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HeliTorque :: View topic - Carb Icing
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HeliTorque Forum Index » Flight Dynamics

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Heliwhore
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PostPosted: Sun Feb 15, 2009 12:41 pm    Post subject: Carb Icing Reply with quote

Has any one ever had carb icing in the air (at least that they knew about). Most of us had it on the ground during start-up I suspect, but in the air what were the first signs that you had it.

I have my own theories here, but would you increase or reduce power whilst heating it out?
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PostPosted: Sun Feb 15, 2009 2:18 pm    Post subject: Reply with quote

Good topic whore!

There has rarely been a day this winter where during the carb heat check at start-up I haven't noticed signs of carb icing, however I can't say I have experienced it in the air. Would it be noticeable if the governor is there subtly opening the throttle to compensate for the drop in RPM? Probably not.

I have had the carb heat pretty well full hot on certain days and only got about 12 max indicated on the gauge though.

If I did experience rough running in flight though I wouldn't reduce power - the throttle valve is apparently never more than 60 open even at full power so if there's a chance of ice being present I don't think I'd want to risk the chance of that valve closing and contacting the ice surrounding it.

What are your theories?

Sarah
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PostPosted: Sun Feb 15, 2009 3:00 pm    Post subject: Reply with quote

Thats my thoughts as well, also by closing the valve further we would be increasing the venturi effect and therefore lowering the temp further.

However if it so far advanced as to us noticing, I'd be surprised if the butterfly wasn't already frozen in position by that stage. In which case keep the throttle wide open so you have a chance of the engine keeping running when you drag a load of ice/water through it.

Next question, would we get a rise in MAP, or a drop, during icing?
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PostPosted: Sun Feb 15, 2009 3:40 pm    Post subject: Reply with quote

I had the onset of it on climb out of le touquet in france. ground weather was quite warm 15C and not much moisture. le touquet is set in a valley and a little up the estuary. as we climbed out towards the sea i noticed there was a sea mist forming a little above the cliffs, i pulled the carb heat out a little more and noticed the carb gauge had dropped into the yellow and on pulling full carb heat and levelling out early there was a drop in manifold pressure probably due to all the warm wet air going through the engine. Check your dials people - glad i did Very Happy
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PostPosted: Sun Feb 15, 2009 4:26 pm    Post subject: Reply with quote

We should get a rise in MAP as the governor opens up to compensate for the less air flow, hence why we could all miss the signs, but thats only the beginning eh? What happens next?

Animal, I suspect your drop in MAP was the gov winding back down once ice had melted. Well caught.
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PostPosted: Sun Feb 15, 2009 4:33 pm    Post subject: Reply with quote

You may not notice a change in MAP. As the icing increases the governor will open the butterfly further until it is at full throttle to maintain the MAP required for maintaining RRPM. Then you will see varying MAP, by that point it is the least of your worries because the engine is going to stop soon. This is providing the butterfly is not frozen. My 2 kroner.

I know a fellow FI who had carb icing in the hover once over grass covered in heavy dew. Rough running engine and a very rapid loss of power. Aircraft settled on to the ground and then the engine stopped. When he went to start it again the throttle was fully open as the governor had masked the icing.

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PostPosted: Sun Feb 15, 2009 5:06 pm    Post subject: Reply with quote

Thats the point I was getting at. After the compensation in MAP from the gov, we're probably likely to see a fall in MAP as the ice slowly cuts off the airflow. Agree with you HH that dials are the least of your worries at this point, and time is prob best spent looking for a field.
I can only assume this, and hope not to find out myself.
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Last edited by Heliwhore on Sun Mar 15, 2009 8:43 am; edited 1 time in total
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PostPosted: Sun Feb 15, 2009 5:28 pm    Post subject: Reply with quote

Although carb ice can occur during any phase of flight, it is particularly dangerous when you are using reduced power such as in a descent. You many not notice it during the descent until you try to add power.

Indications of carb ice are a decrease in engine rpm , or manifold pressure, the carb air temp gauge indicating a temp outside the safe operating range and engine roughness.

These symptoms can occur in order, or combination thereof. Throttle should remain constant to observe the result of corrective measures. If the throttle is changed, the pilot can mask the ice melting and exacerbate the problem.

I have had it in flight. I noticed the drop in MP first since the governor compensated for the loss of rpm. Since I fly by the "numbers', any change is quickly picked up in my panel scan. Full carb heat must be applied as using partial heat allows the ice to melt and re-freeze further down in the carburetor, and chokes the engine completely. Carb heat will then be useless.

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PostPosted: Sun Feb 15, 2009 6:58 pm    Post subject: Reply with quote

The best thing is to avoid getting into this situation in the first place. Try to keep the carb heat gauge at 10-15 during normal flight, and don't just pull it full hot the moment you're going to descend, think ahead and if you can give it 20-30 seconds on full hot before lowering the collective - the heat needs a chance to do it's thing. Imagine how closed that throttle valve is going to be at 15"MAP if it's only about 60 open at full throttle. Best to play it safe.

