PA 28-161 Carb heat on the approach

[Genghis the Engineer]

Apologies for an intrusion into the FIs forum since I'm not an instructor, but since I've certified a few of the aeroplanes that you ladies and gents teach in I hope that I'll be forgiven.

The following is a quote from the PA28-161 POH (at the top of page 4-15), it is referring to the approach and landing.

The mixture control should be kept in full "RICH" position to ensure maximum acceleration if it should be necessary to open the throttle again. Carburetor heat should not be applied unless there is an indication of carburetor icing, since the use of carburettor heat causes a reduction in power which may be critical in the case of a go-around. Full throttle operation with carburetor heat can cause detonation.

In my experience this is different advice to that taught by most FIs. I have a suspicion that this is not because of any superior knowledge they have about how PA28s work, but because of a read-across from Cessnas which DO suffer icing during an idle approach in appropriate conditions.

Thoughts anybody?

G

[hugh flung_dung]

I don't want people to have to think about which aircraft type they are in and whether or not it may be prone to carb ice. Nor do I want them to be on the approach at 500 ft (with carb heat cold) and find that they can't get power when they need it because they're iced-up.

A simple way round this is always to apply carb heat as part of the pre-landing checks and to leave it on until very short finals or until a go-around is initiated. This gives the best protection against carb ice and avoids the other issues mentioned in the POH extract. The only time not to do this is when the air is VERY cold because carb heat could then move the intake air into the icing region.

HFD

[FlyingForFun]

I agree with HFD.

The normal advice is always to follow the POH. In the case of the quote you give us, though, Piper have very kindly told us the reason for the instructions they give, which enables us to use their reasoning, and vary their advice in such a way that we still satisfy the reasons behind their advice albeit in a different way.

In the quote you give, the reason given for not using carb heat is because of "bad things" (loss of power, and detonation) which can happen on the go-around. So teaching people to use carb heat on the approach, but remove it immediately before adding power on a go-around, still satisfies the reasons behind what the POH says, and creates pilots who have habits ingrained into them which will work on just about any aircraft.

FFF
--------------

[Say again s l o w l y]

Read the POH. If it says do something, then there is usually a good reason for it. If it says don't then there will be an equally good reason for not doing it.

Simple really!

I don't like teaching people a "one size fits all" method of flying. Yes there are many things that are the same on all a/c, but there are an awful lot that are totally different. Cue arguments on the "standard" spin recovery technique......

Pilots should be taught to fly that particular aircraft and have it explained why a particular machine is different from another. Follow the POH and you won't go far wrong and you certainly shouldn't end up doing anything totally stupid.

[BEagle]

SaS - absolutely correct in my view.

On our PA28s, we apply the carb heat downwind, then do the pre-landing checks, then put the carb heat back to cold where it stays for the rest of the approach.

On rolling out of the final turn, as the hand moves off the flap lever after selecting full flap, a final check is made that the carb heat is at cold.

You are far more likely to need full power for a go-around than you are to collect carb ice in the minute or so from downwind. If you think you are, then I suggest you land as the weather must be very unpleasant!

[unfazed]

Ghengis -


Strictly speaking the POH is the Gospel and I do not wish to open the "carb heat on or off debate prior to landing"

However......from my experience of sticking to the POH as a student it wasn't worth the trouble when trying to "diplomatically" advise the all knowing Instructors of the advice of the POH (The people who made the aircraft !). Can remember one instance with a very abrasive and miserable instructor where I lost my rag and told him to look in the bloody handbook !....unfortunately his answer was that he didn't give a toss what the handbook said carb heat must be on for final approach (very exasperating).

My advice to students is to get in the habit of putting it on at the end of the downwind (to allow the heat to do it's job prior to power reduction), leave it on all the way down to the ground and if you decide to go around then get rid (ala Cessna). Main reason is to stop them having to argue the toss with an instructor or worse yet an examiner (probably won't be able to enlighten all of the instructors and examiners in the UK)

Detonation may be possible but I haven't ever experienced it due to carb heat being on.

My guess is that many FI's have assumed that it should be on and haven't bothered to read the POH properly.
Hope this helps !

[mad_jock]

I agree with BEagle and SaS and teach what the POH says for the aircraft being flown.

And if a student gets bollocked by an instructor/ examiner for doing whats in the POH. Thats what chirp is for. Or if you get a fail for doing what the POH says A letter to the CAA chief examiner should solve the problem. But I am sure a letter from the CAA asking why they are teaching/ examing outside the POH will be met with suitable ill humour and grace.

MJ

[unfazed]

Mad Jock

In an ideal world maybe

But life is too short for writing letters to the CAA so it's a hell of a lot easier to stick the carb heat on.

Anyway it was yours truly who pointed out this little known fact to Genghis not so long ago so I am a supporter of do what the POH say's, but if you can save yourself a lot of aggravation without endangering safety then why not

Not ideal I know but then what is.........

No sod it ! why don't we get FI's and Examiners to read the POH and do it by the book !

[mad_jock]

Don't worry m8 come the revolution. And we get BEagle incharge of PPL training and examining at the CAA all these twats will get told.

[Whopity]

Students should be taught to read the Flight Manual and operate to it.

One size fits all is utter nonsense.

[Genghis the Engineer]

Or at-least start by having instructors read the POH and operate to it.

Follow that by having students NOT bollocked for failing to follow a POH procedure?

And then finally have everybody flying to, and using, the POH.


But what do I know, I've never been an FI, nor likely to be.

G

[mad_jock]

gengis you are suffering under the belief that doing a FIC will actually impart any great skills to you in teaching flying. It makes you very good at getting various colours of white board pen all over your clothes and drawing pairs of eyes. It has very little effect on your ability to teach anything.

Unfortunatly the course presumes a level of common sense and inate ability. About 90%. The 10% added by the FIC is how to break straight and level into bits.

The actual learning how to teach someone is done with students money in the air by experence. From your previous posts I reckon your about 90% there to being a FI.

Have you thought about doing the class rating instructors course? Very cheap only 3 days and no theory pre course.

[Genghis the Engineer]

Have you thought about doing the class rating instructors course? Very cheap only 3 days and no theory pre course.
An interesting thought, and thanks for the complement. Presumably you mean this? (http://www.ontrackaviation.com/CRI(SE).html). Interesting idea; probably not for the next year, but could potentially fit in conveniently with some other professional activities after that.

G

[mad_jock]

It allows you to be a FI in SEPs for anything which isn't for intial license issue.

