PA 28-161 Carb heat on the approach
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Sans Anoraque
Read the post again I would not dare be so bold as to decide on which method was best but more to show the pro’s and con’s of each method.
The certification standards for GA aircraft carb heat systems JAR 23.1093 require a temperature rise of 56 deg at a 60% power setting now here is the problem, nobody knows how long the carb heat will need to be applied before the ice melts, Lycoming give 30 to 120 seconds where as the CAA AIC give 15 seconds (there is now a study into this commissioned by the CAA/GASCO). But as can be seen with a power setting of less than 60 % on approach the chances of clearing the ice in time to make a difference is not possible.
With ref to a rich cut.
I recommend a little reading up on what happens in a carb with ice building up on the venturi.
Bookworm
The detonation talked about in this instance is not mixture related it is the introduction of significantly hotter air greater than 60 deg into the cylinder head.
Hung flung dung
If you were to increase the outside air temperature at your local airport by more than 60 deg you could expect to suffer from detonation, unless you can reduce the power needed.
Bearing in mind we are talking about all aero engines here and with the lower compression engines fitted to the PA28/C150 it is a very rare event.
Shortstripper
This is quite incorrect oh is it. Read the following from Lycoming and GASCO
http://www.lycoming.textron.com/main...tionIcing.html
http://www.flyontrack.co.uk/carbice2004.htm
Detonation and its causes is a complex subject but I think you have missed the point, I am talking about detonation caused not by a change in mixture but by the extra heat put into induction system when carb heat is left to hot and full power is applied.
No I am not confusing detonation with rich cut as one you will not notice but the other will give you a major adrenalin rush. Stumble is something I do after a night out with the lads.
Read the post again I would not dare be so bold as to decide on which method was best but more to show the pro’s and con’s of each method.
I dont see how the exhaust manifold can get so cold,so quickly that it wouldn't still warm the air.
With ref to a rich cut.
I recommend a little reading up on what happens in a carb with ice building up on the venturi.
Bookworm
What is the proposed mechanism for "detonation"? Other than "it says so in the POH"? It seems counter-intuitive to me. The richer mixture would seem to move further away from the detonation regime.
Hung flung dung
If you were to increase the outside air temperature at your local airport by more than 60 deg you could expect to suffer from detonation, unless you can reduce the power needed.
Bearing in mind we are talking about all aero engines here and with the lower compression engines fitted to the PA28/C150 it is a very rare event.
Shortstripper
This is quite incorrect oh is it. Read the following from Lycoming and GASCO
http://www.lycoming.textron.com/main...tionIcing.html
http://www.flyontrack.co.uk/carbice2004.htm
Detonation and its causes is a complex subject but I think you have missed the point, I am talking about detonation caused not by a change in mixture but by the extra heat put into induction system when carb heat is left to hot and full power is applied.
No I am not confusing detonation with rich cut as one you will not notice but the other will give you a major adrenalin rush. Stumble is something I do after a night out with the lads.
Last edited by Lomcovack; 1st Dec 2005 at 05:42.
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Lomcovack
Both very interesting articles and like I said I'm no expert. I still think detonation is very unlikely to occur where carb heat is accidently left on for two reasons. Having descended the cylinder temperatures are unlikely to be very hot, also the mixture will be richer with carb heat applied. Both these things are opposed to detonation. However, you are correct I suppose if carb heat is then left on significantly into the climb as temperatures rapidly rise and yes, then detonation may occur due to very high cylinder head temperatures and hot air applied. I was thinking you had meant detonation would occur as power was applied for the go around ... which is where it is unlikely. Hopefully by the time you are established in the climb you would have realised carb heat was still applied due to the lack of performance.
Understanding is everything, and I'm sure from what you have said that you do understand. I had just misread how you had applied that understanding to your previous post.
SS
Both very interesting articles and like I said I'm no expert. I still think detonation is very unlikely to occur where carb heat is accidently left on for two reasons. Having descended the cylinder temperatures are unlikely to be very hot, also the mixture will be richer with carb heat applied. Both these things are opposed to detonation. However, you are correct I suppose if carb heat is then left on significantly into the climb as temperatures rapidly rise and yes, then detonation may occur due to very high cylinder head temperatures and hot air applied. I was thinking you had meant detonation would occur as power was applied for the go around ... which is where it is unlikely. Hopefully by the time you are established in the climb you would have realised carb heat was still applied due to the lack of performance.