I have flown with examiners who have insisted you are unlikely to get carb icing in the hover and that you shouldn't use carb heat in the hover or below 500'. As Haggis' colleague had the misfortune of finding out, this is not true. I am not comfortable using no carb heat in the hover at this time of year, and I certainly don't want to find out the answer to this debate the hard way.

Next topic, the bain of my life... carb heat assist. Discuss...

Just as you lift into the hover the darned thing goes back in. You can't take your hand off the lever to pull it out and if you land and pull it out it's just going to do the same thing again.

As another discussion point, the carb assist has a "lock" to stop you using carb heat all together. My understanding of this is that in Siberia the OAT is so low that introducing carb heat would actually bring the carb temperature into the range where the carburettor would be more susceptible to carb icing and having the carb assist going in and out would make flight more hazardous. Do Robinson give an indication of how low the OAT needs to be for this to be an issue? I thought it was something like -8C or below but I can't remember. I don't think it would ever be an issue in this country but it would be interesting to know.

Sarah
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PostPosted: Sun Feb 15, 2009 7:08 pm    Post subject: Reply with quote

Totally with you on carb ht asst, what a nightmare.

It was only put in to show that Robinson was doing something about the whole Carb Ice problem, the fact is that we now are stuck with a system that neither works or doesn't work, if you know what I mean.

I've never heard the Siberia reasoning, thats an interesting take. Always figured it was for very hot dry places.
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PostPosted: Sun Feb 15, 2009 7:12 pm    Post subject: Reply with quote

An excellent article from CASA on carb icing with an icing probability chart.

http://www.casa.gov.au/fsa/2004/dec/32-33.pdf

and links to the Carb Icing related events on Helicopter Safety

http://www.helicoptersafety.org/genericaccident.asp?Keyword=Carb%20Icing

I'll dig some more out and link to them from that page later.
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PostPosted: Sun Feb 15, 2009 7:15 pm    Post subject: Reply with quote

Nice links Veeany.
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PostPosted: Sun Feb 15, 2009 7:16 pm    Post subject: Reply with quote

My source on the Siberia thing was Dick Sanford, who incidentally agrees that it's a bad idea and has tried to reason with Robinson no-end to give it the boot.

Anyway I spend most of my time flying R22 Beta's, so it generally doesn't bother me. However I do fly the R44 with carb assist and that IS a nightmare.

Remember, the temp in the carb can be as much as 30C colder in the carburettor than the OAT. First you've got the whole vaporisation process (think of getting out of a swimming pool and feeling cold as the water tries to evaporate of your skin), then you've got the drop in pressure causing a further temperature drop (opposite effect of a bicycle pump getting hot when you pump up the pressure). Remember the pressure in the carb is already much lower than ambient (what does the MAP gauges read when the engine is stopped? Now look at what it reads when the engine is running? Remember 1013mb = 29.92 inHg). Lowering the lever makes this worse because it lowers the manifold pressure even further.

Now when is it ever hotter than 30C in the UK? We wish!! Every day you fly in the UK could be a carb ice day. Take it seriously.

Sarah
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PostPosted: Sun Feb 15, 2009 7:31 pm    Post subject: Reply with quote

WhirlyGirl wrote:
I have flown with examiners who have insisted you are unlikely to get carb icing in the hover and that you shouldn't use carb heat in the hover or below 500'. As Haggis' colleague had the misfortune of finding out, this is not true. Sarah


Oh it surely is not! In the Schweizer 300 for example, during reduced power operation or autorotation (practice naturally )below 18" MAP, full carb heat is to be used regardless of the CAT gauge reading. ( mainly because the CAT is only reliable ABOVE 18" MAP.)

In hover or cruise flight, above 18" MAP, when CAT is inside caution range (-15c to +5c), carb heat is applied as necessary to keep it outside the range.

One place you don't want the engine to quit is in the hover. Actually "no place" is more preferrable. However, more copters suffer damage, and pilots, in a rollover or bounce landing because of the limited time to reach, closeness to the ground, and yawing moment.
Use the carb heat as prescribed in the POH.

Each aircraft has it's own peculiarities when it comes to carb heat. Know them, use them, and be safe.

AB
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Last edited by afterburner on Sun Feb 15, 2009 7:35 pm; edited 1 time in total
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PostPosted: Sun Feb 15, 2009 7:32 pm    Post subject: Reply with quote

WG, I am with you on the cold OAT and no heat required. In really cold temps any moisture in clear air freezes already.

As for not using carb heat below 500ft... that's just crazy, especially from an examiner of all people.

I used to teach at 300ft on the approach reset the carb heat control to the required position for the hover, so you finish in the hover with approx. 10-15C. That was a recommendation made to me from Derek Jones.

Dick is a man of incredible knowledge. Did you do the safety course with him?

HH
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Last edited by haggishunter on Sun Feb 15, 2009 7:58 pm; edited 1 time in total
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