Carb Ice

[Guest]

Anyone have any good stories, advice, or comments about carb. icing? Is the preferred method to apply the carb heat for a couple of minutes then push it off and practice an auto or do you apply it before the auto and push it off on the power recovery... and what about in normal operations? It's been awhile since I've had to do that.

[jehh]

For known pratice autos, we turn carb heat on before entering the auto.  Whoever is not flying takes it off 150 to 200 feet above the ground.

 

For surprise pratice autos, of course you can't turn the carb heat on or it isn't a surprise.  The CFI cutting the power first makes sure the student properly enters the auto, then applies carb heat while the student controls the aircraft.

 

Jason

[jehh]

BTW, I've never personally experienced carb ice, so I honestly don't know what it feels like.  Hopefully I never find out.  

[jimbo2181]

We put on full carb heat before entering the auto.  We dont even screw with it after that mostly because the carb heat assist pulls it down when you add power anyways.  My normal procedures are to apply on all approaches and a lot of times during cruise because the r22 with the weight we have it loaded with always seems to want to hover right around 18 inches manifold pressure.  Also for all takeoffs except for extremely craptastic conditions we turn the carb heat off.  Other than that we just keep it out of the yellow and follow the POH.

 

Cooper

 

Also never had carb ice problems.  Will avoid them at all costs.  I know on my first solo I just kept the carb heat on all the time so I wouldnt forget

[Jesse James]

In 1971 I was taking a check ride for a new job with Omniflight in Janesville, Wisconsin.(long before there EMS days) Don Bradburn told me to do an auto in the G-2 we were flying. I rolled the throttle to idle and the engine quit because of carb ice. Don took the controls and did a very nice full-on.

I have been told that if you have some carb ice and you put the carb heat on full, it will expand the ice in the carb and cut the air off altogether. I have did a lot of cold weather flying in 47's and never did run with full carb heat. Jesse

[Austria]

dear jesse!

please could I get some more informations on what you mentioned about applying carb heat while in the initial stage of carb ice!

 

I would appreciate it greatly..

 

(what about an engine outage at 10,000ft because carb ice, apply full carb heat and would it help without an air/fuel mixture flow to get rid of it and restart..?)

 

thanks, Andy

[Robbie Driver]

I always make sure we have full carb heat before and during an auto.  I will usually knock it down after we level the ship.  For normal operations we follow the POH, I have students do a downwind check (gauges green, no warning lights, and pull full carb heat) before they begin an approach and are still at over 18", just so they dont forget.  And just keep it out of the yellow above 18".  The only exception to the rule is when we are doing hover work and the carb heat would cause an excessive amount of power being pulled, buts thats only when there is a CFI in there anyway.

 

During a throttle roll off, I will usually do it on downwind after the student has pulled carb heat.  Or if there is a large temp/dew point spread then I will just give them a roll off and pull carb heat after they have sucessfully entered.  

 

I have personally never had any carb heat problems, had a carb temp guage go bad, thats it though.

 

Sorry to go off topic but Jimbo I have a question for you.  How do your Beta II's perform at Quantum.  I know you guys get some pretty good DA's.  Just interested at what you normally pull in a hover, how much fuel do you take on per flight?  We operate Beta's and even with a 2000 DA you have to keep your hovering to minimum until you burn off some fuel.  We are thinking of selling one of the  Betas and buying a BetaII.  

 

Thanks,

RD

[CJ Eliassen]

When you pull carb heat, you raise the density of the air into the engine and you are allowed to pull more power.  A way to check this is put it in a hover on a nice calm day.  Pull carb heat but maintain altitude.  That increase in MP does not increase your horsepower, so you can legally pull it.  The DA charts do not represent maximum MP, they represent maximum power.  If you change either your pressure altitude, or temp, you have to adjust your limit MP.

[jimbo2181]

CJ- thats all fine and good until you run into full throttle because you have the carb heat on and are at high altitude.

 

Robbie- the beta II's do great, especially for the heat.  It is weird at the difference that some of the helis pull.  All we have is beta IIs but one of them pulls 23" (24" 5 min max) and another at the same time and same weight only pulls 21".  We are thinking the MP gauge may be broke on the high one.  Anyway that kind of gives you your explanation right there.  Normally pulling 21-22" in a hover with a 24" 5 min max.

 

Worst problem we run into hovering is that when its 115 degrees out after about 20 minutes of hovering the MR temp light will come on.  We had quite a few days of 5-6000' density altitudes and they handled it like a champ.  I think we are selling one of ours if you are interested.

[CJ Eliassen]

Better to run out of throttle because you have carb heat on then run out of throttle because you have ice.  Then you lose more power pulling the carb heat to melt the ice.

[jimbo2181]

We use carb heat until the last 50 feet or so of an approach, once you are there you normally above 18 so you push the carb heat down and continue on the approach.

[C of G]

CJ,

Just wondering if there is a way to quantify how much of an increase in mp you are legally alowed to pull with the addition of carb heat, and if you might provide a reference for it. Thanks in advance.

[CJ Eliassen]

You can do it the way I mentioned above, or while in a hover, you can take carb heat temp reading with carb heat on and subtract from that the carb heat off temp.  This will give you the change in temp.  Add this to OAT to get the density of the intake air.  

 

The references are the DA charts displayed in the limitations section of the POH.

[RDRickster]

With respect to Jesse's comments, at certain LOW air temperatures - applying carb heat is contraindicated in the Robbie (for example).  As long as you keep the needle BELOW the yellow arc, you should NOT apply carb heat.

