Carb heat/icing

Gooney · Post by **Gooney** » Tue Mar 09, 2004 3:21 am

HI Guys

Something that has worried me a little is that of the Carb Heat.
I recently flew from sunny Cape Town to a rain Plettenberg bay in South Africa, which was my aircrafts inaugural flight to her nesting grounds after varriving from the USA 3 weeks prior. Well after 2 hours flying I caught up with the cold front, and moderate rainfalls with temperature around 59F. Well as you can imagine you start to imagine things after a few hours of solo flying with worsening weather, yes the engine certainly sounded different.
Well I definitely noticed a drop of about 50 RPM (yes I had the throttle friction nut so tight, I just about needed a wrench to loosen it up) Now the question I have is how long ( in the cruise) do you need to have the carb heat selected to hot for in order to ensure that the ice melt. I have heard so guys say 15 minutes others 1 minute; can anyone give me references or good practical input?

Another hassle I had was that the carb heat keeps on sliding out, how can one ensure it stays in when not required or do I have a defective cable?

Another question is that of drop in RPM on your power check, when selecting the carb heat to hot, I had a drop of about 50RPM at the most, is this the norm? I have heard guys say that one should use carb heat below 1500RPM (airborne), as below this figure if conditions are right, you will most probably pick up carb icing? Also if you are in a long descent, they say apply a little power then select carb heat to hot, as the exhaust would have cooled too much at idle in order to melt the ice, is this true?

Look forward to some good info.

Tue Mar 09, 2004 12:42 pm

I use carb ice every time I make a decent with a Continental engine. Now if I''m in a slow decent from say 3000 to 2000ft with just slightly reduced power I probably would not put on the heat.

My experiance is that you should see more than 50 rpm with the 170 carb heat setup. I'd compare your ducting with the original setup to see if its been modified in some way.

I've not had a problem with the carb heat going off on its own. I can't remember if there is a rachet at the carb heat side or if the cable just relies on friction to maintain position.

rudymantel · Post by **rudymantel** » Tue Mar 09, 2004 1:12 pm

Carb ice can form in cruise flight, but it's not real common. If I see a drop in rpm I'd apply full carb heat and lean the mixture (as carb heat richens it). After maybe 5 minutes I'd richen the mixture and push in the carb heat. If it was ice, it should have melted and you should see normal power/rpm. I'd watch it carefully to see if rpm drops again. If so, I'd again apply full heat and lean and fly like that as long as the same weather conditions prevailed.
Carb ice is rare in temperatures over 70 deg F. Here in south Florida I seldom use carb heat with outside temperature of more than 70-75 deg.
I use it before landing if there's significant moisture and/or it's cooler than 70 deg. FWIW,
Rudy

Joe Moilanen · Post by **Joe Moilanen** » Tue Mar 09, 2004 3:40 pm

Here in the NW I've encountered carb ice in all kinds of conditions. Part of the reason that I probably get it more than most is due to the climb prop that I'm using (80/43) as the engine develops less manifold pressure at cruise than with a cruise prop. I can almost feel it coming on and if if carb heat is applied soon enough 15 seconds or so usually takes care of it. If you have a manifold pressure guage you can sometimes detect it there first as a slight drop in pressure.

Joe 4518C

AR Dave · Post by **AR Dave** » Tue Mar 09, 2004 4:40 pm

Know what you're talking about Joe! I've found myself caught between (can't lean out anymore) and (can't enrich anymore) without the engine dying. This range gets tighter and tighter, until the carb finally chokes off. Doesn't take me long to realize what's going on now. From engine almost dying, to applying Carb heat, choking sputtering, & cleared, didn't take more than 15 secs. Sure sounds rough though!

zero.one.victor · Post by **zero.one.victor** » Tue Mar 09, 2004 5:12 pm

In my limited experience,carb heat clears up carb ice in less than a minute--usually 15-20 seconds is enough. I'm kinda in the habit of occasionally pulling out the carb heat every once in a while in cruise flight,just in case. Don't usually pull it on for more than 30 seconds or so til I'm entering the pattern to land. Or doing a low-power (1700-ish rpm) observation pass or orbit.

Eric

rudymantel · Post by **rudymantel** » Tue Mar 09, 2004 9:42 pm

You guys in the NW sure do have a lot more experience with carb ice and heat than we warm weather birds !
Rudy

