Miss at altitude

[wolftimj]

I have a 97 Bravo with a 400 hour new engine. Runs great, starts easy, pulls hard, nothing odd on the EDM during flight or run up. Last couple flight level runs I had a intermittent miss at 21,000 or higher. It wasn't constant enough to show on the EDM, but can feel it occur 2 - 5 times per minute. As is quite clearly spelled out by Lycoming, I have not tried a mag check at altitude, and won't.

Anyway, anyone else have this problem before and then found the solution? Lycoming service docs suggest a mag problem. Anyone?

Tim Wolf

[garytex]

At high altitude there is less insulating property in the air as it is less dense and harness, plug, and magneto problems will show themselves there first.  This could be a heads up for you to do some ignition maintenance

[jetdriven]

pressurized mags?

Tim, I have not heard of this. Has Lycoming specifically banned the checking of mags inflight?

[wolftimj]

Bravo has pressurized mags. 

Obviously need some repair, looking for known problems, hence this thread. 

Note of caution: Do not attempt to do in-flight mag checks for high altitude miss diagnosis. Very often high altitude misses turn into crossfires. Crossfire is when the electrical energy from a miss-fire finds a path of least resis- tance in one of the electrodes next to it. A mag check while attempting to diagnose which magneto is miss/crossfiring can lit- erally blow the exhaust system into pieces. This is especially true of turbo-charged engines. Mag checks at even low cruise power settings will over temp most cylin- der and exhaust components.

 

 

[jetdriven]

You win, Tim.  How about a 30" of MP LOP mag check at 2500'?   Enough to stress the ignition system but not enought to cause damage?

[testwest]

I would start by checking the center electrode resistance of your spark plugs, if they are more than 5000 ohms it may be time for new plugs. If you want more background info on this, head over to Beechtalk, there are scores of posts on the subject. Good luck!

[Kwixdraw]

Tempest recommends the ohm check for their plugs to check for resistance issues.  You might want to check your leads too. Trace them individually and see if you can see any indication that they have been arcing to nearby ground sources. You might want to have a look at your service records and run them past your most trusted engine man. Little things might pop out to him if he's looking for other possible causes.

[johnggreen]

Tim,

It has always been my "understanding" that Bravo mags will begin to give trouble at about 400 hours from rebuild.  Now, remember this is hearsay, shop talk, and perhaps an "old wives tale", but it came to me from mechainics with years of maintaining Bravos.

I don't take any chances.  After 300 hours, I have mine rebuilt or install rebuilt mags.  It is my experience that if you have a mechanic truly knowledgeable with the mag, the parts are only $30 or $40 and it takes maybe an hour.  That is a lot cheaper than the rebuilt mags.  The last time I had mine done, January 2012, I did not have access to such a mechanic so I "paid the piper" and bought rebuilts.  Don't remember the price but about $400 each comes to mind.

That may not be palatable, but it beats being 600 miles away from home with a dead mag.

First thing I would suggest is to check your logs and determine how many hours since the rebuild.

Jgreen

[Kwixdraw]

"Not showing up on the EDM"?   Maybe it's showing up in a way you're not looking for.  A CHT or EGT difference or something.  Does the problem go away if you are operating ROP?  I'm thinking that if you have an ignition problem you'll find something definate like carbon tracking or a bad plug and it would get worse as the condition progresses. I'm wondering if you wouldn't get this syptom with an injector or upper deck seal problem that isn't letting the injector atomize fuel as well as possible. LOP you might not be able to get a good consistant burn and be feeling it more than being able to read it on the EDM.  Just brainstorming. I'm sure curious to find out what it turns out to be.

[jlunseth]

High altitude miss is not something to mess with.  Get the mags checked by a A&P and in the meantime don't go to 210 and don't be running any cruise mag checks.

Garytex's explanation is correct.

