Carb Heat in the Circuit
 

The carb heat comments in the bounce bounce thread reminded me of a question I wanted to ask you guys!

When I did my initial training I was always thought to do a carb heat check on downwind, to clear any ice that might exist. Carb heat would then remain off (cold) for base and final, until next the next downwind check.

Then the club changed ownership, and a checkout with a new instructor took serious issue with this, and insisted that carb heat was applied a couple of seconds before reducing power for base. It would remain on until either on the runway and applying full power, if if I wished I could remove it on short final. Old habits die hard, and it took me a while to get used to this, but I could see the logic and eventually got used to it.

Then more recently, I joined a new club, and their procedure was the same as the first school..ie carb heat as part of down wind checks only, and cold at all other times in the circuit.

I'm just curious as to which is the more common practice.

Thanks,
dp

Carb Heat
 

I first apply carb heat during my downwind checks, then back to off. When I have turned base I then apply carb heat again before reducing power. I keep carb heat on until 300-400 feet when I then turn it off so I have full power available for a go around.

I would guess this is one of those subjects where everyone knows the "correct" way of doing it, and will try to shout down any other methods ;)

Personally, I was taught to apply carb heat as part of the downwind checks, and either remove it on short final, or else either on go-around or when clear of the runway (depending on instructor).

I don't fancy the idea of a quick check on downwind, then leaving it off for base and final - I have flown aircraft which, at the low power settings used on base/final, would certainly get carb icing in this short time without carb heat applied in normal UK weather.

On a related note, it is scary how easy it is to forget habits. Having learnt to fly in a PA28 with carb-heat, then moved to a PA18 with carb heat, and occassionally various other aircraft with carb heat, I now mainly fly two aircraft which don't have carb heat controls (one uses the supercharger to heat the inlet air, and the other is fuel injected). On the rare occassions when I find myself with a carb heat control, it is very easy to forget to use it :eek:

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

G'day Dublin,

I do carb-heat on/off as part of downwind checks. I also then re-apply carb-heat when reducing power to commence descent (i.e. base leg) as I would for any descent! Then, I have to remember (at 200~300') to take carb-heat off in case full power needed for go-around (as per another current thread!).

I am very surprised if people don't use carb-heat on final..! This is one of the most vulnerable phases of flight: you are LOW and you are SLOW... AND the throttle is close to closed, ideal for forming carb-ice in atmospheric conditions which - lets face it - are common in the British Isles and Eire at most times of the year!

Think of the consequences of losing power on short final - when not established for a glide approach: power was planned to be needed to reach the threshold... so you will almost certainly hit the hedge or worse: at Wycombe, where I trained, we have to cross the busy M40 motorway on short final, and dropping in across 6 lanes of traffic would hurt I reckon :ooh: !

But maybe others have a different view: I'd be very interested to learn the rationale behind not using carb-heat during descent through base and final.

Cheers!


Andy :cool: (sunglasses on as I am in Florida at present...)

Good question dp.

When I was first checked out on a PA28 I was taught to use carb heat only while doing down wind checks. I did this during a check out on a C152 and the engine stopped on short final. Fortunately I was able to reach the runway. Since then, I leave CH on until I'm on the ground although, in my forthcoming biennual training flight, I will probably have to remember to change to cold on short final.

I always understood that the Continental engine on the Cessna was much more prone to icing than the Lycoming.

pulse1 - Ouch! You were lucky mate... Leaving carb-heat on until on the rwy is definitely a better idea than having it off after d/wind - as long as you remember to take it off on the go-around. Well done you for making it!

Andy

I was taught to apply Carb Heat when I reduce the RPM in my PA-28 to below 2000 rpm, so when I turn base and have reduced power to 1800 RPM that constitutes carb heat, if you dont reduce power to below 2000 rpm then I guess there is no need?
I would have thought that the more sensible option would be to apply it on final for the reasons Andy stated, there would be nothing worse than to screw up your approach and landing then needing full power if approaching stall etc only to find you have insufficient power due to carb icing, dont think this would go down well with the insurance companies :)

D.

I was taught to apply carb heat during the downwind checks and leave it on until applying full power for the go around or touch and go or until clear of the runway.

However, one day when the wind was 14 knots straight across the runway and pretty gusty, not student solo weather in a 152, instead of scrubbing the lesson I chose to do some circuits with the instructor so as to practise cross wind landings.

I think I might have frightened him slightly on one of the attempts - he managed to go around OK, but made me turn the carb heat off just before landing for the rest of that trip so that he'd have a bit more power if he needed to do it again. So for him killing the carb heat just above the ground was not normal practice but was something you might want to do in bad weather.