Understanding is everything, and I'm sure from what you have said that you do understand. I had just misread how you had applied that understanding to your previous post.
SS
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This has been a wonderully enlightening topic so far... The views expressed across the board have all been justified and in some cases differtences have been based on real experiences.
But the thing I cannot understand is, why in this day and age are we worrying about a venturi causing so much trauma?
With the advent of fuel injection and computerised 'black boxes' surely there must be an aftermarket solution. Does anyone know of a retrofit injection system that would be compatible for the standard lycoming engines?
If not, would it be worth getting someone to look at building one?
The cost of a retrofit would definitely be outweighed by the enhanced flight safety and allow people to concentrate on the flying aspects a little more, rather than engine management in difficult phases of flight.
A training aircraft is supposed to be simple, why not make it both simple and safe for all stages of flight.
A390
But the thing I cannot understand is, why in this day and age are we worrying about a venturi causing so much trauma?
With the advent of fuel injection and computerised 'black boxes' surely there must be an aftermarket solution. Does anyone know of a retrofit injection system that would be compatible for the standard lycoming engines?
If not, would it be worth getting someone to look at building one?
The cost of a retrofit would definitely be outweighed by the enhanced flight safety and allow people to concentrate on the flying aspects a little more, rather than engine management in difficult phases of flight.
A training aircraft is supposed to be simple, why not make it both simple and safe for all stages of flight.
A390
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Thanks for posting those two articles Lomcovak - very interesting as ss said.
I've noticed, from this thread and from talking to pilots, that British trained pilots tend not to run carb air 'hot' all the time on approach, preferring instead to cycle carb heat on downwind and leave it off for the approach.
The only times I've experienced carb ice is when carb air is set to cold. So why not leave it hot on approach? Nothing I've read in this forum suggests it's bad for modern engines in flight, except in very dusty conditions.
At least one person has commented that, when going around with carb heat applied, he first puts carb air to cold and then applies power. In this country a CPL examiner would have grounds to fail a candidate who, when asked to go around, first put carb heat cold and then applied full power, instead of maximising power as a first action. It's all about minimising further height loss!
I wonder could you tell us where you read that, or which aircraft you experienced it in Lomcovak? I've had carb ice in a variety of circumstances, including an O360 that was actually below 1000rpm due carb ice with throttle closed, and carb heat fixed the problem within seconds. I think it's quite important that pilots have confidence that carb heat will fix carb ice if applied promptly.
Sorry about the long post...
O8
I've noticed, from this thread and from talking to pilots, that British trained pilots tend not to run carb air 'hot' all the time on approach, preferring instead to cycle carb heat on downwind and leave it off for the approach.
The only times I've experienced carb ice is when carb air is set to cold. So why not leave it hot on approach? Nothing I've read in this forum suggests it's bad for modern engines in flight, except in very dusty conditions.
At least one person has commented that, when going around with carb heat applied, he first puts carb air to cold and then applies power. In this country a CPL examiner would have grounds to fail a candidate who, when asked to go around, first put carb heat cold and then applied full power, instead of maximising power as a first action. It's all about minimising further height loss!
On finals we notice a slow drop in rpm, indicative of carburettor icing, Is carb heat going to clear any ice, not at these power settings.
Sorry about the long post...
O8
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OK, Lomcevak, perhaps it's because I'm thick, but I don't think you are explaining the pros & cons, I think you're suggesting something that makes no sense.
So why would you put CH to cold? (I presume we're still talking about PA28s & Lycomings)
So there we are on the downwind leg at a medium power setting and our check for carb icing shows nothing significant.We put the carb heat to cold
The detonation talked about in this instance is not mixture related it is the introduction of significantly hotter air greater than 60 deg into the cylinder head.
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I wonder could you tell us where you read that, or which aircraft you experienced it in Lomcovak? I've had carb ice in a variety of circumstances, including an O360 that was actually below 1000rpm due carb ice with throttle closed, and carb heat fixed the problem within seconds. I think it's quite important that pilots have confidence that carb heat will fix carb ice if applied promptly.
Bookworm,
As I said in my answer to Lomcovack, I think it is very unlikely detonation would occur and I believe you are correct to think leaving carb heat "hot" is (whilst not suggesting it's the right thing to do) moving away from detonation regime as you say. He/she? is right though, that if the engine is over heating, then hot air could induce detonation. However, in the context of this subject I think it's a bit far fetched to expect detonation by leaving hot air on by mistake, unless the subsequent climb is very steep, under full power and under extreme conditions!
