CTLS accident

[Jim Meade]

The report smacks of presumptions based on conventional technology. Maybe it is right, but it is a little too pat to suit me. I'd like to see more discussion of the likelihood that the Rotax will develop carb ice in the situations cited. The NTSB wants to close the book. They don't always care that the ending suits the plot line.

[Roger Lee]

It said they thought they lost fuel flow. I don't see carb ice doing this to two very hot carbs at the exact same time. It wouldn't loose flow. I believe something else was at the root.

[Mike]

The carb heat kit we use is ON all the time .. may not be practical in the heat that some of you fly at but you can have it switchable ...

 

http://www.skydrive.co.uk/proddetail.asp?prod=CH-912-3

[Dan Kent]

The carb heat kit we use is ON all the time .. may not be practical in the heat that some of you fly at but you can have it switchable ...

 

http://www.skydrive....p?prod=CH-912-3

This is standard equipment on my Sting.

[Jim Meade]

The carb heat kit we use is ON all the time .. may not be practical in the heat that some of you fly at but you can have it switchable ...

 

http://www.skydrive.co.uk/proddetail.asp?prod=CH-912-3

 

Do you find carb ice a problem with the 912ULS?

What does this do to your power? I'd think you'd have to reduce the fuel flow to maintain the optimal fuel-air mixture?

[Tip]

How does carb ice influence 0 psi fuel pressure?

[FlyingMonkey]

I have heard that the 912 is not normally susceptible to carb ice, and some of the posts here back that up. Can anybody tell me the technical reason why that is? Is it because there are two carbs which would both have to ice, the design of the Bing carbs, the higher RPM we operate at, or something else? When I was training in Florida, I was told that I'd never need to worry about turning on the carb heat in that environment, and operating in similarly warm Georgia I suspect that's generally the case here as well. But I don't know that would be true here in the winter, where temps do get into the 30s, 20s, and sometimes lower.

 

And yes, I know you can get carb ice at much higher temperatures than that.

[Mike]

I had my CTSW 912s engine stop due to carb ice when closing throttle to idle over airfield (luckily )descending . i fitted carb heat kit and never had a problem since in 3 years now i haven't noticed any power drop probably because the kit heats the carb body and not the air going through it

[FastEddieB]

When I was training in Florida, I was told that I'd never need to worry about turning on the carb heat in that environment, and operating in similarly warm Georgia I suspect that's generally the case here as well. But I don't know that would be true here in the winter, where temps do get into the 30s, 20s, and sometimes lower.

 

I recall having been taught, and teaching, that in humid air the temperature in the carb can drop by as much as 60° f, meaning it could be a factor on a 90° day. On really cold days it's less of a factor, since the high relative humidity required is less likely - though still possible.

 

My one known carb ice encounter was in a Cub out of Homestead, FL on a warm day. It was insidious because I had to keep advancing the throttle to maintain power and it felt for all the world like my throttle was creeping. Eventually I had full power but the rpm was still way down. When I put on the carb heat the engine pretty much quit as it ingested a slug of water, but did come back before I had to put her in a field. On landing back at Homestead, the carburetor still had frost on the outside.

 

That said, I do not routinely use carb heat in my ROTAX- powered Sky Arrow - the checklist does not call for it and it has not seemed necessary. Maybe the proximity of the carbs to the exhaust has something to do with it.

 

As a reminder, when you do decide to try carb heat as a solution to a low-power situation, the SOP is to always use FULL carb heat - partial can actually make things worse and you will never have more heat available as power decreases due to the ice.

 

[Jim Meade]

As a reminder, when you do decide to try carb heat as a solution to a low-power situation, the SOP is to always use FULL carb heat - partial can actually make things worse and you will never have more heat available as power decreases due to the ice.

Hmmm. I agree with you 100% - for a conventional engine on first indication. But, I've seen injunctions to fly a C182 with partial carb heat. Is it the same for a Rotax? The POH says use carb heat as required. Does that mean use it when there is icing? When icing is possible? And does it mean use it full on or just so much as seems to keep the engine operating?

 

It looks to me like we need to do some more reearch. I take Mike's warning seriously as I, too, had pretty much discounted carb heat in the Rotax.

[FlyingMonkey]

Eddie, it is true that humidy must be present for icing. I have read that the primary determinant of carb icing is the temperature/dewpoint spread. I think on humid Georgia days that spread is usually pretty wide, but I have seen it tighten up here on occasion. Usually it's in moist, cool conditions like a wet fall day. Again, I'm no expert and am just parroting others' wisdom on this...I have very little experience with carb heat and carb ice. I'd love to get some good guidance on this, I'll happily add it to my checklist if it improves safety.

[Ed Cesnalis]

Andy, humidity plays a big part, you need humidity to close the dew point spread. The venturi effect closes the spread, so its mostly humidity that's at issue. Some designs are highly susceptible to icing, what makes the rotax or bing less susceptible is a good question.