So you could do check rides, 1 hour with instructor flights, Any retraining for anyone working up for a test after being out of 2 year check. Instructor for people out of 90day check for pax carrying. People coming back from the US and needing local instruction before going solo etc etc

here is the on track web listing for it

http://www.ontrackaviation.com/CRI(SE).html

edit to add yes.

And i can put you in touch with someone who has done the course with them. And alos i believe the course hours etc will go towards a full FI if in the future they drop the CPL theory side of things. And it looks pretty easy to maintain as well. None of this flogging round the country going to seminars and the like.

And another bonus is that when ever you go places with another pilot 2 people will be able to log the hours. And alos most plane insurance will cover instructors flying them as well as the named pilots.

[TheOddOne]

Our Group has always taught (and it stood me in good stead for the CPL GFT many years ago):

1. Downwind checks include - note RPM,application of full carb heat at cruise power, note RPM drop, apply carb heat for a count of 10 elephants, carb heat cold, ensure RPM returns to original value.
2. Turn base. At the point at which you wish to start the descent, apply carb heat and reduce power to achieve the rate of descent required; reduce speed to below flap limiting, apply flap as desired, adjusting power & attitude to achieve ROD & speed.
3. Retain carb heat for all the time that power is below cruise until VERY short final, apply full cold, ready for the go-around. If a go-around is initiated earlier in the approach (some muppet pulls onto the runway ahead of you) then carb heat full cold BEFORE applying power.

It may sound silly, but I speak these (and all my other checks) out loud, even solo, so I don't forget them!

The above has been good for all the 'normally' aspirated a/c I've flown, various marques of Rallye, PA28s, Cessnas, T67.

Cheers,
TheOddOne

[FlyingForFun]

Ok, it seems I may be in a minority here. But can I please turn the issue on its head for a minute?Carburetor heat should not be applied unless there is an indication of carburetor icing(Italics added by me.) What indication of carburetor icing should we be looking for? The only indication I know of is a decreasing RPM. But we are told to look for this decreasing RPM at a time when we are decreasing power and then shortly afterwards decreasing speed - both of which, with a constant-pitch prop, will result in decreasing RPMs which might mask any indication of carb ice, possibly until it's too late.

As I said in my first post, the advice in the POH will always result in the aircraft being operated in the "correct" way, but is it not possible that there is more than one correct way? And, as intelligent and reasonably experienced pilots, if we are given the reasons for doing something, is it "wrong" to suggest another way of operating an aircraft which achieves the same result? Maybe in a way which is "better" suited to the environment in which we operate?

FFF
--------------

[CaptAirProx]

I think the best advice was given by hugh flung_dung and flying for fun.

Yes we are all agreed that followeing the POH is the way to teach and operate realistically. And we must ALL note and remember that most engine failures it seems are due fuel starvation or suspected carb ice. If one also adds into the equation the CAA safety sense info and that excellent chart on potential carb ice conditions, one will find that on nearly ANY occasion you are approaching in a Warrior or similar, that icing COULD/MAY exist.

By selecting carbheat just incase, you are covering your arse regarding what really happens in the real world (read the AAIB bulletins) and the CAA reasoning and advice. And by ensuring you deselect carb heat either on short final or on application of full power, you have respected the POH.

From my point of view, I would rather see somebody operating to the POH with an eye for "other" outside problems that exist. Remember, the POH is written to one extreme, and it takes the wise man to read between the lines and enter into the spirit of the POH and work with it.

Someone made the point here too that they didnt agree with one size fits all etc. Well that maybe true, but you have to consider that we are teaching/refering in most cases here to the SEP(L) Class Rating. We must give the student a firm and solid grounding to "get away with it" in any SEP (L) that they are legally allowed to fly. So sometimes, people teach these techniques that are not dangerous, or opposing the POH but if ingrained, will stop the poor sod from killing themselves in something far less forgiving...Such as the good old Continental engined Cessna's/Luscombe's and the like. There is simply not enough scope/time in the PPL to train for everysingle aircraft that can be flown within the rating priviledges.

[unfazed]

Excuse me if I am wrong but I was under the impression that this forum was for professional flying instructors to discuss aspects of their profession ? It is a tad tedious to log on and see Mad Jock ranting on and slagging off instructors.

Rant by all means, but why not go elsewhere to do it ? As someone who has learned from true professionals who had tons of experience of military and civvy flying I for one definitely do not fit your perceived notion "illusion" of what flying instructors are.

Sod off and be a saddo elsewhere !

[hugh flung_dung]

If a stude (or experienced pilot) does something that's different to what I would prefer I will ask them why they did it that way - providing it's safe, achieves the aim and they know why they're doing it then it's OK. Skinning cats and pink pussies come to mind again.

With carb heat we have to consider the impact of the alternatives during the approach.
- C/Ht on:
potential detonation at high power: very damaging, the pilot is not aware it is happening, only relevant in go-around so avoided by selecting C/Ht off with full power
risk of raising very cold air into the icing range: avoided by engagement of brain and looking at OAT gauge
lack of air filter: only normally an issue on the ground
reduced power available: avoided by selecting C/Ht off with full power
- C/Ht off:
risk of icing: only detected when late in the approach and power needed - no options - crash burn die :ugh:

To me the balance seems to be in favour of selecting C/Ht on during the pre-landing checks (please don't call them downwind checks) and then selecting off when very short finals or when initiating a go-around.

HFD

[Say again s l o w l y]

We must give the student a firm and solid grounding to "get away with it" in any SEP (L) that they are legally allowed to fly.

God no.....
What we need to teach is an understanding that people can't just jump into any a/c and "give it a go." Whilst the CAA have given us this catch all SEP rating, it doesn't mean that we are all competent to fly every single SEP class machine.

I hope that FI's aren't telling people methods that will just let them "get away" with anything.

We aren't just teaching the mechanics of how to push and pull flying controls and get the thing safely off the ground and back down again (though this obviously helps!), but also the mindset that ALL pilots should have. (Licence type being an irrelevance here.)

General principles are all well and good, but specifics are very important. Therefore, teach to the POH and explain why you are doing so, or why you may disagree with it and teach something different, but always use the POH as a starting point.

None of this "one size fits all" garbage.

[Whirlybird]

What we need to teach is an understanding that people can't just jump into any a/c and "give it a go." Whilst the CAA have given us this catch all SEP rating, it doesn't mean that we are all competent to fly every single SEP class machine.


