Mag Check Failure

[ltdriser]

I noticed the other day after a flying for a couple hours and leaving my friend’s airport when I checked my mags I had one mag without a 100rpm drop. There was no drop. Instantly thought well hell, p lead issue…probably came loose or something. The odd part was mag checks at the other airports were fine all day.  
 

Fast forward a few days I go out to fly and and confirm I’m not going nuts. I do my mag check on the ramp before taxiing out and sure enough mags check out just fine. I fly for ~hour and bring it in and decide to check while hot, no drop on the R mag ignition position. At the direction of my mechanic, I switched the ignition switch to OFF and confirmed the engine kept running. Hot Mag. 
 

Have you guys ever seen a mag act normal when cold then act up when warm? Is that generally a p-lead thing, internal mag issue, or ignition switch? 

[PT20J]

Switch. If it's a Bendix switch there is an AD 76-07-12 to check that it shuts off off. https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/C14960A415D956BD86256E520053A53E?OpenDocument

[N201MKTurbo]

If it is the switch, take it apart and clean and polish the contacts. Don't lose the springs and balls. If it isn't the switch, it is most likely a bad P lead.

[steingar]

The other thing you can do is turn the engine off at the key but leave the fuel open at idle.  If the mag really isn't grounded the engine will keep going. New squawk for me, that's what happened last night (after we flew back from the shop).  Back in she goes.  Can I legally fly with this, or do I have to use my deeply dysfunctional home shop?

[jaylw314]

1 hour ago, steingar said:

The other thing you can do is turn the engine off at the key but leave the fuel open at idle.  If the mag really isn't grounded the engine will keep going. New squawk for me, that's what happened last night (after we flew back from the shop).  Back in she goes.  Can I legally fly with this, or do I have to use my deeply dysfunctional home shop?

That is essentially what you're supposed to do for the Bendix switch AD  Well, they also say you should jiggle it in the off position to make sure

[ltdriser]

It seems like the plane could skate by the AD pretty easily as I have only noticed it after running for an extended amount of time. Normal mag checks after taxiing for 10 minutes do not show the issue. 

[ltdriser]

Well, checking the P-Lead operation this morning is showing basically what we have experienced. Normal operation when cold. 
 

Left P-Lead is grounded in OFF and R. 

Right P-Lead is grounded in OFF and L. 

No real obvious signs of p-lead degradation. There’s a connector with the heat shrink failing on R p-lead but I think that’s more coincidence. 
 

I was thinking I’d maybe see some resistance in L P-Lead to ground which may manifest as an open circuit when hot. Didn’t really see that. 

[jaylw314]

On 9/3/2021 at 3:00 PM, ltdriser said:

It seems like the plane could skate by the AD pretty easily as I have only noticed it after running for an extended amount of time. Normal mag checks after taxiing for 10 minutes do not show the issue. 

The AD is not just about the switch failing, it's about the switch failing unpredictably.   IIRC there were some failures when the grounding connection in the switch would be just poor enough to work normally, but fail to ground occasionally when the switch was turned a certain way.  When I do it, I turn the switch to off firmly first, then jiggle it while the engine is winding down.  I know I don't have to because it's done at annual anyway, but I figure it's a good idea to do check it occasionally

[PT20J]

If you buy a new date coded switch, you get rid of the recurring AD.

 

[MikeOH]

16 minutes ago, PT20J said:

If you buy a new date coded switch, you get rid of the recurring AD.

 

@PT20J Would you happen to know the date code beyond which the AD does not apply?

[PT20J]

21 minutes ago, MikeOH said:

@PT20J Would you happen to know the date code beyond which the AD does not apply?

Per the AD any switch that has a 4 digit date code is exempt. My 1994 J has a 4 digit date code. Not sure when they started using them.

Skip

[RLCarter]

I check the switch for grounding after each flight on both the Mooney and the Cessna due to me turning the props once they are both in the hangar (kinda tight, so turning the props gives me piece of mind I won’t catch a blade in the boys while walking around . Wasn’t the switch AD issued due to a Piper Tomahawk that failed ending in a fatal off field landing?

[ArtVandelay]

I check the switch for grounding after each flight on both the Mooney and the Cessna due to me turning the props once they are both in the hangar

Turn them backwards.

[RLCarter]

1 hour ago, ArtVandelay said:

Turn them backwards.

