Use of carb heat

[SNS3Guppy]

My physics teacher, and my experience.

You're asserting that your physics teacher was a flight instructor, and that he taught you about carburetor icing?

You've developed more carburetor ice during approaches, and thus know by experience that configuring the airplane for landing causes carburetor ice?

Are you familiar with the term "non-sequitur?"

It has to do with the venturi effect, and the fact that as you are going slower in climb or during approach, the venturi effect is more pronounced.

Now this is interesting. I was getting a little bored with this topic, as it's been fairly beaten to death, but this is different.

You're asserting that going slower causes carburetor ice? You're asserting that going slower causes a "more pronounced venturi effect?"

Have you spoken with your physics teacher lately?

Bernoulli, as passed down through your physics teacher, would tell you (if he were here to do so) that where an increase in velocity occurs, a proportional drop in pressure will be found. Thus, it's not a decrease in velocity which causes a "more pronounced venturi effect," but an increase in velocity. In other words, going slower does not a "more pronounced venturi effect" make.

Call your physics teacher. He would like to hear form you.

Combine this with having a smaller opening in the carburetor, as a result of reducing throttle the "whole" is smaller in the throat, and you have the widest range of temperatures for carburetor icing during the approach (not necessarily approach configuration, but when you've got flaps out and gear down, you're slower)

It's a hole, rather than a "whole," but we're still stuck with that whole physics thing, you see. That engineering degree isn't shining through in your description of the carburetor. The "widest range of temperatures for carburetor icing" occur because of a "smaller opening in the throat," you say. Again, interesting. Given that the temperature drop is proportional to the velocity of the airflow as a function of the pressure drop, you're still fixed on the concept that reduced velocity (and let's not forget, aircraft configuration, a true mystery player here). You're asserting that flaps and gear make for a slower airplane, which makes for a "wider range of temperatures for carburetor icing."

I'm really curious about this wider range of temperatures. Somehow, we have two disconnected concepts here. One is that being configured somehow increases the "range of temperatures for carburetor icing." The other is that being slower somehow also increases the "range of temperatures for carburetor icing."

Hopefully you'll understand that going slower doesn't increase the potential for carburetor ice. Flying an approach at a slower airspeed doesn't increase carburetor icing potential, and flying at a lower engine RPM (with a slower attendant airflow velocity through the carburetor, as well as a reduced mass airflow) doesn't increase carburetor icing potential, either. As well as neither one increasing carburetor icing potential, neither one increases the "range of temperatures of carburetor icing."

Let's examine this a little more closely. The ability of ice to form is a function of the properties of the moisture which will be doing the freezing. Unless one manages to alter the chemical properties of that moisture, then the "range of temperatures" at which ice formation will take place doesn't change. To suggest that a change in the "range of temperatures" takes place, one must acquiesce to the concept that ice freezes at higher temperatures, and lower temperatures (unless you're asserting that this range of temperatures only trends downward, but that's a problem because the temperatures aren't getting colder in that carburetor when the engine speed slows [and airflow velocity slows], and certainly not when the airplane slows). Are you suggesting that a chemical change in the moisture entering the carburetor through induction air has been chemically altered by reduced engine speed, reduced airspeed, or the mysterious magic of configuring the airplane? Say it isn't so.

Let's remember that the internal combustion engine, insofar as the carburetor is concerned, is a big vacuum cleaner. It's a suction machine. The amount of airflow through that machine increases as the speed of the machine increases; run the engine at a higher RPM, and you'll get a higher airflow through the engine with the throttle open, or a greater "suction" with the throttle closed. Therefore, at higher power settings, we have greater airflow (and faster airflow through the engine), and at higher airspeeds with the throttle closed, we have greater differential pressure across the throttle plate, with higher airflow over the idle jet (bypassing the throttle plate as a calibrated leak), and thus the greatest icing potential when diving at idle at high speeds (not slow) and when operating at higher power settings.

You said you knew about carburetor ice based on experience (and your physics teacher). How much carburetor ice experience have you had at high power settings?

I studied engineering at University, and worked for many years doing flow analysis of liquids for flood control districts.

Did the flood control districts get a lot of carburetor icing? How about the university?

[barit1]

My father and I had the good fortune of meeting Max Conrad, who set several light-aircraft distance records and made a good living ferrying Piper aircraft trans-ocean. He won the 1964 Harmon Trophy.