Post by **GAHorn** » Tue Mar 09, 2004 11:01 pm

Ditto what Eric said.
Preflight, during runup, check carb heat. You should see a 100 or more drop when heat is applied, and the rpm should return to that run-up rpm previously selected when it is removed. (I make my run-ups at 1700 but some folks use 1800, while the Owner's Manual suggests 1600. Inspect your carb heat system visually at each oil change. An insect nest can cause sufficient blockage not only to give misleading indication of carb heat function, but can actually prevent a successful takeoff should it dislodge during takeoff/in flight. A mud dauber nest caused the loss of a C-172 recently here in central Texas only two days after the annual inspection wherein carb heat box was completely rebuilt.)
In flight, take note of the rpm you have selected for cruise. Monitor that rpm. If you see it fall off more than 25 or so in level flight, then pull full carb heat on. You should see a 100 rpm or more drop. Watch that new rpm. If it remains at the 100 reduction (or whatever you got upon carb heat application) for more than 15 seconds, then you had no carb ice. (you had some other reason for the rpm loss, such as attitude change, altitude change, throttle position change, etc.)
But if you HAD carb ice, the original 100 rpm or so drop will "recover" or increase 25 rpm or more as the ice melts off. (this takes about 15 sec's or so, never more than 30 secs) The engine will run rough as it ingests the melted ice, then it will run smooth again.
Push the carb heat back in/off. Observe that original cruise rpm is re-established. (or reset cruise rpm at that moment when you are certain of the no-ice condition just obtained.) Keep monitoring that cruise rpm.
If you see any loss of rpm again, re-apply carb heat as before. If this becomes especially frequent, (more than once every 15 minutes or so) then consider changing altitude or flying with carb heat on continuously (with re-leaning for the new carb-heat-ON condition.)
OK< back to normal> Flying along at cruise, you decide to descend. Before reducing power for descent, apply full carb heat and observe the results especially looking for any rpm recovery as any ice is melted. If no ice is found, re-apply descent power setting for the remainder of hte descent, but remember......OBSERVE the selected rpm and MONITOR it for any loss of rpm. Any carb ice will cause a loss of selected rpm, and that is your first warning of ice.
Prior to reducing power out of the green arc (below 2200) for landing, apply full carb heat for the approach to the runway and landing. Remember in case of go-around to push throttle in FIRST then push carb heat off. (or if you're practiced/comfortable push 'em both in simultaneously.)

Caveat: This landing approach procedure is a fail-safe method that I teach student pilots. It's not what I do personally. In personal practice, I check for carb ice before power reduction below 2200 (bottom of green arc), and if no ice is found, I push carb heat off for the remainder of the approach. This technique is not widely subscribed to by other instructors.

Dave Clark · Post by **Dave Clark** » Wed Mar 10, 2004 1:17 am

Hey Eric how 'bout AZ 15% or less humidity sometimes? Sure makes you lazy on the carb heat but then so does the Lycoming. I have to relearn in the Spring when I come up. But then I have to relearn a lot of things like rain gear, sweaters, long pants, water falling from the sky, etc but the worst is girls with bulky clothing

Joe Moilanen · Post by **Joe Moilanen** » Wed Mar 10, 2004 3:09 am

Here in the NW I also check carb heat after mags on runup and leave it on until full takeoff power is reached on takeoff run and then push the carb heat in. The time it takes to check traffic, line-up,etc. is all it takes to accumulate ice in the right conditions.

Joe
4518C

N1478D · Post by **N1478D** » Wed Mar 10, 2004 4:37 am

But Joe, aren't you introducing unfiltered air for an extended period of time?

Joe Moilanen · Post by **Joe Moilanen** » Wed Mar 10, 2004 5:00 am

I don't do it in dusty conditions or on unpaved strips unless every thing is saturated. The butterfly in the airbox is usually so non-precision that you can see daylight around all sides of it anyway that it seems to me that if any dust is present, some will probably be ingested anyway. The sloppy airbox sep-up always has bugged me. Probably a good reason to make sure that your filter is super clean so that the majority of the intake air will take the path of least resistence ( through the filter).

Joe
4518C

N1478D · Post by **N1478D** » Wed Mar 10, 2004 1:45 pm

Keep forgetting that every place isn't dusty like Texas, and windy.

zero.one.victor · Post by **zero.one.victor** » Wed Mar 10, 2004 4:23 pm

Joe Moilanen wrote:Here in the NW I also check carb heat after mags on runup and leave it on until full takeoff power is reached on takeoff run and then push the carb heat in. The time it takes to check traffic, line-up,etc. is all it takes to accumulate ice in the right conditions.

Joe
4518C

Joe,that's a darn good idea! Maybe I'll incorporate it into my TO drill. And dave,you're right re: the bulky clothes factor. The one thing I miss about living in So Cal is the scantilly-clad girls!

Eric

zero.one.victor · Post by **zero.one.victor** » Wed Mar 10, 2004 4:36 pm

Ian,as I recall told us you were buying a 140. Am I right? I would advise you to look in the owner's manual & see what it sez regarding rpm change with carb heat applied at run-up. What exhaust system does it have? As I recall,they came with a pancake muffler arrangement similar to the ragwing & A model 170,but a lot of them have been converted to the round Hanlon-Wilson mufflers off the Cessna 150. In the case of the 170,I think the pancake muffler provides less heat resulting in less of an rpm drop when applying carb heat than with the round muffler arrangement. If you have the round mufflers you might get more heat & experience more of an rpm drop than the manual indicates--just that much better for getting rid of the ice!
How was your airplane delivered to South Africa? I'm assuming in a container with the wings removed. Hopefully whoever assembled it got everything rigged correctly. How's she fly?
Here's wishing you lots of happy flying with your new airplane!

Eric