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Temporary AOG last night


bradp

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We were bringing the plane down from TTA back to Wilmington last night with a stop off at RDU to pick up my wife and little ones (baby’s first flight).  92-F OAT and a short turn time of 25 min.  Got the family loaded up and the plane would not start hot no matter the start state.  Plane is a single impulse coupled A3B6 slick mag.  Last time I had an issue like this it was tracked down to a bad mag that needed to be rebuilt.    

- tried the typical hot start of touch nothing method

- tried again boosting mix off and touching nothing 

- tried as a questionable flooded start throttle open

- tried purposefully flooding 

The Tac air mechanics heard my sputtering and tried to help me out (BTW the TAC air folks at RDU we’re amazing in every respect to help me get my family settled and get us back home safely).  Despite some logical repetition and a couple of hits of throttle mix and boost, tried it again a half hour later to give her some time to cool down, but just ran the battery down. So we gave up and charged the battery overnight. Plane started up just fine cold the next morning. 

I’m  thinking 

1) it could be the $hitty slick mag secondary winding having breaks.  Or the points have worn on these mags (~300 hrs) and the e gap is out.  Typical symptom.  I’ll start with spark because it’s easy to evaluate as it’s low hanging fruit.  I didn’t perform a LOP mag check in the air today.

2) if not spark fuel.  I know as of a month or so ago my compressions are just fine.  Only weird thing was this morning I was getting a little wobble on my fuel pressure gauge at idle.  I’ve had this happen when the fuel pressure line has needed to be blown out before so I’m not totally convinced it’s an issue with the engine driven pump (800 hrs) or fuel servo but I’m willing to entertain both possibilities. 

Many troubleshooting ideas would be much appreciated.  I had this exact symptomatology about 4 years ago and it was a bad mag.   Ran for 4 years and she was never an easy hot start but Id never failed getting her started by the 3rd try.  Similar symptoms now.  It’s to the point where I’m considering putting slick start / dual impulse couplings / switch to bendix mags if I’m going to end up AOG.  Was going to participate in the operation air drop today but don’t have confidence I’d get Bessie started hot...

Thanks.  

 

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  • 2 weeks later...

Long story short- couldn't get the impulse coupled mag to jump an air gap of more than 1/8".  No quick bleed down on the fuel pressure after boosting, making me think the diaphragm is intact - next time I can get the plane started, will look to see that there's no vapor in that line.  Mag pulled and sent to Aaron Williams @ Select in Lancaster TX (DMax's preferred guy, I believe).  We did open the mag prior to send out... observed the following on 285 hr since IRAN Slick mag with who knows what parts:

The condenser wire was pinched in between the housing screw and the metal frame (incorrect assembly from the last Mag service I had done at a shop in Florida that sent me a mag with a bad coil the first round, and this was the replacement mag - cannot recommend PM for details).  The lead's insulation was mostly chafed and I couldn't tell if I could see conductor or not through the dirt.  I don't think this is the failure mode per-se... but E gap was significantly out like 10-degrees.   Had some wear on the high voltage coil tab, and the distributor electrodes (which may be in spec for age).  Aaron will go through the 500 hr inspection items and let me know what was in spec or not.

If the insulation to the condenser was part of this weak spark problem, you could hypothesize that intermittent short to ground for the condenser would fail to charge it as the points open, resulting in increased point pitting wear and possibly an out-of-E-gap scenario.   A weird possibility but I'm not convinced.  I didn't take apart the points assembly to look at this as by this point the decision was made that the mag needed to be serviced. 

Anyway after messing with the condenser lead and adjusting E-gap I got the thing to fire across a half-inch air gap to a crap ground source - which is it's best performance to date.  

As a reminder, if you call lycoming technical support with a starting issue, they will give you the minimum air gap from spark plug lead (plug off) to a good ground to be able to fire the engine.  If I recall correctly it's supposed to jump an air gap of 3/8" to be able to fire the cylinder.  It's a great poor man's test of whether the ignition system is intact or not.  If messing around with propellers remember to make sure fuel is OFF and at least top plugs OUT.  

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4 minutes ago, bradp said:

Long story short- couldn't get the impulse coupled mag to jump an air gap of more than 1/8".  No quick bleed down on the fuel pressure after boosting, making me think the diaphragm is intact - next time I can get the plane started, will look to see that there's no vapor in that line.  Mag pulled and sent to Aaron Williams @ Select in Lancaster TX (DMax's preferred guy, I believe).  We did open the mag prior to send out... observed the following on 285 hr since IRAN Slick mag with who knows what parts:

The condenser wire was pinched in between the housing screw and the metal frame (incorrect assembly from the last Mag service I had done at a shop in Florida that sent me a mag with a bad coil the first round, and this was the replacement mag - cannot recommend PM for details).  The lead's insulation was mostly chafed and I couldn't tell if I could see conductor or not through the dirt.  I don't think this is the failure mode per-se... but E gap was significantly out like 10-degrees.   Had some wear on the high voltage coil tab, and the distributor electrodes (which may be in spec for age).  Aaron will go through the 500 hr inspection items and let me know what was in spec or not.

If the insulation to the condenser was part of this weak spark problem, you could hypothesize that intermittent short to ground for the condenser would fail to charge it as the points open, resulting in increased point pitting wear and possibly an out-of-E-gap scenario.   A weird possibility but I'm not convinced.  I didn't take apart the points assembly to look at this as by this point the decision was made that the mag needed to be serviced. 

Anyway after messing with the condenser lead and adjusting E-gap I got the thing to fire across a half-inch air gap to a crap ground source - which is it's best performance to date.  

As a reminder, if you call lycoming technical support with a starting issue, they will give you the minimum air gap from spark plug lead (plug off) to a good ground to be able to fire the engine.  If I recall correctly it's supposed to jump an air gap of 3/8" to be able to fire the cylinder.  It's a great poor man's test of whether the ignition system is intact or not.  If messing around with propellers remember to make sure fuel is OFF and at least top plugs OUT.  

Yeah, last year when I had REALLY hard starting, my mechanic said he couldn't get it to jump more than 1/4" on bench testing, so I can't imagine how you got it to start at all with that mag.  The E-gap was also off, but I can't remember how many degrees he said

IIRC, E-gap can and does change over time solely from cam wear, although I have to imagine the evidence of poor quality work is not reassuring.

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