Commonsense attitude to Carburettor Icing

[Centaurus]

Extract from BASI Journal No.7 Autumn 1990.
Carburettor Icing:

Look again at the first little fact: `Ice is COLD and needs HEAT to melt it`.
So why one day that is conducive to icing, do we reduce power then pull carb heat on? Stupid really - we pull the power which reduces the heat available! The FAA in an Advisory Circular said: `Heat should be applied for a short time to warm the induction system BEFORE commencing descent.` Ever thought of putting the carb heat on as part of the base checks? Then you will be applying heat BEFORE power reduction.

And our second little fact: `somewhere you have a CLUE to tell you ice will form or has formed`.
.Did you get a definite RPM drop on the engine run?
.Did you check the carburettor icing probability chart to see if today is a good day for carb ice?
.Did you monitor your RPM, manifold and temperature gauges and determine what the indications could mean?
.Did you clear the carb of ice BEFORE you reduced power?
.Do you apply power FIRST in a go-around - then put carb heat to OFF?
............................................................ ......................................

The above recommendations were published in 1990. 27 years on they are still current.:ok:

[gassed budgie]

And who would've thought that in the high tech world of aviation we'd still be fiddling with carburettor heat deep into the 21st century.

The last time I remember having to apply carby heat in a motor car was in the grand fathers '51 split windowed VW beetle. Or perhaps it was the great grandfather's Modet T!

[Lead Balloon]

That Journal article was really active-voicey and used words like "stupid", so I was too confronted and offended to work out what it meant.

(I still carry that carbie icing probability chart, even though I usually fly injected engines.)

[Aussie Bob]

Do you really think it makes that much difference? Carby heat then reduce power or reduce power then carby heat? All of 5 seconds difference.

My O360 has a carby temperature gague and when I reduce power in the circuit, the carby temperature increases without carby heat, usually to well within the green arc. Ice in cruise is not uncommon, as noted by a drop in MP. Ice on approach, never seen it in the last 8 years of ownership.

[MagnumPI]

Early on whilst learning to fly there were two separate accidents near my home airfield which were both due to the engine stopping and insufficient height to make it to the strip.

Both pilots elected to put the aircraft down in paddocks. In both instances the aircraft were destroyed but crew and pax walked away pretty much uninjured - very lucky!

The engines in both aircraft were tested after the accidents and worked fine. Seeing these happen reinforced to me the importance of using carb heat. We were always taught (Cessna aircraft) left to right application, ie. carb heat on when turning base, before throttle pulled back. Or carb heat on when TOD, before pulling throttle back. :ok:

[Ex FSO GRIFFO]

AAhhh, the use of 'that' carby heat knob....

Many moons ago in the NW of WA, a fairly new CPL fella was doing some mustering in a C-150. He had been taught to pull the carby heat knob whenever operating at low level.

So he did this for the week or two of the exercise, and later when the 100hourly was carried out, it was found that the 'new' engine had to be scrapped, due dust abrasion / scouring in all 4 cylinders.

It had never been pointed out to him, that the use of the carby heat, delivers 'hot unfiltered' air, directly into the carby....complete with all present dust particles....and these dust / dirt particles at his 'low level' had 'done the deed' to the donk.

Just sayin' is all.....

Cheers:ok:

[kaz3g]

The C65 in the Luscombe was particularly prone to ice and the whole circuit often had to be flown with carby heat on.

Kaz

[PDR1]

And who would've thought that in the high tech world of aviation we'd still be fiddling with carburettor heat deep into the 21st century.

The last time I remember having to apply carby heat in a motor car was in the grand fathers '51 split windowed VW beetle. Or perhaps it was the great grandfather's Modet T!

That's because the problem was been engineered-out by having bimetal-strip-actuated selectors in the intake trunking that select warm air from above the exhaust manifold when the carb air temp drops too far, plus water-heated manifolds. Most aero engines are air-coooled these days, so the water-heated manifold isn't an option. Automated hot-air selection would become a safety-critical item needing intesive effort to design, certify and maintain - a manual "carb heat" selector is probably a far more cost-effective option.

