improper use of carb heat in IMC?

[cointyro]

A squared illustrates my confusion.

Should I use carb heat if I suspect ice at risk of being accused by the NTSB as making a "pilot error"?

"It should not be applied if ice is already suspected as it would melt and enter the engine." I thought this was exactly what the carb heat was supposed to do... melt the ice.

Better to make an attempt at clearing the air/fuel intake and risk possible engine failure than to watch as your RPMs and power is choked off by ice, no?

So use carb heat when suspect known icing conditions; when in landing / approach, AND when suspected ice has already accumulated, right? Thus, when is it ever pilot error to use carb heat... avbug mentioned using heat might raise your susceptibility to icing. What temp/dewpoint envelopes make it more likely to encounter ice when using carb heat than when not using carb heat?

Thanks all!

 

[tarp]

Everybody is probably dancing on the head of the pin because of the weird science of carb icing.

I've had carb icing on a beautiful sunny 90F day, 45F days (lots) and yet been absolutely free of carb ice in clouds at 33F. You can get it anywhere! Now you can be sensible. 45F wet and dreary and there's a good chance that you'll see some carb icing. Same goes with summer days where you have to cut a hole in the air to breathe due to humidity - there's a lot of moisture for the carb to grab. Change that engine from full power to idle (Cessna's) and you're asking for trouble.

My old sage of an instructor who looked for trouble in every way he could to teach us "real" lessons actually got me to carb ice one day. His advice was: "use it all or don't use it". This was in direct contradiction to the manual which said: "Use it full to defeat the carb icing, then use the minimum of partial heat to maintain best power without an increase in icing". And then finally, I was in Tennessee at a pretty small airport. I had just landed, but there was some commotion around a wise old flyer. A student just asked about carb ice. His comment was: "Pull that thang on and git outta there" (as his hands indicated a full fledged dive for the ground).

So, my intellect has assembled all those thoughts together. At the first sign of carb ice (to me I get the creepies when I've made 2-3 power increases to hold altitude and airspeed) - turn that carb heat on and I mean pull it full "ON" (though slowly to avoid Avbug's and my own cable in your lap syndrome - lucky me, the engine kept running). The next move is not to "find the carb heat value that keeps the engine running at max RPM". NO, the next move is to change something about WHERE you are flying. The most obvious change is altitude and since you are fighting a loss of power, lower may be the only alternative. Another thought is that the carb icing started about 5-10 minutes ago - how about a 180? Another item is to be aware of the ground underneath you - mountain upslope or crossing a body of water may be the reason for the atmospheric change and hence the carb icing. Finally, you're goal is to turn that carb heat full "off". The "finding a setting to keep the engine running at max RPM" is only a means to an end - i.e keeping power long enough to get out of trouble.

Now I will never forget my 90F encounter with carb ice. Luckily I had some altitude over the ground. But I do remember I pulled that control all the way on and the engine just coughed, spluttered and bucked. I knew it was running because my heart had stood still, so there was no noise except for that engine trying to find air in all that water. I lost about 700ft of altitude before the engine roared back to a reasonable sound although now running about 125 RPM slower due to the carb heat. I descended and decided to go find an airport and get a coca-cola. Me and that C-172M just needed a break for awhile.

The first NTSB report is woefully lacking in info. My only take is that since probably hundreds and thousands of pilots have successfully used carb heat to get out of carb heat situations, then the manufacturers and the NTSB believe they have a real winner. However, as stated before, you are at the mercy of mother nature and despite all your best efforts, she may have cooked up something that you the pilot and that airplane just can't cope with. 99% of the time, you're going to win. On the 1% you don't win, then make a decent off-airport landing.

 

[TXCAP4228]

I only read what was posted. Somebody may have said this but it bears mentioning again. Improper use of carb heat could refer to not having used it until it was too late.

Better to keep the ice from forming by leaving carb heat on while in prolonged flight in IMC or in rain during cruise than leave it off and choke the engine with water when it melts.

Anybody disagree?

 

[cointyro]

Thanks Tarp for your great post, it illustrates more about carb icing. Actually, thanks to all those who have contributed, including avbug. I've still a few clarifying questions though.

Prophylactic use of carb heat in known carb icing conditions makes logical sense to me... but I'm a newbie.

Then, if you have carb heat full on, you have no recourse if you finally do start picking up ice, do you?

Plus I was somewhat thrown by Avbug's comment that carb heat can actually speed up carb ice accumuation. Well crud, if this is true, when do we use it prophylactically and when not?

I can just envision this: you're IMC on IFR approach at 2,000 feet AGL, 1,000 feet to go before breakout. As a normal landing procedure you pull full carb heat. Unbeknowst to you, ice has been accumulating. So the heat floods your fuel/airstream with water. You lose the engine, are still IMC, and are well short of the runway environs and the IFR glidepath ... what do you do?

Or am I misunderstanding the nature of carb ice accumulation... will there be a definite drop in RPMs as ice accumulates, even if it does so very gradually?

 

[tarp]

Cointyro:

You just love this, don't you. Allright I'm going to show my age and my butt all in one post.

My age because I'm going to admit that I owned a 1961 Plymouth Valiant with the old slant six engine. Now Chysler made one heck of a little car that drove like a tank. You had enough room to throw a picnic in the engine compartment and the only air pollution equipment was a little wire mesh in the oil breather cap - it was all simple technology.