But the point to be made is his use of carb heat in optimizing fuel consumption. He used a substantial amount of heat in cruise to ensure fuel was completely vaporized before entering the cylinders. Thus he could lean the mixture further, there being less/no unburned/wasted fuel. I do not know his exact technique, but I believe it can be discovered in his books and magazine articles.

[SNS3Guppy]

Partial carburetor heat is the most effective use, but it does require accurate carburetor air temperature instrumentation.

The use of full carburetor heat in most light airplanes is not a function of necessity, beyond that required due to lack of instrumentation, and lack of pilot training (and experience). It becomes an all-or-nothing proposition, in which excess heat is often used, beyond that which is appropriate to put the carburetor in the ideal temperature range.

Lack of instrumentation is a function of economy, and full carburetor heat becomes a simple act of pilot-proofing the airplane, or more specifically, protecting the airplane from the pilot.

[darkroomsource]

Carburetor ice is more likely in approach configuration? Really? Who told you this?

Your lack of understanding of fluid dynamics is apparent.
Please, do not continue to teach your students to believe that you or they are smarter than the engineers who wrote the POH.

There is significant evidence that an fluid, be it air or liquid, when flowing through a venturi has a lower STARTING temperature if the slower it is flowing thorugh the opening. The increase in speed through a venturi in relation to the STARTING speed increases in inverse relation to the starting speed.
Therefore, the air flowing through a venturi is colder when an aircraft is in approach.

From my experience, working with fluid dynamics.

Additionally, from my experience, in flight, I have encountered carb ice far more frequently in climb than in cruise, when the aircraft is flying slower.

I have encountered carb ice, and for that matter, airframe ice, in clear air, on a sunny day, in the desert. And I know others who have also.

However, as you have clearly stated, you know more than anyone else about anything on the face of the planet, so I am through with this discussion.

I would suggest to any readers, that if the POH says to use Carb Heat, then do so, and teach your students to do so. It is meant as a preventative measure (unless you are actually experiencing carb ice), and does not impact the performance of the plane enough to justify not using it. Never mind that your insurance company will probably refuse to cover you if you teach a student to ignore the POH.

[Genghis the Engineer]

Originally Posted by darkroomsource

Your lack of understanding of fluid dynamics is apparent.
Please, do not continue to teach your students to believe that you or they are smarter than the engineers who wrote the POH.

Oh dear, I'm going to regret getting involved here I'm sure - however for the record I have a commercial pilots licence, a PhD in aerospace engineering, and have written (well, technically, led the writing of) several aircraft operators manuals, with about 1000 hrs in piston engined aeroplanes, most of that with carburated engines, and probably 400 of those in something with another venturi for some reason or another.

There is significant evidence that an fluid, be it air or liquid, when flowing through a venturi has a lower STARTING temperature if the slower it is flowing thorugh the opening. The increase in speed through a venturi in relation to the STARTING speed increases in inverse relation to the starting speed.

Err, no, at subsonic speeds (certainly below 0.6M, and probably below 0.8M) the flight speed of the aeroplane has no significant impact upon the inlet temperature. Particularly because on many aeroplanes, the intake isn't pointed into the free airstream anyhow, it's either sat inside still air inside the engine compartment, or through the prop disc.

It's too late at night to work through Bernoulli's equation, but that particular relationship is also a little more complex than you make out.

Therefore, the air flowing through a venturi is colder when an aircraft is in approach.

No, for several reasons. Firstly, airflow is a function of engine speed, which tends to be fairly low on approach. Secondly air temperature is a function of altitude, which tends to be at its highest on approach.

Finally being on approach, and being in the approach configuration are not the same thing. A flapless approach is in the cruise configuration on a fixed gear aeroplane, and offers no significant protection against carb icing.

From my experience, working with fluid dynamics.

Additionally, from my experience, in flight, I have encountered carb ice far more frequently in climb than in cruise, when the aircraft is flying slower.

In what types? - whilst I'm not denying the potential for carb icing in any mode of flight, this is very definitely not my experience.

I have encountered carb ice, and for that matter, airframe ice, in clear air, on a sunny day, in the desert. And I know others who have also.

