Checking mags in flight

[HartParr]

I have always been hesitant about checking mags in the air, but no more-or I learned about flying from that. I went on a 300 mi flight the other day and about 60 mi from destination my engine started to run a little rough, lost about 100 rpms. Picked an airport and ran through everything I could think of. Engine smoothed out-wanted to make it to an airport with mechanic on field so turned on course again, rough again. By this time I eliminated water in fuel and started blaming it on bad fuel. Smooth again then rough, then smooth. Landed and inquired for the mech and started making arrangements for draining fuel. He said I should try a mag check-back in, run up, great check 100 rpm drop each side. Called home mech, he said should have tried mag check when running rough and urged a check of plugs first then mags. Turned out two of the bottom plugs had cracked insulators. Four new plugs later I was on my way on an uneventful trip home. I hope everybody reading this doesn't have to learn things the hard way like me.            '66E

[triple8s]

I had a similar occurance last summer, was on a trip from KDKK to KDKX and after an hour or so at altitude the engine would "hiccup" one time and then run smooth for about 10 minutes and do it again. Well I fiddled with the mixture and it didnt do it for awhile but about 20 mins from home it did it once again, after I landed I thought I'd check it later. Next time I went to the airport I did preflight and run up, all was fine so I headed out thought I'd try to duplicate the hiccup. After half a day of goofing off, flying here and there I finally discovered it would only hiccup after being above 5000ft for about 30 minutes. I then did a mag check at altitude and discovered a very bad miss when running on the left mag. I also disovered the bad miss would disappear if I was flying below 3000ft. Moral of the story? Air is an insulator and at altitude the air is thinner(less dense) so the ignition system was breaking down as the engine temp increased and as the air became less dense. I had just had new spark plugs installed and evidently one of them was bad. I called AS&SS told them I recieved a bad plug and I recieved a new plug and gasket in two days no questions asked. Once the new plug was installed the problem was gone for good.

Mike

 

 

[Gone]

In flight mag check is part of my checks at top of climb.  I have never detected anything from it, but it feels comforting to know that this is the best stress test my mags will have during a flight.  If they pass it, I am good.

[HartParr]

I don't have an engine monitor and am interested in adding one to the panel. Would that (intermitten spark) have shown up with cooler EGTs?

[DaV8or]

Quote: HartParr

I don't have an engine monitor and am interested in adding one to the panel. Would that (intermitten spark) have shown up with cooler EGTs?

[Lionudakis]

HartParr, If you had an engine monitor, It could, depending on how intermittent, especially at high power. Do a mag check in flight, any egt that nearly disapears or behaves differently is a good indication of a problem. Your checking the integrity of the whole ignition system under a load. A monitor isn't in my future yet, but a smooth engine on each mag in flight lets me know everything's ok. 

[Shadrach]

Quote: HartParr

I don't have an engine monitor and am interested in adding one to the panel. Would that (intermitten spark) have shown up with cooler EGTs?

[KSMooniac]

A LOP mag check at cruise power in flight is the best test for an ignition system.  It will reveal weak plugs or mags (or mag wires) long before a pre-takeoff check at lower power on the ground.  

An engine monitor will allow you to return to the mechanic and say "my #3 spark plug on the left mag is faulty" instead of "my engine is running rough."  This could save you HOURS of diagnostic time that you pay for...  An engine monitor is a great bang-for-the-buck instrument, and required equipment for me.