The one thing that slant six refused to do was run on what I would call Irish mist mornings with the temperature right at 42-43 degrees. One of the first of these mornings, the car just died right there on the road. I being the all-american male, ran around the front, popped the hood and zipped off the air cleaner. Looking in the carb throat I saw this light frost layer on the throttle plate and lining the walls. It was really neat. I learned that a shot of starter ether (yeah we had dangerous things like that back then) would melt the frost right off and I could get another start out out of the engine. If I could get the temp to just raise 1 degree (inside the carb), the frost disappeared.

Turning to airplanes and learning of carb icing, I ran into an old nemesis. The good news, airplanes had this carb heat thing. The bad news, airplanes used updraft carbs which other than this carb heat device would not receive any other engine heat.

However, carb ice really only develops in a very small range of temperatures (inside the carb). My guess (and this is only a farmer's guess) is that carb ice probably forms somewhere between 20F and 34F or about the same temps where you would expect to see frost forming on fields/grass in the morning. Again I'm talking about temps inside the carb - outside temps could be in a huge range.

If the temps get really cold, you end up with ice crytals that won't stick to the carb bady unless YOU put heat in there and raise the temp back into this "frost" range.

As to your "prophylactic" or defensive use of heat - I only use it in two occasions: 1.) As part of my landing checklist when in the pattern and 2.) when that old Valiant Irish mist and 42-43 degrees start creeping on my temp gauge. And the second doesn't mean I'm in clouds, it's just that dense, dewy air that feels cold and wet and sends a tiny shiver up my back. I'm sorry I can't explain it better, but I know when I get that feeling, I'm just gonna nudge that carb heat on for a little defense while I get my carbureted machine out of that temp range.

Other than that, I don't use carb heat defensively. It comes on as a response. One thing I always like in Cessna's is a good throttle lock. If I didn't move the throttle, then there shouldn't be pitch (sound) and RPM changes. If there is a change, then it's time for me to start testing - if I put the carb heat on and there is more than a 100RPM drop, I probably started icing. If the RPM's pick back up to 100 below my previous setting - it was definitely carb icing.

My reaction is now to get out. If aloft, I want lower or drier air. In your scenario, if low to the ground and I didn't let the icing get to the point where the engine is running rough, my speedy application of carb heat should make the difference and allow me to make the field. If I wasn't sensitive to the airplane, my lack of action could have made a no-win situation.

Flying down your approach, I should have run the Landing Checklist - Carb heat On. If as you say, I didn't know that I had loaded up on ice in the carb, there's going to be quite an RPM drop - but I should have noticed. A 90 kt, flaps 10 configuration in an old C-172 is going to require 1900RPM to fly right down the pipe. If I've got the throttle against the firewall trying to hold 1900 RPM, there's something wrong.

There's a 100% solution for carb ice and that's to fly injected or turbine equipment. If not, then you have to live with my 99% rule and every so often someone is going to put themselves in the NTSB reports.

 

[bigD]

Prophylactic use of carb heat in known carb icing conditions makes logical sense to me... but I'm a newbie.

I dunno about you guys, but I don't think I EVER thought I'd hear someone use prophylactic and carb ice in the same sentence!

Sorry cointyro - I know the manner in which you're using the word is perfectly fine...I'm just being goofy.

 

[avbug]

My point was that the NTSB cited the use of carb heat because there was nothing else they could pick on. If you read the reports, very often it comes down to pilot error. It may be that the pilot did everything right, but it's still pilot error.

Don't be confused, that's just the way it is. A deer runs onto the runway. Pilot error. Failure to avoid the deer. Contributing factor, a deer on the runway. Contributing factor, two knots of crosswind. Contributing factor, pilot was wearing plaid.

Wing separates from airplane in flight. Three fatalities. Proximal cause, pilot error. Failure to maintain control when right wing separated from aircraft, misapplication of rudder when left wing separated from aircraft, failure to prevent airplane from striking petroleum refinery. Contributing factor, loose wings. Contributing factor, pilot wasn't "getting any" for two weeks prior to this event. Conclusions, get some before flying airplanes with loose wings, watch your rudder(s) when the wings go.

Engine quits due to carburetor ice. Pilot error: decision to fly on a day when carb ice might possibly form. Contributing factor, pilots' use of carburetor heat. Contributing factor, pilot's use of airplane in sky. Contributing factor, pilot's failure to remain on ground. Contributing factor, pilot's failure to wear plaid.

It's either pilot error, pilot born under the wrong sign, or wearing plaid.

Oh, and the big one: failure to file a flight plan. That bastard.

 

[cointyro]

Tarp, thanks again. A great post. I really appreciate it.

You stated "You just love this, don't you. Allright I'm going to show my age and my butt all in one post."

I hope you didn't interpret my posts as being harrassing or intentionally querulous.

I know that you and avbug know where it's at, with so many hours aloft. I was taking advantage of your momentary attention to the post by airing my questions. Always better to ask than to never ask and wonder, I figure.

Thanks for your time and the illustrative stories.

Regards,
Dan / cointyro

 

[cointyro]

Avbug... I dig your sense of humor.

With no experience with NTSB reports I didn't realize they are prone to leave it to pilot error when nothing else arises.

I interpreted the original post to mean that the pilot had done something wrong because he used carb heat... which was contradictory to what I was reading in my training materials.

Now I have a much better idea, thanks all.

 

[Iceman21]

Avbug's sense of humor is amusing and it really demonstrates the NTSB's findings in almost every report I have read.