engine skipping at 11000 ft

[tyrefoote]

I have experienced minor  [one hiccup every couple of minutes] engine skipping at 11000 ft with all mixture settings ? I descended to 9000 and it ran smooth.  

Engine is IO 360 A3B6  [2 seperate mags]  with 1000 hrs since new. on a  M20J

any thoughts on this  ??  thanks always Ty

[takair]

I have had hickups at altitudee with bad plugs or ignition leads.  How many hours since plugs were gapped and cleaned?  Also have had a slug of water cause that.  Either way, it's un-nerving.

[MooneyMitch]

Greetings tyrefoote:

Below is a link from Mike Busch on this very topic.  It may be of some help to you and could be the cause of your misfire.

http://www.avweb.com/news/maint/182843-1.html

[Seth]

I agree with takair. Most likely there is an insulation crack on one of the spark plug cables. Denser air acts as an insulator at lower altitudes but as the air becomes thinner at high altitudes it looses the electrical insulation properties. This is a common problem on radars that use magnetrons.

José

 

[acronut]

Yea it definitely sounds like an ignition problem.  I had the same thing happening in another airplane.  I climbed higher and it got worse.  Did a mag check at altitude (lower power setting than cruise) to figure out which side it was.  Replaced the plugs, then the mag leads before I realized the Mags hadn't been overhauled in over 1200 hrs.  It ended up being the Mag carbon arching at altitude.  I believe Mags are recommended overhaul at 500 hrs.  If you have an engine analizer you could narrow it down to which cylinder.  My guess is that it will get much worse the higher you climb for troubleshooting problems.

Kendal

 

[thinwing]

tyrefoote..what mitch alluded to is classic high altitude missfire.Bendix magnetos resist this a little better than slicks because of their physical size.Mike Busch uses photos of a slick magneto that suffered a total meltdown of the distributor tower...these photos came from Sacramento sky ranch Jon schanner.What happens is that at alitude.the air is a poorer insulator.If your sparkplugs are set with excessive gap,or if carbon dust has built up arround the distributor tower...the spark produced by the magneto takes the easiest route to ground...instead of thru the plug it arcs across the distributor to maybe another plug or even through a bad capacitor.Besides the misfire..this condition can cause preignition..an engine destroying event.Since suspected arcing is already going on...pull both mags for inspection/overhaul.Than after install and retiming regap all plugs and last check condition of ignition harnesses..good luck..kp couch

[John Pleisse]

I'd bet all the tea in China on the mags. My experience has been small hiccups being indicative of larger issues. How are your cold starts? Longer than usual? I too have the A3B6 (Trophy 212) and I go through Slick Mags every 400 hours. I never mess with points, couplings and rebuilding because you just end up paying full boat....now.......or later. I doubt the altitude has much to do with it.

[sleepingsquirrel]

I'm going out on a limb again (as squirrels are want to do) because I'm confused at the discussion that rarified air is a better conductor of a high voltage arc than denser air.  I'm thinking that it may appear so because the charge in the cylinder is also  becoming more rarified at higher altitudes ,creating higher resistance to the arc jumping at the plug electrode. This would explain any other weak insulation point being the a place to lose energy to ground by arcing. Turbo charging effectively keeps the cylinder charge at a denser more conductive environment for the arc to jump at the plug electrodes.

"Here's the problem: Air is a pretty good electrical insulator, but its insulating capability (dielectric constant) varies with pressure. The higher the pressure of the air, the better it insulates — the lower the pressure, the easier it is for electricity to pass through it (in what we call a spark)."

"Dielectric strength in kilovolts per inch (kV/in): "

Material*         Dielectric strength 

=========================================    

Vacuum --------------------- 20            

Air ------------------------ 20 to 75      

Porcelain ------------------ 40 to 200     

Paraffin Wax --------------- 200 to 300    

Transformer Oil ------------ 400           

Bakelite ------------------- 300 to 550    

Rubber --------------------- 450 to 700    

Shellac -------------------- 900           

Paper ---------------------- 1250          

Teflon --------------------- 1500          

Glass ---------------------- 2000 to 3000  

Mica ----------------------- 5000          

never mind! I've been forced to learn something.

[takair]

Hi Squirrel

I think it's not that rarified air is a better conductor, but that it is less of an insulator.  This allows arcing in places it's not supposed to.  For example, instead of jumping the spark at the plug (higher pressure-more insullation), it jumps from the harness to the airframe or engine.  Or it jumps internally in the mag.  Basically it never makes it to the spark plug.  Many turbo-charged planes use pressurised mags for this reason.  I think you alluded to the basic principal above.

[sleepingsquirrel]

Thanks, I got my insulator/conductor values reversed. I've been pondering this and just couldn't pull out of the spiral.

