Hey Paul...  Ready for one of my long-winded (like to hear myself talk) answers ?

Generally; in the big give-n-take of things.. the only disadvantage to leaving carb heat on while cruising, is economy. Since the thoughtful pilot always cruises at something between 65% and 75% power; the power lost from card heat can be made up for, with just a tad more throttle.

Now.. in theory, the heated, less dense air would require an adjustment to other than normal mixture settings.. but not really. Whatever method you use (EGT or even just your "ear"), it would work out OK.

I suppose (depending on the engine configuration), the heated air "could" result in higher than normal CHT, but I gotta believe that would be minimal. And there would probably be a plug-fouling issue; if you didn't allow for mixture differences.. Worst-case scenario would be the heated air AND the leaner mixture AND higher power-setting, resulting in an engine running on the hot side.

Now of course.. most of my R/W experience is in Warriors and Skyhawks.. I'd imagine that there are some airplanes out there where extended carb-heat presents a problem.. I just don't know about them. Off the top of my head, I'm thinking maybe, above and beyound the heat, there might be considerable air-flow reduction.. but again; you could prob counter that with more throttle, again, paying for it in fuel use.

In summary... It would be a deviation from accepted practice.. so I'd advise against it. Consider it situational... And that opens the can of worms..  If you're flying when conditions would have you pondering leaving carb heat on.. you probably should be on the ground  

Just to add, while the wheels are on the ground, the carb heat should be off as it is unfiltered air that is being drawn into the engine with carb heat on.  That would be a particular problem in dusty or dirty conditions, or if you didn't do a FOD walk on the runway before takeoff or landing.

Also, the only 152 I've flown (because I don't fit in one either), is an Aerobat, and the carb heat is used in the same way in that as it is in the 172.

Another 2 cents for you, Foz...

All of the above is true, but in RL it would also not be a good idea to leave the carb heat on in a 150/152 (which I know you fly often in the sim) during takeoff! That's when the difference in power output may be critical.  Even more so, of course, with an even smaller engine.
If there's ice in the carb as you begin your takeoff roll, in a 150, you will know right away (low tach reading or no increase in rpm), and you should be able to abort safely. To me, that mindset is more valuable for safety than planning to use carb heat on the takeoff roll and climbout. A 150 or 152 needs all the hp it can generate for takeoff.

If conditions are so bad as to require carb heat just to prevent icing while preparing to take off, you might want to cancel your flight.

That being said, sometimes carb heat can be used while warming up in very cold weather (remembering about the air filter)... not so much to prevent ice as to just help warm up the engine.

A word about conditions that my require carb heat: some days it's obvious, some it's not so obvious. Only time I ever picked up carb ice (in a 172), it was very cold out, but it was a pretty dry day- it took me by surprise. What was most surprising was that I was at a cruise power setting...  it's "not supposed to happen" when you have the tach well within the green arc. I guess in that instance, the cold made up for the relatively low humidity level. But even in warm weather, with no clouds visible, it can get you.

So... what does this actually mean? Takeoff aside, can you leave it on? I never have. Pitot heat, maybe, but not carb heat. My reasoning is that the most dangerous time to get ice is during the approach, when you are getting lower. It takes a few tense moments to clear carb ice... including the brief period when the power drops off even more when you turn on the heat. In cruise, it's alarming but no big deal (I was able to hold my altitude that one time, and it cleared up very quickly), but at pattern altitudes and airspeeds it could be a very bad deal. So the normal method of making carb heat part of the pre-landing checklist ("abeam the numbers" or whatever) is sensible. You might also want to turn it on if you are throttling way back to descend from cruise, as a precaution (it would suck to restore power as you level off from your descent, only to find there is no power).

Also, unfiltered air may not be so good even while aloft... or the carb heat door mechanism may get stuck... etc... I only use it pre-emptively when I think I'll really need it -which is whenever I throttle back to the bottom of the green arc, or beyond. and of course, if I do pick up ice in cruise, I'll leave it on!!

...excellent news. Chums...

I'm afraid I'm paranoid about Carburettor Icing, due to my experiences in Ground vehicles....

..so I always apply the same precautions in my Carburettor fitted Sim Aero Engines...(like my little Cessna 150/152).... ..!

Run the engine up to max revs on the runway with the brakes on, to test for any problems, apply Carb heat and watch/listen for rev drops, (around 100/150 RPM if all is working fine).

Then make sure the Carb heat is OFF for a full power take-off.

Apply Carb heat prior to reducing or closing the throttle opening, (maximum Carb Venturi Suction, chilling the Carb body), especilly when on the approach to land.....Then shut it off prior to landing, in case a full power aborted landing/go-around/ touch-and-go is required!

All the time listening for gradual changes in the engine revs, and watching the gauges, which may be heralding a build up of Ice in the Venturi Tube!

There you go Chums.....Its just a shame the the MS Flight Sim doesn't replicate similar conditions for the Trainee Sim Pilot!...... ...!
 
Paul...G-BPLF...and my Trusty little Aerobat...with a cosy, warm Carburettor... ...!

Lots of unexplained GA Piston Aircraft crashes due to Carb Icing, which has melted, and disappeared at the time of examination!

On my rare, occasional flights aloft in the Club's Cessna 150/150, the accompanying CFI always has a grin on his face when he sees my hands constantly playing tunes on the Trim Wheel.....and fiddling with the Carb Heat Knob!.. ... ...!

I automatically do the same thing in the Sim Cessna as well... ...!

Paul...G-BPLF... ...!

DaveSims wrote on Apr 27^th, 2009 at 2:05pm:

I did the same thing, except when I was reaching for the throttle while I was in the pattern.  I went through my normal downwind checklist, which finishes with throttling back, but I put my hand on the mixture instead of the throttle, ready to pull it out, even with my thumb pushing in the button thing on the end.  I stopped myself and stared at what I was doing for a second before I actually realized it.

I did that once, too, abeam the numbers.

I am also a bit addicted to the trim wheel.

I ve perfected it to the point where i can notice / move the trim wheel 1/10ths of an inch lol. the slightest 10 fpm, slight nose up or down adn that trim wheel moves .1 to.2 inches to compensate lol. keeps me busy on long cross countries, along with checkpoint spotting and airspace scans.

Also while on topic, I use the carb heat for power off stalls and for engine out sims.

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We need more wannabee Commercial Passenger Jet Sim Pilots LEARNING on the Trainers....FIRST!... ... ...!