My aircraft gave me a little excitement last week. It has the radiator hot water
permanently circulating around the inlet manifold ends of both carburettors,
which heats the downstream ends just beyond the throttle butterflies and also
hopefully the butterfly spindles to conduct residual heat into the butterfly
valves. Many aircraft with Rotax 4 stroke engines use this method to prevent
carburettor
ice build-up and I've never heard of one with this modification which
suffered from it.
There's been a quite well-known spate of sinking floats, possibly due to a batch
of poor plastic from which the floats are made reacting with ethanol (who
knows?)
in the unleaded fuel. I recently acquired 4 spare floats and I changed one
which was half sunken. I'm told that if only one float in a bowl is doing its
job properly, then a richer mixture will happen.
We all know that carburettor ice can happen in air temperatures up to around 25C
and the warmer the air, the more moisture it can hold. Rich fuel / air mixture
makes icing more likely, due to increased fuel evaporation cooling the metal
parts. Also in an carburetted aircraft, long descents with the throttle only
partly open greatly increase the possibility of carburettor ice. But this has
never been reported in a Europa to my knowledge because of the permanent
radiator
water circulation as described above.
On the day in question I had just changed worn out propeller blade tape and
balanced
the carbs to get rid of an annoying vibration in the cruise. I took off,
climbed ahead then turned downwind in the airfield circuit pattern (ACP) to
accelerate
to cruise speed and check for vibrations. All seemed well during an
extended downwind leg, so I closed the throttle to reduce below flap limiting
speed then did a descending curve around onto final approach, using a little
throttle
while making a long flat approach. It was an exceptionally warm humid
day!
I decided to make a "touch and go" landing before opening throttle wide to climb
back into the ACP. At about 300 ft while retracting the remaining half of flap,
the engine suddenly went "BRRRP" and shook violently for about 1 or 2 seconds
before becoming smooth again. I immediately considered either landing in a
nearby farm field or landing with a slight downwind on the runway if the engine
didn't die. But because there seemed to be plenty of smooth power restored,
I made a very short tight ACP and landed normally.
After discussion with another pilot who shares my 'plane and witnessed my swift
return from the ground, we agreed that carburettor icing was the most likely
cause, when a lump of ice probably broke free and went through the cylinders.
I removed the top engine cowl and looked for other possible causes and then
removed
the float bowls. And there it was! Another one of the floats had sunk and
thereby allowed a richer mixture to worsen the risk of ice.
So on that day all the possible circumstances combined to allow ice to form in
one carburettor, not least being that the throttle was closed and then slightly
open for about 2.5 minutes. I replaced the float and did several more ACPs
without
incident. However I can do without that kind of excitement, if the truth
be told.
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