WaynePierce

Left mag check kills engine

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I experienced something I've never dealt with before and looking for your expertise.

During run-up yesterday on the trip back home when I did the mag check and switched to the left mag the engine lost all power. Right mag checked fine. At first I thought I had a fouled plug but that didn't make sense because it wasn't running rough, it just sounded like I lost all power. I eventually got the left mag to check with about 100 rpm drop so I took off. Everything was fine, plenty of power for take-off and the flight home was uneventful until I parked it in the hangar... another thread. Another thing I noticed was my tach was bouncing when close to idle. It was fine at 1800 and it was fine in the air, but close to idle it was not steady at all. The engine sounded fine, it was not sounding like the tach was reading. This may be completely unrelated but I don't know.

Have any of you had a bad ignition switch? I'm wondering if this is what I was experiencing with the loss of power, maybe going through the left mag to the "off" position? Even though I didn't go pass the left mag indent... Sound familiar to anyone?

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I'll look through the logs and find out... New to me plane, 1,680 hours total time, I'll report back.

 

Found where they replaced the Mags at 1325.2 so theoretically the 500 hour test should have another 200 hours.

 

Edited by WaynePierce
checked logs.

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Wow, that's a low-time J.

You mentioned you "got the left mag to check with a 100 rpm dropoff".  What did you do?

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21 minutes ago, WaynePierce said:

I'll look through the logs and find out... New to me plane, 1,680 hours total time, I'll report back.

 

Found where they replaced the Mags at 1325.2 so theoretically the 500 hour test should have another 200 hours.

 

Actually only 145 hours (1680-1325=355).

 

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11 minutes ago, jaylw314 said:

Wow, that's a low-time J.

You mentioned you "got the left mag to check with a 100 rpm dropoff".  What did you do?

Low time, yes. That's one of the gambles. It's got only about 400 hours on a rebuild but hasn't been flown a lot in the last couple of years.

Nothing different. That's why I thought maybe the switch was going south.

Edited by WaynePierce

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6 minutes ago, LANCECASPER said:

Actually only 145 hours (1680-1325=355).

 

Math in my head is not good. So is this normal in a Mooney to have to replace a mag between 400 and 500 hours? I've read where some folks have replaced their mags with something different but I don't recall what that was. I'll have to research some more....

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I'm assuming with a rebuild engine, you have the two separate Slick mags, correct?  Common (but not universal) opinion is that their quality sucks more than the Bendix single-drive magnetos.  I had to get my left mag internal timing adjusted after only about 200 hours, it was hard starting and died on taxi a few times.  So it is fully possible that the left mag needs to be IRAN'd.

Sure, the switch can be the culprit too.  If it's an intermittent problem, it might be hard to catch, but someone else found a loose lead on theirs.

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I have a dual mag J and made it all of 285 hours before the mag needed another “500 hr”  IRAN.  Worn cam.    Slicks fans are made of plastic instead of metal.  

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If your engine is an IO-360A3B6D you’ve got a Bendix dual magneto.  If your engine is an IO-360A3B6 you’ll have 2 separate magnetos.  In either case further investigation is required.

Start with the simple and work to the difficult. Spark plugs, high tension test of the wires, then the magneto.

Tachometer is driven from the magneto switch.

Clarence

 

Edited by M20Doc

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2 hours ago, WaynePierce said:

It was fine at 1800 and it was fine in the air, but close to idle it was not steady at all.

When you say "it was fine in the air" did you do a mag check enroute.  If you didn't, I suggest you consider the value of doing a mag check under power.  That will generally give you more information on the health of your mags (and wires) than the pre-flight mag check.

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2 hours ago, WaynePierce said:

I experienced something I've never dealt with before and looking for your expertise.

During run-up yesterday on the trip back home when I did the mag check and switched to the left mag the engine lost all power. Right mag checked fine. At first I thought I had a fouled plug but that didn't make sense because it wasn't running rough, it just sounded like I lost all power. I eventually got the left mag to check with about 100 rpm drop so I took off. Everything was fine, plenty of power for take-off and the flight home was uneventful until I parked it in the hangar... another thread. Another thing I noticed was my tach was bouncing when close to idle. It was fine at 1800 and it was fine in the air, but close to idle it was not steady at all. The engine sounded fine, it was not sounding like the tach was reading. This may be completely unrelated but I don't know.

Have any of you had a bad ignition switch? I'm wondering if this is what I was experiencing with the loss of power, maybe going through the left mag to the "off" position? Even though I didn't go pass the left mag indent... Sound familiar to anyone?

If fine in the air means that you took off and the plane ran okay- then okay. It sounds like you lost the left mag on the ground and then flew home on one mag. 

I’ve lost mags twice in an IO360 A3B6 J.  Both times I didn’t find it until the runup on the next flight.  First time it was coming back to home base. Second time I was stranded away from home. Wasn’t fun, but I caught a ride home from a transient plane headed to Dallas. 

Looking at engine monitor data for the previous flight, it was clear in the data where I lost the mag. In both cases I had pulled power back, and the mag failed when I pushed the power back in.

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+1 on Clarence’s observation... the tach is piggybacking on the mag working...

-1 the tach is telling you something is wrong.

-1 the mag croaked on you during run-up.

Could be as simple as a broken or loose grounding wire...

Planes don’t perform very well without both mags.  There are no performance charts for single mag operations...

You are not alone doing this...

I did this once in a rented C152.  After the flight, I realized how much risk was added by flying on one mag... a subtle hint was the max altitude I could get was about 2k’...