SS
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With internal temps too high, you can get pre-ignition this certainly can lead to detonation.
Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?
The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!
Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?
The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!
With internal temps too high, you can get pre-ignition this certainly can lead to detonation.
Anyone got an engine monitor on a carburettor engine?
Personally I haven't done any testing of a Lycoming to see if this is a problem and I very much doubt anyone else here has, so why would you not believe a manufacturer if they state that such a problem could exist?
http://www.avweb.com/news/columns/186216-1.html
and an Aussie coroner that was rather more persuaded by Braly and Deakin than Lycoming!
...not to mention some personal experience of reading things in POHs that don't seem to make physical sense.
The onus is not on FI's to justify the POH, but for those who disbelieve it to prove why they think themselves to be correct. With impirical data as well please!
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The actual heat and pressure sensing inside a cylinder is a very difficult thing to do. All sorts of things can lead to problems and having a high temp inlet charge is one of them.
Just because an engine seems to be producing less power, doesn't necessarily mean that it's running cooler. It could just be less efficient and running hotter in certain areas, again a very difficult thing to measure without very high tech equipment, a dyno and then a rip down to see what has been occuring.
This was exactly the sort of thing we used to do at uni with car and bike engines of different designs and there were very often results that were unexpected if you looked at the engine in the simplistic way that most pilots would. There were certianly a few that didn't fit the model I had predicted.
Just because an engine seems to be producing less power, doesn't necessarily mean that it's running cooler. It could just be less efficient and running hotter in certain areas, again a very difficult thing to measure without very high tech equipment, a dyno and then a rip down to see what has been occuring.
This was exactly the sort of thing we used to do at uni with car and bike engines of different designs and there were very often results that were unexpected if you looked at the engine in the simplistic way that most pilots would. There were certianly a few that didn't fit the model I had predicted.
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That was always my understanding and pretty true overall, but there maybe a situation that doesn't fit that, for example having hot air coming into the cylinder and the a great glob of fuel stuffed in.
I think the problem is if you were to leave the carb heat set to hot with full power applied, rather than a momentary issue just after power application.
If you have all the heat being produced by having the throttle wide open AND carb heat hot, then I can well imagine problems, especially as the more power you have applied, the hotter the exhausts will get, so the hotter the carb air will become....... I can definately see this potentially leading to pre-ignition, which then goes to detonation. And this being at relatively low speed, so the cooling airflow isn't massive.
I think they are covering their backsides about prolonged application of full power and carb heat hot, the next question is how long would it need to be to before this could become apparant?
I can just imagine the engineers sitting around a table trying to answer that and then someone saying, "forget it, let's just write the POH to avoid this situation in the first place."
I think the problem is if you were to leave the carb heat set to hot with full power applied, rather than a momentary issue just after power application.
If you have all the heat being produced by having the throttle wide open AND carb heat hot, then I can well imagine problems, especially as the more power you have applied, the hotter the exhausts will get, so the hotter the carb air will become....... I can definately see this potentially leading to pre-ignition, which then goes to detonation. And this being at relatively low speed, so the cooling airflow isn't massive.
I think they are covering their backsides about prolonged application of full power and carb heat hot, the next question is how long would it need to be to before this could become apparant?
I can just imagine the engineers sitting around a table trying to answer that and then someone saying, "forget it, let's just write the POH to avoid this situation in the first place."
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No they definately are linked and pre-ignition is definately a pre-cursor to detonation.
Detonation is when the mixture inside the cylinder explodes rather than burn in a controlled manner. You can often get multiple flame fronts and lots of other nasties. Basically the temps and pressures inside the cylinders go up massively and can cause problems to the head gasket, pistons, piston rings, sparkplugs and all the bearings. Not good really!
Pre-ignition is where there is a hot spot that causes the mixture to ignite before the spark plug fires, this can lead to big problems such as trying to turn the engine backwards as well as localised heating, which in turn can burn out piston etc.
Pre-ignition can cause or exacerbate a detonation problem. But pre-ignition is far more dangerous to an engine than detonation. Detonation can be lived with for a while before an engine gives up the ghost, wheras pre-ignition can kill an engine very, very quickly.
Pre-ignition can be caused by all manner of things, from carbon deposits or anything that causes the engine to run hotter than usual. ie having carb heat selected at full power.
