Carb Icing on Rotax 503?

[tviljoen]

I had an incident on a flight today which I think might be carb icing. The temperature was about 8 deg C and the cloud levels were low, at times 300-400 ft. On approaching Microland and turning finals to land for the Saturday breakfast, the engine spluttered and lost nearly all power. I had been using full power before that and had just reduced power for the descent and landing. I tried different power settings but could not get the engine to run smoothly for a period of 15-20 seconds, and was looking for a suitable landing spot in case I could not make the airfield. Then the engine recovered and ran smoothly again and I could land. After the breakfast the engine ran normally again and I was able to make the flight back to Petit. I had also noticed some minutes before the incident, that the EGT on one cylinder had run a little high (700degC or so, whereas the other had kept at ~600 deg).
On investigation at Petit, it was found that the air filters were dirty and needed cleaning. The spark plugs were near the 25 hour replacement interval and were changed. The engine seems to run better after this.
I heard at the airfield that Rotax two-strokes would not ice-up under SA conditions, and the problem must be something else. However, only the ice theory would account for the problem occurring and then clearing up.
I looked around on the web, and found this explanation for icing, if I understand it right:

A BBC weather prediction said that the RH today was 34% (http://www.bbc.co.uk/weather/5day.shtml?world=0082)
A site which works out the dew point from RH and temperature says that with 34% RH and 8 degC air temperature, the dew point is - 6deg. (http://www.decatur.de/javascript/dew/index.html)
From a carb icing prediction graph (http://ibis.experimentals.de/images/car ... assl14.gif), shown below, with a -6deg C dewpoint and 8deg air temp, the situation appears to fall in the yellow area - "Light Icing - cruise or descent power". This appears to predict what happened.

Comments?

[Wargames]

Interesting story,

2 questions:

1. When last did you change your fuel filter??

2. When decending on finals, and on idle, what is your idle speed. Mine is approx 2200 static and on finals about 3200.

Regards,

[Ian]

I had carb icing on a high descent (around 25% power) in the Berg on a 582 a few years ago, similar conditions with low temp and moisture in the air, no cloud tho. Eventually I opened and closed the throttle and the ice went through (a real long 16 seconds ). Researched and found pictures as well, so yep, you do get carb icing on the 2 strokes. What's nice is you can open and close the throttle and clear it, I also always varied my settings when flying at low power in such conditions after that ....
cheers ian

[tviljoen]

Wargames wrote:
Interesting story,

2 questions:

1. When last did you change your fuel filter??

2. When decending on finals, and on idle, what is your idle speed. Mine is approx 2200 static and on finals about 3200.

Regards,

1. Fuel filter is approx 30 hours old.
2. My speed is also about 2200 static. I'm not sure what the revs are on finals but could easily be ~ 3200. Why?

Rgds

[Wargames]

Hi TV,

The questions I asked was probably the wrong ones, but here goes:

1. I am assuming that you have the BING 54 carbs on your 503 as most of do, and that in itself is carb that is not that prone to carb icing, because it works with a piston, and not a valve as most other carbs work with. The issue is actually that the carb does not provide "handles" for the ice to cling to, to create a very bad case of carb icing. Most ice that will be formed, will by design be sucked into the engine, and heated up to melt down, and thus the carb piston itself, does not freeze in a fixed position. You will mostly see that Icing will happen at low speeds, when the piston is lowered, but if you open the throttle, the cable will force the piston open again, and hence shut down your carb icing problem.

2. With regards to your reference to your temperatures: 700 is very high with a 100 diff, and that made me think that maybe there was a blockage in your carbs jets, that could have leaned it out to create such high temps. Because your carbs is essentially the same, if it was icing, your temps should have stayed the same, because of the same effects of icing in both.

Just my thoughts on this. Might be a bit of both.

I had a case once where 1 jet was blocked, and my engine had no power and stuttered along.

Regards,

[Morph]

Carb icing does happen on Bing 54 carbs. The conditions must be right. Don't always assume that since the ambient temps are low that icing will be present. Icing requires humidity and the dewpoint to be very close to your current flying temp. A cold winter's day in Gauteng is typically dry and a summers day is very humid. I got taught HHH, Hot, High and humid, a nice hot wet day, you take off and as you climb the temps drop. Eventually you get to dew point where the water in the air becomes visible. Being humid there is a lot more water. This water gathers in the intakes and as further cooling occurs, due to continued climb, the venturi effect of carbs, and the presence of cool fuel you get icing occuring.

The reason they talk about Bing 54 being less susceptable is that the throttle cable motion pulls the piston open. So if you get a bit of icing opening the throttle will usually dislodge it. Unfortunately the ice must go somewhere, and through the engine is the only place, so you will hear the motor run rough and cough etc. Other planes including the 912 engines use a spring to open the carb and a cable to close it. If you had icing here you cannot force the carb open. In these cases it is best to have carb heaters.

The highest probability for carb icing is as Ian said, during a long decent. Here it is best to decend in say 5 min intervals, opening the throttle every now and again. if you get rough running from the icing going through the carbs, then increase the frequency of increasing throttle.