Carb Heat in the Circuit

[dublinpilot]

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

[Maxflyer]

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.

[FlyingForFun]

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
---------------

[Aussie Andy]

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...)

[pulse1]

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.

[Aussie Andy]

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

[Deano777]

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.

[Gertrude the Wombat]

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.

[strafer]

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?)

[Say again s l o w l y]

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!

[flyingwysiwyg]

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

[BeauMan]

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.

[Flyin'Dutch']

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

[Monocock]

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:

[PA38]

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:

[Final 3 Greens]

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.

[singaporegirl]

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! ;) )

[Whirlybird]

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?

[Say again s l o w l y]

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.

[max roll rate]

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

[jezbowman]

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!!!

[LowNSlow]

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

[Whirlybird]

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.

[Say again s l o w l y]

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.

[Northern Highflyer]

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.

[Whirlybird]

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

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.

[Aussie Andy]

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

[pulse1]

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

[Kingy]

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

[genius-747]

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

[shortstripper]

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

[singaporegirl]

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:

[BigEndBob]

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.

[DubTrub]

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.

[IO540]

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

[Say again s l o w l y]

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!;)

[dublinpilot]

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

[LowNSlow]

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!

[Say again s l o w l y]

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!

[shortstripper]

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

[Chuck Ellsworth]

Shock cooling is a very serious consideration on air cooled engines.

Maybe you should read the engine handling cautions on the Wright 1820 for starters.

Chuck

[Say again s l o w l y]

On a massive great '40s radial hanging out in the breeze, I reckon it would be a problem, I'm talking about on (slightly!) more modern enclosed lycomings and continentals as fitted to the majority of light a/c. These don't operate anywhere near the power levels of the big radials and the engine handling characteristics are very different I assume, also they usually operate at much lower speeds, so the cooling effect would be much less.

I wonder whether the idea of 'shock' cooling comes from the days of the radials where it was a problem, but like many aviation myths continues to this day.

[Shaggy Sheep Driver]

The Gipsy engine as fitted to the Chipmunk has 'warm' and 'cold' air positions. It is actually the 'warm' that is filtered, and 'cold' that is unfiltered, and normal practice is to run the engine continuosly in 'warm'. The 'warm' air is drawn from alongside the cylinders, and the 'cold' from an outside air scoop.

RAF Chippys were permanently wired in 'warm', but because we often operate from short strips we like to be able to use 'cold' for take off for those few extra bhp. Once established in the climb, it's into 'warm' and there it stays until the next take off. Go-arounds in 'warm' are not a problem.

SSD

[shortstripper]

I think you'll find the old big radials operated at no more revs than most horizonally opposed types ... in fact usually less. They also tend to give better performance than their horsepower implies as they normally turn a nice big diameter prop at a slower speed than your typical lyc or cont.

IM

[Say again s l o w l y]

True, but big horse power implies big heating. Lots of cylinders and multiple rows, high alts(therefore v.cold temps) and higher speeds does seem like a recipe for problems.

RPM isn't really relevant in this, rather the specific output of the engine and the relative thermal inefficiency of all piston engines shows that the bigger and more powerful the engine, the larger the heating effect.

Combining the two things (high engine temps and a colder ambient temp due to the issues above) I can understand that radials will suffer the problem of shock cooling but I don't think that the relatively tiny lycomings and conti's will suffer compared to the different animal that the radial is due to their different design and utilisation.

[Chuck Ellsworth]

Well lets examine cylinder head temps.

What difference in cyl heads temps will you find between all aircooled aircraft engines?

If you own the airplane you are free to ignore shock cooling with large power changes, actually the proper description is thermal shock and it works both ways, ie. adding power and reducing power.

But hey, have at it and ignore physics, but I would not allow anyone to operate any engine of mine without consideration for thermal shock among other things.

Anyone who thinks there is a quicker thermal change after a normal shut down than when reducing power to say idle at flying speed may well be advised to do a little research on the topic, thermal shock is not a myth it is a fact of physics.