The engine does make a difference. In Florida's humidity, a 150 is almost guaranteed to generate ice on descent with no CH. As others have said, I was taught in the Warrior to just give it a quick blast on downwind.

I personally leave CH on until about 100ft agl. You don't want the carb too hot in case you need full throttle a few seconds later. (Or would the extra fuel chucked into the carb just soak up the excess heat anyway?)

It does depend on what a/c you are in. Look at the POH and work from there. In Cessnas the carb control and throttle can be easily used at the same time, so on go arounds that is what I would recommend. Other types PA28's, robins etc. If you have time, carb heat first and then power. But I must stress that ONLY if you are initiating a g/a before getting any where near the runway.
Usually if you have forgotten to put the carb heat away on finals (can't believe that ever happens!!:rolleyes: ) Apply full power and then immediately check if the carb heat is cold or not.

I always (try to) get my students into the habit of thinking about the carb heat whenever they consider power. As stated on another thread, carb heat is a large chunk of power missing especially on C150's with a knackered old Lycoming!

As most others have said,

Flying Cessna 152 / 150, I was taught 10 seconds burst of carb heat on downwind checks (unless evidence of icing, in which case leave it on) and then carb heat on when setting up for descent before reducing the throttle. Carb heat to cold at 300' on short final in case full power was required for a go-around.

I was also taught that carb heat should always be on when the engine RPM is below the green or if it is drizzling or raining.

On top of this the engine should be warmed during glide approaches or PFLs. Just my 2ps worth.

F - Wyg

Much like many others have said, I give ten seconds of carb heat during my downwind checks as I'm in cruise power, then turn it back off.

As soon as I turn base, I begin my descent and the carb heat goes in then to guard against potential icing while I'm only at 1700rpm. Carb heat then comes back off at 300 feet in case I need to go around.

Carb heat on during the downwind leg until on the deck.

Just have to bear in mind that some engines completely do not like full throttle (especially if applied to swiftly) with the carb heat out. May give rich cut.

However chances of that are less likely (and unless it stops - easier to cure) than icing up when you go to 'cold' on finals.

Depends on what you are happiest with for your particular mount.

Injection takes the guesswork out of the equation.

FD

I stand to be corrected but here's my theory.

Carb heat is only properly effective when the engine is hot. Therefore it is not as effective to apply the heat once the power has been reduced as the amount of heat given off to combat ice is less than if cruise power has been applied for several minutes before its introduction.

There are some days when icing is very prevalent, there are others when it is not.

The facts are clear and proven though. If carb ice HAS built up it takes a lot longer than most people think to clear it. Up to 40 seconds in fact. I don't believe that one would wish to find carb icing on base leg and have to wait 40 seconds for it to clear whilst in the latter stages of the final approach.

Basically, I perform the check downwind and if there is signs of icing, redo the check just before pulling power back on base.

If the revs did not rise after applying downwind then I consider it safe to slide from base to finals without another application.

I remember learning to fly 14 years ago and performing the carb ice check "just because I thought it was right to do so". Since then I have found myself in numerous situations where applying the carb heat has given a 300 rpm rise in power with some serious coughing and spluttering of the engine to boot.

As far as I'm concerned, keep applying it even if it's habitual or seems unecessary at the time.



:ok:

I have just done a GFT to re-validate my PPL after 2 years absence, and was told to put the carb heat on when descending on the dead side (PA28), and keep it on till 300 feet.
When I queried this the examiner said it was an engine manufactures directive. :confused:

Just as a practical aside, if a steep approach is made (say 5-6 degrees), then if the engine does stop, there should be enough energy left inf the airframe to make the runway.

I like to arrive at a 1 mile final around 700' agl for this reason.

Gulp! I was never taught to apply carb heat on the downwind leg at all. As part of the field approach checks, yes, but not in the circuit until base leg, just before reducing power. And then off at about 300ft agl. (I was very bad at remembering to do this until my instructor said I owed him a pint for every time I forgot - an amazingly effective strategy! ;) )

I don't quite understand why people are so reluctant to use carb heat. In this country, carb icing is likely at almost any time of year, in almost any weather. Apart from when you need full power, for example for a go-around, to my knowledge it can't do any harm when used for short periods. And at low power settings carb icing is more likely. So why not use it, just in case? If in doubt, why not use it? What's wrong with putting it on on downwind, and leaving it till 200ft or so on final? Am I missing something here?

I know what you mean Whirly. It may come from a belief that using carb heat would damage an engine due to over heating! I've heard this more than once.
I wouldn't recommend that you use it all the time though (unlike in an R22 where you could and can also use partial carb ht) simply due to the fact that the carb air is usually unfiltered and in straight and level flight light aircraft engines need quite a bit of power, again unlike an R22 which has a derated engine, so has an excess of power compared to it's fixed wing brothers.