25 years ago I built a Westfield 7SE kit car with a modified Ford Crossflow engine (roughly 160bhp, breathing through a pair of Webber 40DCOEs). To fit this lump in the car the carb intakes and air filters were largely outside the bonnet. When I first took this car onto a motorway I found that it started misfiring with reduced power after 5 miles or so of "cruise" running at 85-90mph^h^h^h^h^h^h^h^h 70mph ("cruise" power for a performance car would be less than 20%, so the throttles would be nearly shut), and if I continued the car would come to a standstill on the hard shoulder about 3 miles later. Looking under he bonnet I could never find anything wrong, and after 2-3 minutes the car would restart and drive normally.

This continued for several months and it had me metaphorically tearing my hair out until one day when it stopped for some reason I popped the cover off the front air filter to see if it was somehow clogged. Looking down the choke of no1 cylinder I saw a huge ice accretion almost completely blocking the choke. There was a similar one in the no.2 choke, and so I popped the other filter cover off and sure enough there were smaller ice blocks there as well. They all just melted away as I watched.

The webber manifold had no provision for heating, and the carbs were thermally isolated from the manifold anyway, so I ended up making a beaten-aluminium cover which extended the bonnet over the carbs and forced them to breath warm air from behind the radiator. After that the problem never recurred.

PDR

[Band a Lot]

AAhhh, the use of 'that' carby heat knob....

Many moons ago in the NW of WA, a fairly new CPL fella was doing some mustering in a C-150. He had been taught to pull the carby heat knob whenever operating at low level.

So he did this for the week or two of the exercise, and later when the 100hourly was carried out, it was found that the 'new' engine had to be scrapped, due dust abrasion / scouring in all 4 cylinders.

It had never been pointed out to him, that the use of the carby heat, delivers 'hot unfiltered' air, directly into the carby....complete with all present dust particles....and these dust / dirt particles at his 'low level' had 'done the deed' to the donk.

Just sayin' is all.....

Cheers:ok:


A value for money instructor there!

[PDR1]

Why would they have scrapped the engine when a set of replacement barrels, pistons and rings would be far less expensive than a whole new engine?

Just sayin'...

PDR

[Band a Lot]

Why would they have scrapped the engine when a set of replacement barrels, pistons and rings would be far less expensive than a whole new engine?

Just sayin'...

PDR



Since when did the Carb Heat Air bypass the carby?

Just askin'

And just sayin

A exchange engine during a mustering season may be the fast option required.

[RadioSaigon]

Since when did the Carb Heat Air bypass the carby?

Might have missed something -but I can't see where anyone Has said that! Selecting Carb Air HOT does bypass any normal air-induction filtration, usually taking hot air from adjacent to the exhaust manifold, to the carb. Quite counter-productive to have the hot air "bypass the carby" -at several levels, I would think.

[R755]

Why would they have scrapped the engine when a set of replacement barrels, pistons and rings would be far less expensive than a whole new engine?

Just sayin'...

PDR
So you don't think the dust and debris would get all through the engine in a hundred or so hours? Top end only, eh?
Maybe so, but I will bet against you on that one.
Twenty hours running is around 5,110,560 litres of dusty air for a 200 cubic inch, 4 cycle, engine at 2500 rpm. That's grade A, tasty, red dust.

[PDR1]

Might have missed something -but I can't see where anyone Has said that! Selecting Carb Air HOT does bypass any normal air-induction filtration, usually taking hot air from adjacent to the exhaust manifold, to the carb. Quite counter-productive to have the hot air "bypass the carby" -at several levels, I would think.


I think he's suggesting that the dusty air would have somehow damaged the carb.

PDR

[PDR1]

So you don't think the dust and debris would get all through the engine in a hundred or so hours? Top end only, eh?


Well that's the only part of the engine that is normally fed by filtered air, so yes. Unless you're suggesting that dust particles large enough to not be immediately scrubbed into the oil filter can somehow get past the rings

PDR

[Aussie Bob]

Very few aircraft provide filtered air to the hot air intake. I had one once, a 1963 172 that used a second Bracket air filter for the hot air. It got surprisingly dirty.