SAS, spot on. :ok:

As a helicopter instructor, perhaps my life is slightly easier, since a separate rating is required for each helicopter type. But if students ask me, for instance, if the R44 is different from the R22, I tell them there are similarities, but it's definitely different, and you can't expect to just jump into an R44 if you've learned on an R22. They seem to be able to understand that. It's not a complicated thing to explain.

So why aren't f/w instructors dojng the same thing, or not all of them anyway? When I got my PPL(A), I was given the definite impression that flying a different type was no big deal, and that the hour (only one hour, mind you!) required with an instructor was almost a formality. As for the POH, it was never mentioned, and I never even knew that such a document existed. And since then, if I've ever asked to read a POH, it's quite often treated as a top secret document to be kept under lock and key, and definitely not to be read by mere PPLs!!! And I happen to know my experience is not unique.

Most students learn on one type only. What on earth is wrong with teaching them the correct use of carb heat - and everything else - for that type? You then tell them that other types will be different. Is that so complicated?

As for instructors who insist things should be done "their" way, for no good reason, my opinion of them would get this post banned! But these days, for an easy life, I don't argue on check rides and similar - I just do as I'm told. After all, I don't think such people have would enough intelligence to understand if I told them that egotism has no place whatsoever in flying instruction.

[CaptAirProx]

Wow, sorry guys, I was using a loose term there regarding "Getting away with it"

Regarding the carb heat, HFD has explained it all now. Thanx.

Regarding getting away with it, would one advocate not teaching someone how to prevent a wing drop in a Warrior cos it hardly ever does? I would imagine one teaches the wing drop recovery regardless as it will be needed in their future flying types. So my point being, yes show them the POH at what is allowed and not allowed then go through the merits as explained above and before as to why we actually do it slightly differently with the obvious caveats etc.

As to not questioning your instructor during a checkride, thats a great shame. As generally thats when we can then discuss WHY we are doing something. Maybe you the checkee has a better idea, or has heard of a better idea. Great great shame if one doesn't like to share experiences.

[Whirlybird]

CaptAirProx,

As to not questioning your instructor during a checkride, thats a great shame. As generally thats when we can then discuss WHY we are doing something. Maybe you the checkee has a better idea, or has heard of a better idea. Great great shame if one doesn't like to share experiences.

If that was directed at my post, I was referring to "instructors who insist things should be done "their" way, for no good reason". Under normal circumstances, I'd entirely agree with you. But if I end up flying with some (so-called) instructor who demands that carb heat be pulled or not pulled in a particular position in the circuit, and when queried, gives as his reason "because we always do it that way; you should know that" or something similar, I'm not going to waste my breath arguing. Such people, in my experience, think they're demi-gods and won't listen to mere PPLs. Though if I mention I'm a rotary instructor it does tend to tone down the egotistical strutting just a tad. But normally, yes, I'm all for discussing things and sharing experiences and us all learning something. :ok:

[BEagle]

"I would imagine one teaches the wing drop recovery......"

What's that when it's at home?

[CaptAirProx]

BEagle, lets not complicate things! I just hope my point was obvious rightly or wrongly.

Whirly, I guess it was, I should look back to see who wrote what, but just finished the late shift and well you know. But yes I very much see your point there! I guess this is the exact problem we are all discussing from the outset. People who teach with really no thought as to what the hell they are teaching. Sad but its true. I will not pick somebody up for something unless I have a bloody good reason for why they should'nt do it, or at least pass my view as to why I see it as potentially at odds with the grand scheme of things.

Lets face it we aren't all perfect and the world isn't black and white. Carb heat on/carb heat off. Use your brain, knowledge and experiences with a little bit of predicted luck and come up with the best course of action for the day.....Captaincy

[foxmoth]

There are many areas in aviation that there is not one right way of doing things, this is one of them, in these circumstances IMHO it is for the CFI to lay down how his school will teach it, the instructors should then discuss with students what is in the POH and any differences if the school chooses an alternate method - checkouts should accept any method as long as it is safe - the main thing here surely is that carb. heat is at cold when power is reapplied.

[Whirlybird]

My last biannual check flight, I took control of this situation right from the start. I was flying our group-owned ancient C150, and I know its ideosyncracies - like getting carb ice at any stage of flight if it's even remotely possible. I announced to the instructor before flight that the aircraft was extremely prone to carb icing, and that I would do all the pre-takeoff checks with carb heat on, push it in to taxi, then give it a blast before take-off. In the climb, I told him, I would level off every 500 ft and apply carb heat. In the circuit, I would pull it on downwind, leave it out, and not push it in unless I needed to go-around. This, I told him, I had found by experience was what worked, as we'd had carb ice in the past on this aircraft at every stage of flight!

He said that was all absolutely fine. I don't suppose he really had much choice, since I didn't ask, I told him! Nevertheless, I suspect some instructors would have taken exception to my doing that.

Sorry if anyone thinks this is a digression.

[CaptAirProx]

No you were being Captain, thats what instructors should be pleased to see in someone!

[markflyer6580]

I am not an instructor however,I was taught to leave it on all the way down having put it on at the start of the descent.
I now put it to cold at 200ft ish in case of a low go around.
Recently on a short hop with another ppl and the a/c owner he suggested I turn it off around 700 ft. I'm sure it can't make to much difference as long as it is cold before initiating a go around. PA28 by the way.:D

[Say again s l o w l y]

If the carb heat is cold BEFORE you apply full power, then this is fine, but I don't like arbitrary figures such as 200, 300 or 700 ft for when to put carb heat away.
What happens if you need to initiate a go around at a greater height? Hopefully people don't leave the decision to the last minute and only go around at low level. If you routinely put the carb heat away at 300 ft and then find yourself needing to go around at 450 on the approach, what do you do then? Will you make sure the carb heat is cold before applying full power?
If so, fine, but the vast majority of people I have flown with always seem to miss putting the carb heat to cold, simply because they have been trained to set it to cold at a set figure rather than when needed. Often this isn't a problem, but on some a/c this can be an issue.

So to reiterate, read the POH for the type you are flying and use that as a base, if you are going to do something different, understand why and make sure you use procedures that will work for that particular machine and understand the consquences of doing something not prescribed in the manual.

[markflyer6580]

I always put it to cold before applying power,the reason for the change was: when doing touch and gos during training I found I would be messing about looking down for flaps carb heat etc whilst doing 60kts down the runway swerving all over the place! Its one less thing to do! :ok:

[Whirlybird]

I'm sure it can't make to much difference as long as it is cold before initiating a go around. PA28 by the way.

