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There is quite a debate about this! I'm a student, have gone solo. My first instructor insists the carb heat goes off/COLD on the approach ('ready for go-around' if needs must! And to prevent dirty unfiltered air entering on landing). My second instructor goes along with the POH and insists the carb heat stays on/HOT until landed (saying icing is more dangerous than dirt).

I LIKE the idea of keeping the carb heat on because if a go-around is needed all I have to do is remember to push in the throttle AND carb heat (and retract flaps, 30 to 20 degrees,etc.) no matter what height. I learn that ONE move. But if I'm trained to take carb heat off, say at 300', and learn only 'throttle/flaps' for go-around, I'm worried I'll FORGET carb heat's HOT if I do a go-around from 400'! See what I mean?

What's the best method for a student to learn?

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    $\begingroup$ The POH or checklist is typically the "bible" when it comes to operating the aircraft. Don't rely on muscle memory here, if you ever fly something different, the procedure may be different. I'd only be worried about unfiltered/dirty air entering the carb if operating out of an unimproved strip (dirt/grass). $\endgroup$ – Ron Beyer Nov 1 '17 at 3:38
  • $\begingroup$ Thank you Ron. It IS grass. Presumably that influences instructor number 1 more than number 2. I am a novice, so it's hard for me to decide, but I favour carb heat on for safety reasons. The manufacturer MUST have had a good reason to favour carb heat on, even on grass. $\endgroup$ – MissMoira Nov 1 '17 at 20:10
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The approach and landing phase are the times when carburetor icing is MOST likely to occur.

Once a carburetor has ice, it may take a period of time for full application of carburetor heat to clear any ice. Furthermore it is possible to have enough ice accumulation that application of full throttle and carburetor heat would be insufficient to melt carburetor ice, and little or no additional power would be available at go-around.

So in short, not applying carburetor heat puts you at unacceptable risk of loss of power in the event of a go-around, and potentially even in the event of the need for additional power to compensate for wind shear on approach and for glide path adjustments.

Having examined the risks of no-carb-heat on approach, we should look at the secondary consequences. The operation is contrary to the POH, and the POH is the manufacturer's best guidance for the safe operation of the plane. The practice of asking yourself, "What would the accident review board have to say about this?" is a healthy tool for evaluating decisions. To deviate from the POH, and from conventional wisdom and practice in a critical phase, where there is little recovery time, presents an unreasonable risk on your flights, and is scarcely defensible. Without an articulable and compelling argument as to why EVERY time you make an approach and landing, the carburetor heat should be off, you are hard pressed to justify a deviation from the manufacturer's guidance.

Ok let's go to an extreme, and say that you are landing in high winds at a desert strip with blowing sand across the runway. In a situation like that, and experienced private pilot might elect to turn off carb heat just prior to landing to prevent unfiltered air with sand from being ingested into the engine. On the other hand, if the wind is blowing that hard, the possibility of a missed approach and go-around is even higher.

There are circumstances when carb heat may be left off on an approach. If the aircraft is equipped with a carb temp gauge, and the carb temp has been verified to be above 32F, then it is commonly considered acceptable to avoid carb heat. Some planes equipped like that will also have a warning alert to announce temperatures which are conducive to carburetor icing. That alert is often coupled with an ice detector sensor in the carburetor. Likely your 152 has neither a carburetor temperature gauge, nor a carburetor ice annunciator.

Let's also talk training risks, since you are a student pilot. Your operations for some time will be as a single pilot, and your procedures should be designed to minimize risk. One way to accomplish that is to simplify your procedures so that they can be accomplished by rote. Having the same procedure each time, without the need to collect data and make evaluations, helps simplify your piloting chores. You are not flying complex aircraft with complex pilot chores, and you do not have the benefit of a second crewmember to make observations and read you checklists.

Most CFIs, DPEs (pilot examiners) and FAA Operations Inspectors, will expect that you follow the POH and the manufacturer's checklist and procedures. In the case of the 152, that will be to turn on carburetor heat prior to the reduction of power (in the pattern). On balked approach, the procedure is to apply full throttle, and at most altitudes, verify mixture rich, and then remove carb heat. If the removal of carb heat causes a reduction in power (or RPMS) then carb heat is to be reapplied.

There may be other aircraft with different procedures, but you are not flying them. You are flying a 152, and it would be best practice and a defensible position to fly in accordance with the manufacturer's published procedures.

If you were taking your private pilot check ride with me, and you failed to apply carburetor heat in a C152 lacking a carburetor temperature gauge and/or ice sensor, you would not pass the check ride.

Finally, let me just add a story...on a crisp fall day, without exceptional humidity, a student/instructor were flying in the pattern doing touch and gos. The airport was surrounded on all sides by quarry, with a road leading to the flight school. That road also had the power lines to operate the stone handling and crushing quarry equipment. The plane was a new C152, the first year that they were available. It was later found that the cable had slipped in coupler on the valve for carb heat. You know where this is going. The student was mis-aligned on her approach, and when she applied full throttle for a balked approach, insufficient power was available to establish a climb. The instructor took over, and had to weave the wings between 13.2kV three-phase power lines on each side of the road, to land the plane on the road to the flight school.

Why adopt practices which will unnecessarily test your exceptional flying skills?

