Carb heat above ~ 10,000 ft

[Narwhal]

Quick question: could using carb heat at higher altitudes for an NA recip cause the mixture to be too rich even at a very lean mixture control setting?

The reason I ask is that I was climbing at about 10,500 the other day with carb heat on (ambient 20 F, some clouds nearby, C182B with an O470) and the engine stumbled pretty drastically. Mixture was leaned for about 1500 EGT and CHT was about 380. I hadn’t touched any engine control in a few minutes when it happened, was running full throttle, so about 19” and 2450. When I turned the carb heat off it immediately got better.

Thanks.

 

[A1Topgun]

Yes. Carb heat affects mixture. Makes it more rich.

 

[Salty]

Warm air is less dense than colder air. No matter your mixture setting, carb heat is going to reduce the performance of your engine.

 

[Sluggo63]

It's been a while since I've flown a carbureted engine (first world problems, I know). But I remember hearing not to use carb heat in the climb with full power since the carb is being heated by the engine and you'll only rob power since it's not likely to form ice with all that heat. That might have been only for Continentals (or Lycomings... I really can't remember). Someone with more knowledge than I will be along shortly to correct me, I'm sure.

 

[Jeff767]

You need to give us a bit more info on where you had the mixture. 1500 on the EGT does not mean much. Where you were relative to peak EGT is more important. Were you leaning in the climb? Were you leaning to best power? Do you have a fuel flow indication? Did you suspect carb ice?

 

[dtuuri]

...the engine stumbled pretty drastically.

Sudden changes don't sound like a rich mixture to me. Maybe swallowed a slug of H2O? Or the heat box flapper "suddenly" came loose, blocking the intake air, then stowed tight again when you went back to the normal position? A rich mixture might have fouled a plug "suddenly", but that wouldn't immediately clear, IMO, but I guess it could.

 

[AA5Bman]

I’m gonna say no - you can always lean out farther to override the richening effect of carb heat. How do I know? Well, ask yourself this question: with the carb heat on at 10,000, would the engine stop if I pulled the mixture to idle cut-off? Of course. So I don’t think the issue is a too-rich mixture that couldn’t be fixed by further leaning. That said, I’m not a mechanic, but I suspect it’s not a too-rich mixture at all, and if it were my plane, I’d be suspicious of a too lean mixture (see below) or ingesting a carbon deposit or a bit of water.

Why carb heat at 10,000’ with an ambient temperature of 20 degrees? Provided you weren’t in the clouds (I.e. assuming dew point was less than 20 degrees), you’re in a zone of almost zero carb ice risk. I have a similar engine (high-compression IO-470S) on my C205, and if the ambient temp is 20 degrees and the cowl flaps are open, there is no way I’m at 1500 EGT and 380 CHT in climb - I’d be way cooler all around and, I assume, way richer. It actually sounds too lean to me which is another potential cause of rough running.

 

[Dbarbee]

Curious what your reasoning was for using carb heat in the climb, especially if you weren’t actually in the clouds.

It’s my understanding that carb ice is most likely to form in the venturi when the throttle plate is partially closed and the pressure drop, and resulting temperature drop, is the lowest.

At full throttle the pressure drop should be minimal and engine heat at its highest making carb ice unlikely. I suppose maybe if you’re in IMC under ideal conditions you could get some ice forming in the venturi even at full throttle. But then again, I don’t have anti-ice or deice equipment so I wouldn’t be flying in IMC with temps near or below freezing anyway.

I’m not challenging your decision to use carb heat in a climb, just looking to learn something I may not have considered.

 

[kgruber]

Just check it. Don't leave carb heat on.

 

[WDD]

It’s my understanding that carb ice is most likely to form in the venturi when the throttle plate is partially closed and the pressure drop, and resulting temperature drop, is the lowest

This is my understanding as well. Carb ice occurs because you ingest moist air (above freezing) and then cool it below freezing by closing the throttle plate, where the drop in pressure drops the temp (PV=T). This is how you can get carb ice on a humid 70 degree day in summer.

At ambient 20 degrees, at that altitude, and outside of clouds there isn't going to be moisture in the air for you to form carb ice. If you were flying through dark clouds with super cooled water droplets, then you have a lot of problems besides carb ice LOL!

I would guess that you stumbled the engine because you starved it for air.

 

[AA5Bman]

I would expect a mixture induced roughness to be more gradual.

I agree with your comment here, and it's part of why I don't think it's a gradually too-rich mixture.

On the other hand, if you go a tiny bit too lean on the LOP side, you can get an immediate roughness. But I would say generally not when going slightly richer and richer (due to climbing) on the rich side of peak. I have some O-470 experience, but not a ton, but A) if your peak is actually 1700 degrees EGT, you're obviously not LOP, and B) wow that strikes me as suuuuuuper hot! Could just be probe placement, but is that right?

Due to the rapid onset, I'm going with water (in the fuel or due to a tiny bit of carb ice) or ingesting a fouled plug carbon deposit kind of thing. Or an ignition problem like a mag going bad - I don't know much about them, but that's an often-cited cause of unexplained roughness.

Also, acknowledging that the O-470 is a known carb icemaker, you're still in a super low risk zone. I personally wouldn't run carb heat in that scenario unless I suspected I actually had ice.

 

[Matthew Rogers]

Why carb heat at 10,000’ with an ambient temperature of 20 degrees? Provided you weren’t in the clouds (I.e. assuming dew point was less than 20 degrees), you’re in a zone of almost zero carb ice risk.

Well, one day I climbed my C150 to 13,500’ in the winter on a cold, clear day. Temp at the surface was 20 degrees and interestingly, temp at 12,000’ was also 20F. I was at full power the entire time and didn’t consider carb ice a risk, but at high altitudes, the little engine was only producing 60% power max at full throttle. I got pronounced carb ice at the top of the climb and descended down to an airport for a coffee and to double check if anything was wrong, as I was just in the air to kill some time. Had to tell the delta controller that I was 4 miles south at 13,000’ inbound for landing.