There is a phenomenon called "Detonation Induced Pre-ignition" so the two are definately linked.
One final thing. If an engine is detonating, what happens to the EGT?
It goes Down.
Detonation is when the mixture inside the cylinder explodes rather than burn in a controlled manner. You can often get multiple flame fronts and lots of other nasties. Basically the temps and pressures inside the cylinders go up massively and can cause problems to the head gasket, pistons, piston rings, sparkplugs and all the bearings. Not good really!
Pre-ignition is where there is a hot spot that causes the mixture to ignite before the spark plug fires, this can lead to big problems such as trying to turn the engine backwards as well as localised heating, which in turn can burn out piston etc.
Pre-ignition can cause or exacerbate a detonation problem. But pre-ignition is far more dangerous to an engine than detonation. Detonation can be lived with for a while before an engine gives up the ghost, wheras pre-ignition can kill an engine very, very quickly.
Pre-ignition can be caused by all manner of things, from carbon deposits or anything that causes the engine to run hotter than usual. ie having carb heat selected at full power.
There is a phenomenon called "Detonation Induced Pre-ignition" so the two are definately linked.
One final thing. If an engine is detonating, what happens to the EGT?
It goes Down.
Last edited by Say again s l o w l y; 2nd Dec 2005 at 10:05.
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Ok, but isn't detonation more likely to lead to pre-ignition, than pre-ignition to detonation as you previously posted? Pre-ignition used to be quite common in cars when you could buy various star rated fuels and spark plug technology wasn't so good. You KNOW! when you get pre-ignition and can't ignor it. So how would you go from that to the much milder and less noticable detonation?
Sorry ... I'm getting a bit off topic here, and only really being a bit pedantic, but it's hard to resist (although I'm probably wrong) lol
SS
Sorry ... I'm getting a bit off topic here, and only really being a bit pedantic, but it's hard to resist (although I'm probably wrong) lol
SS
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Detonation is MORE likely to lead to pre-ignition than pre-ignition leading to detonation, but it does also go the other way sometimes!
The fact is, that you often wouldn't notice it as the engine has just eaten itself due to pre-ignition!
Pre-ignition is classified as anything that causes the mixture to ignite before the spark goes off, this usually around 20 deg BTDC and peak pressure being at 14 deg ATDC.
If the mixture ignites before the spark plug fires, then it is pre-ignition. Detonation is usually the "end gas" from a normal burn exploding due to lack of octane and causing a pressure spike.(one that doesn't last that long)
This "end gas" could for instance be ignited by a hot spot on something like a hot exhaust valve, so the hotter the engine, the hotter the end gas, the more likely it is to detonate and as you get detonation, you get more heat, so the cyle continues until something breaks.
The fact is, that you often wouldn't notice it as the engine has just eaten itself due to pre-ignition!
Pre-ignition is classified as anything that causes the mixture to ignite before the spark goes off, this usually around 20 deg BTDC and peak pressure being at 14 deg ATDC.
If the mixture ignites before the spark plug fires, then it is pre-ignition. Detonation is usually the "end gas" from a normal burn exploding due to lack of octane and causing a pressure spike.(one that doesn't last that long)
This "end gas" could for instance be ignited by a hot spot on something like a hot exhaust valve, so the hotter the engine, the hotter the end gas, the more likely it is to detonate and as you get detonation, you get more heat, so the cyle continues until something breaks.
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long time ago before I met aeroplanes I had met Physics and chemistry and alot of it from an atmospheric view point so I support:
The POH with adaptation to the current circumstances.
There would be appear to be quite alot of evidence that the risk of not apply carb heat enough far out weighs the risk of hyaving it on when you go around.
Personally I teach you cannot have enough carb heat going down wind and whether you need to go back to cold is small fry (you must get rid of any carb ice before the final approach starts) Then carb heat must be on for low power settings as a less then effect pre ventive measure (but at least its step in the right direction). Then to handle the original question I tie the issue odf carb heat on/off to the final stage of flaps decision. i.e do I need it/not need it - m\ake decision and then then carb heat cold
The POH with adaptation to the current circumstances.
There would be appear to be quite alot of evidence that the risk of not apply carb heat enough far out weighs the risk of hyaving it on when you go around.