On a PA28 it probably doesn't make much difference - though I'm not an expert on the type. But on a C150, for instance, putting the carb heat to cold on final may well mean you'll get carb ice! SAS is right - read the POH for the type you're flying, and don't assume that other aircraft will be the same.

[Genghis the Engineer]

This is, after-all, why we have a POH on the shelf marked "PA28-161" or "C150F" or whatever, and not marked "SEP".

G

[mad_jock]

I think on the pipers its something to do with the Air intake and the exhaust. The air intake pipe runs next to the exhaust and gets heated anyway. On the semniole the air intake goes over the top of the exhaust so the carb heat is next to useless in normal conditions. As the air is only marginaly hotter than the normal air.

[Whirlybird]

This is, after-all, why we have a POH on the shelf marked "PA28-161" or "C150F" or whatever, and not marked "SEP".

We do? Really? That's news to me! I thought the POH was hidden away under lock and key, heavily disguised as something else and with no labels, and that you needed to sign the Official Secrets Act in triplicate and get it witnessed by the flying school owner, CFI, and an least three other people before you were allowed to take a look at its hallowed pages. At least, that's roughly how it's been in every flying school I've been to.

[Genghis the Engineer]

When I was young and innocent (well, young anyhow) I accepted that, nowadays I have a simple personal rule - if I haven't read the manual, I won't fly as PiC (and frankly am very unhappy flying dual).

What made me so firm on the issue? An unhappy personal circumstance some years ago, I overran a runway, bending an aeroplane. The club sent their licenced engineer up to collect me and survey the damage, he came bearing the POH and handed it to the insurance man saying "see, pristine, I keep it locked away so that the pilots can't mess it up".

So, I went away and BOUGHT an original company manual before the aeroplane was repaired. Guess what, I'd been briefed in the WRONG short field landing technique, had I flown it by the book I'm 99% certain now I'd have landed safely.

I'm less young and less innocent now, and also a much safer pilot. But it does irritate me badly how often I have to demand the POH. Young and impressionable pilots should be sat down and ordered to read and digest it cover-to-cover. (And instructors should be made to do so at gunpoint if necessary).

If schools really don't want to risk their nice shiny POH getting messed up there's a simple solution. It's called a photocopier.

On which subject let's have a poll and see what's going on out there. (http://www.pprune.org/forums/showthread.php?s=&threadid=198088)

G

[A390]

This post has raised a number of very valid points but for me, it has highlighted how important the POH is for each aircraft type.

Why aren't the POHs for each and every aircraft published on the net, so every pilot (regardless of experience or ability) can have access to this knowledge.

Surely it would be in the interest of all light aircraft manufacturers to reduce accidents / incidents in their products. Flight safety would be the real winner here and perhaps the corporate knowledge of the aviation community may increase.

Some US flying Schools have published pdf files of copied POHs on their own web sites but it would be much better if we could have the real thing.

I must be one of the lucky ones, as I have never experienced carb icing, although I have seen a number of aircraft in difficulty with it - even on the taxi out... I'm sure that everyone has forgotten to set carb heat to hot at some stage in their flying career, I know I have, although it always makes me sit down and think about it afterwards.

Perhaps the biggest lesson here is that there is always more than one way to skin a cat but we should all follow the correct technique for that particular aircraft type and ensure that all our students understand why.


A390

[mad_jock]

Another requirement which would be very easy to implament on a pratical side is the CAA making it a requirment that all manufactors have to publish the POH on the web in PDF format.

Would think that it will meet some political noise due to the POH's being so bloody expensive to buy. But in one fail swoop it would increase safety. Even if they published the British certified ones on the AIS website. Paper trail to prove you had read it. Its a win win on the safety case. But I think quite a few schools wouldn't like it or the document suppliers.

MJ

[CaptAirProx]

Of interest, the PA28 suffers carb ice quite badly in the morning on grass after a heavy dew whilst at taxiing power......

Bet that ain't in manual?

My point?

As long as we all are aware of the limitations of a system/aircraft POH, whats the harm in using a method that pretty much covers your arse across the board.

[Say again s l o w l y]

The original POH checklist has you checking the carb heat twice before takeoff for that exact reason. On the ground on a damp day is always when you are most likely to get carb icing, it's also potentially the most serious, trying to takeoff from a shortstrip with carb ice would have to be a bad idea!

I have made the original check list available in the club, but I actually don't like the lay out and sequencing. Somehow it just doesn't seem to fit the way we operate, so most people use the checklists sold by a certain large pilot supplies company. Personally I'm not a fan of these either, but each to their own, there are different checklists available if need be.

I think having POH's available on the web would be a great idea, instead of sitting people down with it before we go flying, then they could read it at leisure at home. I have no idea how to do it, bit if I could I'd get the POH's of all the different types we have put up on our website. If anyone has any copies in electronic format that won't break any copyright rules, then I'd definaltely appreciate a copy.
(PA 38, Pa28 181, C172 XPII and PA28R)

If you have a 'system' that you operate to, that has no problems when put against the POH of all the different types, then go ahead, but we are teaching people, so we should give them the correct info from the start. If they wish to change this in the future after they have a licence, then so be it, but at the initial stages, we must all teach the correct methods. It is just laziness not to. Would you also advocate changing types every lesson, because afterall, they are all SEP so they are all the same to operate.
No? Well why teach students to fly and operate each aircraft in the same way when they all have their own idiosyncracies.

[mad_jock]

And some of the commercial checklist are actually wrong.

For example the the AFE PA38 checklist states that you set the rpm to 1800rpm for power checks the POH says 2000rpm.

[Say again s l o w l y]

A very valid point. Can anyone explain why there would be such a difference? Surely the AFE checklist used the POH as a guide?

[CaptAirProx]

Say again,

I don't disagree with anything you have said. If we take the original post, it asks, why do we seem to all teach something at odds with the POH. And then the suspected conjecture as to why this maybe.

Yes there are I am sure many instructors out there that may teach only one scenario with carb heat without thought that the stude may not appreciate the limitations of a POH.

But I fail to see why we should not use carb heat on approach because it does'nt say so in the POH. We have to teach the system being used (old fashioned carb etc)....through Tech exam, and instructor input. This generally implies that in pretty much any engine you will every come across that as a rule of thumb, carb heat ON whilst at low power (approach) is a wise thing to do. BUT also to teach the stude that as always, check with the aircraft POH before operating such a device.

We as instructors have to also be realistic and realise that quite often most things we tell our students get lost in the haze. So I find that its best to drum at least some form of discipline into that person so that they have some method that 'should' work in most cases. Balance of risk.