[FL235]

Change of emphasis, perhaps, but I saw a 150 go into Moreby harbour probable cause carby ice on a hot png day. Humid tho. He was beating up boats along the reef on reduced power to get good looks at crews, when he opened the throttle the response was noted by absence. Trying to climb was the last mistake, he wallowed up to about 200', stall, spin, splash. Of course you can't get ice at 90+, can you?

[Band a Lot]

I think he's suggesting that the dusty air would have somehow damaged the carb.

PDR



Any that think dusty air wont effect a caby, its surfaces (and areas), bearings, bushes and orifices - will just do a Top.

[PDR1]

Sure, dust will accrete, but it won't erode anything much. The only moving part that's exposed to this unfiltered airflow is the throttle butterfly, and I struggle to see how dust in a fast-moving airstream is going to find its way into the butterfly bushes. All the other moving parts are exposed to the dust to the same extent whether the carb heat is selected or not, so they are irrelevant.

You might get some blocked jets (although the air pressure is, by definition, pushing against the incoming dust rather than sucking it into the jets), but even if you did it would only need an overhaul (strip & clean) and wouyld not justify scrapping the carb.

So I still don't see it.

PDR

[Aussie Bob]

Feel free PDR1, to feed dust into any engine you own. I fail to see how this dust in the carby ****e can be extracted from Griffo's post, which is about dust damaging an engine.

It had never been pointed out to him, that the use of the carby heat, delivers 'hot unfiltered' air, directly into the carby....complete with all present dust particles....and these dust / dirt particles at his 'low level' had 'done the deed' to the donk.Hot unfiltered air into the carby ..... then on into the engine. Not in my donk, even if it doesn't hurt the carby!!!!!! How else is the dust going to enter the engine if it doesn't go through the carby? I call bollocks to what you are saying anyway, dust will damage a carburetor eventually.

[Band a Lot]

Sure, dust will accrete, but it won't erode anything much. The only moving part that's exposed to this unfiltered airflow is the throttle butterfly, and I struggle to see how dust in a fast-moving airstream is going to find its way into the butterfly bushes. All the other moving parts are exposed to the dust to the same extent whether the carb heat is selected or not, so they are irrelevant.

You might get some blocked jets (although the air pressure is, by definition, pushing against the incoming dust rather than sucking it into the jets), but even if you did it would only need an overhaul (strip & clean) and wouyld not justify scrapping the carb.

So I still don't see it.

PDR

Erode one side of a fuel jet and that makes a larger nozzle size and makes many changes that the carb was designed for.

Mostly I have found worn butterfly bushes - these neither give full power or a proper idle - without other adjustments if possible to do it.

[Clare Prop]

Amazing how many people who previously flew Cessnas would use the same technique for application of carb heat in a PA28 or PA38. When asked why this was done instead of following the POH the response 99% of the time was simply "My last instructor told me to"

Learning by rote is about the highest level of understanding in many instances.

[LeadSled]

Folks,
Long time ago, now, but we always fitted warm air filters, where available, either as an OEM option, or an STC.
I was always impressed with the difference in an engine at overhaul in the UK, vesus Australia, we still had warm air filters there, but there was never anything like the dust we got in our VH- aircraft. Indeed, in western NSW and FNQ we often did filter and oil changes at 50 hours, for air filters, sometimes less, depending on how fast they loaded up.
UK versus AU certainly showed up in overhaul costs.
Tootle pip!!

[Centaurus]

When asked why this was done instead of following the POH the response 99% of the time was simply "My last instructor told me to"

Happens with other items such as Cessna 152/172 before landing items eg Brakes, Undercarriage down, Mixture Rich, Master Switch on, Magnetos both, Fuel, Harness and Hatches etc
Yet the only before landing checks in the manufacturer's POH (Cessna 172 for example)
are: Seats, belts, Harnesses secure, Mixture Rich, Carb heat on.