What sort of relative humidity? This sounds highly unlikely to me; I don't dispute that you may have had an engine problem - but where did your humidity come from to form ice? Lack of humidity is a usual feature of the air over deserts.

However, as you have clearly stated, you know more than anyone else about anything on the face of the planet, so I am through with this discussion.

I'm pretty certain that's actually a fellow called Bill Brooks, who has one degree more than me, about 5 times my flying hours, and has designed and tested getting towards 20 aircraft types. SNS is a bright fellow, but he's not Billy.

I would suggest to any readers, that if the POH says to use Carb Heat, then do so, and teach your students to do so. It is meant as a preventative measure (unless you are actually experiencing carb ice),

That really does depend upon the aeroplane. For example the POH for the PA28-161 that I used to fly, specifically recommended NOT using the carb heat on approach unless carb icing is experienced. So that is cure, not prevention.

and does not impact the performance of the plane enough to justify not using it. Never mind that your insurance company will probably refuse to cover you if you teach a student to ignore the POH.

You're being a tad contradictory here - you've said you get carb icing in climb, but in that mode of flight carb heat will most definitely reduce performance in many (although not all) aeroplanes.

And I don't think that anybody has proposed ignoring the POH, so much as understanding it, and the aeroplane, and making a reasoned judgment on the basis of knowing as much as possible.

G

[Big Pistons Forever]

Originally Posted by Genghis the Engineer

,
And I don't think that anybody has proposed ignoring the POH, so much as understanding it, and the aeroplane, and making a reasoned judgment on the basis of knowing as much as possible.

G

It is sure nice to see someone who can take a 5 page thread and distill its essence into 3 lines. Well done

[SNS3Guppy]

Additionally, from my experience, in flight, I have encountered carb ice far more frequently in climb than in cruise, when the aircraft is flying slower.

It's a shame that you're done with this conversation, because you really need to explain how you were previously able to tell us that being configured for landing produces the widest range of carb icing, and that in your experience, carburetor ice is more likely in approach configuration because your physics professor told you so.

The problem is that now you tell us that it's not really the approach configuration, or the idle descent to approach, but in the climb where you've experienced carb ice the most.

Given that this is contrary to what your physics professor taught you, and contrary to what you previously told us has been your experience, I'm very curious to know which manufacturers recommend using carb heat in the climb. After all, you're adamant that these are the folks with all the degrees (in carburetor icing??), and that they know best. The problem is that I don't think I've ever seen a manufacturer call for carb heat in the climb as part of the climb procedure. Are you now telling us that all the engineers and the manufacturers are wrong?

Does your experience teach us this?

It's really a shame that you won't be a part of this conversation any more, because I'm entranced by your latest turn. If by any chance you reconsider, I await with rapt attention and baited breath. You're fascinating.

[LH2]

as a result of reducing throttle the "whole" is smaller in the throat

Perchance you mean the "hole"?

I would not normally point that out, but...

I studied engineering at University, and worked for many years doing flow analysis of liquids for flood control districts

Well, given your spelling and your manner of expression, I think the above statements are liable to qualification, if not outright questionable.

And just what is your flying experience, btw? (not that I'm interested)

[CREAMER]

I learned a good lesson today. In future I will be keeping carb heat applied until landing.

I fly an aircraft with a Continental A65 which has a reputation for being susceptible to carb icing. The conditions today were Temp -1deg C, Dew point -2.5deg C and RH 89%.

I initially flew two circuits. I gave a good dose of carb heat throughout most of the circuit but went to cold in the last 200 feet (for that one in a million go around). My first touch and go was uneventful. On the second circuit, I reduced to idle for the touchtown. The touchdown was uneventful but a few seconds later the engine stopped dead. I turned off the runway and restarted the engine without difficulty.

My first idea was that the idle speed was set too low but when stationary the engine idled ok.

I took off again, did another uneventful touch and go but again, on the second, the engine died after touchdown. I decided to give up after that, thinking that I would probably have to make some adjustment.

After discussion with two other instructors and having ascertained the conditions, we deduced that icing must have caused the problem. In fact, others had been experiencing icing on slightly more advanced engines today.

I was surprised at how quickly the ice must have formed and I am glad that I did not reduce to idle at about 100 feet (for some reason).