Then that is only my opinion and everyone is welcome to their own way of doing things. :ok:

Chuck

[shortstripper]

Ok look at it this way,

Power is limited by the ability of the engine to get rid of heat. Take any size engine and stick it in a poorly designed cowl and you will find that you can only sustain high power settings for so long before the temps hit red and power has to be reduced. So you could say that the cowl or cooling ability of the engine itself (fins, water cooled? ect) are what limits the sustained power an engine can deliver. Now take your big radial. Sure, it may put out more power than a "typical" cont or lyc, but it's cooling will be designed to take this into account. Assuming it is cowled properly it will run no hotter than a properly cowled cont/lyc so that aurgument is dead! sorry

Now take any aircooled engine and consider where most heat is generated ... the cylinders and heads. The case and oil system are designed to keep internal heat at a controlled level as best as possible and generally do ... but this is much harder for the aforementioned sticky out bits. Therefore we try to either stick them out in the airflow with deflector baffles or within a pressure cowl. On the ground with no airflow through the cowl or down through the fins, the whole engine will cool at a similar rate. OK so the cylinders/heads will be hotter, but they will cool at a "similar" rate (cylinders may cool slightly quicker I guess as they will have a greater surface area). However the cooling differential will be nowhere near as great as when they have a good airflow over them. It is this cooling differential that causes the problem. I suppose ( but don't know for sure ) that a/c like YAKs with closeable fins to reduce airflow through the cowls might have addressed this to a certain extent, but normally this doesn't apply. So, when you have really heated the engine in a high power climb (re glider tugging) and you suddenly reduce power and dive, the engine case will not cool anything like as quick as the exposed heads, and unfortunately, due often to poor cowl design (in many cases) nor will the heads. The cast iron cylinders between the two shed heat very quickly in the middle but the areas near the two ends more slowly due to proximity of ally case and head and also poorer airflow. Anybody who has ever had anything to do with cast iron wil know this uneven heat/heat loss will eventually cause a crack!

As I said before, In most "normal" operations Say again Slowly may be correct that this is not a terrible problem. But in any extremes like tugging, para dropping ect it is very real. Also as I have said, it is something you should gaurd against whatever, so it pays to practice good proceedure anyway and be sympathetic to your engine ... be it in cooling, leaning, use of carb heat ... whatever!

:ok:

IM

[Kingy]

Chaps,

Surely a hard worked club trainer would be subjected to potential shock cooling problems when hacking endlessly around the curcuit, think how many EFATOs and PFLs these aircraft do too. Yet their engines often get to TBO due to regular use and good maintenence...

Just food for thought.

Kingy

[Final 3 Greens]

Well Chuck has about 29,500 more hours than me ;) , but I'd like to make three small observations about shock cooling from experience.....

1 - There are engines that are not big radials that require gentle treatment, e.g. Lycoming and Continental TSIO as used on light aircraft.

2 - I had a share in a light aircraft where we were gentle with the engine and it ran well past TBO and was still in good condition when we sold it

3 - If engines are handled in a cautious manner from early in training, there is less risk of 'regression' with potentially catastrophic results later on ...... I nearly 'chopped' the throttles on a turbo engined Navajo at 10,000' when flying for the first time ... thank goodness that the instructor was alert and had strong forearm which stopped me :mad: otherwise I hate to think of the consequences

[Say again s l o w l y]

Thank you short stripper, that was the point I was trying to put across, in normal ops with a 'normal' flat four aircooled engine, I don't see any issues, but in the examples outlined above, I can very much understand how it could be a problem.

What I meant about the difference with large radials is not that there operating temps are different, rather that when the power is reduced, because of the very way they have to able to get vast amounts of air through them to keep their operating temps sensible, that same ability would cause them to cool faster than a well cowled flat engine. My only experience of radial engines is in a Yak 52/18 and even then it is limited, but the cowling fins had a great amount of importance put onto them.

Despite my comments about whether shock cooling is really an issue in normal light a/c, I do operate them as if there was a problem. Since I'm not really convinced one way or the other because of no actual test data, (I'm still sure about the drop off rates in a recently shut-down engine, but I can't find the results of an experiment from university that I was part of. A slightly different thing really, as it was measuring temperature differentials across an water-cooled car engine) I operate with due caution and teach the same.