Mags can and will fire in either direction, the right wing of the Cessna is directly over the Mooney prop. A prop strike can be pricey when a wing causes the stoppage 

[Guest]

AD 76-07-12 covers the Bendix ignition switch.  Whether the AD applies by part number and date code, don’t they teach doing a live mag check prior to shut down?  I was taught this decades ago.

Clarence

[ArtVandelay]

Mags can and will fire in either direction

But assuming you have an impulse coupling, it won’t, and the magnetos need to be rotating at a certain speed in order to generate enough power to create a spark.

[RLCarter]

51 minutes ago, ArtVandelay said:

But assuming you have an impulse coupling, it won’t, and the magnetos need to be rotating at a certain speed in order to generate enough power to create a spark.

True, but I have seen a Piper Cub start and run by just moving the prop, and recently a BT-13 kick back while just moving the prop, both were pretty scary so I don’t chance it. 

[A64Pilot]

6 hours ago, ArtVandelay said:

Turn them backwards.

If you have a carbon blade dry vacuum pump. turning it backwards can lead to early failure.

 

[ltdriser]

3 hours ago, M20Doc said:

AD 76-07-12 covers the Bendix ignition switch.  Whether the AD applies by part number and date code, don’t they teach doing a live mag check prior to shut down?  I was taught this decades ago.

Clarence

I’m not sure where I missed this during training but I’ve never been taught to check mags after flying, only during initial run up. I will say though, after all this I will be checking at the end of my runs. Who knows how long I’ve been pushing the plane back with a hot mag. 

[Ragsf15e]

1 hour ago, ltdriser said:

I’m not sure where I missed this during training but I’ve never been taught to check mags after flying, only during initial run up. I will say though, after all this I will be checking at the end of my runs. Who knows how long I’ve been pushing the plane back with a hot mag. 

It’s a good habit to be in even if it just reminds you to think twice before moving the prop.  You understand better how the mags are “turned off”, the possibility of hot mags, and the danger. There are some new setups with switches for the mags and push button start that are awesome, but possibly easier to accidentally leave a mag switch on.  I had a student shut down (mixture pull), half-hazardly finish the shutdown checklist, hop out, and get the tow bar to move the airplane.  No key in that airplane as it has mag switches, which were both still on.  I should have double checked her during the checklist, but thankfully my spidey sense made me check the switches before we moved the prop to hook up the towbar. I hate that airplane!

[PT20J]

3 hours ago, RLCarter said:

True, but I have seen a Piper Cub start and run by just moving the prop, and recently a BT-13 kick back while just moving the prop, both were pretty scary so I don’t chance it. 

I don’t think the little mags on Lycoming and Continentals will generate enough voltage to fire without a pretty good spin UNLESS the impulse coupling is cocked and the mag is hot. In that case a small prop movement will fire the mag. The BT-13 has a R-985 with big mags without impulse couplings and I’ve heard of people hand propping R-985s back when I was flying Beavers.

Nonetheless, I do a check to see that the ignition switch kills the mags before shutdown, I put the key on the glareshield where I can see it from outside, and I turn the prop backwards 1/8 turn to connect the tow bar.

Speaking of tow bars, I was in the shop checking on the progress of my GFC 500 installation and saw a nice A36 with all three blade tips mangled. Started with tow bar attached. Mechanic said it was the second one in a month — the other was a Mooney.

Skip

[EricJ]

28 minutes ago, PT20J said:

Speaking of tow bars, I was in the shop checking on the progress of my GFC 500 installation and saw a nice A36 with all three blade tips mangled. Started with tow bar attached. Mechanic said it was the second one in a month — the other was a Mooney.

Skip

My insurance guy told me once that tow bars are a big source of prop strike claims.

 

[PT20J]

27 minutes ago, EricJ said:

My insurance guy told me once that tow bars are a big source of prop strike claims.

 

One of my biggest fears.

[RLCarter]

I’ll fess up that I left the towbar on the 172 once, flew three approaches and was landing back at my home field when my safety pilot noticed the shadow, one of my best landings every, Got very very lucky that the handle had collapsed and didn’t get into the prop. Its now on my checklist in 3 places “towbar removed”

[Vance Harral]

56 minutes ago, RLCarter said:

I’ll fess up that I left the towbar on the 172 once

A local flying club with a pair of 172s where I am an affiliated instructor requires the towbar be left attached to the airplane when parked in the hangar.  This makes my standard instructional admonition that, "the towbar may only be attached to the aircraft when the other end of it is in your hand", an actual violation of club rules.  Sigh.  It is an unbelievably stupid practice in what is otherwise a great flying club.