I was reading the Europa Flyer on Ruedi Vogal's engine failure in HB-YJF.
Excellent airmanship making a safe forced landing.
By the sounds of the failure, I must bring up a recurring problem with the
912S with XS firewall forward and its exhaust problem.
I highly suspect an exhaust leak may be another culprit to consider, as wit
h red Aeroquip or Parker fire sleeve, fuel vaporization is rare in the XS c
owl with the boost pump on for takeoff. I have had two recent experiences
with engine roughness due to exhaust leaks and left to continue the engine
would have failed at full power. Exhaust leaks don't show up during initia
l ground runs. When the engine is cowled up, shortly after takeoff, if the
exhaust leak is on either aft cylinder or its downpipe, the exhaust will j
et out of the leak onto the float bowl and boil the fuel out of the carb in
a very short time. This causes roughness then failure normally not a pur
e just dead silent engine.
The post flight analysis usually is seen as small amounts of soot on the ca
rb body or bowl, upon disassembly, the carb may have a sticky lacquer type
feel due to the baked fuel residue. It is really frustrating as during any
engine roughness emergency we normally do the following:
Aux Boost Pump - On
Fuel selector - switch to unused full tank.
Reduce power-As required to obtain smooth operations.
Land as soon as possible.
This throttle reduction action refills the float bowl at reduced power (as
the jet of exhaust is now diminished) and there is no evidence of a boiling
incident. Drat.
I did a poor job of explaining in my Troubleshooting guide on how to look f
or and solve an exhaust leak. Look for the tan soot trail out of the junct
ion of the pipe to cylinder head joint or the ball socket at the muffler fo
r sure, but one should also make a heat shield to protect the carb from th
e possible future exhaust leaks. I am not a fan of the 914 drip tray on th
e older models we all suffered with, but take the idea from the new tray de
sign for the 914. A simple bent piece of aluminum attached to the foot wel
l will work also. I personally have my clients fab a tray, bolted to an Ad
el clamp on the engine frame and to the 5mm threaded hole in the cylinder h
ead and shaped to deflect any exhaust leak from the carb and wiring. Crude
but effective.
I apologize for not including this in my miscellaneous ramblings from one n
ight.
Best Regards,
Bud Yerly
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