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  • $\begingroup$ Mongo, thank you. I would prefer to go with the manual, and probably will. Here in the UK I notice there are more pilots who go carb heat cold for the landing. It must be something that's got into the teaching system over the years. $\endgroup$ – MissMoira Nov 1 '17 at 20:12
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    $\begingroup$ @MissMoira, it depends upon the equipment, but in the case of a Cessna 152, the data is clear. Furthermore, my understanding is that at least from a climate standpoint, in UK you are more likely to have carburetor icing than in many other parts of the world. You are to be commended because you have demonstrated one of the most important characteristics of a PIC, and that is to know your equipment, and seek out the facts in managing that equipment. FWIW, I fly on turf fields all the time, and the risk vs the benefits of deselecting carb heat on landing are not tipped by a turf field. $\endgroup$ – mongo Nov 1 '17 at 21:40
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If in doubt, follow the POH instructions. For example:

BEFORE LANDING
1. Seats, Belts, Harnesses -- ADJUST and LOCK.
2. Mixture -- RICH.
3. Carburetor Heat -- ON (apply full heat before closing throttle).

BALKED LANDING
1. Throttle -- FULL OPEN.
2. Carburetor Heat -- COLD.
3. Wing Flaps -- RETRACT to 20.
4. Airspeed -- 55 KIAS.
5. Wing Flaps -- RETRACT (slowly).

AFTER LANDING
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.

That's very clear: carb heat on for landing, and off only after landing or when going around.

FWIW, I was originally taught (in a C152) to turn carb heat on when throttling back on downwind, and turn it off again on short final in case of a go-around. However, no other instructor I've flown with does that and I now follow the POH procedure.

Since your instructors seem to differ on this it might be a good idea to find out what your local examiner prefers before you take your final checkride. That way you can be sure to do what he expects and avoid a debate about carb heat or worse, fail the checkride.

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  • $\begingroup$ Pondlife! Thank you. There does seem to be a difference in the approach (ha! good pun!) of some instructors. I prefer to go with the manual. And as for the examiner.... perhaps the examiner should listen to me and all these replies! $\endgroup$ – MissMoira Nov 1 '17 at 20:15
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    $\begingroup$ @MissMoira I'd rather say that if the examiner fails your check ride because you follow the procedures specifically laid out for the maneuver in question in the POH for the aircraft you perform the check ride in, then the onus is on the examiner to explain why they're failing you! They just might possibly have a reason for doing so, but they had better be able to explain it in light of what the POH says, whatever that is. Surely they shouldn't be failing you for, say, slowing down to below VFE before fully extending flaps, so why should carb heat during landing be different? $\endgroup$ – a CVn Nov 1 '17 at 22:07
  • $\begingroup$ Thank you Michael. I've only just gone solo so am a long way from the final exam. Costs limit me to one lesson a week! But I will continue to listen in to the debate - and there IS a debate. Believe me, there are a number of pilots and instructors who swear by carb heat cold BEFORE landing. I've met them. I wonder how it all started - this decision to go against the manual? It seems be more prevalent in the UK. Why??? $\endgroup$ – MissMoira Nov 2 '17 at 10:19
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Your instincts are right on this and your first instructor is wrong, you should not turn off carb heat on approach for very good reasons:

  • Icing: on approach your engine will be on low power, with the carburetor butterfly valve mostly closed, which is when most ice will accrete on your carb surfaces. Your instructor says to close carb heat at 300 feet, that's about a minute from touchdown probably. A carb isn't going to fully ice up in a minute, but it could ice up enough to rob you of enough takeoff power that it's a problem. Consider, you are doing some pattern work, each time you turn off carb heat on approach and each time you build up a bit more ice. You do your carb heat check on each downwind leg but it doesn't get completely melted, and you might not realize it. Gradually you may build up enough ice to lose a serious amount of power, with fatal consequences
  • Distraction: Final approach is not the time for unnecessary steps, and turning off carb heat is unnecessary. You want to concentrate on your speed and touchdown point, not fiddling with carb heat, which will take your hand off of the throttle at a critical phase of flight. Sure, it doesn't sound like much of an action but you want to be as sterile as possible on final approach and landing

As for your instructor's view on ingesting dirty air on final, I don't know what planet the person is on but the air aloft is plenty dirty too, you aren't saving the engine much by turning off carb heat 300 feet above the ground. In any case, it's one of the reasons airplane engines have such frequent oil and filter changes. My advice would be to get a different instructor, yours seems to have some strange and possibly dangerous misconceptions.

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  • $\begingroup$ Thank you, GdD. I've done a ton of googling and it seems more common here in the UK to be taught carb heat cold before landing. A debate rages though, with a good number on instructors defy the carb-heat-cold brigade. $\endgroup$ – MissMoira Nov 1 '17 at 20:18
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    $\begingroup$ I've never heard anyone telling to go carb heat cold in the UK, but your experience may vary. With cool, wet conditions that prevail over here more carb heat = better. $\endgroup$ – GdD Nov 1 '17 at 20:28
  • $\begingroup$ The chief flying instructor is a carb heat cold man. The other instructor, who I am now with, lands with carb heat on, and I am now doing so. But I have had a number of opinions come in (I keep asking) from pilots agreeing with the chief. I'd say it's about 70% carb heat hot, 30% carb heat cold. But I'm still counting! And it varies with country. $\endgroup$ – MissMoira Nov 2 '17 at 10:23
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Leaving carb heat on results in a loss of power due to less dense hot air passing through the carburetor. I've never landed a plane at 85% power. That is the risk of having it on for a go around. I would check it while I was raising flaps.

The risks of carb heat off are obvious, and clearing a carburetor of ice can take over a minute. Carb heat can be applied when plane drops out of cruise and is on lower power descent to landing. No need to wait until you are in downwind.

As far as when to take it off, under most conditions, it's wheels down, flaps up, carb heat off, taxi off runway, or power, flaps reduced, carb heat off for go-around. Then in pattern, downwind, power down, flaps down, carb heat on, turn to base.

So carb heat can go with power reduction, but, under favorable conditions, in cruise too. Don't wait until you need to climb!

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