 

With respect to C.J.'s comments, I seem to remember the same thing from the Safety Course.  They recommend that you assume a hover and apply carb heat to the proper temperature before take-off (this assumes some fairly cold weather).  You can base some of your calculations based on THAT temperature (air intake - albeit from carb heat).  However, I think that running carb heat for EXTENDED periods of time can cause engine issues such as excess build up of carbon (so I've heard)?

 

With respect to auto's, I learned the same way above.  Carb heat applied as part of checks prior to auto, the CFI/SIC closes carb heat 50-100 AGL.  However, for surprise auto's (throttle chops), the CFI usually applied carb heat before the procedure.  Even though the student may see it coming, I think that is a safer course of action.  Having said that, there have been many times when my CFI slid his hand down to pull carb heat without me seeing him at all.  At those times, I was usually looking for traffic, looking for forced landing areas, looking out to check wind direction from trees/smoke/fields, etc... it only takes about 1 second to pull carb heat.  Sometimes, I'd see him and knew it was coming; however, about 75% of the time I was caught by surprise (as I should).

[C of G]

CJ,

 

I am aware of the DA reference, but am unfamiliar with the addition of carb heat as increasing your power limit as the placard refers to OAT, not CAT.  I would figure by your reasoning that the power limit would also decrease as the CAT temp decreases  through increased velocity with an increase in throttle/butterfly aperture.  On the safety course I attended it was mentioned that carb heat did increase the limit man press., but at the time I didn't per sue the matter further, and I have been wondering about it since.  Thanks for your input. :down:

[CJ Eliassen]

You cannot use CAT as the temperature for determining air density in the carburator.  You have to use ambiant.  However, when CAT rises due to carb heat, you are in effect raising the ambiant air temp going into the carb.  Of course you don't know what that temperature is, but you can determine is by finding how much your CAT gage raises, and adding that to ambiant.  The CAT measures the air after fuel has absorbed a great deal of heat, and the reduction in pressure has created a temp drop from the ambiant temp.  

 

My DA charts do not reference OAT.  They just simply have the temp in degrees celcius.  

 

You are not increasing you power limit.  You lose power when carb heat is applied, so you pull more MP to regain that power up to 131 HP takeoff on the Betas and 124 HP MCP for all models.

[C of G]

Thanks for your reply.  

I mis-spoke when I mentioned an increase in power limit, as I am aware it is a MP increase.  I don't mean to belabor the issue, but.......my DA charts also only ref.  ambient temp., but all performance charts do seem to refer to OAT, most notably those (Beta Models) on p. 2-10 and 2-11 which are obviously in the limitations section, not the performance.  Now it is all too possible that I don't know my way around the R22 POH all that well anymore, so maybe I'm missing something.  What I'm wondering, though, is if your reasoning also works in reverse to decrease the Limit MP as the CAT temp drops below OAT.  I haven't noted what that difference is, but the next time I find my self in the Robbie, I'll try.  As for the increase in CAT temp., I must admit I've never noted the actual number increase, but if I am to believe part 27.1305, it must be able to increase that at least 60 deg F/15c, so can I then pro-rate my Limit MP that much??  If so, I think I'd find the occasional vertical take off that much easier.  Thanks in advance..............again. ??? P.S.  Enjoy the Labor Day weekend if you are fortunate enough to have it off!!!

[CJ Eliassen]

You don't use the CAT as an OAT gage.  The limit MP settings are based on intake air temp and not manifold temperature.  If, for example, your CAT in a hover shows -15C with the OAT at 0C.  When you pull carb heat, you see a rise of the CAT to 15C.  This is a 30C rise.  Add 30C (The differece) to 0C (the OAT) and you get 30C. Use 30C in your DA chart, not 0C.  A much easier way to figure this is to put it in a hover, look at the MP gage and lets say is 22 inches and your limit is 23 inches.  When you pull carb heat it jumps up to 23.2 inches.  Add the difference of 1.2 inches to your limit so your new MP limit is 24.4 inches.  This is not 100% accurate, but thats the same way they figure MCP limit.  You subtract 1 inch from the limit, yet 1 inch of MP is not a direct correlation of 7 HP.  It it were, the engine should produce 203 HP at 29 inches and we know this is not true.  As you get higher on the MP gage, each inch produces slightly more power then the preceding inch.  But its still close enough for government work.

[Winnie]

I scared the heck out of my student once in Napa, Calif. (APC). We had just practiced slope landings over a ditch, next to the runway, (wet and fairly cold day) and I was going to chop him over the grass, but I decided to wait, and then over the runway, I rolled the throttle off, the student did a proper landing, and then said: "why wont the rotor spin up?? "

To which I humbly replied: "because the engine stopped!! ??? "

 

Had to get on to the tower to explain why the rotor was coasting down, and that we had to restart, but it was no big deal. It got me to respect the environment more, and to learn a LOT more about carb ice!!

 

It was in 1999 or early 2000 in a S300CB.

[highspeedflyboy]

Sometimes, during photo flights in the R22 (Beta), I have found it necssary to closely monitor the CAT (Carb Air Temp) guage and periodically apply Carb Heat during summer days with high humidity.  A rough running engine or variation vibration would be a good hint that you might be needing Carb Ht.  Under certain (light weight) conditions, the MP may be less than 18", necessitating application of full Carb. Ht. (since guage is placarded about reliability below 18" MP).

 

Email me with any thoughts:  primaries@hotmail.com

[Bart]

Hey Winnie,

 

I used to work in Napa (American Canyon actually) at a warehouse right on Hwy. 29...I saw a Schweitzer 300 fly over just about everyday!  Must have been you, huh?

 

I started my fixed wing training at Napa and actually worked at the Bridgeford Flight Shop for about 4 months part time.  Now I am training in an R-22 in Los Banos, CA...have about 13 hours so far...great fun!

 

Just thought I'd let you know

[Bart]

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