I am sure that with the aircraft stationary, there was enough residual heat from the engine to melt ice without the super cold airflow there.

On reflection, I don't think that there is a good reason to go to cold on final approach. If you have to go around you can put the carb heat to cold then. I don't even remember being taught to do this when I did my PPL in 1980. I do remember it being taught when I did a refresher in 1997. I guess it's one of those faddy things that crept in. Sadly it may cause more trouble than it solves.

I hope this helps and apologise if I have repeated too much of what is in previous posts.

[FormationFlyer]

Ive resisted until now...due to the large amount of hot air being exchanged - and exasperation that this debate goes on year after year after year. Sorry folks but 15 years of this debate (for me) is madness and *still* pilots lose engines due to carb icing - I suspect this is directly down to the community and instructors failing to teach to the published recommendations.

The CAA's guidance is straight forward and presented in two excellent documents - which I'm surprised to note haven't been mentioned.

AIC Pink 077/2009 Induction System Icing on Piston Engines as Fitted to Aeroplanes, Helicopters and Airships
and
Safety Sense Leaflet 14 Piston Engine Icing

Also the Lycoming service bulletin previously stated in this thread is excellent.

Icing conditions are present in the UK 100% of the time - its a question of severity rather than existence.

The recommendations are very clear. Carb heat on until touchdown.
It does say..."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 heat be returned to COLD at about 200/ 300 ft on finals." Unfortunately many pilots/instructors seem to believe (wrongly) that their aircraft is one of the exceptions. Why I have no idea.

The arguments about the go-around and loss of power on Cessna or Piper aircraft I believe is ridiculous - and such debates Im convinced are downright dangerous as we appear to be encouraging pilots not to err on the side of caution.

In aircraft where you can increase power and put the carb heat cold simultaneously do this simultaneously (C152 etc). In aircraft that this is not possible in (PA28) Apply power FIRST then move the carb heat to cold. This takes about 1/2 - 1 sec. The loss of 50-200rpm in this time is is reality inconsequential - the aircraft will as a MINIMUM fly level and in all but rare cases will climb with the resultant power on every aircraft I have flown. Comments about detonation (the dreaded carb heat and full power) in the space of 1/2 - 1 sec are also unfounded - especially as the engine itself will have been cooling (to some degree) during the descent anyway.

CREAMER - you situation is not unusual. Over the years I have come to know 2 instructors who have had engine failure actually on final approach in PA28s with Lycoming engines. I know another instructor who had engine failure on final approach in a Cessna 152 again with a Lycoming. All 3 made it to the runway thankfully.

I recommend that all pilots stop this continental vs lycoming myth as well - carb icing happens on majority of engine installations - play safe - use carb heat!

Personally I will teach in accordance with CAA recommended practices, and encourage ALL pilots to do the same.

I have yet to have an engine fail due to carb icing. Nor am I aware of any of my students having lost power due to carb icing and I plan to keep it that way.

[SNS3Guppy]

I have yet to have an engine fail due to carb icing. Nor am I aware of any of my students having lost power due to carb icing and I plan to keep it that way.

No need to apologize for lack of experience. Simply because you haven't experienced it yet isn't an indictment upon those who have.

Icing conditions are present in the UK 100% of the time - its a question of severity rather than existence.

An excellent observation, and one that applies outside the UK, as well.

The recommendations are very clear. Carb heat on until touchdown.

A blanket statement, and therefore only applicable some of the time.

Not all situations require it, not all aircraft require it. Some manufacturers recommend against it, as we've previously discussed here. In aircraft utilizing a carburetor air temperature gauge, the use of carburetor heat less than, or in excess of that required to place the carburetor air temperature in the proper operating range is inappropriate, and wrong.

I fly an aircraft with a Continental A65 which has a reputation for being susceptible to carb icing. The conditions today were Temp -1deg C, Dew point -2.5deg C and RH 89%.

Sounds like ideal carburetor icing conditions.

I learned to fly in a J-3 cub. One had to lean forward to pull the carburetor heat, and lean forward to push it off. Doing so in a go-around wasn't practical, really. Our policy was to apply carburetor heat prior to the power reduction abeam the numbers, for fifteen seconds, then shut it off for the descent and approach.