[Jim Meade]

CT, I don't understand what you wrote. To get ice, you have to have the temp-dew point spread come together. The Bernoulli principle in the carb venturi involves a drop in pressure which leads directly to a drop in temperature, so the temp comes down to the dew point and moisture precipitates out in the form of ice.

So, does the Bing have the same or similar venturi effect that Marvel Schebler or Zenith carbs do?

Pipers were more tightly cowled than Cessna's and had the carb in a warmer spot, so they were typically not so likely to get carb ice.

[FastEddieB]

Along the lines of unintended consequences...

 

...imagine that a manufacturer completes all their testing and determines carb heat is not required in normal operations.

 

And now imagine that radiant heat from the exhaust is a major factor in keeping the carbs warm.

 

And now imagine I wrap my exhaust (which I did), dropping the amount of heat radiated into the cowling, thought to be an overall good, with zero perceived downside.

 

But now I suddenly find my plane much more susceptible to carb ice.

 

Lots of speculation above. Just mentioning it because sometimes the most seemingly benign modification can have unintended drawbacks that were not foreseen.

 

I am aware that FD provided an LOA for exhaust wrapping, so apparently they do not see it as an issue.

[Ed Cesnalis]

Jim,

 

Andy seems pretty focused on temp not humidity. The temperature is adjusted in the carb but the humidity has to be present. I live in a very arrid area, not much carb ice here but when I flew a Skyhawk I always used it so I wouldn't have to adjust when I went to the coast.

[FastEddieB]

 

So, does the Bing have the same or similar venturi effect that Marvel Schebler or Zenith carbs do?

 

They must, right?

 

 

Pipers were more tightly cowled than Cessna's and had the carb in a warmer spot, so they were typically not so likely to get carb ice.

 

I remember that on the Lycomings involved the carb was mounted to the oil pan, which kept it warm. So, neither my Traveler nor my Tiger called for carb heat routinely as most Cessnas of the time did.

 

As seen here, but on a Cardinal:

 

[FlyingMonkey]

 

I remember that on the Lycomings involved the carb was mounted to the oil pan, which kept it warm. So, neither my Traveler nor my Tiger called for carb heat routinely as most Cessnas of the time did.

 

 

It's interesting the solutions engineers come up with for problems. It seems though, that attaching the carb to anything hot might hurt engine performance, as the air charge is always warmed...like having partial carb heat on all the time.

[Ed Cesnalis]

I think it is the ratio of the amount of air the motor pumps ( displacement * compression ) and the number and size of the venturis.

 

Rotax is small but hi compression with 2 carbs. I bet 1 carb would be 4 times as likely to ice? just guessing.

[Dan Kent]

Here's the FAA's direction on carb icing. I discovered this when I was training here in Houston not in my Sting and asked my instructor why we weren't using carb heat. He said in the 1700 hours they had put on the Rotax carb icing had never been a problem, but he started training me on turning in on on downwind.

CE-09-35 carb icing.pdf

[Jim]

I think it is the ratio of the amount of air the motor pumps ( displacement * compression ) and the number and size of the venturis.

 

Rotax is small but hi compression with 2 carbs. I bet 1 carb would be 4 times as likely to ice? just guessing.

 

I don't know. What I've read about carbs says that regardless of size, they all have to have a restriction to establish a specific air velocity. It would seem to me that a little carb would need an even littler restriction and might be more prone to ice.

[FlyingMonkey]

I don't know. What I've read about carbs says that regardless of size, they all have to have a restriction to establish a specific air velocity. It would seem to me that a little carb would need an even littler restriction and might be more prone to ice.

 

Or the higher velocity air could move through it fast enough that ice does not have time to form. I could see that going either way.

[sandpiper]

Here is another CTLS accident. In Iran!

 

http://airfactsjournal.com/2013/07/i-cant-believe-i-did-that-9/?utm_source=airfacts&utm_medium=email

[Ed Cesnalis]

variable venturi type carbs are less prone to icing.

[Ed Cesnalis]

It said they thought they lost fuel flow. I don't see carb ice doing this to two very hot carbs at the exact same time. It wouldn't loose flow. I believe something else was at the root.

 

Ice develops then is often consumed and develops again. The big picture is an incremental increase in the amount of ice and number of carbs involved.

 

Who said the carbs were 'very hot' and that the froze at the exact same time?

[Roger Lee]

After you land next time pull the cowl off and grab a carb. Hold it as long as you can. (maybe a second or two)

 

The pilot stated that he lost fuel flow and icing wouldn't actually cause that. Carb ice in a variable throat carb is less likely to happen than a standard fixed throat carb, but it can happen. It happens more often in the UK verses places like our Southwest US states because of the right conditions existing more often. Cool temps around 70F +/- and water saturated air. Many carbs don't get very hot because of their location and cowling setup, but a CT's carbs get quite hot.

 

You can't have a fuel flow loss if only one carb is affected. They would both need to be totally blocked to loose total fuel flow.