Hi peeps
i think you are all a bit right but know your engines ! The carb of lycoming engines is recessed in to the sump so gets heat from the engine oil , a conti is stuck out in the elements below the sump very prone to carb ice , not saying any 1 is wrong just passing on info lol

I was taught stick it on whenever the tachometer is out of the green arc (on the C152 that's about 1900 rpm), push it closed about 100ft above the tarmac and on the ground have it open when doing the idle power check, keeping it open until I want to move off (yes carb icing can form on the ground, and half way down the runway is not a good time to find out your not getting full power!)

My biggest problem is remembering to do this since I've been flying the fuel injected C172R - the carb heat is replaced with a panel light dimmer knob!!!

Say Again You are more likely to find a knackered old Continental in a 150 and a knackered old Lycoming in a 152. Not being pedantic but the latter generates carb ice at a reasonable rate, the former generates more carb ice than power at low revs for the reason given be Max Rate . Most times in a 152 you'll get away with a quick squirt of carb heat on the downwind leg. A 150 on a humid day can generate loads of ice by the end of the downwind leg and needs a good dose of heat to clear it.

As jezbowman says carb ice can and DOES form on the ground. The pre-takeoff carb heat check isn't just to see if the carb heat is working, it's also to check if you have carb ice!!! If the engine runs rougher than usual, it'll be the carb ice folks. Don't agree with keeping it open on the ground though. Unfiltered air in a dusty environment (or even not so dusty) does terrible things to cylinders and piston rings. Far better to do your checks and then, if you've been sitting for a few minutes, run up to 1500 or so rpm to get some heat into the exhaust and give a quick shot of carb heat. If there is some rough running keep the carb heat on until it clears. If there is nothing more than a slight drop in the rpm you are clear to go :ok: As Whirly implies, it's not there to decorate the dash panel, USE IT. I seem to recall the CAA saying that carb ice causes more CFIT (controlled flight into terrain) accidents than any other single cause (I stand to be corrected on this).

Don't be confused by the OAT, as strafer points out it is common in Florida which generally is a tad warmer than Blighty!!

As the air contracts in the carburettor throat the temperature drop can be as much as 50 C. It has to be a VERY warm day for there not to be -ve temperatures in the carb throat. Couple that with a touch of humidity and you have a lovely carb ice cocktail. Unless the carb heat is used, silence WILL eventually ensue (apart from the screams from the left hand seat!).

Ramble mode OFF

I've had carb icing on the ground FREQUENTLY in the R22! :eek: It's fairly common on the first flight of the day to find that when you check the carb heat the RPM drops, then rises a LOT as the carb ice melts...sometimes enough that you have to reduce the throttle to prevent an overspeed.

I've also had the engine stop due to carb icing , on the ground, in both a C150 and an R22. In the C150 I'd been out with a safety pilot as I was a bit rusty, came back to let him out, and was about to go flying. I had to wait ages to taxi, the engine started rough running, I closed the throttle (no idea why!) , and it stopped!!! I refused to fly the aircraft, until an instructor assured me it was carb icing. Apparently the position of the carb in the C150 is different from that of the C152 which I usually flew, making it more exposed and more prone to carb icing. So now I use that carb heat little but very often!

In the R22, we'd been out flying, on a day I would have thought was too cold for carb icing - temp below zero and cold and dry. I was shutting down, closed the throttle, and the engine stopped. :eek: Again, I was assured by a multi-thousand hour instructor (since I wouldn't believe it from anyone else) that it was carb icing.

That's why I repeat what I said...in this country carb icing is possible in almost any conditions, and frequent 30 second blasts of carb heat when you don't need full power are not going to do any harm, and could do a great deal of good.

I know there's already more in the PPL syllabus than most students can cope with, but perhaps we need more emphasis on when and why you get carb icing, and less insistence on exactly where in the circuit you put it on and off, with instructor A insisting instructor B's way is wrong etc etc. I spend years with an incomplete knowledge of how it all worked (not saying I know it all now, by any means), and I'm sure I'm not the only one.

LownSlow, Doh!!:O you are absolutely correct, shows how often I fly a C150!

What's nice about the R22 though, is that it has a prominent carb temperature gauge and the ability to use partial carb heat to maintain at least 15 deg.C. I've only ever had carb ice on start up in an R22.