Comment: Note seats secure in the POH but not mentioned in "what my last instructor told me to". Note nothing either in the POH about brakes, undercarriage or master switch or mags on. I recall asking one instructor why he taught Masters and Mags as part of the before landing items. Because they start with "M" he said.
PUFF checks on final. Whatever mnemonic takes your fancy. All inventions of some instructors who blindly adhere to 'My last instructor told me to"

[MagnumPI]

Centaurus, I was staying in a town on business and spontaneously went for a bash in a C152 late one summer evening. As I was unfamiliar with the area I went with an instructor who showed me the sights. It was an awesome flight over the coast and I asked if we could return to the field for a few touch and goes.

On turning base I completed the checklist from memory and verified against the written one in the aircraft (as is my habit). Naturally before pulling the throttle back I applied full carb heat.

On very short final I went to push the carb heat to COLD. The instructor balked at this and immediately pulled it back out to ON. I didn't say anything and touched down, retracted the flaps, then went to push the carb heat back to cold again before applying full throttle. He practically smacked my hand away and said "leave it on". He left the carb heat on until approx 300ft AGL and then he set it to COLD.

I was confused by all of this. His explanation was that it was their standard operating procedure and that it reduced the likelihood of carb ice accumulation during the critical take-off phase. Clearly there was no arguing with him so we finished up the next circuit with a full stop.

Who was right or wrong in this situation? The C152 POH says that any normal takeoff should be with carb heat COLD, and as part of the BEFORE LANDING checklist it should be set ON. It doesn't make mention of if it should be set to COLD on short final!

[Aussie Bob]

Who is correct here? The engine rebuilders when they marvel at the worn cylinders and wonder where the air filtration system went wrong. Stupid instruction, stupid policy.

[IFEZ]

Certainly not the way I was taught. Aside from the various other reasons given in previous posts, you also get rid of the carby heat on short final so that you have FULL power available should you need to go around or if you're doing a touch & go you have FULL power for takeoff. http://cdn.pprune.org/images/smilies/thumbs.gif

[Lead Balloon]

He practically smacked my hand away and said "leave it on". He left the carb heat on until approx 300ft AGL and then he set it to COLD.

I was confused by all of this. His explanation was that it was their standard operating procedure and that it reduced the likelihood of carb ice accumulation during the critical take-off phase. Clearly there was no arguing with him so we finished up the next circuit with a full stop.

Since the use of carburetor heat tends to reduce the output of the engine and to increase the operating temperature, carburetor heat should not be used when full power is required (as during takeoff) or during normal engine operation, except to check for the presence or to remove carburetor ice.

Read more Carburetor Heat (http://www.flightlearnings.com/2010/03/03/carburetor-heat/)Seems an odd trade off: A certain reduction in take off power to deal with the remote risk of ice having accumulated during the approach and preventing the engine from developing full power.

It seems to me that if carbie heat has been applied properly before the reduction in power, the probabilities of ice accumulating during the approach are extraordinarily low. However, it should be noted that the rote-learned procedure is usually to apply carby heat immediately before pulling the throttle. But it takes time for carby heat to heat the carby. If the probabilities of icing are high, carby heat should be applied up to 30 seconds before the throttle is pulled.

[A37575]

The C152 POH says that any normal takeoff should be with carb heat COLD, and as part of the BEFORE LANDING checklist it should be set ON. It doesn't make mention of if it should be set to COLD on short final!

The C152 POH (mine is April 1981) states before landing carb heat on (apply full heat before reducing power). For baulked landing the POH states throttle full open then carb heat to cold. However the aircraft is doing a take-off (touch and go) which is just a fancy baulked landing and the POH states carb heat should be cold. Therefore to ensure full power is available for the take off your instructor was wrong to leave the carb heat on during the take off run and subsequent climb to his personal choice of 300 ft In any case where did he dream up 300 ft as the height to place the carb heat back to off?

As another example of instructor personal ideas confounding common sense, I was undergoing a dual check in a Navajo. The instructor insisted the cowl flaps should be set to fully open on final in case (in his own words) we had to go-around. The fact that it would take us only 45 seconds to get back to circuit height went over his head and the CHT was never going to be a problem in that time.

[Clare Prop]

It is up to the operator if they want to add things like brakes off to pre-landing checks. It's quite another to ignore the POH and just lazy to adopt a one size fits all instead of reading the POH for different types.