Lets remember, there are cowboys and charletens (spelling?). Cowboys are easy to spot and generally get back on track. The latter pull the wool over your eyes and bugger off into the wild blue yonder of PPL and never listen a word that was advised before.......Manuals? Who needs Manuals? I can fly? ITs a piece of P**S! Maybe just maybe that stupid fellow, will remember, as it was drummed into him, that carb heat (might) be a good idea on approach. He has a higher chance of getting it right than he has of getting it wrong as most types as said favour this technique.

We all, I'm afraid, are not perfect! I can vouch for that.

[Say again s l o w l y]

There are a great many muppets out there who shouldn't be in charge of wheelbarrow, let alone an a/c.

FI's aren't to blame for this, but we must all try and see and quosh this sort of blase attitude early on. I don't want to take any of the fun away, just get people to think about the consequences of their actions.

I have seen a large number of gash pilots, one or two of whom have passed through my hands at some point and despite my best efforts, when they are away from supervising (prying really!) they get up to all sorts of idiocy. Unfortunately thses people aren't members of our club so I'm limited in my power over them and only usually get to 'have a go' on the 2 year flight. Not really enough and anyway they are on their best behaviour then anyway.

:mad:

[unfazed]

I'm limited in my power over them and only usually get to 'have a go' on the 2 year flight.


I think that the above comment is very telling ! It would be great to hear the "rebel" PPL side of the coin.

[Genghis the Engineer]

Interesting poll I started!

So far, of the PPLs who have responded, just under 50% are saying that either they're not in any way encouraged to read the POH before flying, or the document is actually kept out of their reach.

I'd like to say that I found that shocking, but frankly it's about what I expected. What it is in my view, is bloody dangerous!

Is there an FI here who will admit to teaching or releasing to fly solo pilots who have not been encouraged to read the POH? If so, what is your justification for this?

G

[homeguard]

The possesion of the POH is of course a requirement of the C of A and invalidates the C of A if it is lost. So, it is treated like gold. Part of every pilots training must include reference to the POH and where to find info as with the AIP, ANO, AIC's and Notams etc.

Some manuals on older types are not a bound item but an assortment of papers found in a binder that requires particular care and protection from careless marauding hands - but just the same MUST always be available to every pilot.

All of our checklists are produced by ourselves for a particular aeroplane, none are generic. The checklists contain from the manual - actual copies where possible of performance data; TO run/TO distance tables, climb, cruise, descent and landing data, W & B and emergency procedures - nothing may be included that is at odds with the manual. Where the POH makes no specific recommendation then the CAA safety factor figures are used.

Carb icing is of course IMPORTANT but that is not to ignore the dangers of an engine running rich! Over use of carb heat with many installations may lead to rough running (so lack of power)and even to a rich cut when aggravated by a hot summers day. Knowledgable use of the mixture is also part of the engine management scenario. Carb icing should not be briefed in isolation to the full story

[FlyingForFun]

I've never yet come across a school where the POH is not available, and if I did I would refuse to use the school. I've sometimes met with confused looks and "I'll find someone who knows" when I've asked for the POH, but I've always managed to get a copy of it.

At my school, the POH for all our aircraft are in a drawer behind the counter. Anyone can ask for them at any time, and I frequently pull them out and show students around them, but I can only think of one PPL who has ever asked for the manuals. We don't allow them to be taken home, and I wouldn't expect to be able to take a manual home with me, but we do sell copies of our types' manuals although these won't have all the supplements for the specific aircraft concerned.

FFF
----------------

[Charlie Foxtrot India]

Hmm mad_jock, my PA38 POHs say to do the run-ups at 1800 RPM....?

One thing that hasn't been mentioned here is the fact that the hot air is unfiltered so to have it hot on the ground in our dusty climate causes wear and tear on the engine.

If you "check for ice" downwind, then put it cold and leave it there, you are avoiding the possibility of trying to go around with it on "hot", also the possiblity of forgetting to put it back to cold prior to landing, and filling your engine up with dust on our Aussie runways.

Because I have found any small distraction on final and the stude almost always forgets to put it to "cold".

Agree one size should not fit all, and I have had many debates about "generic" checklists and still can't see one good reason for them; here in Aus we have a "Single engine below 5700kg" endorsement but obviously poeple need a check ride before getting into something they haven't flown before. Also, the POH has to be kept inside the aircraft and they need to read it before they fly the aircraft!

[2R]

When all else fails read the instructions POH.
Questions that this has raised ;

What is carb icing ?

How do you recognize indications of Carb icing ?

When would expect to get Carb-icing ?

How do you get the most power from the engine if using the carb -heat during those conditions most conducive to carb-icing ?

If you cannot answer these questions do not fly a piston banger stick to turbine engine (s)

:} :}

[mad_jock]

Unfortunatly i don't have acess to the POH any more.

The difference was noted in a Instructor renewal check ride by the examiner.

Is yours for a mark 1 or mark 2

MJ

[shortstripper]

Hmmmm

All interesting stuff! However, (PPL here) aren't you supposed to teach us why we apply carb heat, why we should select cold before power and why we need to understand the principals and not to simply refer to the POH without knowing why we do the things it tells us?

Most of the aircraft I fly have no POH because they are PFA type home builds.

It is only through understanding why we do the things that we do that we can make informed decisions over how we do them. Isn't that your job as instructors to do? If not then we just as well simply go along assuming anything written down in manuals, text books or (dare I say?) POH's are always and infallibly correct?

SS

[Say again s l o w l y]

Yes, that is exactly what you are supposed to be taught.

Monkey see, Monkey do is not acceptable whether it's blindly following what an instructor tells you, or blindly following the POH and not understanding why you are doing something.

Mind you if I had to pick one of the two, I'd rather people blindly followed the POH. Not that I want anyone to do either of course.

PFA types usually have some kind of manual, certainly the engine you are using will have one.

[Lomcovack]

So there we are on the downwind leg at a medium power setting and our check for carb icing shows nothing significant.

We put the carb heat to cold, Turn base and reduce the power to a lower power setting.

On finals we notice a slow drop in rpm, indicative of carburettor icing, Is carb heat going to clear any ice, not at these power settings.

Applying power may make the situation worse and if a go around is required an over rich mixture is possible leading to a rich cut at the worse possible moment.

If on the other hand we flew down the final approach with carb heat hot (would the engine produce enough heat to stop ice build up !) and if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1.