[FlyingForFun]

Porsche use air-cooled engines in many of their cars, don't they? Does anyone know if they have any special precautions to avoid shock cooling?

I have no idea what the answer to this is, by the way, having never driven a Porsche. Not sure whether it's a good comparison, though, since car engines aren't subjected to high airflows at low power settings, as aeroplane engines are in the descent.

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

[Say again s l o w l y]

Not any more, they have to be water cooled due to the enormous heat generated by such high power outputs. An air cooled donk would just melt!!

My sister's VW camper, is definately air cooled!! I don't see any potential probs as the engine is mounted at the back in an enclosed space without a huge amount of cooling airflow, but as it is such a low powered engine she has never had any over heating issues with it.

[Chuck Ellsworth]

Good morning again:

I have a few spare moments so will make a few short comments regarding engine handling.

I have worked all my career as a pilot / mechanic and during the past fifty years have made many observations and learned many lessons by making mistakes.

When operating any aircraft engine you should first read the manufacturers recommendations for that engine and model number.

Three main factors contribute to wear in an engine.

(1) Piston travel

(2) Thermal spikes and or overheat

(3) Bearing and moving parts reverse load changes..

So when operating the engine controls you should strive to move the throttle as smoothly and slowly as practible for any given power change.

As well remember that the lower the OAT the faster the temperature drop will be when reducing power.

That is about as simple as I can describe engine handling. :ok:


P.S.

Note number one piston travel, remember that one when advancing prop RPM especially during the approach and landing.

I leave the RPM at cruise setting during the approach and landing, when the throttle / s are closed for the touch down is when the prop control is moved to full fine.

Chuck

[LowNSlow]

Kingy think of the sequence for a typical training circuit.

1. Full power (2500 rpm ish) for take off and climbout.
2. Reduce power to 2300 rpm (ish) for the downwind
3. Reduce power to 1900 rpm (ish) to attain Vfe
4. Adjust power on base to reach the runway
5. Power off for landing
6. Taxi in at around 1000rpm

All the changes are relatively gradual and the power and temps have a chance to stabilise between changes. This is why a good flying school should realise the TBO on their engines.

As far as I am aware, hotrods like Bonanzas etc need a bit of decent speed management when coming out of the cruise to join the circuit. It's not just slowing down in time, it has to be done in a manner that doesn't over cool the engine.

[Say again s l o w l y]

At the flying schools I teach at we have had no problems with a/c not reaching their TBO's, depite the (relatively) rough handling they are subjected to; lots of power changes, PFL practices, start up and shut downs at least 6X a day. Compare this to most of the private operators at the airfields, who seem to have no end of problems. I feel this is down to under-use rather than the way the a/c are operated.
Private a/c have an easier life as they are generally operated for longer flights, have less take offs and landings, less hard landings(though this may not always be the case!!) just as much maintenance support and are just generally treated better than you average club hack.

Despite this I know of at least 6 a/c at my home field that didn't get anywhere near their TBO's. I know the guy's who fly them and they are not muppets who handle the engine badly, and yet they have had major problems.

I always think that we over emphasise on certain areas such as 'shock' cooling and ignore many of the more serious issues, such as corrosion due to under use, that affect far more people than a relatively unproven 'problem' that is shock cooling on a light a/c used 'normally.'

[Chuck Ellsworth]

What is "used normally" mean to you as far as power changes related to OAT mean Say Again Slowly?

Maybe I just have not understood your methods of teaching engine handling.


With all due respect shock cooling is not a relatively unproven problem, it is a fact.

Chuck

[Say again s l o w l y]

Normal use to me Chuck is tavelling from A to B generally below 8000ft and often in +ve temps. Smooth power changes, not rattling the throttles around constantly, lowish speeds (certainly below 130kts) with maybe the occasional PFL.

It's probably easier to describe conditions that I would call abnormal in respect to most people who read this particular forum.
Paradropping/Meatbombing.
Aero's.
High altitudes.
Instructing, especially exercises 4-10(JAA syllabus) Lots of power changes and climbs and descents.
Any aircraft with radial engines.

