JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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 Posted: Wed Jul 01, 2020 12:56 am Post subject: Carburettor icing with heated carbs |
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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 a 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 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 circuit pattern. At about 300 ft in the climb out, 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 circuit pattern 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 flew several more circuit patterns without incident. However I can do without that kind of excitement, if the truth be told.
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