Reading and understanding the POH is a must but also thinking for yourself in the way the POH is used is just as important.

As Genghis has pointed out with ref reading the POH for the shortfield landing technique.

But a pilot must be aware of the limitations of the POH for example TODR are made with a new aeroplane, new engine and experianced on type pilot, I would not want to see a student pilot with a 20 year old aircraft and 1800 hr engine try to match these figures.

[Sans Anoraque]

Don't agree at all Lomcevak.

Downwind, I don't just check for carb ice, I leave CH on to prevent it. Why would you put CH to cold on downwind? Is carb heat going to clear any ice, not at these power settings. I don't see how the exhaust manifold can get so cold, so quickly that it wouldn't still warm the air.
an over rich mixture is possible leading to a rich cut at the worse possible moment. Is it?

[Say again s l o w l y]

Applying full power quickly with the Carb heat set to cold, can lead to a rich cut. I've certainly experienced it with a student and it is not funny at all, especially at low level over trees........

Carb ice is an isidious thing and can creep up on you at any time, For many people getting it on final approach would often be masked by the constant power changes.

How quickly will an engine ice up? It all depends on type. Some like the C150 with the Continental engine can be pretty bad, whereas other (most PA28's) don't suffer any where near as much.

[bookworm]

if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1.

What is the proposed mechanism for "detonation"? Other than "it says so in the POH"? It seems counter-intuitive to me. The richer mixture would seem to move further away from the detonation regime.

Genghis? You raised this...

[hugh flung_dung]

An interesting question. I've always assumed that detonation is more likely because the intake air is hotter, but does that argument make sense when the engine is approved to operate in parts of the world that are pretty damned hot? I guess the difference is that the mixture would be leaned to cope with the less dense air but this means less cooling fuel.
Mmm. I await the answer with interest.

HFD

[shortstripper]

Oh dear!

My point seems to be proved! All these instructors and yet not all seem to really understand the principals of carb heat and its affect on mixture and engine performance themselves, let alone teach it. I guess the POH should be followed then as most of us are unlikely to be taught the why's anyway!

I've not yet come across a POH for a VW aero conversion btw, but I suppose they may exist?

I'm no expert, and stand happy to be corrected, but here is what I "think" are the correct answers to the last few posts.

Applying full power quickly with the Carb heat set to cold, can lead to a rich cut. I've certainly experienced it with a student and it is not funny at all, especially at low level over trees........ Unlikely, but theoretically correct. Have you ever tried the same thing with hot air set? ... You'd probably get the same thing and I'd hazard a guess that it wasn't to do with the carb heat setting!

Carb ice is an insidious thing and can creep up on you at any time, For many people getting it on final approach would often be masked by the constant power changes. Very very true! and yes, in a prolonged descent the exhaust can cool down enough to allow ice to form as the air sucked over it isn't heated enough.

if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1. This is quite incorrect, a leaner mixture (carb heat cold) is less resistant to detonation than a richer mixture (carb heat hot). Bookworm has picked up on this and is right. You may be confusing detonation with rich cut or stumble?

So?

What's wrong with leaving carb heat on hot all the way down? It's unlikely to do the engine any harm and as for unfiltered air? Well that only really applies on run up (and even then, if you do your mag check first, any dust kicked up will have normally blown away before carb heat check). If you are in the habit of pushing carb heat to cold as you apply power it should never become an issue as it's a non-event and full power is available. Keep all power adjustments smooth and things like rich cut or stumble will also be very unlikely to occur.

SS

[Say again s l o w l y]

Actually, I meant to say "with the carb heat set to HOT", that'll teach me not to pay attention!:O

I tried the same scenario in the same a/c (robin HR200) and it did seem to like rich cutting and a momentary loss of power. It was absolutely fine when the carb air was cold and it required quite a violent push forward of the throttle. If increased at a more sensible rate, then there was nothing untoward noticed whatever the carb heat setting.

With regard to detonation, it could be that Piper were worried about the fact that lots of hotter air than normal going into the cylinders could in theory cause pre-ignition by increasing the temps too much and this then leading to detonation.

As has been said, Detonation is usually caused by too lean a mixture, but other things can cause it. This could be exactly why reading the POH is useful, the manufacturers may know something we don't.

All the PFA a/c I've flown have had Rotax engines, so there are definately manuals for them, as for VW engines........ No idea, they are hateful things in acar or van, let alone in an a/c!

(I've spent far too long trying to get an old camper van back to life after my sister knackered it, what a bag of bolts!)

[Lomcovack]

Sans Anoraque

Read the post again I would not dare be so bold as to decide on which method was best but more to show the pro’s and con’s of each method.
I dont see how the exhaust manifold can get so cold,so quickly that it wouldn't still warm the air.
The certification standards for GA aircraft carb heat systems JAR 23.1093 require a temperature rise of 56 deg at a 60% power setting now here is the problem, nobody knows how long the carb heat will need to be applied before the ice melts, Lycoming give 30 to 120 seconds where as the CAA AIC give 15 seconds (there is now a study into this commissioned by the CAA/GASCO). But as can be seen with a power setting of less than 60 % on approach the chances of clearing the ice in time to make a difference is not possible.

With ref to a rich cut.
I recommend a little reading up on what happens in a carb with ice building up on the venturi.


Bookworm

What is the proposed mechanism for "detonation"? Other than "it says so in the POH"? It seems counter-intuitive to me. The richer mixture would seem to move further away from the detonation regime.

The detonation talked about in this instance is not mixture related it is the introduction of significantly hotter air greater than 60 deg into the cylinder head.


Hung flung dung

If you were to increase the outside air temperature at your local airport by more than 60 deg you could expect to suffer from detonation, unless you can reduce the power needed.

Bearing in mind we are talking about all aero engines here and with the lower compression engines fitted to the PA28/C150 it is a very rare event.


Shortstripper

This is quite incorrect oh is it. Read the following from Lycoming and GASCO
http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/inductionIcing.html

http://www.flyontrack.co.uk/carbice2004.htm

Detonation and its causes is a complex subject but I think you have missed the point, I am talking about detonation caused not by a change in mixture but by the extra heat put into induction system when carb heat is left to hot and full power is applied.

No I am not confusing detonation with rich cut as one you will not notice but the other will give you a major adrenalin rush. Stumble is something I do after a night out with the lads.

