Cherokees and Carb Heat

MidlifeFlyer said:
Yes, in a perfect world, everyone would be to "detect the crab out of carb ice." But since we do not live in one, if there is no mechanical downside, the question becomes one of unnecessary-but-harmless vs. preventing-an-accident.
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In the Piper I fly most, the loss of power with carb heat on is substantial enough to be an issue on takeoff. And the carb heat control is not situated such that you can see it. I worry more about that than the risk of carb ice on approach (where I'd make the field with no engine anyway.) I only use it if I suspect carb ice, per the POH.
 
MidlifeFlyer said:
But your comment interests me. So you don't believe in teaching "best practices" unless they are also in the POH?
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Who says they are best practices? Certainly not the FAA or the Manufacturer. Even the NTSB report you posted says carb heat should be full ON when you are out of the Green Arc. A PA28's Green Arc has a low end near 500rpm.

Descents in most carburetor-
equipped general aviation airplanes can be conducted without adverse
consequences to the powerplant with full carburetor heat on when power is
reduced below the normal cruise flight engine power setting (that is, below
the green arc on the tachometer or the manifold pressure gauge).
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In the long run this argument is moot. If you want to teach your students to fly a piper like a cessna it's not a big deal. Those planes are similar enough. :)
 
I check it when entering the pattern as Part of Gas in GUMPS. if none is detected, I don't worry about it again.

I left it one once, and did a T&G. Do NOT move the throttle forward to quick, it was a hot day, and it didn't run real good for a couple seconds... And yes, I thought I was being slow per the placard...
 
The older Cherokee POHs I have been exposed to are quite lacking compared to a similar model Cessna. I don't really see how applying carb heat is going to heat up the engine much more than normal, honestly. Where does the heat come from? Granted, there might be a significant power loss, but I highly doubt it causes metal warping due to heat.

This was a quote from Lycoming in an old AOPA article:

"If you are wondering how long to continue the use of the [carburetor] heat, it would depend on the icing conditions. If icing happened to be severe, then heat should be used as flight continues into known or suspected icing conditions. We have found in our flight tests on various models of Lycoming engines, and with special detonation pickups, that at cruise power with full heat or alternate air, we have never experienced detonation or damage to the engine. If that is difficult to believe, a turbocharger heats induction air hotter at high altitudes than the typical carburetor heat or alternate air, without causing any detonation or damage to the engine." ? Avco Lycoming FLYER

I realize full power is higher than cruise power, but the only time you would be going full power is in a go around and only if you forget to turn off the carb heat. Even then, it would be momentary. Otherwise, your power settings will be well below cruise the entire time you are using the carb heat for landing ops.
 
drunkenbeagle said:
In the Piper I fly most, the loss of power with carb heat on is substantial enough to be an issue on takeoff. And the carb heat control is not situated such that you can see it. I worry more about that than the risk of carb ice on approach (where I'd make the field with no engine anyway.) I only use it if I suspect carb ice, per the POH.
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Not picking on you specifically, but if the loss of power is substantial, it means that the carb heat is significantly warming the induction air, which means it is needed to prevent or remove carb ice.

z987k said:
In reality it's not really Cessna, it's 6 cylinder Continentals. All the O-360 and O-320 172s out there don't have the problem.
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CFI A&P said:
Needed it a few times in 4 cylinder Continentals too. Once in a O-320 powered PA18 also. Pressure carb versus float carb is also an issue as to whether it is normal to apply carb heat.
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Actually, it's the design of the induction system. The Mighty Gutless Cutless (C-172RG)* draws its induction air through a hole in the firewall on top of the engine (Lyc O-360). Then it is ducted down the backside of the firewall, back through the firewall, and into the sidedraft carb. Therefore the induction air is already considerably warmed by traveling over and around the hot engine, and when you apply carb heat the drop in RPM is barely noticeable. (Yay for Cessna and/or Lycoming!) However, in the C-152 (Lyc O-235)**, the induction air is drawn in through a cowling vent just under the prop, then travels six inches back into the carburetor. When the carb heat is applied to that engine, a noticeable RPM drop occurs. Therefore, that induction system is much more susceptible to carb ice.

In response to the OP, operationally, I prefer carb heat on when power is reduced on downwind to start the descent for landing.

Cessnaflyer said:
Piper said that their carb heat was so much hotter than the Beechcrafts...
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Somebody call Piper and tell them I call Bull****!



*The BE-76 had the same induction system design.

**Or many of the carbureted C-172s. I know the O-320 had the same setup.
 
Krieger said:
Who says they are best practices?
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Well, as Midlife said, I think the whole point of the original question was what is the best practice.

Per the PHAK Chap 6 pages 9 and 10
"Carburetor heat can be used to melt ice that has already formed in the carburetor if the accumulation is not too great, but using carburetor heat as a preventative measure is the better option."
"If carburetor icing conditions are suspected and closed-throttle operation anticipated, adjust the carburetor heat to the full ON position before closing the throttle and leave it on during the closed-throttle operation."
(Something about copy and paste screws the font all up, and I'm not smart enough to figure out how to fix it...)
 
Hammertime said:
The Mighty Gutless Cutless (C-172RG)* draws its induction air through a hole in the firewall on top of the engine (Lyc O-360). Then it is ducted down the backside of the firewall, back through the firewall, and into the sidedraft carb.
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That seems like it fundamentally defeats the purpose of a "firewall". :eek:

I guess I haven't poked around in the engine compartment enough, but wouldn't that same ducting then belch a backfiring engine into the cabin (if the duct/hose gets broken)?
 
Hammertime said:
Not picking on you specifically, but if the loss of power is substantial, it means that the carb heat is significantly warming the induction air, which means it is needed to prevent or remove carb ice.
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Ummm, carb heat will always significantly warm induction air. It is using a heat stove from the exhaust manifold, which in my plane, is warmed with 1300 degree air. Carb heat can and does cause a considerable loss of power, even if there is no ice.

Now, whether it is needed is a different question. An idle descent in warm moist air? Probably. By the time you are midfield downwind, losing an engine isn't really an emergency anymore, and it is probably too late for carb heat to melt much ice.
 
Hammertime said:
Somebody call Piper and tell them I call Bull****!



*The BE-76 had the same induction system design.

**Or many of the carbureted C-172s. I know the O-320 had the same setup.
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The Sport, Sundowner, Sierra and Duchess that used the airframe all had different engines. Our school also tested with different set ups which they hold STCs for. IIRC the distance from exhaust to carb was further in the Beeches.
 
Minuteman said:
That seems like it fundamentally defeats the purpose of a "firewall". :eek:

I guess I haven't poked around in the engine compartment enough, but wouldn't that same ducting then belch a backfiring engine into the cabin (if the duct/hose gets broken)?
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My fault... Wrong choice of words. It's actually the aftmost engine cooling baffle, mounted to the back end of the engine. Still well forward of the firewall...

It divides the engine compatment into a "hot section," and the section where most of the engine accessories are located
 
drunkenbeagle said:
Ummm, carb heat will always significantly warm induction air. It is using a heat stove from the exhaust manifold, which in my plane, is warmed with 1300 degree air. Carb heat can and does cause a considerable loss of power, even if there is no ice.

Now, whether it is needed is a different question. An idle descent in warm moist air? Probably. By the time you are midfield downwind, losing an engine isn't really an emergency anymore, and it is probably too late for carb heat to melt much ice.
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As I mentioned above, it may not always significantly warm the induction air if the induction air is already quite warm from it's route through the cowling. (Note: While your EGT may be 1300, the air going through the carb heat box won't be quite that hot.)

You are right. You should be able to easily make the runway from midfield downwind should you lose the engine. However if you can't perform a procedurally normal go around (say due to traffic on the runway) because of an iced up carb, that becomes a really bad situation real fast. As I quoted from the PHAK, carb heat is intended to be used preventatively. Go around procedures in any carbureted aircraft should always include turning the carb heat off, therefore the power loss shouldn't be an issue
 
Autothrust Blue said:
This one time, as a student pilot.

"Oops better go around"
- BURP -
"Yipe. Oh. Carb heat."
- ROAR -
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LOL. Nice. To be honest, works for me... You got a good scare, and learned your lesson, without dying. Better than:
"Oh, crap! That idiot line guy just pulled out on the runway in the fuel truck, better go around!"
-BURP-
"Yipe. Oh. Carb ice!"
-BOOM-

:biggrin:
 
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