The only engine failure I had in a cub occurred on short final when I pushed the power up too fast because some deer ran onto the runway. Lacking an accelerator pump, one needs to be judicious with throttle application in that airplane, and the result was too much throttle, too quickly, and the engine complained. I never saw any issues with carburetor ice at that operation, in any of the airplanes. That's not to say that it wouldn't have been experienced there or elsewhere, but at no time did the policy of applying carburetor heat before the descent in the patttern, and then shutting it off, cause us a problem.

It can certainly cause a problem in other settings, other aircraft, other circumstances; one shouldn't apply that policy or procedure from that one location in that one airplane (it was standard fare among the other airplanes there, too...aeroncas, cubs, cessna 120's, etc) to your flying. Likewise, however, one shouldn't apply the blanket notion that carb heat must always be used for the descent. Then again, one shouldn't get in the habit of doing idle descents in light piston airplanes, either.

[FormationFlyer]

SNS3Guppy - Agree totally. Don't confuse not experienced carb icing (which I have many times) with not having an engine stopping.

Simply because you haven't experienced it yet isn't an indictment upon those who have.

There are many reasons why engines stop due to carb icing - much of it though IS down to pilot error. Indictment? hardly. I'm not here to put people down. I'm here to make sure we all stay living.

The blanket statement of course comes with the caveat unless contradicted by FM/POH. Don't take my word for it though. Please read the full background material - that is what I am suggesting. I am merely paraphrasing here.

In the J3 you are right - and a sensible move to take the carb heat away at suitable height.

But thats what this is about isnt it? Use the standard advice UNLESS the FM/POH or operational issues mean that the standard advice is not practical. Here you quote one of the exception due to practical operational reasons. I still believe that the number of these exceptions is relatively low - and indeed tend to exist on older a/c types such as the J3.

Why is it though that CREAMER experienced his issue? (please comment creamer). Was this a lack of training in general procedures? Lack of training specifically on type? Was CREAMER's training performed in an a/c that was fuel injected and then they famil'd onto the Cessna and the Famil training was lacking?

I'd love to know. I want to know where this problem could have been avoided - Because CREAMER should never have experienced this issue to start with.

I love the CAA's old slogan. "Safety is No Accident". It was very clever in its double meaning. I don't know why they don't seem to use it so much any more but its one that I hold dear. So I like to focus on avoiding problems and making flying safer.

[DFC]

The situation is very simple.

By all means follow the advice in the POH and don't select Carb heat during the approach.

When you have an engine failure due to carb ice on an approach or in a go-arround then don't blame the manufacturer or the author of the POH.

The answer will simply be that it was your fault because clearly the conditions existed for carb icing and that is why it formed. By not following the advvice in the POH to have hot air selected in such conditions you caused the accident.

99% of people having problems making a go-arround have overloaded the aircraft fro the conditions. Every aircraft shoulld have a suitable balked landing performance - that is with gear down and flap for landing. While having hot air selected may reduce this performance temporarily until it is selected cold, the fact that the aircraft can climb with drag flap means it will climb a lot better with approach flap.

So who is now going to ignore the POH advice and only use partial flap settings to land?..........oh sorry, plenty of people doing that already...........and plenty more loading aircraft to such an extent that the balked landing performance is nil.

Pipoer are very clear, if the conditions are suitable for formation of ise then have the hot air selected during approach. Clearly if you don't and the engine fails it is your fault for not recognising the situation.

This is the situation in the UK for most of the year.

[CREAMER]

In my case, I probably got caught out because my training said that I should select carb heat at 200 feet. This was on club aircraft (C152) during a refresher and Instructor Course.

Translate this now to an Aeronca with an A65 in fairly typical winter conditions very conducive to carb icing. You just can't do it. In fact, if I fly a C152 again I will still keep carb heat to touchdown. If the CAA are saying this as well then we have little choice (in court).

[FormationFlyer]

I just want to point out that the original question here was one of instructor standardisation in a school in the UK. It was a discussion by an instructor who wanted to consult fellow instructors and examiners.

How did we end up with comments about some dratted 'physics teacher?'. Laughable - and detracts from the importance of this thread.

For those who are not flying in the UK or used to permenant carb icing conditions the biggest cause of engine failure in the UK is carb icing, followed by mismanagement of fuel systems. In other countries where carb icing conditions are mild most of the time I would imagine fuel mis-management is probably a bigger issue.

