A carb heat trick

[Big Pistons Forever]

Originally Posted by A and C

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.

Originally Posted by 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.

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.

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]

Originally Posted by Step Turn

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]

Originally Posted by Step Turn

.

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]

Big Pistons

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]

Originally Posted by Vilters

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]

Originally Posted by A and C

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 , 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 , 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...ml#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!!

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.