Mag Not Grounding

[Kenny Taylor]

I ran into a situation with a substitute instructor yesterday where the left mag on the Cherokee wouldn't ground out. The previous renter squawked it, so we took it out to verify. Sure enough, no RPM change when selecting the right mag. We brought it back in and added another squawk. In hindsight, I should have switched it to OFF to see if it kept running.

Does failure to ground a mag make it unairworthy? What FAR would that be covered by?

 

[Gary Ward]

I ran into a situation with a substitute instructor yesterday where the left mag on the Cherokee wouldn't ground out. The previous renter squawked it, so we took it out to verify. Sure enough, no RPM change when selecting the right mag. We brought it back in and added another squawk. In hindsight, I should have switched it to OFF to see if it kept running.

Does failure to ground a mag make it unairworthy? What FAR would that be covered by?

You should be checking that every time you shut the engine down. Hasn't your instructor taught you that?

 

[Kenny Taylor]

Negative.. I was taught to lean out, then keys off/out. The checklist we're using doesn't mention it either:

https://www.airtreknorth.com/uploads/4/7/2/4/4724302/piper_140_checklist_revised__1_.pdf

Of course that checklist doesn't ever mention turning the mags on, either. I'll start checking the mags on shutdown from now on.

 

[Bell206]

Does failure to ground a mag make it unairworthy?

The existing open discrepancy write up made the aircraft unairworthy. Did you fly the aircraft when you took it out to verify?

 

[Kenny Taylor]

Nope. Did the run-up, noted the issue, taxied back.

 

[Bell206]

Nope. Did the run-up, noted the issue, taxied back.

Okay. Now if you took it out to verify and the RH mag grounded as required would you have taxied to the runway and gone flying?

 

[flyingron]

Turning the switch OFF only tells you that the P-Lead is connected. It doesn't do squat to guarantee that the mag isn't left hot with the keys out. There are a lot of crappy old mag switches out there. I had one in my plane that you could turn it to OFF all day long and the engine would die, but you pulled the keys out and one mag would go hot.

Not being able to shut off a mag *IS* an airworthiness issue. It needs to be addressed.

 

[GeorgeC]

You should be checking that every time you shut the engine down. Hasn't your instructor taught you that?

I thought turning both mags off while the engine is running could cause a potentially damaging backfire when you turn the mags back on.

 

[mondtster]

I thought turning both mags off while the engine is running could cause a potentially damaging backfire when you turn the mags back on.

It will definitely create a loud bang and potential exhaust damage if you perform the test at run up speed. There is little risk in doing it at idle. Some folks perform an ignition check at idle after a flight, many do not.

 

[Dan Thomas]

It will definitely create a loud bang and potential exhaust damage if you perform the test at run up speed. There is little risk in doing it at idle. Some folks perform an ignition check at idle after a flight, many do not.

We did it at the flight school with seven airplanes for many years, at idle, with never a bang. But at runup RPM? Potential for exhaust sytem damage. If one inadvertently switches off in the runup, leave it off, close the throttle and let it die, then restart.

 

[flyingron]

I thought turning both mags off while the engine is running could cause a potentially damaging backfire when you turn the mags back on.

Only if you do so immediately and at a non-idle power setting. Then you do what Rod Machado calls the click-click-click-click-BANG student mag check.

It's not a backfire actually, it's a big blast in the exhaust manifold which may damage the that.

 

[Dan Thomas]

It's not a backfire actually, it's a big blast in the exhaust manifold which may damage the that.

Afterfire. https://www.beamng.com/threads/afterfire-vs-backfire.49197/

 

[Clip4]

Nope. Did the run-up, noted the issue, taxied back.

If you had did the run up and was unable to duplicate, would you have taken off?

When a renter writes up an issue the plane shouldn't be checked out to the next renter until the discrepancy is cleared by an AP.

 

[Albany Tom]

I always learned that for some flying problems, there's a potential risk of breaking a rule, and a potential risk of crashing. You address both, but focus on safety first.

The reason you need to be able to ground both mags is to make sure they both work before you take off. I'm not taking off unless I have a good run up. Not doing that, among other things, cost several lives and a B-17 a few years ago. But yeah, it's probably against the regs, too.