Personally I teach you cannot have enough carb heat going down wind and whether you need to go back to cold is small fry (you must get rid of any carb ice before the final approach starts) Then carb heat must be on for low power settings as a less then effect pre ventive measure (but at least its step in the right direction). Then to handle the original question I tie the issue odf carb heat on/off to the final stage of flaps decision. i.e do I need it/not need it - m\ake decision and then then carb heat cold
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Listen to Mad Jock, he knows what he is talking about.
In general with those older engines (non fuel injected) the Lycomings, because of the position and type of carburetor and air ducting, do not need precautionary carb heat. Use it to clear suspected ice, but not to prevent ice, since the air is already heated. The Continentals do need precautionary carb heat, since they have a more direct induction system and the carb is below the engine (updraft) in a position where it is colder, usually and more susceptible to icing.
The ducting is also different in the PA 28 compared to the Cessna models, even for the same engine.
Another point though is that to clear ice you need the control in hot before the engine loses power, and on approach there is probably not much heat available so it is better to clear the ice on downwind in all types or at least check to see if there was any ice at that time, and if it is present, keep the power up as long as you can on approach, avoiding idle power until the landing is assured.
A further point I did not see here is that unless you have a gauge to read the carb temp, use full heat only; partial heat might just move the temp into the icing range.
In general with those older engines (non fuel injected) the Lycomings, because of the position and type of carburetor and air ducting, do not need precautionary carb heat. Use it to clear suspected ice, but not to prevent ice, since the air is already heated. The Continentals do need precautionary carb heat, since they have a more direct induction system and the carb is below the engine (updraft) in a position where it is colder, usually and more susceptible to icing.
The ducting is also different in the PA 28 compared to the Cessna models, even for the same engine.
Another point though is that to clear ice you need the control in hot before the engine loses power, and on approach there is probably not much heat available so it is better to clear the ice on downwind in all types or at least check to see if there was any ice at that time, and if it is present, keep the power up as long as you can on approach, avoiding idle power until the landing is assured.
A further point I did not see here is that unless you have a gauge to read the carb temp, use full heat only; partial heat might just move the temp into the icing range.
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In the U.K we only teach the use of full carb heat, never partial.
In fact the only machine I've ever flown that you can use partial carb heat at any time in the U.K is the R22 helicopter. (Derated engine and a carb temp gauge)
In fact the only machine I've ever flown that you can use partial carb heat at any time in the U.K is the R22 helicopter. (Derated engine and a carb temp gauge)
Extract from Cessna 152 manufacturer's POH and C172N.
Balked Landing. 1. Throttle wide open. 2. Carburetor Heat..COLD. I suggest this would equally apply to a touch and go landing on other than a dirt or grass strip - a touch and go being nothing more than a go-around procedure.
In other words the aircraft manufacturer recommends you leave the carb heat fully on for the final approach and go-around and then take it off after you have applied full power. This ensures as far as possible that the burst of heat from the exhaust shroud as the throttle is opened wide minimises the possibility of ice problems at a critical stage.
The manufacturer would have conducted engineering studies during the test flying stage and found that no adverse effect on the engine would occur if the recommendation was followed. The Cessna 152 POH states that after landing checks include carb heat to off - meaning the carb heat is on for the landing and landing roll - not placed to off on final just in case a go-around occurs.
A wise pilot also applies full carb heat for several seconds on the ground immediately prior to applying full throttle for a normal take off if carb icing conditions exist. Testing the carb heat during a run-up will not necessarily ensure that ice has not built up after taxying to the runway for take off - especially if you are at a holding point for several minutes.
Balked Landing. 1. Throttle wide open. 2. Carburetor Heat..COLD. I suggest this would equally apply to a touch and go landing on other than a dirt or grass strip - a touch and go being nothing more than a go-around procedure.
In other words the aircraft manufacturer recommends you leave the carb heat fully on for the final approach and go-around and then take it off after you have applied full power. This ensures as far as possible that the burst of heat from the exhaust shroud as the throttle is opened wide minimises the possibility of ice problems at a critical stage.
The manufacturer would have conducted engineering studies during the test flying stage and found that no adverse effect on the engine would occur if the recommendation was followed. The Cessna 152 POH states that after landing checks include carb heat to off - meaning the carb heat is on for the landing and landing roll - not placed to off on final just in case a go-around occurs.
A wise pilot also applies full carb heat for several seconds on the ground immediately prior to applying full throttle for a normal take off if carb icing conditions exist. Testing the carb heat during a run-up will not necessarily ensure that ice has not built up after taxying to the runway for take off - especially if you are at a holding point for several minutes.