POH for Tomahawk and Warrior state in section 4:


APPROACH AND ANDING
Carburetor heat should not be applied unless there is an indication of carburettor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go-around. Full throttle operation with carburettor heat on can cause detonation.


Section 4 of POH for C152 states:
BEFORE LANDING
1. Seats, Belts, Harnesses -- ADJUST and LOCK.
2. Mixture -- RICH.
3. Carburetor Heat -- ON (apply full heat before reducing power).

AFTER LANDING
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.

BALKED LANDING
1. Throttle -- FULL OPEN.
2. Carburetor Heat - COLD.
3. Wing Flaps -- RETRACT to 20°.
4. Airspeed -- 55 KIAS.
5. Wing Flaps -- RETRACT (slowly).


So, similar to the "in case of a go-around" reasoning that people give for opening cowl flaps on final, where for example C182 POH says to open them for a baulked landing; it shows that people are just parroting rather than learning.

[Ultralights]

go with the aircraft/engine ops manual.

Rotax manual in my aircraft, Use carby heat when required to keep airbox temp at 21Deg C. so, in winter, its not uncommon to fly the entire flight with carby heat on... and the reason rotaxe's should always have an airbox, so heated air is added before the filter.

only ever had carby ice once in a rotax, on a long descent from 8500 to sea level mid winter. and of course, i forgot to keep carby heat on in cruise.


And what exactly are you trying to do when "Clearing the carby" clearing it of what exactly?

[Jabawocky]

It is up to the operator if they want to add things like brakes off to pre-landing checks. It's quite another to ignore the POH and just lazy to adopt a one size fits all instead of reading the POH for different types.

POH for Tomahawk and Warrior state in section 4:


APPROACH AND ANDING
Carburetor heat should not be applied unless there is an indication of carburettor icing, since the use of carburetor heat causes a reduction in power which may be critical in case of a go-around. Full throttle operation with carburettor heat on can cause detonation.


Section 4 of POH for C152 states:
BEFORE LANDING
1. Seats, Belts, Harnesses -- ADJUST and LOCK.
2. Mixture -- RICH.
3. Carburetor Heat -- ON (apply full heat before reducing power).

AFTER LANDING
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.

BALKED LANDING
1. Throttle -- FULL OPEN.
2. Carburetor Heat - COLD.
3. Wing Flaps -- RETRACT to 20°.
4. Airspeed -- 55 KIAS.
5. Wing Flaps -- RETRACT (slowly).


So, similar to the "in case of a go-around" reasoning that people give for opening cowl flaps on final, where for example C182 POH says to open them for a baulked landing; it shows that people are just parroting rather than learning.

First of all there is never a go around that requires so much power that the carby heat will be the defining difference. But lets just play this out.

If the POH is written such that we can assume pilots can learn to do things correctly, it SHOULD read like this.

BEFORE LANDING
1. Seats, Belts, Harnesses -- ADJUST and LOCK.
2. Mixture -- LEFT ALONE IN A LOP CRUISE SETTING or at a ROP Cruise position.
3. Carburetor Heat -- ON (apply full heat before reducing power).

AFTER LANDING
1. Wing Flaps -- UP.
2. Carburetor Heat -- COLD.

BALKED LANDING
1. Throttle -- FULL OPEN.
2. Mixture RICH THEN Carburetor Heat - COLD.
3. Wing Flaps -- RETRACT to 20°.
4. Airspeed -- 55 KIAS.
5. Wing Flaps -- RETRACT (slowly).



And yes folks that is how it should be done! Zero harm to engine and airframe, less chance of icing and less than the POH method. Does everything it should and nothing it shouldn't and yes I have generated the data personally.


LAST OF ALL......A MASSIVE OLD WIVES TALE TO KILL OFF.

Please do not start saying the OEM should know........This one is BULL**** of the highest order. I have participated in more detonation testing than anyone on PPRUNE, on the worlds most advanced aero-piston engine dyno. I declare this to be an OWT of the highest order.

Full throttle operation with carburettor heat on can cause detonation.