As far as checking before shutdown, the only time I check the mag switch on shutdown is if I'm flying a plane that isn't shutdown with mixture.

 

[flyingron]

If you can't test each mag individually, how do you know if they are both working? You can't finish the preflight check without it. Climb out is not the time to find out whether your engine is able to produce full power.

 

[Dan Thomas]

The reason you need to be able to ground both mags is to make sure they both work before you take off. .

That's only one reason. Another is the need to be able to shut off a rogue mag. Mags have plastic distributor gears in them, plastic for obvious electrical reasons, and they age with heat and wear with motion and can eventually slip so that the spark goes to wrong cylinders instead of the right ones. Imagine the spark igniting the air/fuel mix when the piston is near the bottom of the intake stroke: it sets off the mix, and with that intake valve still open it sets off all the mix in the intake manifold too, meaning that none of the cylinders get anything but exhaust. Now how important is it that we be able to short that mag, in flight, and get the engine running right on the surviving mag?

This is something not taught in PPL flight instruction. It should be. There are way too many mags out there with 40 years and 1500 hours on them just waiting to do exactly that. Maintenance matters, not of just the mag but of that P-lead wiring and the switch.

 

[pfarber]

I've never been told or shown how to do a mag check at shutdown. Its not in any POH I've ever read. Mag check during run up? Sure, in every POH I've ever read.

Every POH said to shutdown by going to idle and pulling the mixture, master off, ignition off.

Where did this mag check thing come from? Cessna/Piper didn't put it in thier manuals.

 

[Gary Ward]

I've never been told or shown how to do a mag check at shutdown. Its not in any POH I've ever read. Mag check during run up? Sure, in every POH I've ever read.

Every POH said to shutdown by going to idle and pulling the mixture, master off, ignition off.

Where did this mag check thing come from? Cessna/Piper didn't put it in thier manuals.

I learned to fly at a med sized school so I flew with 5 different instructors during stage checks and a couple of them left the flight school to go to ATP school while I was there. I learned something different from each instructor.
A P lead check was one of them during shutdown. I have heard/noticed plenty of other planes doing the same thing during shutdown out on the ramp.

My plane has a ignition switch AD that my AP/IA would list on my annual each year. I ordered a 18 dollar kit for it. It was easy to do and I got a diode installed on my starter solenoid out of the deal to protect my ignition switch.

In hindsite I am glad I got to fly with so many different instructors in the last few years. I flew with 3 different CFII during my instrument training. Now my best bud fly's with me and he is a check airman for SW airlines.

Here is the new contacts and fresh lube for my ignition switch to hopefully head off any future problems with it. It was working just fine before I opened it up.

I think if you see blue you know the switch has been upgraded? At least on ACS switch.

 

[pfarber]

I learned to fly at a med sized school so I flew with 5 different instructors during stage checks and a couple of them left the flight school to go to ATP school while I was there. I learned something different from each instructor.
A P lead check was one of them during shutdown. I have heard/noticed plenty of other planes doing the same thing during shutdown out on the ramp.

My plane has a ignition switch AD that my AP/IA would list on my annual each year. I ordered a 18 dollar kit for it. It was easy to do and I got a diode installed on my starter solenoid out of the deal to protect my ignition switch.

In hindsite I am glad I got to fly with so many different instructors in the last few years. I flew with 3 different CFII during my instrument training. Now my best bud fly's with me and he is a check airman for SW airlines.

Here is the new contacts and fresh lube for my ignition switch to hopefully head off any future problems with it. It was working just fine before I opened it up.

I think if you see blue you know the switch has been upgraded? At least on ACS switch.

Cool story bro. Now show me a POH that says check the mags at shutdown.

People do all sorts kf crazy things to AC becuase reasons. None of them good or anyone knows why... But hey, the CFI said so lol

 

[Gary Ward]

Cool story bro. Now show me a POH that says check the mags at shutdown.

People do all sorts kf crazy things to AC becuase reasons. None of them good or anyone knows why... But hey, the CFI said so lol

No one said you need to do it. You do your thing and I'll do mine. You did learn something though...right?