People should use the carb heat more judicsiously, especially on the ground. Low power settings, high humidity (dew on the ground) and mediumish abient temps. Sounds like a recipe for carb ice if ever I heard of one. I have only had carb ice form once in flight, but at least 50 times on the ground, usually in the morning, but not always. As Jez has mentioned, when do you actually need full power most? At T/O, so having a restriction in the carb and a potential stoppage just because the carb ice check was only to see if it worked, not if you have any. Madness!!:\ Check lists should be written so as there is no doubt that you must check both function AND for ice. Instructors themselves need to make sure students understand this. Alot of people do seem to think of the carb heat as a distraction and for some reason are loath to use it. I still cannot really understand why, but it is something I point out and make sure is understood with everybody I fly with.

You don't however need to use 30 seconds of carb heat every time. A good ten seconds should allow you to see whether any has formed. If you get rough running and the RPM picks up, then leave it on until the engine runs clear. One way of noticing if you have carb ice on a fixed pitch aircraft is if the RPM reduces slightly for no reason, often this is due to the throttle slipping against the friction or a change of speed, but why take the chance, apply carb heat just to be sure.

Whirly, I hope when you say you use carb heat "little and often" that you don't mean you use partial carb heat in a fixed wing. I'm sure it isn't, but I don't want there to be any misunderstanding by anyone.

In a PA28 I apply carb heat in any descent where the rpm is low.

In the circuit carb heat goes on as part of the downwind checks and stays on until finals at around 300 - 400 ft. Then it goes off so that I have the power should I need to initiate a g/a.

Good point SAS. I didn't realise that was ambiguous. No, of course you should apply full carb heat or none in a f/w aircraft. I meant that I apply FULL carb heat, for 30 seconds or so, at frequent intervals - before take-of, when doing cruise checks, when realising I've been forgetting to do cruise checks, in any descent at all, if the RPM appears to reduce for no reason, and if I haven't done it for a while so maybe a blast wouldn't do any harm.

I wonder if SingaporeGirl and I had the same instructor - did you train at Wycombe where "KA" also told me I owed him a pint every time I forgot to remove carb heat on short final: seemed to work!

Andy:O

Got my "training flight" with an instructor tomorrow - I just hope he doesn't get any ideas about doing that, it could get expensive.

Being serious for a moment folks,

After transitioning from Lycoming powered A/C to one with a small Continental be aware! the 'A' and 'C' series in particular, really, really do ice up for a pastime. We're talking full carb heat every 5 mins in the air for 30 seconds. A good blast before pulling the power for a descent, then full on for the whole descent, curcuit and approach.

I personally leave it on all the way in, but some dump it on short final.

Kingy

Hey dp,

Seen as I am a Dublin man myself I can only persume you did your training in Weston, and then it was an instructer in the NFC that set you straight about the carb heat!

And you now say you have moved to another club... makes me wonder is it a club based in Dublin Apt?...if it is I am a member of that club also drop me an email: [email protected]


Anyway Carb Heat:

I was thought from the guys in NFC to:

-carb heat check as part of landing/downwind checks (heat for aprox 10 secs)

- full carb heat on either base/finals once the RPM guage is out of the green arc. Carb heat stays on until the wheels touch down.

-full cold then applied on the ground for either a touch and go or full stop

As a rule of thumb I use carb heat when ever the Rpm guage is out of the green arc, it makes sence because if its out of the green arc, you are not using max thrust, therefore your not climbing, your also not cruising because that is done within the green arc, therefore you must be decending or flying slow.... Carb heat is always a good idea in these phases of flight in my opinion.

G74

Applying carb heat for extended periods certainly does no harm ... except as has been pointed out, in dusty conditions ect as the air is unfiltered. I suppose it might just richen the mixture, which I guess could lead to fouled plugs ... but you'd need it on all the time to do that! Tigermoths and a few other types actually have a flap that moves as throttle is reduced to automatically apply hot air, so I'm sure they wouldn't if the warmer air was somehow dodgy!

So, (again as has been pointed out) know YOUR engine and it's particular need for carb heat.

IM

Aussie Andy
No, I trained at Biggin, rather than Wycombe, but there's clearly a cabal of instructors who use the 'leave carb heat on and you owe me a pint' ruse to eke out their drinking money! (Probably mentioned in that little book on useful patter for instructors.) :rolleyes:

There have been a few accidents where pilots have claimed to put carb heat back to cold on short final to find the engine stops over the runway or goaround.
I,ve always taught in 150/152 to keep carb heat on until rolling on touch and go. Always treat the 152 as if it is a 150 just in case it is flown one day.
It takes no more effort to apply carb heat cold than full power.
I have hardly ever had carb icing in PA28, but perhaps because its always checked every 5-10 mins.
Good time to demo carb icing is on an early morning start on the PA28. After 1 min put carb heat hot, rpm falls then after few seconds rises. Carb cold and the rpm has risen 100-200 rpm.
could be just the effect of warming the carb or damp in the inlet tract being sucked into carb producing ice in cold carb.