There is a myth that PA28s don't carb ice - they do (ive had it in IMC, ive had it on the ground before night flying during the taxi etc). There is a myth that C150/152 on continentals are terrible but you are OK on a lycoming - you arent. Ive had carb icing on C152 with Lycomings.

The point about I was making about the instructors who had engine failures is this - all 3 of them without exception now take carb heat to the deck. I like to learn from others experiences - I read GASIL and associated accident summaries religiously. I go to a CAA safety evening every year - even though some might expect that as an experienced instructor and examiner I might not. Personally I recommend everyone attends - they are good (and as a side note it was nice to see David Cockburn's hard work recognised).

DFC - absolutely.
CREAMER - thanks for the info. Most interesting.

In the UK the most common training aircraft are PA28s, C150/152/172 and Robin aircraft. The CAAs advice holds good for all these aircraft.

I believe its an absolute must that we instructors standardise our teaching practices - particularly within the same school. I believe the basis for that advice should start with the CAA AIC & Safety sense leaflet further modified by the FM/POH and engine manufacturers advice - as pointed out (quite rightly) every aircraft is different. What we as instructors must teach and prepare students for any aircraft - but equally we must give the 'basic' advice on the cautionary side.

[DFC]

To look at the issue from a different perspective.

When descending from 5000 to say say 2000ft does anyone not use carb heat when the power is reduced - and keep it hot until power is increased?

So there is absolutely no perceived problem with quickly establishing full power if an urgent climb is required at 3500ft - to avoid another aircraft for example?

If someone is teaching to put carb heat cold at 300ft in case of go-arround then what about the go-arround from 600ft?

How many instructors here teach that the power off fully developed stall or any other stall exercises where low power is used should be entered with carb heat cold - just so that max power can be established on recovery?

If the stall warner goes off at 350ft (for those that select cold at 300) do you not simultaneously reduce the AOA and apply full power?

Many actions in these critical situations such as reduce AOA, full power (throttle full, carbheat cold) have to be automatic to be effective.

[SNS3Guppy]

When descending from 5000 to say say 2000ft does anyone not use carb heat when the power is reduced - and keep it hot until power is increased?

Of course I don't use carb heat as part of a routine, normal cruise descent.

Are you making idle descents?

In other countries where carb icing conditions are mild most of the time I would imagine fuel mis-management is probably a bigger issue.

Carburetor ice is a geopolitical issue, now?

[johns7022]

Guppy...you must be here to confuse the kids.....

OP: Power out of the green...in a decent...carb heat on....

I would elaborate...but Guppy and I will get into it.

[FormationFlyer]

Carburetor ice is a geopolitical issue, now?

Dear oh dear. SNS3Guppy that is just a silly comment that suggests belligerence. Shame because I'm sure you have a wealth of real experience to share instead of unwieldy attempts at points scoring.

I thought we were experienced instructors and examiners on this forum and therefore did not need explain the basics behind such a comment in detail, but as you obviously feel the need to have me justify the context of the comment Ill indulge you this time.

Whilst Carb icing is *not* geographically confined by country NAAs are and thus accident/incident statistics etc are as well. The statistics are collected by the NAAs and thus it follows that the most common cause of engine failure may differ between countries for a variety of reasons.

Additionally advice and guidance issues forth from those NAAs to be applied by the pilots and on aircraft operated within their jurisdiction. This may have an effect and thus may effect the statistics for a given NAA.

Where I refer to carb icing conditions being mild I was asserting that it was unlikely that carb icing is likely to be the most significant cause of engine failure. Additionally whilst carb icing is a global issue the prevalent conditions in a given country will be influenced by their geographical location. In the UK we experience high humidities as any air mass inbound to us tracks over water.

I hope this clears up any possible confusion that I may have caused.

[SNS3Guppy]

The statistics are collected by the NAAs and thus it follows that the most common cause of engine failure may differ between countries for a variety of reasons.

Given the commentary here and on the private pilot forum, I'd say that the sampling on this site clearly shows the reason for the geopolitical difference isn't climatic, but the lack of proper training. Poster after poster has acknowledged the scant receipt of carburetor training, systems training, or even the availability of a pilot handbook during their training, rental, or use of the airplane.

No wonder that incidents occur given the lack of proper training.