 

[triple8s]

I had the same thing happen to me (but little 4 cylinder) anyhow, about 5 -10 minute intervals a little burp which started at cruise 9500 and up. After discussion with my AP he said replace the plugs. I had already started pricing a mag OH, he said start with the simple things first. Turned out it was a bad plug, I couldnt find "which" cylinder/plug on the monitor because it was at a long interval. Its a wonder there isnt a chamber to put spark plugs in with a high voltage supply to operate the plug and a pump to lower the pressure to check for these things. Plugs are cheaper.

[jetdriven]

Did you check the resistance below 5K ohms at the center? 

Also, I fail to see how a low altitude low power inflight LOP mag check is going to blow anything up. The book says dont do that at altitude. But doesnt say "never while inflight".  That will show your bad plug.  TN Bonanza guys do this every leg.

[johnggreen]

If you do a mag check on a Bravo at any altitude, you will see TIT's sky rocket. Be careful.

Jgreen 

[jlunseth]

Just wanted to identify some special issues if you do a mag check at cruise in a Mooney turbo.  Unlike the turbonormalized Bonanza guys, our aircraft depend on turbo even at sea level. 

If you conduct a cruise mag check and one side turns out to be bad, the engine quits.  In that event, your first reaction, which would be to switch back to "Both," would be wrong.  You must pull the mix to idle and the throttle to idle, let it sit there for awhile, then in my aircraft you would put the throttle to 1/3rd, pull the prop to minimum RPM, and slowly add mixture back until the engine begins to run.

If you were to conduct the check in my 231 above 12000 the manual says that the engine likely would not start.  Remember, the turbo is no longer operating since it has no exhaust gas to turn the compressor vanes, and therefore the engine is operating as a normally aspirated engine that has a low compression ratio.  Above 12000, the mixture would be too rich to start, in all likelihood.  So the procedure, if the first effort to start above 12000 fails, would be to wait until the aircraft is below 12000 and use the air start procedure.  Once the engine starts, it is cold because it has been at idle for a time.  So it is necessary to add power back in slowly to allow the engine to warm up.  If you don't, the engine will go from CHT's in the 160-180 range, to CHT's in the high 300's in very short order.  Have not tried the LOP cruise check myself, but have seen these temps and this issue during steep spirals for commercial, you do need to give the engine some time to warm up.  It gets cold fairly quickly.  This means operating for awhile at MP's that will not allow the aircraft to maintain altitude.

I don't fly a Bravo and don't have a Bravo manual, but it is the same type engine, the procedures and altitudes might be somewhat different but probably not alot.

I am not saying don't do it (try the cruise LOP test).  But I do think it would be wise to do it a few thousand feet up, but probably not over 10,000 MSL, in a turbo.  Those are my thoughts.

[wolftimj]

Just a follow-up. I started this thread after experiencing a high altitude miss. The problem was corrected. 7 of the 12 plugs had higher than acepatable resistance (from 10k ohm to over a million). Replaced all 12 plugs with new Autolite fine wires. Problem corrected, verified up to FL250. All the plugs that were bad were Champion fine wires with about 400 hours on them. Mags looked at and timing rechecked but nothing out of spec there.

All advice, right and wrong, appreciated.

Tim Wolf

[N9453V]

Glad you got it resolved!  Things like that can be very annoying.

[carusoam]

Nice details,

Thanks Tim.

Does it make sence to measure resistance of the plugs at annual?

Best regards,

-a-

[wolftimj]

I think I'm going to measure them all each time they are cleaned since it takes about 10 seconds to do. I try and clean mine every third oil change or about 100 hours, or about twice a year.

Tim

[stevesm20b]

I would recommend checking the resistance on the plugs anytime you have them out. I had an interminttent miss in my O-360. Pulled all the plugs, checked them on a spark plug tester. All the plugs checked out good on the tester. Cleaned and gaped them. Reinstalled them and still had a miss. Pulled them out again and measured the resistance and found one of them was over 5000 ohms resistance. Replaced the plug with the high resistance and the miss was gone.