 

[Dan Thomas]

I've never been told or shown how to do a mag check at shutdown. Its not in any POH I've ever read. Mag check during run up? Sure, in every POH I've ever read.

Every POH said to shutdown by going to idle and pulling the mixture, master off, ignition off.

Where did this mag check thing come from? Cessna/Piper didn't put it in thier manuals.

There is a LOT of stuff taught that is not in POHs. Does the POH teach you what knots to use to tie down the airplane? Does it even tell you to tie down the airplane?

We have resources, resources written because people were injured or killed. Just because you don't see something in the POH, or weren't taught it, doesn't mean that it's not valid. After all, a significant percentage of pilots don't under stand carb ice or angle of attack either, and it kills some of them. Ignorance is not a preferable state of mind. Many commercial operators have this procedure in their checklists. We did.

Here's something from an aviation safety publication:

In 1977 the FAA issued Airworthiness Directive AD 76-07-12 requiring all aircraft using magnetos with certain Bendix ignition switches be tested for proper grounding function every 100 hours. The AD states, “With the engine at normal idle, rotate the switch key or lever through the Off detent to the extreme limit of its travel in the Off direction. If the engine stops firing, this indicates an airworthy switch.”

Many affected aircraft may have had their ignition switches replaced, though the FAA subsequently issued a similar AD in 1994 affecting certain Teledyne Continental Motors (TCM, formerly Bendix) magnetos. However, pilots still perform this check prior to engine shutdown because it is a foolproof way of ensuring that both magnetos truly are grounded when the ignition switch is in the Off position.

The Off position mechanically causes both the Right and Left magneto P-leads to engage, grounding both magnetos at the same time. But what if the switch is broken and the Off position doesnt ground the mags? Its possible the magnetos may ground independently during the run-up check but that one or both of them could still be “live” with the ignition in the Off position at shutdown.

Grounding Check

As a Student pilot, I was taught to do a grounding check at shutdown to ensure that when and if I needed to move the propeller by hand (for instance, to attach the tow bar) I wouldnt accidentally start the engine and make salsa out of myself. I did this test faithfully for a while, but after several loud, hair-raising backfires that resulted from me fumbling to get the ignition key quickly back to Both from the Off position before the engine quit, the procedure faded from my repertoire. I started to wonder if I was doing more harm than good. Besides, I was very careful not to move the propeller too much, avoiding the telltale click of the impulse coupling-a wound-up spring that, when let go, snaps the magneto into action and generates a spark that, if the correct fuel-air mixture is present in one or more cylinders, could cause ignition. But how could the engine start if Id shut it down by pulling the mixture to idle cutoff?

After my recent hot magneto discovery and subsequent conversation with several mechanics, Ive decided to revise my engine shutdown checklist.

“Once you shut the engine off with the mixture, you may not have completely shut off the fuel flow,” Hughes advised. “What youve done is reduced the fuel flow to the point where whatever was going into the engine was not sufficient to support combustion. With Cessnas [and other aircraft whose fuel tanks are above the engine -Ed.], that fuel system has constant gravity pressure trying to feed fuel to the engine. If the mixture is not fully shut off, fuel can seep past the seals and enter the cylinders or get into the carburetor area, in sufficient quantity that if there is an ignition or compression source it could start the engine.”

A query of the NTSBs online database for events involving a “hot mag” or “hot magneto” returned just one hit in the last 20 years, though presumably many unintentional engine starts that do not result in injury or damage go unreported. On May 15, 1991, the pilot of a Cessna Model 177 Cardinal in Greensboro, N.C., noticed during engine start that the starter was operating but not engaging the engine.

“He turned off the magnetos and battery and exited the airplane to investigate,” the report states. “He did not chock or secure the airplane. He turned the propeller some, then began to walk away. The engine then roared to life. The airplane proceeded across the ramp and collided with a parked Beech Model 77 Skipper. An inspection of the airplane revealed that the left mag would not ground out with the switch, resulting in a hot mag condition.”

https://www.aviationsafetymagazine.com/features/the-other-grounding/

That guy's "hair-raising backfires" during the mag check was likely due to doing it at 1000 or 1200 RPM. That's wrong. Throttle completely closed and it won't do that. In many thousands of flight school hours we never had it happen.