[shortstripper]

Lomcovack

Both very interesting articles and like I said I'm no expert. I still think detonation is very unlikely to occur where carb heat is accidently left on for two reasons. Having descended the cylinder temperatures are unlikely to be very hot, also the mixture will be richer with carb heat applied. Both these things are opposed to detonation. However, you are correct I suppose if carb heat is then left on significantly into the climb as temperatures rapidly rise and yes, then detonation may occur due to very high cylinder head temperatures and hot air applied. I was thinking you had meant detonation would occur as power was applied for the go around ... which is where it is unlikely. Hopefully by the time you are established in the climb you would have realised carb heat was still applied due to the lack of performance.

Understanding is everything, and I'm sure from what you have said that you do understand. I had just misread how you had applied that understanding to your previous post.

SS

[A390]

This has been a wonderully enlightening topic so far... The views expressed across the board have all been justified and in some cases differtences have been based on real experiences.

But the thing I cannot understand is, why in this day and age are we worrying about a venturi causing so much trauma?

With the advent of fuel injection and computerised 'black boxes' surely there must be an aftermarket solution. Does anyone know of a retrofit injection system that would be compatible for the standard lycoming engines?

If not, would it be worth getting someone to look at building one?

The cost of a retrofit would definitely be outweighed by the enhanced flight safety and allow people to concentrate on the flying aspects a little more, rather than engine management in difficult phases of flight.

A training aircraft is supposed to be simple, why not make it both simple and safe for all stages of flight.


A390

[Oktas8]

Thanks for posting those two articles Lomcovak - very interesting as ss said.

I've noticed, from this thread and from talking to pilots, that British trained pilots tend not to run carb air 'hot' all the time on approach, preferring instead to cycle carb heat on downwind and leave it off for the approach.

The only times I've experienced carb ice is when carb air is set to cold. So why not leave it hot on approach? Nothing I've read in this forum suggests it's bad for modern engines in flight, except in very dusty conditions.

At least one person has commented that, when going around with carb heat applied, he first puts carb air to cold and then applies power. In this country a CPL examiner would have grounds to fail a candidate who, when asked to go around, first put carb heat cold and then applied full power, instead of maximising power as a first action. It's all about minimising further height loss!

On finals we notice a slow drop in rpm, indicative of carburettor icing, Is carb heat going to clear any ice, not at these power settings. I wonder could you tell us where you read that, or which aircraft you experienced it in Lomcovak? I've had carb ice in a variety of circumstances, including an O360 that was actually below 1000rpm due carb ice with throttle closed, and carb heat fixed the problem within seconds. I think it's quite important that pilots have confidence that carb heat will fix carb ice if applied promptly.

Sorry about the long post... :ok:
O8

[Sans Anoraque]

OK, Lomcevak, perhaps it's because I'm thick, but I don't think you are explaining the pros & cons, I think you're suggesting something that makes no sense.
So there we are on the downwind leg at a medium power setting and our check for carb icing shows nothing significant.We put the carb heat to cold So why would you put CH to cold? (I presume we're still talking about PA28s & Lycomings)

[bookworm]

The detonation talked about in this instance is not mixture related it is the introduction of significantly hotter air greater than 60 deg into the cylinder head.

Okaaaaayyyyy, so why does "significantly hotter air" increase the likelihood of detonation?

[shortstripper]

I wonder could you tell us where you read that, or which aircraft you experienced it in Lomcovak? I've had carb ice in a variety of circumstances, including an O360 that was actually below 1000rpm due carb ice with throttle closed, and carb heat fixed the problem within seconds. I think it's quite important that pilots have confidence that carb heat will fix carb ice if applied promptly.

Well Lomcovak is correct there. Do not rely on carb heat to clear ice if the engine has been operating at reduced power settings for any significant amount of time! I've had carb ice build during low power cruise and carb heat really struggled to clear it. That was with a C90 which is very prone to icing, but it does prove his/her point!

Bookworm,

As I said in my answer to Lomcovack, I think it is very unlikely detonation would occur and I believe you are correct to think leaving carb heat "hot" is (whilst not suggesting it's the right thing to do) moving away from detonation regime as you say. He/she? is right though, that if the engine is over heating, then hot air could induce detonation. However, in the context of this subject I think it's a bit far fetched to expect detonation by leaving hot air on by mistake, unless the subsequent climb is very steep, under full power and under extreme conditions!

SS

[Say again s l o w l y]

With internal temps too high, you can get pre-ignition this certainly can lead to detonation.

Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?

The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!

[bookworm]

With internal temps too high, you can get pre-ignition this certainly can lead to detonation.

But does hot air lead to higher "internal temps"? I'd be surprised if it did, as the power is substantially decreased. I'd wager the CHT would go down on application of carb heat.

Anyone got an engine monitor on a carburettor engine?

Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?

http://www.avweb.com/news/columns/186015-1.html
http://www.avweb.com/news/columns/186216-1.html

and an Aussie coroner that was rather more persuaded by Braly and Deakin than Lycoming!

...not to mention some personal experience of reading things in POHs that don't seem to make physical sense.

The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!

Empirical data and scientific principles, yes.

[Say again s l o w l y]

The actual heat and pressure sensing inside a cylinder is a very difficult thing to do. All sorts of things can lead to problems and having a high temp inlet charge is one of them.

Just because an engine seems to be producing less power, doesn't necessarily mean that it's running cooler. It could just be less efficient and running hotter in certain areas, again a very difficult thing to measure without very high tech equipment, a dyno and then a rip down to see what has been occuring.

This was exactly the sort of thing we used to do at uni with car and bike engines of different designs and there were very often results that were unexpected if you looked at the engine in the simplistic way that most pilots would. There were certianly a few that didn't fit the model I had predicted.

[BigEndBob]

Have noticed the EGT drops on carb heat application.
I assume warmer air less dense =richer mixture=cooler exhaust gas =less power.

[Say again s l o w l y]

That was always my understanding and pretty true overall, but there maybe a situation that doesn't fit that, for example having hot air coming into the cylinder and the a great glob of fuel stuffed in.

I think the problem is if you were to leave the carb heat set to hot with full power applied, rather than a momentary issue just after power application.

If you have all the heat being produced by having the throttle wide open AND carb heat hot, then I can well imagine problems, especially as the more power you have applied, the hotter the exhausts will get, so the hotter the carb air will become....... I can definately see this potentially leading to pre-ignition, which then goes to detonation. And this being at relatively low speed, so the cooling airflow isn't massive.

I think they are covering their backsides about prolonged application of full power and carb heat hot, the next question is how long would it need to be to before this could become apparant?
I can just imagine the engineers sitting around a table trying to answer that and then someone saying, "forget it, let's just write the POH to avoid this situation in the first place."