The vast majority of private pilots in the U.K do not own their own aircraft and due to the way he syllabus is often taught, they don't tend to use the mixture control so CHT's are often relatively low, because they don't care about hom much fuel they use or often understand the whole issue of leaning the mixture correctly.

Personally I'm not sure what you mean about power changes related to OAT

I teach my students so that they understand all the issues that relate to the care of an a/c engine from mixture control, shock cooling, proper warming before applying high RPM's, problems that can arrise if the engine is too hot at shut down, different procedures when running in an engine, use of carb heat, hot and high issues as well as making sure they have a good basic knowledge of how an engine actually works. As well as the specific issues regarding each an every engine. If I'm missing anything out, please tell me. It's always good to get some feedback from those who are more experienced.

[Chuck Ellsworth]

Thanks for the explination on what / how you teach engine handling.
I had problems with your thoughts on thermal shock, by OAT I am refeering to the lower the OAT the slower you reduce power to a given setting that will ensure the engine does not cool to low.

Do you teach the relationship of carb heat to ensure that the mixture is not to lean?

Chuck

[shortstripper]

Just a few things to note ...

Lots of Porsche engines are aircooled ... the 358 is basically a beefed up VW with better heads.

Well used engines often make TBO where under used ones often don't. This isn't so much to do with engine handling as it is to do with internal corrosion. A well run, well used engine will outlive a well used or little used but well run one. Make sense? Well heat will cause the breakdown of oil and cause certain acids to be released ... this is bad. Lack of use alows any water vapour to remain within the engine ... this is bad, also certain parts such as cam followers, pushrods ect are held tight to other parts in one position for extended periods ... this is bad. Poorly run engines suffer more wear by misuse than well run ones ... this is bad. However, accept in catastrophic circumstances this may not be as bad as a a little used engine that isn't properly inhibited or at least run regularly.

Hopefully that makes sense?

Any more contentious myths? ... or none myths?;)

IM

[Say again s l o w l y]

I have to say Chuck that I don't, Since the vast majority of flying here is done in a smallish temperature range, neither very high nor very low (also due to the fact most people stay below 5000' the vast majority of the time) It has never been an issue, but it is something I will certainly brief my students on, so if they do end up flying in different places, they may have a chance to get it right.

We certainly do teach about the change in mixture due to carb heat, it could be catastrophic if you didn't! if somebody descended from high alt without richening the mixture and with carb heat applied.... Detonation city!!
We generally teach as much as possible about the workings of engines and aircraft, I find it makes a much safer pilot, to be honest every one that learns to fly should be made to strip an engine down and descibe the parts of it and the problems that can arise before they get their licences issued!!

[Chuck Ellsworth]

S A S :

Do you teach your students to use carb heat to check for to lean a mixture, when leaning the mixture?

Chuck

[IO540]

I've been away for a couple of days and yes I seem to have started something off :O

As the owner of a very expensive IO540-engined plane I have read everything I can find on engine management generally and shock cooling specifically, and have not found anything other than the same old stories being regurgitated by everyone, with no supporting data whatsoever. Nobody has any data to support a CORRELATION between shock cooling and cracked cylinders - never mind an explanation of what exact mechanism would cause it which accounts for anomalies like the supposed lack of problems caused by shock HEATING on takeoff for example, and the fact that most schools don't get cracked heads despite doing PFLs all the time.

Except one: a year or so ago I read an article by a commercial glider towing pilot, who over 20 years or so has worn out or damaged many engines. He found that they used to get cracked heads regularly, until they modified their operating procedures so that instead of cutting power abruptly after releasing the glider they allowed the CHT to stabilise at its CRUISE value (which doesn't take all that long, a minute or less), and after that they could cut power all the way back (and descend quickly) without any problems.

So, based on this bit of data, it would appear that shock cooling does not happen unless the CHT is above a certain point to start with.

For someone who wants a bit of reading, see

http://www.avweb.com/news/columns/182146-1.html

for some relatively respected stuff on engine management.