P-leads are famous (to mechanics) for fraying and breaking at the magneto, especially if they're not properly secured to prevent vibration from flopping them around. It's best to find that busted lead with an idle mag check rather than at the next runup or when someone disturbs that prop and get chopped.

 

[Therookguy]

You should be checking that every time you shut the engine down. Hasn't your instructor taught you that?

I only do this once every 4 or 5 flights. Worried about damaging my engine.

 

[Therookguy]

I can vouch for the engine starting with the mixture lever all the way out. I get nervous at FBOs sometimes because I've had my engine nearly come to a stop then bolt back to life 3 or 4 times before it finally stops. Happens 40% of the time. Engine doesn't wanna stop, which I guess is a good thing.

 

[Gary Ward]

I can vouch for the engine starting with the mixture lever all the way out. I get nervous at FBOs sometimes because I've had my engine nearly come to a stop then bolt back to life 3 or 4 times before it finally stops. Happens 40% of the time. Engine doesn't wanna stop, which I guess is a good thing.

Sounds like you’re shut off needs adjustment.

I have done it religiously for 1100 hours now on 2 different 0-320 motors. I fly 250 hours a year and have never had it backfire. I am doing it at shut down at 1000 rpm’s. My motor shuts off clean the first time I pull the fuel.

yours needs adjustment.

 

[Therookguy]

Sounds like you’re shut off needs adjustment.

I have done religiously for 1100 hours now on 2 different 0-320 motors. I fly 250 hours a year and have never had it backfire. I am doing it at shut down at 1000 rpm’s. My motor shuts off clean the first time I pull the fuel.

yours needs adjustment.

It's possible that full-lean isn't actually getting full-lean. It was idling a bit on the lean side and had it richened a bit so perhaps that's why.

 

[flyingron]

The problem is that the check isn't good enough. It's possible for the mag to pass the "turn it off" test and still not ground the mags all the time.

 

[mondtster]

I can vouch for the engine starting with the mixture lever all the way out. I get nervous at FBOs sometimes because I've had my engine nearly come to a stop then bolt back to life 3 or 4 times before it finally stops. Happens 40% of the time. Engine doesn't wanna stop, which I guess is a good thing.

What aircraft and engine?

 

[JScarry]

I

Where did this mag check thing come from? Cessna/Piper didn't put it in thier manuals.

I always do a mag check just before shutdown because it lets me know if there are fouled plugs. I’d much rather know of an issue when I have time to fix it rather than in the runup area when I want to go somewhere. That very issue happened to the my Cherokee just this week. The guys flying it haven’t been leaning enough and the bottom plugs were fouled.

 

[JScarry]

I can vouch for the engine starting with the mixture lever all the way out. I get nervous at FBOs sometimes because I've had my engine nearly come to a stop then bolt back to life 3 or 4 times before it finally stops. Happens 40% of the time. Engine doesn't wanna stop, which I guess is a good thing.

If you have a carburetor you probably need to have it rebuilt/replaced. Mine was like that for years and the only way to get a clean shutdown was to run it up and then pull the mixture. I finally spent the $1,600 and put in a new carb and the problem went away. Also fixed my intermittent fuel pressure gauge issue at the same time.

 

[Albany Tom]

I wasn't going to jump back in, but I will after reading the "salsa" comment. I don't check mags on shutdown. I suppose there are some reasons to do that. One of them isn't, though, that you might want to move the prop and the mag might be hot. To me, every prop is always hot. And I say that having hand propped a plane probably 300+ times. Just because a mag is off in one moment does NOT mean it's not hot the next. I treat it exactly like the safety on a gun, which is to not believe it at all. Some mags, some planes, it just takes a little click for them to fire right off. I've had it happen when I was prepared for it, but not expecting it. It will wake you right up.

 

[Dan Thomas]

It's possible that full-lean isn't actually getting full-lean. It was idling a bit on the lean side and had it richened a bit so perhaps that's why.

That wouldn't do it. The idle cutoff shuts off all the fuel to both the idle circuit and main nozzle.

I would get the mechanic to check to see that the mixture control lever on the carb is moving all the way between its stops. If it's not, it's a rigging problem.