Some interesting comments, here.

I particularly agree with those folk identifying the differences between the Lycoming & small Continental engines (the latter being more prone to ice). Incidentally, the 6-cyl Cont fitted to some 172's has the same issue.

My little A-65 powered Cub has no intake filter, so the dust issue for carb heat in/out does not arise.

I would suggest that anyone flying an aircraft that has such low power that a go-around with carb heat hot causes performance concerns to consider changing aircraft. Marginal performance as seen on some 100hp Rallye's & Pups leaves little margin for error. However, the return of carb heat to cold at the same time as applying full throttle can also be learned.

Further, lean cuts that can happen on some carburettors (those without an accelerator pump) when going from idle to full power can be avoided by keeping the carb heat hot until the transition from the idle jet to the main jet is achieved.

Well with all these amazing combinations of ways of applying carb heat, shall we start a discussion on engine shock cooling? :O

Shock cooling, what a load of old b*lls. The speed of temperature reduction at shut down is far higher than in any descent and engines don't generally crack crankcases every time they are used. If you dipped a hot engine into liquid nitrogen, then you may have an issue.

I'd like to know if anybody has an impirical data on this phenomenon, rather than supposition. I'm wiling to have my mind changed if it can be proven, but as I've never seen any engine issues relating to 'shock' cooling I feel it is one of the old aviation myths that has been sustained from the days of crap materials in the engines

IO540, what have you started. A good bit of stirring if ever I saw one!;)

Thanks folks!

Looks like I'm in the minority with having virtually all my instructors insist on c/h on downwind only.

Incidentially all the a/c I have flown have been with Lyc's, and the instructor that told me to use c/h on base and final as well as d/w would have been more used to the cont's in 150's.

dp

Ahhh, shock cooling, that old chestnut
 

Say again does an engine cool more rapidly after shutdown? I wouldn't have thought so. The issue with shock cooling (if indeed it exists) is the sudden change in temperature between the cylinders and the block.

After taxiing in and running the engine for a few minutes at 1,200 rpm or so to clear the plugs and equalise the engine temps we turn the engine off. The cowling acts as an insulator and the cylinder heads and the block cool down at more or less the same rate. Compare this with a paradropper (a bad one):

Max power climb to drop height, takes 10 minutes say. Engine at the higher end of the normal temp range. Cylnders nice and hot, oil nice and hot. Get to the top of the climb. Reduce power, loonies jump out, stuff nose down to go and get the next lot. Now you have cold air being forced over hot cylinders due to the "efficient" ducting in the nose. Cylinders are producing minimum power and therefore minimum heat. They cool rapidly. Meanwhile the engine block is out of the breeze coming through the nose and is also being kept warm by the engine oil that was heated on the way up. The temperature differential between the heads (cold) and the engine block end of the cylinder barrel (hot) will cause contraction on one end and (relative) expansion on the other. Repeat this cycle on a busy dropping (or glider tugging) weekend and see how long your cylinders last. It's no coincidence that a lot of para clubs opt for turbine power when they can afford it (better ROC too).

Normal operations shouldn't cause major problems it's the extremes mentioned above that keep the cylinder manufacturers in business!

All engine parts will be subjected to a large cooling airflow not just the pots, and it isn't as if the cylinders actually get cold, with the prop windmilling the pistons are stil compessing the air, combustion is still going on so heat is still being produced. The airflow through out the cowling must be fairly even, or you would be burning the rear cylinders all the time.

Genghis, do you have any knowledge or data to back up either side.

LowNSlow, the meatbombing brigade do push the a/c fairly hard, Do the engines have shorter lives? If so is that due to the fact they spend their lives at high power settings, or is there some truth to 'shock' cooling theory.
The engine temp dropoff rates are very high after a shut down, especially in the out lying cylinders away from the crankcase. I have never seen an aircraft with a split cases due to heating, but quite a few due to vibration, specifically out of balance alternators! That is an expensive problem!

It's the heads that tend to crack. We had it happen once at a gliding club where I flew.

Airflow through a cowling is more to create high to low pressure around the cylinders and help cool air flow down between them, the case itself is fairly well insulated in comparison.

I reckon you're probably right in normal operations. However, it pays to be cautious and if you are mechanically sympathetic it just goes against the grain not to act to avoid any chance of shock cooling. Like leaning the mix ... do it right and it will extend engine life and pay dividends ... do it wrong and it "could" cost you dear. Finesse is everything!

IM