[Say again s l o w l y]

No I don't Chuck, we generally teach (depending on whether the a/c is fitted with anEGT gauge) to bring mixture back until you get rough running and then richen by 1-3cm depending on type. Or go to peak EGT and richen until temp drops by 25 deg F.

IO540, my point exactly.

[Chuck Ellsworth]

How many cylinders are probed to read EGT?

If after leaning the mixture by either method that you use what would you do if as a final check you applied carb heat and saw an increase in power?

Chuck

[Say again s l o w l y]

Probably be worried something was wrong!!

Seriously Chuck, I've never even thought about this, I assume you are implying that the mixture would be too lean. We never teach people to leave the mixture at it's most efficient, as it would be too easy for inexperienced chaps and chapesses to make an expensive error. We definately teach for people to err on the safe side with regard to leaning. Especially as most never go above 3000ft anyway!!

It's a good technique and I'll try it out tomorrow. Can't believe I'd never heard of it before!

[Final 3 Greens]

Chuck

May I join in? I'd enrichen the mixture a little, on the basis that hot air is less dense than cold air and the fuel air mixture is therefore slightly richer with carb heat 'on.'

[ShyTorque]

I too think that Chuck is checking for a weak mixture by applying carb heat, which gives a lower charge of air. A weak mix is far worse for the engine than too rich, hence that check.

There was a problem with cracked Lycoming cylinder heads during my time instructing on RAF Bulldogs, put down to pilots not giving due consideration to the effects of rapid cooling during the descent.

[Say again s l o w l y]

ShyT, was it ever proved? Or was it a convenient scape-goat. Did you continue to have these probs when procedures were changed?? (I'm assuming they were)

[Chuck Ellsworth]

The use of carb heat as a check for to lean a mixture is the poor mans multi function mixture check and it is dependable...

If you find that you get no drop in power or an increase in power when you apply carb heat, that is a bullet proof indication the thing was to lean...hot air is less dense therefore hot air enrichens the mixture.

Normally you get a drop in power with carb heat due to the hot air enrichening the mixture.

So if the mixture is to lean carb heat will correct the to lean mixture back to normal...thus the increase in power..

Poor mans check and works every time to ensure you definately are not to lean.

See how nice I am sharing all these simple things with everyone.

Its usually the simple things that get us. :O

That and thinking with the wrong head. :ok:

Chuck

[Say again s l o w l y]

Thanks for that Chuck, have you any other gems that I can use to try and make myself look intelligent in front of students!!:p I need all the help I can get most of the time!

[Chuck Ellsworth]

SAS :

No problem.

Any time I see a topic that may need some suggestions I am more than happy to share little things.

Chuck

[ShyTorque]

Say again slowly,

We were regularly operating up to 10,000ft and so leaning the mixture was taught from an early stage of training. Our type of flying was in the abnormal category according to your earlier classification but to be honest I've never heard it called that before.

Procedures weren't changed as such. Pilots were reminded of the importance of complying with the manufacturer's advice already in place. It was important that QFIs were fully informed and fully understood the importance of engine warming etc, as many military fliers of my era had been trained entirely on jet aircraft and only converted to piston engines later on in their careers. :ooh:

[Say again s l o w l y]

Interesting way of doing it!!

It isn't really 'abnormal' flying in any context other than maybe in way the 'normal' pilots who frequent this forum generally operate their a/c.

Did the reminders actually do anything about the number of cracked cylinder heads?

[ShyTorque]

Say again Slowly

I don't know but then I was a pilot - you would have to ask an engineering stats person. ;)

[IO540]

Chuck

If you find that you get no drop in power or an increase in power when you apply carb heat, that is a bullet proof indication the thing was to lean...hot air is less dense therefore hot air enrichens the mixture.

Is this really true?

The reason carb heat application reduces the engine power (shown by an RPM drop on a non-CS prop engine) is that the air is warmer, which reduces the efficiency of the engine.

The operation of the carb, or the fuel injection unit, should reduce the fuel flow approximately correctly to maintain the air/fuel ratio.

(I say "approximately" because both metering systems measure a mixture (no pun intended) of volume flow and mass flow when it comes to air, but they do measure mass flow accurately when it comes to the fuel.)

[FlyingForFun]

IO540,

There's another thread about carb heat, in which I explain that the fuel flow does not decrease when you apply carb heat (or at least, not appreciably). However, I think that when Chuck says: "Normally you get a drop in power with carb heat due to the hot air enrichening the mixture" he is over-simplifying things slightly.

The application of carb heat reduces the amount of air going into the engine, which (ordinarilly) reduces the power output.

Because the fuel flow doesn't change, you also get a richer mixture. Now, if the mixture was too lean to begin with then the lean mixture would cause a lack of power (as well as engine damage). In this case, when you apply carb heat, not only will you get a reduction in power because of the lower air flow, you will also get an increase in power because of the more efficient fuel burn due to the more appropriate mixture - and the increase in power may well be more than the decrease in power, resulting in a net increase.

At least, I think that's what Chuck is getting at.

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

(PS - Chuck, hope you don't mind me jumping in. Explaining it myself usually helps me get it clear in my own head, though.)

[IO540]

FFF

I don't believe your explanation is correct (but I don't know for sure)

The normal operation of a carb (or a fuel metering unit) is that it measures the flow of air (actually it measures a parameter which is a combination of volume and mass flow) and dispenses the fuel accordingly.

So if you reduce the airflow, the fuel flow must also reduce. But do you actually reduce the airflow with carb heat? I would bet that whatever air flow parameter is measured by the carb does not reduce - what merely happens is that the air is warmer which reduces the combustion efficiency.

In the present thread context, the question is why the application of carb heat fails to drop the RPM (assuming non-CS prop) if the engine has been leaned "excessively". I don't know the answer to this. I suppose that if the engine was leaned to (or past) peak EGT, and if applying carb heat really made the mixture richer, then the RPM would either not charge or actually increase a little. Can one get carb heat to INCREASE the RPM on an engine which has been leaned far enough?

So it's an interesting question.

Re leaning: contrary to very common belief, leaning does not cause engine damage unless it is run at high power, say 75% or higher. Lots of people are mortified of leaning during taxi for example - but it is completely and utterly impossible to do engine damage at 10-20% power or whatever it is. Lycoming authorise peak-EGT operation (the worst temperature case actually) up to 75% power. Detonation occurs before peak-EGT (about 50F rich of peak), right where people are likely to be cruising if using the carb heat method to check for correct leaning! But fortunately detonation cannot occur at power settings below 75% typically. Leaning past peak EGT is no danger because all the temperatures actually fall again. It is fine to lean way past peak EGT on the ground for example.

[FlyingForFun]

IO540,

From my post on the other thread (http://www.pprune.com/forums/showthread.php?s=&threadid=106871) that I referred to:The fuel flow into the carb is dependant on the volume of air passing through it. The volume of air, for any given throttle setting and engine speed, will not change if you apply carb heat. Therefore, the fuel flow will be identical.

What does change, though, is the density of the air. Warm air is less dense, therefore there is less mass of air going into the engine (or, to put it another way, fewer molecules). If there are fewer molecules of air, then you can't produce as much power, since you need the air for the fuel to burn.Once you accept this, the idea of carb heat increasing the power on an over-leaned engine makes sense, as per my earlier post on this thread.

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

[ShyTorque]

IO540,

I don't think this statement is correct:

"The normal operation of a carb (or a fuel metering unit) is that it measures the flow of air (actually it measures a parameter which is a combination of volume and mass flow) and dispenses the fuel accordingly".

I don't think a carb is so clever as to measure mass flow. Fuel is drawn through a jet or series of jets, depending on the barometric depression in the venturi. The depression is merely proportional to airflow velocity.

Also, you said that detonation cannot occur below 75% power. What basis of fact do you have for that? In my experience, piston engines can suffer from detonation at much lower power settings, depending on the load on the engine.

Hence the requirement to remember "RPM max, mixture fully rich, carb heat cold".