Simply cannot happen. Not unless you advance timing well out of spec, run on crappy low grade mogas, have oil temps in the 245dF range, CHT's in the 440dF or much higher range (unlikely prior to landing) and you are at 34" MAP with a mixture accurately set at 35-40dF ROP. :ugh:

If anyone at Piper can explain how a pilot can do that in one of those planes (Chieftain is another matter but still take a bit of effort) and even if they did it would be so mild to begin with the GA and subsequent climb to circuit height would barely clean the cylinders let alone damage anything. :rolleyes:

So......any more OWT's need busting? :ok:

[Aussie Bob]

So......any more OWT's need busting?Yep! Use of carby heat on the ground. My personal opinion is that it should be avoided wherever possible unless the carby hot air intake passes through an air filter or your engine is making ice on the ground.

[Clare Prop]

Hey whoa Jaba, I'm just the messenger here.


OWTs to bust?
Well, there's the people who turn the PA28 electric fuel pump off at 300 feet after take off :eek: "Because you will wear it out",

The one about the MP number should always be lower than your RPM number "Or you will blow up the engine"

The ones who fully open cowl flaps on final no matter what the CHT is "in case you need to go around",

The one who told me "a localiser will only work correctly if the runway number is put on the top" :bored:

That you must NEVER use a rotating card on the ADF "in case it gets stuck"....

Putting a variable pitch prop to coarse on a single engine piston to "glide further in the event f an engine failure"

The signature on part 3 of a maintenance release is always signed by the pilot in command...

[Lead Balloon]

Putting a variable pitch prop to coarse on a single engine piston to "glide further in the event f an engine failure"Errrm, I think you will find that's true, not an OWT.

[Clare Prop]

If the engine has failed there is no oil pressure to act against the spring and it would go to the stops, most will default to full fine.

[Lead Balloon]

That's quite an overstatement, Clare. If the engine has stopped or lost all its oil (and oil pressure is necessary for the pitch control on the specfic propellor) then: Yes.

But if the engine has "failed" because of fuel starvation or exhaustion (the usual cause) or (unusually) both mags have failed, the propellor is windmilling and the resulting oil pressure is perfectly adequate to drive the prop to full coarse. And it makes a profound difference to glide range.

[Tinstaafl]

I think the significant part is "...in case of an engine failure..." ie not yet trying to glide. After the failure then, yes, prop to coarse, otherwise as required i.a.w. the POH/AFM.

[Old Fella]

Seems to me that the post from Ex FSO Griffo was perfectly reasonable. He said the engine was scrapped due to dust abrasion/scouring in all 4 cylinders. Maybe he really meant just that, the cylinders had to be scrapped. Seems many have over-read Griffo's post. Regardless, why use unfiltered air in a mustering environment. Not too much time spent at low power settings I suspect.

[Lead Balloon]

I think the significant part is "...in case of an engine failure..." ie not yet trying to glide. After the failure then, yes, prop to coarse, otherwise as required i.a.w. the POH/AFM.I'm afraid I don't understand the distinction you're trying to draw, Tin.

[Jabawocky]

Hey whoa Jaba, I'm just the messenger here. :ok: Yep, and I was addressing the issues....not you :ok:


OWTs to bust?
Well, there's the people who turn the PA28 electric fuel pump off at 300 feet after take off :eek: "Because you will wear it out", :ugh:

The one about the MP number should always be lower than your RPM number "Or you will blow up the engine" :ugh:

The ones who fully open cowl flaps on final no matter what the CHT is "in case you need to go around", :ooh:

The one who told me "a localiser will only work correctly if the runway number is put on the top" :bored:

That you must NEVER use a rotating card on the ADF "in case it gets stuck"....

Putting a variable pitch prop to coarse on a single engine piston to "glide further in the event f an engine failure" :} Ohh dear!

The signature on part 3 of a maintenance release is always signed by the pilot in command...


And no doubt there are more!

[Clare Prop]

No mention of the prop in the PA28-236 or C182 power off landing procedures.

[triton140]

PA28-201 POH is confusing - no mention of the prop in the power off landing checklists, but the amplified procedures do mention prop control in full DECREASE rpm.

Was taught to move prop to full coarse in case of engine failure, and had it demonstrated that it did in fact increase the glide.