Not knowing performance, and not knowing what killed one mag, might kill the other... could be something wrong with the ignition switch...

It is OK to land on one mag... but the rules pretty much dictate T/O on one mag is not a good idea...

Just rethink that going forwards...

Most people get their mags OH’d in 500 hour intervals... capacitors and plastic gears don’t do well in the nasty environment they live in...

PP thoughts only not a mechanic...

Best regards,

-a-

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Thank you all for your input, experiences, and knowledge.

I do have the A3B6D engine so it sounds like a mag replacement is probably in order...

Mag check en route sounds very scary...

This may have been my most expensive 100.00 burger run, ever... maybe it's just a ground wire but time to find a mechanic to take a look at it.

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A replacement is not necessarily required, but it sounds like a good IRAN or O/H may be in order. My J did the same thing at 404 hours on the mag, and once it came back from being O/H'd, I have not had a problem since. The Bendix dual mags are fine IFF (for you math junkies) you maintain them properly including cap, wires, etc.

The more I have flown my plane, the more I have come to understand when it is telling me something is not right. Sounds like yours is trying to say something to you.

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37 minutes ago, Oldguy said:

The more I have flown my plane, the more I have come to understand when it is telling me something is not right. Sounds like yours is trying to say something to you.

Owning your airplane is so much more than just convenient. It's a safety factor... because of this very thing. Well said John.

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2 hours ago, WaynePierce said:

Thank you all for your input, experiences, and knowledge.

I do have the A3B6D engine so it sounds like a mag replacement is probably in order...

Mag check en route sounds very scary...

This may have been my most expensive 100.00 burger run, ever... maybe it's just a ground wire but time to find a mechanic to take a look at it.

Weird, I figured if you had a 400 hour rebuid, it must have been an A3B6, bad guess on my part.

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I've had this plane for just 6 weeks so I have SOOO much to learn. First complex time and trying to get my head around all the nuances of and engine monitor, manifold pressure, rpm, GUMPS, etc. I'm just glad I have this board to help me through this learning process.

 

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4 minutes ago, WaynePierce said:

I've had this plane for just 6 weeks so I have SOOO much to learn. First complex time and trying to get my head around all the nuances of and engine monitor, manifold pressure, rpm, GUMPS, etc. I'm just glad I have this board to help me through this learning process.

 

If it's just the expected internal wear on one of the mags, the repair could be as little as $250-300.  Not too much more than a couple hamburger runs.

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18 minutes ago, jaylw314 said:

If it's just the expected internal wear on one of the mags, the repair could be as little as $250-300.  Not too much more than a couple hamburger runs.

Plus shipping both ways, plus new gasket, new lock washers every time, plus removal and install labor and labor to time the mag. More like 3 or 4 hamburger runs . . lol

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19 minutes ago, LANCECASPER said:

Plus shipping both ways, plus new gasket, new lock washers every time, plus removal and install labor and labor to time the mag. More like 3 or 4 hamburger runs . . lol

Just gotta find an A&P that will bench test the mag and reset the e-gap locally.  If they send it out to a mag shop, yes, that significantly increases the cost.

And don't forget to add the cost of buying donuts for the shop, too :P  Never hurts...

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52 minutes ago, jaylw314 said:

Just gotta find an A&P that will bench test the mag and reset the e-gap locally.  If they send it out to a mag shop, yes, that significantly increases the cost.

I would send it to a mag specialist like Aaron. http://selectaircraft.com/. From Memphis it's probably two day ground to Lancaster TX. In about a week to ten days you are ready to go again and it's done right.

Most A & P's don't open up mags every day. Aaron has seen it all. My personal philosophy is do it once and do it right. That has usually proven to be less expensive in the long run. Not just in airplanes, but any time I've tried to skimp I spend the original "lower" cost and then i ended up having to do it right anyway, so I spend more than if I would have just done it right to begin with. The removal, install and timing labor, gasket, lock washers, etc. should be the same either way. 

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52 minutes ago, LANCECASPER said:

I would send it to a mag specialist like Aaron. http://selectaircraft.com/. From Memphis it's probably two day ground to Lancaster TX. In about a week to ten days you are ready to go again and it's done right.

Most A & P's don't open up mags every day. Aaron has seen it all. My personal philosophy is do it once and do it right. That has usually proven to be less expensive in the long run. Not just in airplanes, but any time I've tried to skimp I spend the original "lower" cost and then i ended up having to do it right anyway, so I spend more than if I would have just doing it right to begin with. The removal, install and timing labor, gasket, lock washers, etc. should be the same either way. 

True, most shops will not do mag work.  Turns out, the IA at my shop has all the bench testing equipment said he will do a lot of repairs in house, but would send it out if there was something more complex.  By that point, of course, we were pretty confident it was an e-gap problem only.

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1) Log check... hours in each mag...

2) 500 hours since last service on the bad mag? Send out to Aaron...

This gets you going the right direction without spending much time proving something has experienced normal wear...

3) Most likely you have a lot more hours than 500... only your log knows.

4) if the maintenance is done recently, how old is the ignition switch? 5,000 hours?

5) switches are rebuildsble...

6) what kills the engine is grounding of the mag... broken ground wires keep the plane running...

7) Start studying details regarding new mags and timing. A few details you want to be knowledgeable on...

8) Do you have a monitor?  Homework assignment... download the flight detail when the mag quit and post it.

Keep asking questions, no flying until you know your next steps...

Best regards,

-a-

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