Carb icing

[rotorrodent]

I know it's a subtle difference, but what reason did you learn for not using carb heat below -10C (or, -4C in the RHC POH)?

 

 

what is carb heat?

 

rotorrodent (just kidding)

[nightsta1ker]

There doesn't have to be visible moisture in the air for there to be enough to get carb ice. It can happen in a desert environment too. I have even heard rumors that Robinson is considering manufacturing R22s with the carb heat ALWAYS in the on position and not controllable by the pilot (take it with a grain of salt, I don't know how true that is). The primary cause of engine failures in R22s is from carb ice, and guess when it happens? On short final, or when executing a practice autorotation. Low power settings means a tighter Venturi at the butterfly valve which means there will be both a bigger drop in temperature, and that it will take less ice build up to choke the engine to death. There is ALWAYS some amount of moisture in the air. It WILL supercool when it goes through the manifold and will solidify as ice on the first solid thing it touches. Adding carb heat at low power settings is the only way to keep the ice buildup, however small, from closing off that gap.

 

Saying that that there are ANY conditions when it is not necessary to add full carb heat at low power settings is foolhardy and you mighet get an unpleasant surprise one fine warm clear day.

The obvious exception is below -4c when any moisture in the air is already frozen and will just be sucked through rather than freezing as it hits the valve.

 

[Flybull]

Nice thread resurection, nightsta1ker!! Only 3 years old!!

 

Just kidding, excellent post.

[eagle5]

The primary cause of engine failures in R22s is from carb ice, and guess when it happens? On short final, or when executing a practice autorotation.

 

I remember my cfi telling me to push the carb heat in on short final! I guess that was a common practice/problem, because when I was at the Robinson course Tim said that if you have it on LEAVE IT ON! If you're worried about needing more power to hover, he also said that when using carb heat, use the CAT gage for determinimg you MAP for the trip.

 

If the carb heat was on all the time, could that fowel out your spark plugs?

[nightsta1ker]

My instructors would always put the carb heat back down once in ground effect. Managing carb heat like this is definitely harder single pilot, which I suspect is a big part of the problem. I have seen and heard enough that I would rather err on the side of caution. You can always get into spots you might not be able to get out of. If you land somewhere and don't have sufficient power to get back out with carb heat applied, set down and push it in. Then try again. Or better yet, plan for it. Before you go fly, note your hover power with full carb heat. Common sense applies here. There are definitely conditions where there it is less likely to happen. But in a low power situation, there IS going to be ice. So carb heat on approach is (as it should be) a requirement. and pushing it back in on SHORT final is usually ok, ice is not going to build up instantly an you will very shortly be using a high power setting.

 

Another thing I would like to add is that it was mentioned earlier in the thread about accidents due to insufficient power on approach. I think this is a bull excrement excuse. You can tell on short final if you are Not going to have sufficient power. If it doesn't feel right, GO AROUND!

Edited April 5, 2012 by nightsta1ker

[nightsta1ker]

Edit: Posted twice on accident!

Edited April 5, 2012 by nightsta1ker

[Dave-R22]

Robinson Safety Notice SN-25 says, 

"When in doubt, assume conditions are conducive to carburetor ice and apply carb heat as required".

https://robinsonheli.com/wp-content/uploads/2015/12/rhc_sn25.pdf