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Carb Heat Box Failure


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Posted

Hello everyone, 

Just a quick heads up that while flying with a student yesterday in his PA-17 powered by a C-85 I had a partial engine failure in the circuit. We were working on runway changes so at time of the failure a bit further out on downwind as usual and unable to return to the pavement, safety concerns with houses, power lines, highways etc. 

Anyhow, as I showed him once more pre-landing power reduction by pulling carb heat prior to throttle reduction the engine nearly died. Also the carb heat cable came out slightly further as it used to. I immediately pushed the nose down a bit, pushed the carb heat back in - no change 500 - 700 rpm rough at best. Reduce speed to best glide, look out below, check fuel, 3/4 tank, fuel valve on, bla bla bla ..while slowly sinking towards unfriendly terrain below. At this point I started cycling the engine and after several cycles it seemed to stabilize at about 1900 - 2000 rpm, ..time to look for a landing site, open terrain below and possibly nurse it back to the airport. No luck, can't maintain altitude so out over a small valley we go to get additional heights in the sinking game. It is also fairly open and has a few frozen ponds (barely frozen) in it that can be used as a last resort scenario. But no power lines, roads, houses, and if needed a bit of room to select the best bad place there is, since there really is no good ones. 

Knowing of an old airstrip in the area and realizing that going back to CZML was a no go we decided to use the old field. It has some fairly tall trees at the approach and a new fence (hard to see near dusk) about 1/3 down, ..never mind its all we got. Over the trees we go, lean it over into a nice slip towards the fence, level off, flare, touch down, roll out, full brakes, watch the ditch, big rock, bull wallows and and we come to halt, ..best of all right side up and not a scratch on the plane. 

After shut down and inspection I put my fingers up the hot air bleed hole of the box and immediately could feel the butterfly valve laying on the bottom of the box. Upon removing the filter it was rather obvious that some really poor workmanship was to blame and a secondary, possible 3rd repair and brazing failed - see photos. It partially occluded the intake and reduced the amount of air needed for the engine to run properly.

So if anyone is using an old carb heat box, they are now 60 years old, things fail, don't be afraid to check it good on your next annual. Look at the welds!! Anytime you pull that filter, give that Butterfly Valve a good hands on wiggle, not just a visual. Ideally spend a couple hundred bucks for a new one and get a peace of mind with it.  And also, anyone telling you that certified engines are better than your Rotax, Subaru or Yamaha, because they never fail, ..I beg to differ. Certified Aircraft engines have problems too and I could go on for some time listing issues here. But the point here is, that a look is good, a hands on wiggle better and new box ideal. So for all those on this forum or maybe your friends that are bombing around with an old C-65, 75, 85, 90 you name it, ..maybe check that old airbox and the welds. Was a first for us, but there is lots of them out there. Stay safe! 

PA17-Carbheatbox.jpg

PA17-brkn butterfly.jpg

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Posted

Glad you got her down safe!  All too often its the little things that get you.  broken exhaust baffles got me in the pacer during a hard climb out with no reserve airspeed.. my outcome was not as nice as yours.

 

Thanks for the heads up, much appreciated!

:BC:

 

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Posted

 But the point here is, that a look is good, a hands on wiggle better and new box ideal.

Just waiting for all the single guys to chime in on this comment... ^_^

Valuable advice. Especially when grabbing, yanking and shaking our 'dainty' little E-AB's might seem counter-intuitive at first or lead to concern of breaking something. But then that's the idea, better on the preflight than inflight! Thanks for sharing.

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