[jlunseth]

The mag is a good answer.  Another possibility is a malfunction in an altitude compensated fuel pump.  Had the misfire happen in the 231, but it was at 22,000.  Gets your attention.

[tyrefoote]

As always, thanks so much for the wealth of information, I love this site.  The mags were serviced at the last annual 10 months [80 hrs] ago by the A/p shop and not sent off to a mag specialist.  I will replace them and harness at annual in 2 months. I think they are Slick 4370- 4372 does that sound right ?

[thinwing]

tyrefoote...slick came out with a new style tower design...it now has ridges cast into the plastic that are designed to better insulate the high voltage spark and also prevent the carbon dust coming from the spinning carbon high voltage contact contaminating the rest of the distributor.Once an arc has occurred the extreme high temperature of the spark causes a pyrolysis of the plastic...this creates a future pathway...these are really hard to see by the naked eye.If interested google web site Sacramento skyranch.Click on knowledge base and than slick magnetos.I think this will answer the rest of your questions.Sleeping squirrel...turbo charging also supplies the bleed air to pressurize the magnetos in my Bravo...the reason is to prevent high alitude missfire...say at 25k ft...sinc kp couch

[jlunseth]

The mags in my plane had both been replaced about 50 hrs. before the missing, and then one of the mags. had been replaced again because it was faulty.  They were both good, just takes a little something wrong.

[thinwing]

jlunseth...so your problem was fuel issue...too rich??sinc kpcouch

[jlunseth]

I don't think too rich.  My plane is a turbo and we were about 125 ROP.  When we got to cruise at 22k the engine just skipped one beat every 15 minutes or so for about an hour.  Otherwise it ran fine.  Then i just touched one of the knobs, don't even remember which one or what I did with it, and the engine started spitting and wanted to quit.  I throttled back a little, put it in an immediate descent, and told ATC we needed FL170 (we were IFR).  Got the clearance right away, we went down to 17k and the engine was fine.  It was not richness or leaness, that was ok (have an engine monitor).  No CHT or EGT issues.  Just the missing.  This was in the middle of a fairly long flight (Glacier Park to Bismarck), and the remainder of the flight at 17 was uneventful. I flew several long trips after that, but stayed at or under FL200 and the problem did not repeat itself.  Willmar initially thought it might be altitude compensated fuel pump not right, but the final diagnosis was the mags, or one of them anyway.  I am picking it up from annual in the next couple of days, so I will out on the O2 mask, take it up to FL220 again in due time and see what happens.

The other thought I had was ice in the fuel.  It is pretty cold up there even in the middle of the summer, probably -10 C or colder.  But it straightened out so fast when we went down to 17k that could not have been the problem.

[thinwing]

jlunseth,....that would have gotten my attention,especially considering the terrain east of Kalispell mt.Lots of grizzlies in those mountains.Last annual we pulled both mags as a matter of course.They had been given 500 hr overhaul during previous 300 hrs.My a/I is a stickler for 100 hr checks on mags that go to high altitude.The downside of pressurized magnetos is that the turbo can pump in whatever debris like water that comes in past the airfilter.I notice Tcm uses a little inline filter to act as water trap...the pressure line on my lycoming doesnt have this feature...any way my mags looked fine with normal wear but the A/I talked me into installing a set of fine wire iridium sparkplugs set to minimum gap and changing to new style slick distributor block.sinc kp couch

[jlunseth]

We were out over the Dakotas somewhere when it happened, probably no grizzlies, but it did get my attention, definitely.  Alot of land and not many airports.  The b!@#$ of it was that the mags had both been replaced in May, then one had failed and been replaced again under warranty.  So they were both brand new mags with less than 50 hours.  This trip was in early August.  I have a guess what happened.  The replacement of both mags was by a good Mooney shop.  The one mag subsequently failing was a defective mag from the factory, it failed in under ten hours from installation.  The replacement of the defective mag. was by a non-Mooney shop where I was having some interior work done.  Found the mag. had failed when I flew the plane to the shop, and did not want to take off with only one mag., so had them do the replacement.  I am going to guess that is the mag that had the problem, and that something in the pressurization system came loose/wasn't properly installed to begin with by the non-Mooney shop.

The more I do this, the more reluctant I am to have anything at all done by a place I do not know very well. Not even an oil change or cleaning the windshield.  Willmar's prices are pretty good.  They make sure they find anything that is wrong with the aircraft.  Alot of pilots resent that thinking the A&P is looking for excuses to replace things.  I appreciate it very much, I am not replaceable, nor are the people who fly with me, and there are enough things to deal with in the air without a malfunction being one of them.