[shortstripper]

Spot on SAS! ......

Except pre-ignition and detonation are quite separate things, and one doesn't necessarily lead to the other :E

SS

[Say again s l o w l y]

No they definately are linked and pre-ignition is definately a pre-cursor to detonation.

Detonation is when the mixture inside the cylinder explodes rather than burn in a controlled manner. You can often get multiple flame fronts and lots of other nasties. Basically the temps and pressures inside the cylinders go up massively and can cause problems to the head gasket, pistons, piston rings, sparkplugs and all the bearings. Not good really!

Pre-ignition is where there is a hot spot that causes the mixture to ignite before the spark plug fires, this can lead to big problems such as trying to turn the engine backwards as well as localised heating, which in turn can burn out piston etc.

Pre-ignition can cause or exacerbate a detonation problem. But pre-ignition is far more dangerous to an engine than detonation. Detonation can be lived with for a while before an engine gives up the ghost, wheras pre-ignition can kill an engine very, very quickly.

Pre-ignition can be caused by all manner of things, from carbon deposits or anything that causes the engine to run hotter than usual. ie having carb heat selected at full power.

There is a phenomenon called "Detonation Induced Pre-ignition" so the two are definately linked.

One final thing. If an engine is detonating, what happens to the EGT?

It goes Down.

[shortstripper]

Ok, but isn't detonation more likely to lead to pre-ignition, than pre-ignition to detonation as you previously posted? Pre-ignition used to be quite common in cars when you could buy various star rated fuels and spark plug technology wasn't so good. You KNOW! when you get pre-ignition and can't ignor it. So how would you go from that to the much milder and less noticable detonation?

Sorry ... I'm getting a bit off topic here, and only really being a bit pedantic, but it's hard to resist (although I'm probably wrong) lol :p

SS

[Say again s l o w l y]

Detonation is MORE likely to lead to pre-ignition than pre-ignition leading to detonation, but it does also go the other way sometimes!

The fact is, that you often wouldn't notice it as the engine has just eaten itself due to pre-ignition!

Pre-ignition is classified as anything that causes the mixture to ignite before the spark goes off, this usually around 20 deg BTDC and peak pressure being at 14 deg ATDC.

If the mixture ignites before the spark plug fires, then it is pre-ignition. Detonation is usually the "end gas" from a normal burn exploding due to lack of octane and causing a pressure spike.(one that doesn't last that long)

This "end gas" could for instance be ignited by a hot spot on something like a hot exhaust valve, so the hotter the engine, the hotter the end gas, the more likely it is to detonate and as you get detonation, you get more heat, so the cyle continues until something breaks.

[Rickford]

long time ago before I met aeroplanes I had met Physics and chemistry and alot of it from an atmospheric view point so I support:

The POH with adaptation to the current circumstances.

There would be appear to be quite alot of evidence that the risk of not apply carb heat enough far out weighs the risk of hyaving it on when you go around.

Personally I teach you cannot have enough carb heat going down wind and whether you need to go back to cold is small fry (you must get rid of any carb ice before the final approach starts) Then carb heat must be on for low power settings as a less then effect pre ventive measure (but at least its step in the right direction). Then to handle the original question I tie the issue odf carb heat on/off to the final stage of flaps decision. i.e do I need it/not need it - m\ake decision and then then carb heat cold

[boofhead]

Listen to Mad Jock, he knows what he is talking about.
In general with those older engines (non fuel injected) the Lycomings, because of the position and type of carburetor and air ducting, do not need precautionary carb heat. Use it to clear suspected ice, but not to prevent ice, since the air is already heated. The Continentals do need precautionary carb heat, since they have a more direct induction system and the carb is below the engine (updraft) in a position where it is colder, usually and more susceptible to icing.
The ducting is also different in the PA 28 compared to the Cessna models, even for the same engine.
Another point though is that to clear ice you need the control in hot before the engine loses power, and on approach there is probably not much heat available so it is better to clear the ice on downwind in all types or at least check to see if there was any ice at that time, and if it is present, keep the power up as long as you can on approach, avoiding idle power until the landing is assured.
A further point I did not see here is that unless you have a gauge to read the carb temp, use full heat only; partial heat might just move the temp into the icing range.

[Say again s l o w l y]

In the U.K we only teach the use of full carb heat, never partial.

In fact the only machine I've ever flown that you can use partial carb heat at any time in the U.K is the R22 helicopter. (Derated engine and a carb temp gauge)

[Centaurus]

Extract from Cessna 152 manufacturer's POH and C172N.

Balked Landing. 1. Throttle wide open. 2. Carburetor Heat..COLD. I suggest this would equally apply to a touch and go landing on other than a dirt or grass strip - a touch and go being nothing more than a go-around procedure.

In other words the aircraft manufacturer recommends you leave the carb heat fully on for the final approach and go-around and then take it off after you have applied full power. This ensures as far as possible that the burst of heat from the exhaust shroud as the throttle is opened wide minimises the possibility of ice problems at a critical stage.

The manufacturer would have conducted engineering studies during the test flying stage and found that no adverse effect on the engine would occur if the recommendation was followed. The Cessna 152 POH states that after landing checks include carb heat to off - meaning the carb heat is on for the landing and landing roll - not placed to off on final just in case a go-around occurs.

A wise pilot also applies full carb heat for several seconds on the ground immediately prior to applying full throttle for a normal take off if carb icing conditions exist. Testing the carb heat during a run-up will not necessarily ensure that ice has not built up after taxying to the runway for take off - especially if you are at a holding point for several minutes.

[pilotbear]

Centaurus,
that is exactly what I believe in and teach when instructing PPL/CPL/ ME/IR and advanced piloting courses. Works in everything from mountain/seaplane flying in extreme in Canada to tomahawk flying in soggy old UK.
Carb heat for 10 seconds at 2000rpm prior to Take off and Carb heat on late in the downwind or when given the localiser intercept heading/base turn; then leave on until landed or after application of full power for missed approach. I always teach the student from early on (so it becomes habit) that full power is preceded by richer mixture and carb ht cold.
The trouble with de-selecting carb heat on finals is it increases the power setting and destabilises the nicely set up approach.

As for partial heat, on the C182 that I fly a lot it there is a recommendation that you use whatever setting keeps the carb above the carb ht range on the gauge, which is sometimes partial at lower altitudes or full at F90 - F110 if in very moist clouds.
However, I don't think I would recommend partial ht if there is no carb temp gauge for reference:ok: