A carb heat trick

[9 lives]

I was talking with a client today, who is the chief pilot overseeing a flock of 172's which are used for survey. One of his pilots (a 1000+hr guy) had just limped away from destroying a 172, following a forced landing. Apparently the engine pretty well ended up in the passenger's seat beside him - lucky guy, other than I suspect he'll be looking for new employment.

There was some discussion that perhaps carb ice had been a factor in the engine failure which preceded the crash. While discussing carb ice topics, I mentioned a trick I was taught, and I was a little surprised that he had not been aware of this, so maybe it's time to present it again.

If you have decided that you need carb heat to deice the carb, you need lots, and you need it quickly, 'cause things are getting worse in the mean time. To get the most of the heat available from the engine, into the carb, after applying carb heat (and worrying more, 'cause now it's really stumbling), lean the mixture to achieve the most lean setting you can manage without making it run worse. You're not developing full power with carb ice anyway, so you're not going to damage the engine by overleaning it. The presence of carb ice will have enrichened it anyway, but leaning it to "normal" and thereafter to peak lean will result in more heat from the exhaust, which turns into the carb heat that you need.

As the carb heat begins to have an effect, consider flying at a reduced power setting (and leaning more if need be), as less volume of air going through the carb at the lower power setting needs less total heat to warm it and melt ice.

If you have an aircraft with a carb air temperature indicator, watch it as you do this, and the affect will be obvious. A CAT indicator is a great investment.

[westhawk]

I have a couple of doubts.

Since peak EGT typically occurs very near to the mixture for best power, a mixture leaner than that might result in a lower EGT and therefore less heat to the carb. I like the idea of leaning to peak EGT but I'd want avoid leaning too much. (though the difference may be small in terms of heat energy transfer to the carb heat air)

At any position less than fully open, wouldn't the pressure and temperature of the air passing the throttle valve be even colder? Possibly contributing to faster icing?

I think perhaps sticking with the recommended procedure of applying full heat as soon as carb icing is suspected would do the most to help the situation. Then of course the mixture may be leaned as required to achieve a smooth(er) running engine. I think the key point is timely application of carb heat before the ice gets too thick.

Anyway that's my initial thoughts...

Oh, a question! Where exactly is your carb air temp sensor installed? Before the throttle valve or after?

westhawk

[India Four Two]

ST,

Great suggestion. I hadn't thought of that "trick" before.

Concerning carb icing, I had never experienced it prior to coming to Canada. The main reason for that was the RAF Chipmunks that I mostly flew in the UK had the carb heat wired in Hot.

My first experience was solo, flying a 172 from Calgary to Banff. There were some clouds ahead with virga underneath, so I thought I would do a carb ice check. When I pulled the carb heat control out, there was instantaneous rough-running that was so bad that my instinctive, inexperienced reaction was to push the control in! :eek:

So the moral of the story here is that students should get their instructors to show them the effects of carb icing on a suitable day, preferably within gliding range of a suitable landing area. ;)

I agree with your comments about the carburetor temperature gauge. I've just started flying a 182 that has one and it is very comforting to be able to glance at the gauge and make sure the needle is not on the yellow arc.

[Big Pistons Forever]

Call me a skeptic on this one.

I have never seen carb ice that did not clear simply by applying full carb heat. The engine may stumble for a bit but it will pick up quickly so by the time you finished faffing around with the mixture the ice will be gone anyway. I live in a temperate Coastal city that has conditions conducive to a very high probability of carb ice for at least 6 months of the year.

The signs of carb ice are clear if you are looking for them starting with a uncommanded reduction in power followed by rough running. There is simply no excuse to let the engine develop so much ice that it stops.

[piperboy84]

I have a bone to pick on the Carb heat usage issue, my friend recently totaled his plane and was incredibly lucky to walk away from the wreckage. Also, I recently read a CAA accident bulletin about a fatal crash in North Wales both incidents happened on final and both were attributed to carb ice.

I understand that instructors and flight manuals alike call for a “check” for the need for carb heat upon approach. Here is my question why not put the carb heat on just prior to entering the pattern and power reduction AND LEAVE IT ON right up to short final and the runway is assured and where if you need full power for a go-around you have it and you will not be getting unfiltered air if landing on a dirt strip.

What’s the downside of leaving it on throughout the entire pattern till short final compared to the risk of picking up icing late on finals after you have "checked" for it back on the downwind and returned it to cold?

[stevelup]

I thought that was standard practice? It's certainly how I was taught...

[BackPacker]

I thought that was standard practice? It's certainly how I was taught..

Same here. Carb heat on as part of the downwind checks, off on short final.

As the carb heat begins to have an effect, consider flying at a reduced power setting (and leaning more if need be), as less volume of air going through the carb at the lower power setting needs less total heat to warm it and melt ice.

I wonder if this is actually true. A reduced power setting also means less power from the engine (duh) and thus less warmth produced. Which in turn reduces the amount of warm air available for carb heat - assuming carb heat is generated in the traditional way, with a cuff around the exhaust.

But there might be another reason not to do this. I'm not an expert, but here's my reasoning. Carb ice is formed due to a temperature drop in the carburetor. This temperature drop can be caused by two things: The pressure drop across the butterfly valve that regulates the MAP, and the evaporation effect of the fuel in the venturi.

The evaporation of fuel, and the resulting drop of temperature, scales more or less evenly with the MAP and thus the power applied. A higher MAP leads to more fuel evaporation, leads to more power from the engine, leads to more heat produced, makes more heat available for carb heat. So assuming you have carb heat applied, it would not matter all that much how much power you apply as these effects cancel each other out.

But the ice formed by the butterfly valve has a completely different, and in fact negative relationship. If you reduce power you do so by closing the butterfly valve. This leads to a lower MAP and thus a higher pressure drop across the valve. This may produce more carb ice, right at the time when the engine is producing less power and thus less heat.

Because of this last effect my gut feeling would be to open the throttle completely in case of carb ice, instead of restricting it. And in fact this seems to be confirmed in POHs and such, which call for full throttle in case of persistent carb ice.

Anyone can confirm I'm right?

[A and C]

First a few words of caution, carb ice is not only a function of the engine and atmospheric conditions it is also a function of intake design my PA28 exhibits less inclination to carb ice than my DR400 but both have the same engine and carb.

I would strongly recommend that cab heat is applied just before the power is reduced to make the approach to the runway ( top of base leg or equivalent position) because at point the heat exchanger has the most heat in it and is capable of doing the most good in terms of ice removal and as the carb is now clear of ice the engine will continue to run and put some heat into the system. The real danger if you don't do this being that if the carb should ice during a glide approach you won't know it has iced until you try to go around when opening the throttle gives you no engine response.

EDIT

Backpacker I agree with you, the most likely time for a carb to ice is with the butterfly valve closed, this position gives a very large acceleration of the air ( so large PX & TX drop ) as it try's to get through the small gap, ad the fuel from the idle jet evaporating and you have the perfect conditions for carb ice.

A major ice encounter !
A few years back when in the cruise at FL090 in solid IMC OAT + 2 the DR400 engine started loosing power I applied carb heat and this slowed the power loss but it was clear that the aircraft was going to have to descend to increase the OAT being as the power was reducing the throttle was left in the cruise position because it would not move and as is the training mantra the mixture was set full rich...............l BIG MISTAKE !!!!! The rpm dropped and the EGT fell rapidly, my next action was to lean the mixture to give me max EGT this gave me the best RPM ( power ) and the most heat in the carb heat exchanger.
The mixture was set a long way lean of normal optimum for the ( rapidly decreasing ) altitude because the carb was so restricted that any more fuel would result in a massively over rich mixture and a decrease in the heat in the carb heat exchanger.
Eventual at about 3500 ft the engine power was restored and the settings retuned to the normal values................. A little later during the final approach to the airfield as we broke out of the overcast I was thinking that the Irish Sea looked very cold and uninviting !

After this incident the carb heat system was checked and a very well hidden crack found in the carb heat valve, the result of this crack was that the engine was only getting partial heat and so the systems ability to clear ice much impaired.

[running dog]

I was taught many years ago that you selected carb heat on as part of the downwind pre landing checks and left it on until within gliding range of the threshold. Only turn it off because its unfiltered air so as not to get dust into the engine.

In cruise select it as part of the regular scan check and if any of the signs as above. Leave it on for at least 30 seconds. If the engine runs rough you are burning water! Wait for it to smooth out before closing.

There should be plenty of heat in the exhaust system to melt the ice unless you are in a long slow descent when warming the engine every thousand feet is required for the engine's health as well as the carb heat!

[FullWings]

If you have decided that you need carb heat to deice the carb, you need lots, and you need it quickly, 'cause things are getting worse in the mean time. To get the most of the heat available from the engine, into the carb, after applying carb heat (and worrying more, 'cause now it's really stumbling), lean the mixture to achieve the most lean setting you can manage without making it run worse.
Hmmm. The logic and physics sort of makes sense but do you really want to add another variable (mixture setting) to the equation when dealing with a non-normal event? As the onset and clearing of carb ice is a dynamic process, the mixture ratio will be altering anyway without any input from the engine controls. Why further complicate the issue with a procedure that’s not in the POH?

IMHO, many icing events could be avoided/mitigated by a) a thorough understanding of the causes, b) regular checking and c) applying the clearance procedure in the correct way, i.e. giving it time to work.

[A and C]

Much of what you say is true but outside the training environment most people descend with some power in a cruise descend rather than a glide, this keeps the engine temp more stable and makes the practice of warming the engine unnecessary.

I also think that the "keeping dust out of the engine" thing that the training industry is so keen on is a myth unless you operate in a desert environment, the path for the air when carb heat is selected is such that in a normal northern European environment not a lot of rubbish will enter the inlet track.

[Gertrude the Wombat]

I was taught many years ago that you selected carb heat on as part of the downwind pre landing checks and left it on until within gliding range of the threshold.
I was taught to leave it on until on the ground (it's easy enough in a 152 to push it in at the same time as the throttle for the go-around or T&G).

[A and C]

While your logic about changing to much at once is reasonable my guess is that if I had not leaned the mixture to get optimum heat in the engine my aircraft ( and may be me ) would be at the bottom of the Irish Sea.

[FullWings]

I suppose the point I’m trying to make is that unless you have information that points pretty strongly towards an alternative course of action, following the manufacturer recommended procedure to begin with is generally a good idea. If it doesn’t work or produces sub-optimal results, then that is the time to experiment.

Boeing have this to say about their jets and I think it contains wisdom applicable to operating most other types of aircraft too:

“It should be noted that, in determining the safest course of action, troubleshooting, i.e. taking steps beyond published non-normal checklist steps, may cause further loss of system function or system failure. Troubleshooting should only be considered when completion of the published non-normal checklist results in an unacceptable situation."

[Discorde]

In cruise select it as part of the regular scan check

We've all heard of FADEC engine controls. The acronym is useful for light aircraft periodic cruise checks:

F Fuel config
A Alternator/ suction
D DI sync
E Engine gauges
C Carb heat

[ChickenHouse]

@ST: I remember from once upon a time at checkride, when I was told by my old instructor to no longer use that "trick". It will do the job i.e. with an O-200 or O-300 in older Cessnas and if you are fully aware of what you are doing, but won't in many other cases - especially younger aircraft. If you encounter carb ice, you have to apply heat asap, and yes, a bit of leaning will help increase temperature (the same lean as it is used in cold winter to heat up morning cold engines), but there is a danger in too excessive carb ice melting - resulting in engine cut off once the ice melts. The instructors I know therefore decided it is not worth the bit of more heat at increased risk.

[funfly]

Isn't it about time carb heaters were standard. I had them fitted over 12 years ago which consisted of a water jacket (engine was water cooled) but I am sure even then electric heaters were available.
Still did the carb checks but it did work and I never experienced carb icing with them.
2015 and aircraft engines are still prone to icing? The one thing that results in ef and the one thing that can be cured.
Unless you have an injected engine when you cease worrying.

[Maoraigh1]

I often lean if a carb heat check indicates ice. I was taught to leave carb heat on until touchdown, but later was told to go to cold air on very short final. I went back to hot air until on the ground after the engine almost stopped when I opened the throttle to turn 180 and backtrack. O200 in Jodel DR1050 in Scotland is an excellent ice-making combination.

[Above The Clouds]

piperboy84
Here is my question why not put the carb heat on just prior to entering the pattern and power reduction AND LEAVE IT ON right up to short final and the runway is assured and where if you need full power for a go-around you have it and you will not be getting unfiltered air if landing on a dirt strip.

What’s the downside of leaving it on throughout the entire pattern till short final compared to the risk of picking up icing late on finals after you have "checked" for it back on the downwind and returned it to cold?


Carb heat should be used in accordance with the engine manufactures manual or when the engine shows signs of accumulating carburettor ice.

There are occasions when using the above method could actually induce carb ice if the atmospheric conditions are right, not one size fits all.


funfly
Unless you have an injected engine when you cease worrying.


Not true, it is possible to get induction icing, hence the fitting of an alternate air source.

[Oldbiggincfi]

Quote

"Unless you have an injected engine when you cease worrying."


Then why bother to have an alternate air supply ?

[Big Pistons Forever]

After this incident the carb heat system was checked and a very well hidden crack found in the carb heat valve, the result of this crack was that the engine was only getting partial heat and so the systems ability to clear ice much impaired.

The efficiency of the carb heat system will be indicated by the magnitude of the RPM drop during the runup. I have grounded several airplanes when they did not IMO appear to be getting sufficient heated air during the runup carb heat check as indicated by a non existent or very small drop in RPM when the carb heat was selected full on. In all cases various problems were found with cracked carb air boxes, deteriorated scat hoses or improperly adjusted carb heat controls.

In my opinion the "carb heat check" is one of the most poorly understood and performed of the runup checks.

To do a proper check I recommend the following. (for your typical C 172 Pa 28 aircraft)

Have runup RPM RPM set for at least 30 seconds to allow time for the exhaust to heat up

1) Apply full carb heat. You should observe a reduction of RPM of at least 100 RPM. This indicates that the carb is getting hot air

2) Wait at least 15 seconds with carb heat full on. If there is no rise in the RPM the engine has no carb ice. If a rise in RPM is noted than take the appropriate cautions.

I often lean if a carb heat check indicates ice. I was taught to leave carb heat on until touchdown, but later was told to go to cold air on very short final. I went back to hot air until on the ground after the engine almost stopped when I opened the throttle to turn 180 and backtrack. O200 in Jodel DR1050 in Scotland is an excellent ice-making combination.

This switching off of Carb heat on short final seems to be a unique to British flight training practice that seems to have been mindlessly passed down through the generations. :ugh:

It is IMO a stupid practice and directly contrary to the direction on what every carburated Cessna SEP POH provides.

Specifically in the POH Before landing Checklist the following direction is given

Quote

Carburetor Heat: ---ON (apply full heat before closing throttle)

Unquote

In the event of a Go Around apply full power and select carb heat cold, simples.

[9 lives]

I gathered a little data today. O-200 in C150M, at 2500 RPM, 58F OAT.

Carb air temp cold air = 42F, apply full carb heat = 62F carb air temp, RPM now 2425. Lean to peak 74F carb air temp, and RPM 2425. I did it three times this way, and in all cases, temps within 2F of those values. If I needed to deice the carb, I would like to have those additional 12 degrees F, which i get just for leaning the mixture.

[Big Pistons Forever]

I gathered a little data today. O-200 in C150M, at 2500 RPM, 58F OAT.

Carb air temp cold air = 42F, apply full carb heat = 62F carb air temp, RPM now 2425. Lean to peak 74F carb air temp, and RPM 2425. I did it three times this way, and in all cases, temps within 2F of those values. If I needed to deice the carb, I would like to have those additional 12 degrees F, which i get just for leaning the mixture.

It would seem to me that blowing 62F air on the frosted parts of the carb would be more than enough to melt the obstruction quickly. I base this observation on carb icing I have experienced in a C 150 shortly after start. At idle RPM on a Coastal winter day the engine will ice very quickly yet applying carb heat only resulted in a modest rise in carb temp, certainly not as high as 62 deg, but the ice clears in seconds. While the carb temp will be hotter when leaned does the extra heat really make a difference ? My personal opinion is not really

However operating an engine for prolonged periods very over rich can be almost as bad as too lean. The broader issue of understanding the effect of carb heat and carb icing on the mixture is an important one and IMO not well understood by many pilots

In conclusion I continue to believe that the most important pilot action is to apply carb heat at the first sign of icing. Adjusting the mixture is much less important than getting that heat on early. I am distressed that there is still a constant drip drip drip of accidents where pilots let carb ice develop to such and extent that the engine stops.

[9 lives]

the most important pilot action is to apply carb heat at the first sign of icing. Adjusting the mixture is much less important than getting that heat on early. I am distressed that there is still a constant drip drip drip of accidents where pilots let carb ice develop to such and extent that the engine stops.

Very true.

It would seem to me that blowing 62F air on the frosted parts of the carb would be more than enough to melt the obstruction quickly.

The 62F carb heat air I had was only 20 degrees warmer than ambient. Or, if I also lean, 30 degrees (F) warmer. If you've foolishly accumulated so much carb ice that you cannot maintain cruising power, you're headed down. Deicing in a hurry might be necessary, the extra 10 degrees F is going to be pretty welcomed then!

[Big Pistons Forever]

.

The 62F carb heat air I had was only 20 degrees warmer than ambient. Or, if I also lean, 30 degrees (F) warmer. If you've foolishly accumulated so much carb ice that you cannot maintain cruising power, you're headed down. Deicing in a hurry might be necessary, the extra 10 degrees F is going to be pretty welcomed then!

I guess we will have to agree to disagree on this one as every time I have put the crab heat on the ice went away pretty fast. Hotter air will obviously melt things faster but I think the practical difference is negligible.

In any case I don't see many reports where leaning the mixture would have been what made the difference from clearing the ice, or suffering an engine failure. What I do see is plenty of engine failures where no heat was applied at all right up to the time it got really quiet....

[A and C]

I agree with all you say but in this case the problem was masked by the fact that the system worked perfectly on the ground as there was no ram air to deform the carb heat valve.

Once the aircraft became airborne the ram air going into the intake deformed the valve due to a crack that was hidden from view by the locking wire that held the bolts for the input shaft secure.

No matter how well a pilot did the pre flight checks of the system this defect would not have shown up on the ground, any visual check by a pilot would not have seen the crack as it was hidden from view and the variables of airborne operation hid the defect until a major ice encounter.

The problem was only found in the hangar after the icing incident referred to above when pressure was applied to carb heat valve to simulate airborne operation, as soon as this was done the crack opened up and the defect became apparent.

I have no doubt that the crack quickly propagated between maintenance checks from something so small that detection was very unlikely to the three inch crack right across the width of the valve that caused the problem, it all goes to show that no matter how careful you are aircraft will invent new ways to kill you.

[phiggsbroadband]

Why be in such a hurry to melt the carb ice? Do you prefer the engine to ingest BIG LUMPS of ICE, instead of smaller amounts.


Each of the local training establishments that I have knowledge of, all say that there are three times when you should apply carb heat... 1. If you think carb icing is likely. 2. Any time the engine is run below 2000rpm. and 3. As the C in the downwind pre-landing BUMFFPITCHH checks.

[Vilters]

When an engine starts running rough due to carb icing, this rough running is partly due to :
a) the airflow in the carb being restricted
b) the engine running too rich

For those 2 reasons, it is good practice to :
c) Apply full carb heat
d) Lean mixture to peak EGT initially, and enrichen again as soon as the engine picks up power when the ice is melting off.

Because of "a" the airflow being restricted, the engine is not pulling full power, so you can not hurt the engine by "d" leaning to peak EGT.


But you have to enrichen again, as soon as the engine starts running normally again.

My personal procedure is :
a) detect carb ice
b) pull FULL carb heat
c) advance throttle to full power
d) lean to peak EGT
e) wait for the engine to pick up power again
f) enrichen mixture again
g) push carb heat in when engine is running normally
h) set power to cruise
i) lean back to cruise mixture

Normal downwind procedure from cruise power:

- Pull carb heat
- Reduce throttle
Speed, gear, speed, flaps, speed turn, and so on.

- On short final, about 200ft, PUSH carb heat IN, check stabilised, if OK land, if not OK Go -AROUND.

- Hands on stick/yoke and throttle till standstill, or till stable climb-out, positive VVI, ready to retract gear, flaps for another go.

THE trick to a good landing or Go-AROUND is : Only ONE item to think about.=> Throttle forward in case off.

That is why the short final check is so important.

At about 200ft on short final, you and the airplane have to be configured and stabilised for a landing, or a Go-Round.

The only difference between them being a SINGLE THROTTLE increase without having to think about anything else.


Below 200 ft ?
One hand glued on stick/yoke.
Second hand glued on Throttle.

And don't you dare touch anything else till standstill, or a positive and confirmed climb out.

[Big Pistons Forever]

THE trick to a good landing or Go-AROUND is : Only ONE item to think about.=> Throttle forward in case off.

That is why the short final check is so important.

At about 200ft on short final, you and the airplane have to be configured and stabilised for a landing, or a Go-Round.

The only difference between them being a SINGLE THROTTLE increase without having to think about anything else.


Below 200 ft ?
One hand glued on stick/yoke.
Second hand glued on Throttle.



And don't you dare touch anything else till standstill, or a positive and confirmed climb out.

I don't get it. If you can move the carb heat control to cold at 200 feet on very short final while flying the aircraft to the touchdown point, why can't you select cold after achieving a

a positive and confirmed climb out

Something that is going to happen pretty early in the go around sequence.

If you do select cold at 200 feet what happens if the engine starts to develop ice and you need to make a late go around with power now reduced due to carb ice forming ?

[9 lives]

I flew my other plane yesterday, O-360 powered. With very similar conditions to my data gathering in my 150, I could rise the indicated CAT by 8 more F degrees by leaning to peak after applying carb heat.

I agree that in the case of carb ice, prevention by pre-emtive action is much more appropriate than cure following accumulation. That said, if cure is what is needed, things are getting worse fast. In that case, I would rather use, I would rather use everything I have to melt the ice in the least time. It was mentioned that more heat would cause a risk of chunks of ice being ingested. I think this is of reduced likelihood, as the heated air is warming the layered ice, rather than the body of the carb, so it will melt it out as it accumulated - in layers. However, comparing the risks of having inadequate power to maintain altitude, vs the remote chance of a chunk of ice going through to an intake valve, I'd rather get the engine power returned, so I'll use all the heat I can produce.

[phiggsbroadband]

Hi Vilters... quote... THE trick to a good landing or Go-AROUND is : Only ONE item to think about.=>
Not too sure I agree with that. In a Cessna 172 with 40deg flaps and maybe a full passenger load, you would struggle to pass over the fence (or trees).
A C172 Go-Around is simply done by your fingers moving right to left over the Flaps Lever, Throttle, and Carb Heat... Lift Up, Push In, Push In.

[Vilters]

In a C-172, on short final, with full load and full 40° flaps?

( With the asumption that carb heat went in at 200 ft AGL)

=> Go-Around

=> Throttle FULL => And KEEP your hand on the throttle to prevent kreep back.

=> Positive VVI => Lock Throttle

No problem to go from 40° directly to 30° (Most is drag anyway). Then Milk the flaps up => 30°, 20°, 10°, 0°

Most, if not all aircraft climb better with a clean wing => NO flap.
=> PoH

After a Go-AROUND desision has been been made? Throttle ALWAYS comes first. Flaps a distant second after positive VVI has been confirmed, then milk them up.
(And carb heat should already be OFF from your short final checklist.)

[abgd]

Looking at the Cessna 172M POH, the checklists suggest carb heat prior to landing, but only mention setting carb heat to cold during a 'balked landing' (go-around) or after landing.

This fits with what I was taught - I'm told that Cessnas can occasionally ice-up within seconds; hence the need to keep carb-heat on all the way to the ground.

[Big Pistons Forever]

The problem was only found in the hangar after the icing incident referred to above when pressure was applied to carb heat valve to simulate airborne operation, as soon as this was done the crack opened up and the defect became apparent.

I have no doubt that the crack quickly propagated between maintenance checks from something so small that detection was very unlikely to the three inch crack right across the width of the valve that caused the problem, it all goes to show that no matter how careful you are aircraft will invent new ways to kill you.

This is a pretty unusual failure mode. Most issues with inadequate carb heat involve a cracked carb air box, broken butterfly valve or a loose/deteriorated scat hose, all of which are pretty obvious.

Getting the carb heat full on should do the job, although many new pilots have been caught out by the fact that often the first thing that happens when you apply full carb heat is the engine runs worse :eek:, this is because the engine has to ingest all the water you just melted. Engines don't like even a little bit of water. The temptation is to push the carb neat back to cold, but this must be resisted. The good news is that the engine will pick up in a few seconds and full power will be restored.

However systems knowledge is always a good thing and this discussions is a great example of this. Under normal circumstances full carb heat should clear very quickly, within 10 or 15 seconds of application of the heat.

If the engine is still showing signs of carb ice with the carb heat full on for an extended period, something unlikely but certainly possible, than more measures need to be taken and this is where leaning to increase carb temps and provide a more favorable fuel to air mixture for the iced engine would be a prudent course of action.

[India Four Two]

although many new pilots have been caught out by the fact that often the first thing that happens when you apply full carb heat is the engine runs worse :eek:, this is because the engine has to ingest all the water you just melted.

BPF,

Yes, I was one - see my post above:
http://www.pprune.org/private-flying/560980-carb-heat-trick.html#post8966722

In my opinion, all pilots should have carb icing demonstrated to them, preferably early in their careers. For many, it is just a rote procedure during the run-up and downwind checks, taught to them by instructors who may not fully understand the process and may not be following the POH instructions.

[phiggsbroadband]

Hi I42, I wonder how an Instructor would deliberately 'Ice-Up' an engine?
Maybe fly through a cloud with partial throttle?
Even at 7000ft a dead engine would be hard to restart before the plane glides to a suitable field.


So I don't think you could 'demo' this safely.

[India Four Two]

phiggs,

Yes, I've been thinking about that too. Possibly, you could do it on the ground on a day with a suitable dew point spread.

I also think there is a need for much better instruction on this topic, including the important point that the engine can run very roughly while it is ingesting the melted ice.

[phiggsbroadband]

I42... Strange thing is that of all the IC engines I have used, I cannot say that any of them has suffered from carb icing. However I did have a Mower cut out on me one day, and would not restart until it had stood for about 15 minutes.... Ran it a bit further, and again it stopped for 15 minutes.
My diagnosis, after seeing bubbles coming back into the fuel tank, was that it was suffering from vapour-lock, caused by the very hot operating conditions, and the fuel pipe near the engine.
Good job we don't have that problem with aero engines.

[Fly-by-Wife]

Engines don't like even a little bit of water.

Not strictly true, as water injection or water-methanol injection was a very common technique in high performance (mainly military) piston-engine aircraft.

By using water injection, the cooling effect of the water allows the fuel mixture to be run leaner at its maximum power setting without detonating.

I think Step Turn is spot on here, more power, more heat and less fuel - a win-win-win situation!! :ok:

FBW

[Big Pistons Forever]

Fly-by-Wife

OK engines don't life ingesting liquid water, which is what happens when the ice melted by the carb heat gets sucked into the engine. Happy now ?

This process and how Water Meth injection systems work are totally different.

[Tankengine]

Phiggsbroadband:
Funny you should bring that up! In years of flying in Northern Australia I never had issues with icing but certainly did with vapour locking! :ooh:
We always went carby heat cold before landing to avoid dust through the un filtered carby heat system on the ground.:ouch:

So, you need to modify your techniques to cater for different weather situations.:ok:

[tecman]

I wasn't sure whether Phiggs was being ironic or not! Indeed there is a vast body of literature on vapour lock in aero engines and those of us flying on mogas in hot climates are well advised to be conversant with it.

As far as carb heat goes, I pretty much subscribe to the BPF philosophy, modified for the dusty Australian situations as described by Tankengine. One of the problems we have in much of Australia is getting people to believe that carb ice is possible in what often look like benign conditions. I've noticed that CASA have had a number of campaigns on the topic and one of their more useful charts (which I'm sure has equivalents in other countries) is:

http://www.casa.gov.au/wcmswr/_assets/main/pilots/download/carburettor_icing_chart.pdf

It's well worth staring at and explains, for example, why partial application of carb heat is usually a bad idea: it's quite possible to push the temperature UP into a severe icing regime.

Having said that, I think we're all agreed that a working knowledge of the fuel/air induction system in your aircraft is a good thing. While all my carb ice problems have been fairly easily resolved, juggling the mixture and carb heat did once get me out of a wasp-induced induction block.

On a slightly different note, I was surprised to see Vilters' insistence of a fully stabilized approach and decision height of 200' in an SEP. All I can say is that, with the tubulence and crosswinds we often get in these parts (and lots of other places), we might never land on that basis. And, of course, there's always the possibility of livestock intrusion or other runway incursion. It's certainly a good idea to go around early if you know you have to do so but it's a dangerous mindset to assume you're prematurely "committed" in an SEP.

Finally, I smiled when looking at the source of this carb heat "trick" discussion. Step Turn, isn't this "trick" the sort of non-POH procedure you've been deprecating in other contexts?

[Vilters]

Sloppy landings come from sloppy approaches.

When I write "stabilised at 200 ft", read that as.

On centerline, on speed, X-Wind corrected, in gear and flaps landing configuration.

On every landing, one should feel confident in the ability to land the aircraft from this "stable" position.

( Even when fighting a wind or X-Wind, one should be "here" )

But, a wind gust, a deer on the runway, anything can spoil your day.

That is why a GO-AROUND has to be a single move from here till touchdown:
And that is Throttle forward.

All the rest can come later.

= > Throttle forward
= > Positive VVI ( Vertical Velocity Indication) the question was asked. DO not touch ANYTHING untill you have a positive VVI.

And then as per PoH, gear, flaps and so on.

[9 lives]

Finally, I smiled when looking at the source of this carb heat "trick" discussion. Step Turn, isn't this "trick" the sort of non-POH procedure you've been deprecating in other contexts?

Fair enough, I knew this would come up at some point :uhoh:. That is the case, and that criticism is fair. This "trick" is on the edge of guidance to operate the aircraft in a manner not described in the Flight Manual. So, a few thoughts as to why I made the post of this edgy topic!...

I do not believe that leaning after carb heat application is counter to any flight manual procedure, and I do believe that it equates to flight manual procedures for higher altitude operation. It is not counter to good airmanship. It is a "touchy feely" thing you can do with a running engine to keep it running. Aside from leaning too much, to power reduction, it can be instantly reversed without affecting the flying characteristics of the aircraft. Retracting flaps on final will affect the flying qualities of most aircraft, and mostly in a negative way, in a low power, approach configuration.

I was inspired to think more about the effectiveness of carb heat systems while assiting a freind with his modification of a C185 to have a carburetted engine. Included in that effort was the requirement to build a carb heat system, which would meet the FAA requirement of:

Sec. 23.1093

Induction system icing protection.

(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30° F.--
(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90° F. with the engines at 75 percent of maximum continuous power;.....................

We had a heck of a time providing enough heat to rise the 90F heat rise, as measured in the carb venturi. We succeeded, but during the process, also developed the leaning trick, and we did describe it in the changed flight manual, as we had to do that to pass the test for the approval. As you can see from the data I provided on two aircraft, one certified, one not, the carb air temperature rise is not even close to the required value for FAA approval, yet the 150 is approved. The FAA considered this 90F heat rise important enough to specify it, so I suppose any action a pilot can take to get closer to it when needed is a good thing - at least as viewed for certification!

I wanted people to think about this. If after thinking about it, you don't agree, don't do it! But, you'd have to agree that people are thinking and discussing it, and for this topic, I opine that discussion which promotes systems understanding is good.

Yes, I'm the flight manual proponent, nanny state guy. Pilots seem to not want to read them much, and I guess that the manufacturers know this, so they try and keep them thin (though not so much for brand new Cessnas!). So little gems of knowledge are set aside (like not slamming the throttle closed on a cold day at altitude, and shock cooling the engine).

But after all is said and done, this rather good discussion was worth taking a black eye in my inconsistency with respect to following flight manual procedure :ouch:. But, this does not mean that I have now gone soft on retracting flaps for a continued landing, nor waterskiing landplanes! :=

[tecman]

ST, the POH point has been gracefully acknowledged and I'm not going to labour it since, as I noted elsewhere, I actually believe that judgements are always needed and go hand-in-hand with an understanding of the POH.

I quite understand your resolution to keep up standards and the need to draw the line. It put me in mind of a piece by Garrison Keillor who linked the installation of airconditoning to coming home and finding one's mother in moral turpitude :)

I don't see myself needing to lean routinely while applying carb heat but, while there was no carb ice involved in the wasp incident I mentioned, experiences like that one underline that information sharing is useful in opening our minds to possibilities which may help in unusual circumstances. But I would just comment that an inexperienced pilot managing to stop the engine while in a confused state will certainly change the flying quality of the aircraft.

It's interesting that there is a new generation of LSA aircraft powered by Rotax (and other) engines in which there is no carb heat installation. A common view is that the position of the air intake source and the nature of the carbs mitigate against icing. I'm sure this is largely true but it was sobering for me when I did once clear some ice on run-up (being a certified variant of an LSA, my aircraft has a conventional carb heat arrangement). I have no way of knowing if this would have been a problem at full power (and maximum engine environment heating) but I'm not sure I wanted to find out. Of course, it could be that making the air induction system more 'conventional' for certification has negated some of the advantages of LSA installations.

[rusty sparrow]

Re BPF 'This switching off of Carb heat on short final seems to be a unique to British flight training practice that seems to have been mindlessly passed down through the generations. :ugh:'

I got my licence on the Australian syllabus and was taught to keep carb heat on until on the ground. When I converted my licence to a UK one, I was told to switch carb hear off at 200'. I've since seen this requirement documented in the order book of a flying club.

I've been following this rule, however a few months ago, climbing out in a Cub behind a C90, I got carb ice at around 300'. I pulled carb heat and recovered power without incident but with a lesson being learned: leave carb heat on until on the ground.

[Johnm]

This practice was used for some aeroplanes that would struggle to climb in a go-around with carb heat on!

[Big Pistons Forever]

This practice was used for some aeroplanes that would struggle to climb in a go-around with carb heat on!

Which is why you select if off after applying full power when you start the go around. If you are incapable of pushing the card heat knob/lever while flying the aircraft in the go around maneuver, then I would suggest there is a flying skill deficit that needs to get addressed.

[TheWrongTrousers]

Can anyone quote me the power loss for full power with carb. heat on?

BTW, a C172N POH I have quotes for inadvertent icing encounter, to lean the mixture to improve carb. heat efficiency, IF used continuously. No real surprise there.

Same POH says to keep carb. heat on, till go-around (turning off after full power is applied) or after landing therefore.

Wallace

[phiggsbroadband]

Quote... Can anyone quote me the power loss for full power with carb. heat on?
Well maybe not in terms of BHP, but on the Power Check, done at 1700 rpm it gives an rpm drop of 75rpm.
So my guess would be a drop of 100 rpm at full throttle, say 2500 down to 2400rpm. ( -4%.)


btw... Why are you using full power, are you in the climb?

[bingofuel]

= > Throttle forward
= > Positive VVI ( Vertical Velocity Indication) the question was asked. DO not touch ANYTHING untill you have a positive VVI.

It could be a long wait on an older Cessna with 40 degrees of flap extended!

[abgd]

The power needed to spin a propeller goes up roughly with the cube of the RPM, so that 4% drop equates to a much greater loss in output - about 12%.

There's also a risk of overheating the engine if you forget to turn the carb heat off.

[Big Pistons Forever]

Quote... Can anyone quote me the power loss for full power with carb. heat on?
Well maybe not in terms of BHP, but on the Power Check, done at 1700 rpm it gives an rpm drop of 75rpm.
So my guess would be a drop of 100 rpm at full throttle, say 2500 down to 2400rpm. ( -4%.)


btw... Why are you using full power, are you in the climb?

That is why you select carb heat off after you have applied full power if you have to go around !

As for the power loss why is there such a huge issue about the potential power loss for the brief time the engine is at full power but before you have selected carb heat off, but no thought given about the power loss you will get if you apply full power to an engine that has already started to develop carb ice because it was selected off on final:confused:.

This is the exact scenario you setting your self up for following the stupid, and apparently universal to UK flight training, practice of selecting carb heat off on final. :ugh:

Under the right conditions carb ice can develop in seconds, certainly in the time the aircraft went from 200 feet on final to the start of the landing flare at a very low power setting.

[Chuck Ellsworth]

When I converted my licence to a UK one, I was told to switch carb hear off at 200'. I've since seen this requirement documented in the order book of a flying club.

If you are taught to do that why apply carb heat in the first place?

If it ices up during the approach you will at least have a chance of getting power back before you hit the ground.

But carb heat off at 200 feet is beyond stupid.

[Tankengine]

So you are going to get carby ice between 200' and the ground after not having it at 200' due carby heat on?:confused::hmm:

I think the turning off the carby heat at 200' is to cater for a possible go around and also so the engine does not injest dust from the dirt strip you are landing on, (perhaps not a problem in green countries like Canada), most carby heat systems bypass the air filters.

[India Four Two]

perhaps not a problem in green countries like CanadaTankengine,
I've flown off several quite dusty strips in Canada and that's just in the south. I've never been anywhere near the gravel strips in the north, which can be very dusty in the summer:

http://cdn-www.airliners.net/aviation-photos/photos/7/6/5/2456567.jpg

This 737 has the "unpaved strip kit" - the most visible parts of which are the gravel deflectors on the nose and main gear, and the vortex dissipator probes under the nacelle leading edges.

[Pilot DAR]

Steady on posters....

Carb heat is both preventative, and curative, and should be used as required, by an informed pilot. The affect on engine power resulting from the use of carb heat, has the same variabilities as those induced by atmospheric change on the engine, because it is an atmospheric change, as the engine sees it!

If you're relying on the exact effect of the application of carb heat, both in temperature change in the carb, and affect on power produced, you may be disappointed. It's just not that precise. How much power does your engine make anyway? We really don't know... The design will have a rated power, but many factors, most certainly condition, will affect that. Even a newly overhauled engine is rarely dynomometer checked for actual power output.

Carb heat is always unfiltered, as it also serves as the "alternate air" which must not been drawn through the air filter. I have once used it, when I encountered freezing rain, the air filter instantly iced over, and the engine stopped. "alternate air" got it running again, for my retreat from those conditions. It had not been a carb ice event, but rather obstructed induction system.

I share the opinion that the pilot should manage power as required, and if this means moving carb heat from hot to cold for a go around, than that should be done. If the go around decision is so short that you just can't get to it, then fly the plane, and check configuration as soon as you can as you begin the climb away.

[TheWrongTrousers]

Read CAA safety sense leaflet 14 on Piston Engine Icing, and note their comments on 200ft Carb. heat off.......

[Tankengine]

CAA of which country? perhaps you could post it here.

[Pilot DAR]

The referenced Leaflet 14 can be found here:

http://www.caa.co.uk/docs/33/20130121SSL14.pdf

[9 lives]

Interesting leaflet. My experience does not lead me to agree with all of it, but it's generally good.

I'm opposed to the notion of procedurally fiddling with carb heat at 200' feet up on final. This is the phase of flight when minimizing "things to do" in the cockpit is good. Rather that pre configuring the aircraft for a slight chance go around, why not leave it configured as described in the POH, and only reselect carb heat on that very infrequent occasion when a go around might be required, you'll have time... The plane will go around with carb heat hot if it has to, and once you have your focus on flying the overshoot, then you can come back to the engine controls!

[Chuck Ellsworth]

I'm opposed to the notion of procedurally fiddling with carb heat at 200' feet up on final. This is the phase of flight when minimizing "things to do" in the cockpit is good. Rather that pre configuring the aircraft for a slight chance go around, why not leave it configured as described in the POH, and only reselect carb heat on that very infrequent occasion when a go around might be required, you'll have time... The plane will go around with carb heat hot if it has to, and once you have your focus on flying the overshoot, then you can come back to the engine controls!

Exactly, why make things more complex than is necessary.

[Big Pistons Forever]

From Leaflet 14

Unless otherwise stated in the Pilot's
Operating Handbook or Flight
Manual, the HOT position should be
selected well before power is reduced
and retained to touchdown. On some
engine installations, to ensure better
engine response and to permit a
go-around to be initiated without
delay, it may be recommended that
the carb hot air be returned to COLD
at about 200/300 ft on finals.
m) Go-around or Touch and Go

The leaflet clearly says for carb heat to stay on until touchdown unless the POH specifically requires the carb heat off. Every carburated single engine Cessna POH says carb heat on for landing.

Some Pipers do have a note in the POH that cautions that "carb heat should not be on for approach and landing unless icing conditions are present"

Having carb heat on until 200 feet as a general practice regardless of conditions would be operating contrary to the POH. In any case if you have the carb heat on during an approach in a Piper because you are experiencing icing, it seems pretty silly to turn it off when you are still in the air.

Grumman POH's have a note in the before landing checklist that says "Carb Heat: As required, leave on if icing conditions exist"

[JammedStab]

The efficiency of the carb heat system will be indicated by the magnitude of the RPM drop during the runup. I have grounded several airplanes when they did not IMO appear to be getting sufficient heated air during the runup carb heat check as indicated by a non existent or very small drop in RPM when the carb heat was selected full on. In all cases various problems were found with cracked carb air boxes, deteriorated scat hoses or improperly adjusted carb heat controls.





Grumman POH's have a note in the before landing checklist that says "Carb Heat: As required, leave on if icing conditions exist"

I rent a Grumman Cheetah in Dallas. Recently I flew one of their models that I had not flown before. It had been several months since my last flight and I only have a few flights total.

During the carb heat check on the run up there was barely any needle movement. I ended up calling the instructor who checked me out and he assured me that it is normal. The owner of the school who seems to be very knowledgeable later said the same. He said that the heater unit around the exhaust pipe is smaller than the Cessna's I am used to.

I have meant to ask about this on a Grumman forum but haven't done so yet. No problems encountered on that flight.

[Big Pistons Forever]

I rent a Grumman Cheetah in Dallas. Recently I flew one of their models that I had not flown before. It had been several months since my last flight and I only have a few flights total.

During the carb heat check on the run up there was barely any needle movement. I ended up calling the instructor who checked me out and he assured me that it is normal. The owner of the school who seems to be very knowledgeable later said the same. He said that the heater unit around the exhaust pipe is smaller than the Cessna's I am used to.

I have meant to ask about this on a Grumman forum but haven't done so yet. No problems encountered on that flight.

I call BS. My Grumman AA1 has a 200 + RPM drop with carb heat selected on and the Traveler I used to fly was the same.

[Pull what]

Returning carb air to cold at 200 feet is a stupid dangerous folk lore procedure started by Cherokee instructors 50 years ago and passed on without question. A small amount of dust in the carburetor wont kill you, carburetor icing can, as accidents show.

[rusty sparrow]

Agreed - the argument is as simple as that. And the folklore that carb icing can't happen at full power is also wrong.

[phiggsbroadband]

Life does not necessarily end when the fan stops turning... You just enter the fascinating realm of 'Gliding'.
If you are flying from one of the ex WW2 airfields, at 200ft you most likely have 1/4 to 1/2 mile of Paved Undershoot underneath you.


''Keep Calm and Stay Flying''

[9 lives]

Can anyone quote me the power loss for full power with carb. heat on?

Yup, I can now! O-550D was 3% at full power and three different power settings I tried today This plane has an EDM 930, which does tell % power being developed.

But I'm not saying that's appropriate data for each engine model....

[JammedStab]

I call BS. My Grumman AA1 has a 200 + RPM drop with carb heat selected on and the Traveler I used to fly was the same.

Thanks for the info BPF. I was told that all the Cheetahs in this flight school are the same. While I do see that you have Grumman experience, it isn't on the Cheetah. Is it possible that there could be a difference. Any Cheetah pilots out there.

[fireflybob]

Some Pipers do have a note in the POH that cautions that "carb heat should not be on for approach and landing unless icing conditions are present"

You could easily argue that "icing conditions are present" almost every day in the UK when you look at the Carb Ice probability chart.

[fireflybob]

This is a good report to read concerning carb icing:-

The investigation did not find any evidence of a failure within the engine but the atmospheric conditions were conducive to carburettor icing.
(https://assets.publishing.service.gov.uk/media/5422fa07ed915d1374000783/Piper_PA-28-140_Cherokee_G-ATRR_06-14.pdf)

Each of the local training establishments that I have knowledge of, all say that there are three times when you should apply carb heat... 1. If you think carb icing is likely. 2. Any time the engine is run below 2000rpm. and 3. As the C in the downwind pre-landing BUMFFPITCHH checks.

If you conduct a straight in/direct approach you invariably won't fly downwind! For this reason when teaching I always refer to them as (Pre) Landing Checks. As the report above shows the practice of selecting hot air in the pre landing checks won't necessarily give you the protection you need. You need to select hot before reducing power for the approach and leave it there until landing and/or going around.

[thing]

Re BPF 'This switching off of Carb heat on short final seems to be a unique to British flight training practice that seems to have been mindlessly passed down through the generations. :ugh:'

I got my licence on the Australian syllabus and was taught to keep carb heat on until on the ground. When I converted my licence to a UK one, I was told to switch carb hear off at 200'. I've since seen this requirement documented in the order book of a flying club.
Interesting. I was taught to fly in UK and it was always carb heat on at pre landing checks until touchdown. Can't see for the life of me why you would be wanting to fiddle with something on short finals, or wonder whether you have or have not selected/deselected it. I've discussed this with friends who were taught to fly at various ATO's and they were all taught carb heat until touchdown.

[Tarq57]

I've flown a couple of Cheetahs.
The carb heat drop on runup (from memory, this was a while back) was quite small. Maybe 50-100 revs.

Try it aloft at cruise power. I did, and the difference was more noticeable.

[memories of px]

Always worth reading other peoples experiences, i think that discussions on carb heat should be always at the fore, where do you think it stands in the list of accident causes? how long do you do a carb heat check for? 5 seconds? 20 seconds?

[cotterpot]

0 seconds - if you haven't got one.
I thought CFIT was more significant

[Meldrew]

I learned to fly in the 1970s on a Continental powered taildragger. I was taught then to leave carb heat on from downwind right through to touchdown. Later, converting to Lycoming powered PA28s they told me to apply carb heat briefly on downwind and then to go to cold for the rest of the circuit and landing. In the light of all the above posts, this advise would have seemed to be very unwise!
I think previous posters are right that some advise seems to be handed down over the years without any re analysis from experience.
Nowadays, flying mainly Cessna 172, I frequently notice icing right after engine start and need carb heat during the first part of the initial warm up.

[mikehallam]

Without reading back over the older years:-

At a dusty or grit covered landing strip, carb. air 'cold' ensures the engine gets filtered air. Whereas 'Carb. heat' bypasses all such protection -

So perhaps it's a case for having more power for going around combined with helping preserve the engine innards. Particularly on circuit bashing/training a/c?

mikehallam.

[Crash one]

I may be wrong but just how much lack of power results from carb heat on versus off?
My Emeraude with its Continental C90 (apparently ice making machine) drops about 20/30 revs with it on, possibly less than that, I can barely see much movement on the dial.
It climbs out over trees on the 600 metre strip, no problem with carb hot, full flap, and over weight.
So I am in no hurry to blindly follow old wives tales passed down over years of " that's the way I was taught".
Yes the carb heat does work because I once picked up ice while taxiing on wet grass, carb cold, it was cleared in a few minutes using hot.
Perhaps this depends on the airframe/engine setup?

[Ebbie 2003]

I get only about a 50 rpm drop on applying carb heat during the run up - used to get about 200 in 172's.

I too have one those carb heat gauges with the small yellow avoid area in which the needle always seems to obstinately remain embedded.

Things would have to get quite desperate for the extra heat of leaning to have an effect - but if all else fails, why not - as the TV ad says "every little helps".

[S-Works]

What is the carburettor of which you speak.......:)