 

[Dan Thomas]

I wasn't going to jump back in, but I will after reading the "salsa" comment. I don't check mags on shutdown. I suppose there are some reasons to do that. One of them isn't, though, that you might want to move the prop and the mag might be hot. To me, every prop is always hot. And I say that having hand propped a plane probably 300+ times. Just because a mag is off in one moment does NOT mean it's not hot the next. I treat it exactly like the safety on a gun, which is to not believe it at all. Some mags, some planes, it just takes a little click for them to fire right off. I've had it happen when I was prepared for it, but not expecting it. It will wake you right up.

This is true, but the shutdown check will catch 99% of the hot mags. That sort of insurance is nothing to scoff at.

 

[Dan Thomas]

I learned to fly at a med sized school so I flew with 5 different instructors during stage checks and a couple of them left the flight school to go to ATP school while I was there. I learned something different from each instructor.
A P lead check was one of them during shutdown. I have heard/noticed plenty of other planes doing the same thing during shutdown out on the ramp.

My plane has a ignition switch AD that my AP/IA would list on my annual each year. I ordered a 18 dollar kit for it. It was easy to do and I got a diode installed on my starter solenoid out of the deal to protect my ignition switch.

In hindsite I am glad I got to fly with so many different instructors in the last few years. I flew with 3 different CFII during my instrument training. Now my best bud fly's with me and he is a check airman for SW airlines.

Here is the new contacts and fresh lube for my ignition switch to hopefully head off any future problems with it. It was working just fine before I opened it up.

I think if you see blue you know the switch has been upgraded? At least on ACS switch.

You wouldn't believe how many times I have found an ACS switch in an airplane, yet the logs showed that the Bendix AD check had been faithfully done for 30 years. Nobody had ever climbed under the panel in all that time to see if it really was a Bendix. Those ACS switches were often shot by that time, being way past the ACS 2000-hour AD overhaul and not having the mandated diode on the starter contactor coil. Arcing contacts (with no diode) burn stuff and can leave debris in the switch that fouls the magneto contacts.

Bendix Switch AD: https://rgl.faa.gov/Regulatory_and_...C14960A415D956BD86256E520053A53E?OpenDocument


ACS switch AD: https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument

ACS Service Bulletin referenced by the AD: https://acsproducts.co/wp-content/uploads/2021/01/SB92-01.pdf

If you have a Cessna, and your logs show that the Bendix switch AD is done every year, better get under there and make sure it really is a Bendix. If it is, look to see if it has the identification specified by the AD that terminates the AD.

Cessna has used ACS switches for a long time, and many old Bendix switches have been replaced with ACS.

 

[Gary Ward]

I wasn't going to jump back in, but I will after reading the "salsa" comment. I don't check mags on shutdown. I suppose there are some reasons to do that. One of them isn't, though, that you might want to move the prop and the mag might be hot. To me, every prop is always hot. And I say that having hand propped a plane probably 300+ times. Just because a mag is off in one moment does NOT mean it's not hot the next. I treat it exactly like the safety on a gun, which is to not believe it at all. Some mags, some planes, it just takes a little click for them to fire right off. I've had it happen when I was prepared for it, but not expecting it. It will wake you right up.

I always turn my propeller backwards so not to trip the impulse coupler in effort to be safer. My buddy won't do his plane that way because he says it could damage his vacuum pump which I don't have. Even if I did have a vacuum pump I would turn the prop backwards if I needed to turn it.

And yes I do treat that propeller like a loaded gun. It helped that I watched a video on U tube where a person got hit by a prop of a experimental plane. You remember something like that.

 

[flyingron]

It's not going to damage the vacuum pump. The vanes aren't flung snug against the housing at the speeds you're moving the prop by hand.

But I always assume it will fire or kick back or something, stand clear and be careful where you put your hands.

 

[Dan Thomas]

The vacuum pump will not break if the vanes are not worn past their limits. Period. Engines kick back. Mechanics turn props back and forth during compression and timing checks. Both Tempest and Rapco sell pumps with vane inspection provisions; takes maybe five minutes to do the check at annual, starting once the pump has about 500 hours on it. This allows you to to get the maximum life out of the pump without risking a failure.