[Chuck Ellsworth]

My reference to using carb heat to check for to lean a mixture was meant for use in small bug smasher type airplanes such as normally used in flight training.

Hot air is less dense than cold air...therefore hot air will make the mixture more rich...

Chuck

[IO540]

ShyTorque

I didn't say a carb measures mass flow (of the air). I said it measures some combination of volume flow and mass flow. It doesn't measure pure velocity either - I am sure air at a given velocity at 100,000ft will suck up a lot less fuel in the venturi than air at 10,000ft at the same velocity would.

If a carb (or a fuel injection system) measured pure air mass flow, that would be great because you could set up a particular engine operating point, e.g. peak EGT, during cruise climb, and just sit there all the way to the aircraft's operating ceiling (as the MP drops off) without any further leaning, and the engine would be running at peak EGT the whole time.

Altitude compensated carbs are an attempt to achieve this.

Car engines that operate at peak EGT have to have a mass flow sensor. This can be done by conventional air flow measurement aided by measurement of pressure and temperature. I don't think any old-style aircraft engine does this though.

The reference to 75% power comes from GAMI's tests on IO540 type engines. I would have expected it to be roughly right for other engines of similar design. It won't be easy to verify because it is said that one cannot tell detonation from the cockpit (pre-ignition is something very different). Yes it will depend on the engine load but that is what power is, at a given RPM. Can you give an example of a current-model engine which can be made to enter detonation at say 50% power?

[ShyTorque]

This is now way off the original topic of carb heat, my apology for that.

IO540,

At 100,000ft a typical spark ignition engine or even a turbine would probably not be running at all.

However, to restate what I said, a simple carb does not measure anything. It just senses the difference between ambient and venturi pressures. The fuel jet is exposed to the depression in the venturi and is drawn out as an aerosol spray, mixed with the air and vapourised, either in the inlet manifold or partly in the cylinder. You could look at this from another viewpoint and say that fuel is pushed into the venturi by atmospheric pressure over venturi pressure.

More complex carbs, as you say, can be designed to compensate for altitude to some extent.

The simple carb doesn't know what altitude is, or what altitude it is operating at. Therefore, if the same differential pressure existed at your theoretical 100,000 feet, the same amount of fuel would be drawn out and into the engine because the fuel metering orifice / jet size is the same. However, because the density of the air at that altitude is a fraction of that prevailing at sea level, the mixture would become massively rich at such an altitude, there simply wouldn't be enough oxygen molecules to react with the fuel. The power output reduces with altitude because the airflow through the engine is less, although the engine is theoretically more efficient with increased altitude.

It isn't correct to generalise about when detonation might occur. Your quoted figures are undoubtedly correct for your engine as they will have been derived from complex experimentation by the manufacturer. It is a complex subject that depends on a number of factors such as cylinder size, combustion chamber shape, mixture strength, fuel quality, ignition timing, charge temperature, CHT etc. I agree it is definitely not the same as pre-ignition, I never said it was. It's not the same as auto-ignition either.

I can't give you an example of an engine that can be made to enter detonation at 50% power as I'm not an engine manufacturer, or someone with access to a library of such data. I do have some experience of piston engines, both building them, modifying them, using them and a while back, teaching student pilots about them. From my own experience, detonation may occur at any power setting if the conditions are suitable, certainly at smaller throttle openings than would give 75% power.

For example, in some car engines, you can hear the detonation "pinking" at very small throttle settings if the right conditions prevail. Opening the throttle further may actually result in the detonation no longer occurring.

[G-Foxtrot Oscar 69]

I always use 20 Secs Carb down wind Ie BUMFICH (C for Carb)

Then as soon as you configure to start descent ie on base if flying cct then Carb on and power back to "descent setting" ie on PA28's approx 1500 RPM.

If you do a go arround must remember to put Carb Off. I have wondered on many occasions why I have sh:mad: t performance in the climb. Oops Carb is still on. I have however found it don't blow up the engine as I was always told. I guess it don't do it much good though and the aircraft don't perform.

I must admit I never really saw the point of downwind carb as you put it on for a good long stint in the descent and final any way but old habits die hard and I ani't risking it now.