one side was ungrounded.

Only badly worn mag switches do that. Sounds like you had one of the AD-affected Bendix switches. I've also seen switches so worn out the key would fall out in flight.

So many troubles are blamed on "faulty" components that should have been replaced a generation earlier. As always, it's all about maintenance. These things don't last forever no matter what anyone expects of them.

I'd like to know how long ago, in years and flight hours, the OP's mags were off for internal inspection.

 

[Tom-D]

Sounds to me like you need an AP for both the mag and the violent nose shimmy issue.

one issue at a time please, less the pilots get confused.