And yet, people still buy pumps without that inspection provision, then run them to failure or replace them at some arbitrary time. I really don't get it. We run engines on-condition by checking the filters for metal and checking compressions and so forth, but we'll run pumps and magnetos and alternators until they just quit, often at some critical or really inconvenient time and place.

 

[Gary Ward]

Bendix AD you posted above says to shut the switch off at idle to check the switch to make sure the engine dies. Step B
Makes sense.

There it is in writing from a manufacturer. Not from some silly CFI...

FYI The switch above in my pictures only had 1700 hours on it. It did not have a diode on the solenoid coil. Which means it should have been opened up and inspected within 100 hours or at annual time a long time ago. That is extra work for the AP and part of why a good annual can be expensive I’d bet.

 

[Dan Thomas]

Bendix AD you posted above says to shut the switch off at idle to check the switch to make sure the engine dies. Step B
Makes sense.

There it is in writing from a manufacturer. Not from some silly CFI...

FYI The switch above in my pictures only had 1700 hours on it. It did not have a diode on the solenoid coil. Which means it should have been opened up and inspected within 100 hours or at annual time a long time ago. That is extra work for the AP and part of why a good annual can be expensive I’d bet.

That's why it had some burned contacts inside it. In your pictures of the kit the diode was there. Yours might be just one more airplane that got the endless Bendix checks instead of the applicable ACS AD. I found very few Bendix switches in Cessnas, mostly in pretty old ones.

Yes, the AD demanded the hot-mag check. At idle. If there was risk of engine damage, they would have asked you to put a mag timer on the mags, rotate the prop (plugs disconnected!) so that both timer lights were indicating points open, and run the switch through its checks while watching the lights. I've done that while troubleshooting suspected bad P-lead wiring at the switch, wiggling the wires to see if there's erratic grounding.

AD stuff can be extra work for the mechanic, alright. Just knowing that there are two switch ADs seems uncommon. That AD doesn't show up on any Cessna model AD listing; it's an appliance that is found in many airplanes. Other overlooked ADs include some United Instruments altimeters, upper shoulder harness adjuster springs https://rgl.faa.gov/Regulatory_and_...a6d4b9c8cc2986256f1200531f70/$FILE/041901.pdf And Amsafe seat belts https://rgl.faa.gov/Regulatory_and_...06DD731840F4677686256A340060C97C?OpenDocument

There are lots of others. It can take hours to review an airplane's components and clear them.

On the subject of seat belts and should harnesses: It was common to find 40 and 50-year-old airplanes with the original belts still in them. The nylon deteriorates with age and might not hold the occupant in the event of an accident. They can look good but be totally shot.

 

[Gary Ward]

That's why it had some burned contacts inside it. In your pictures of the kit the diode was there. Yours might be just one more airplane that got the endless Bendix checks instead of the applicable ACS AD. I found very few Bendix switches in Cessnas, mostly in pretty old ones.

Yes, the AD demanded the hot-mag check. At idle. If there was risk of engine damage, they would have asked you to put a mag timer on the mags, rotate the prop (plugs disconnected!) so that both timer lights were indicating points open, and run the switch through its checks while watching the lights. I've done that while troubleshooting suspected bad P-lead wiring at the switch, wiggling the wires to see if there's erratic grounding.

AD stuff can be extra work for the mechanic, alright. Just knowing that there are two switch ADs seems uncommon. That AD doesn't show up on any Cessna model AD listing; it's an appliance that is found in many airplanes. Other overlooked ADs include some United Instruments altimeters, upper shoulder harness adjuster springs https://rgl.faa.gov/Regulatory_and_...a6d4b9c8cc2986256f1200531f70/$FILE/041901.pdf And Amsafe seat belts https://rgl.faa.gov/Regulatory_and_...06DD731840F4677686256A340060C97C?OpenDocument

There are lots of others. It can take hours to review an airplane's components and clear them.

On the subject of seat belts and should harnesses: It was common to find 40 and 50-year-old airplanes with the original belts still in them. The nylon deteriorates with age and might not hold the occupant in the event of an accident. They can look good but be totally shot.

Race cars get new belts every 2 years!

Mine are 40 years old…there’s always something needed for these damn things. Lol