When should carb heat be turned off in the landing sequence?

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When should carb heat be turned off in the landing sequence?

Unread post by Wayne Boonzaier » Tue Nov 12, 2019 12:43 am

It seems to me that there are two schools of thought here and the answer seems to differ depending who you talk to.

1) Some say that carb heat should be turned on as power is reduced to below the green arc in the circuit or on turning base and turned off once the final approach has started. This is because we may have to do a go around and we want full engine power in that eventuality. ( Ehem how does one do a go around when the engine is spluttering because of carb icing that developed on finals ?)

2) Others say that carb heat should be kept on all the way into the approach and landing and only turned off if a go around becomes necessary. ( Ehem not great to touch down with carb heat still on sucking all the dirt and grime into the engine on the roll)

To me the answer seems to lie somewhere in-between these schools of thought and that is turn Carb heat off on late finals say at 200 ft ? (Ehem but wont we forget to turn off carb heat because its late in the sequence and we have already done our final approach checks ?)

So to those in the know what is the correct procedure ?
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Romeo E.T. » Tue Nov 12, 2019 1:37 am

your question is too "procedural" and not practical

vital questions that need to be asked
1) What engine installation - Continental with its stand alone carburetor or - Lycoming with the carburetor heat soaking on the bottom of the oil sump
2) Outside air temperature ?
3) Outside air humidity ? if the air is dry, can it produce icing ? or could it be sufficiently humid to cause condensation in the reduced temperature of the venturi ?
4) Temperature reduction in the venturi of the carburetor whilst under power (lots of airflow and thus lots of venturi effect) and lots of fuel atomization vs temperature reduction in the venturi of the carburetor at flight idle (much lower airflow and thus less venturi effect) and minimum fuel atomization (the fuel atomization actually causes a lowering of air temp)
5) Effectiveness of the heat shroud around the exhaust, to heat up the unfiltered air, before entry to the carburetor
6) An engine at idle produces less heat, an engine at idle has less air flow demand into the carburetor, is the air actually becoming warm enough to prevent carb icing, or is it only slightly raising the temperature, only to be cooled in the venturi again, and possibly being cooled straight into the "icing range" again ?

one would really need a TAT gauge/probe to be fitted in the carburetor venturi, and judicious use of combination of "power on" (heat generation) and carb heat application, to keep the TAT outside of the icing temperature range (5deg C downwards)

is there any documented "HARD" data from the engine manufacturer, on the combination of atmospheric conditions, engine performance, fuel atomization etc, that will prevent or contribute to carb icing formation.

The instructed procedure, is far to rote to suit all applications of engine variants, engine power parameters, engine heat generation, exhaust shroud variability, atmospheric conditions etc etc
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Slam and Go » Tue Nov 12, 2019 5:24 am

Below 300' I will not be faffing with carb heat as I want my hands free to control and/or abuse some poor student :lol: . It will remain off in the event of a landing or go-around until safe. It is common for flight schools to teach students to consider carb-heat on late downwind and/or base leg. The same schools will then teach a mnemonic for final (I have heard GUMPs, PUFC, Red Green Blue etc) I will let those with more grey hair than me debate whether or not this is good practice.

EDIT:

Upon reflection, I detest the application of carb-heat due to rote learning. I'd rather my student to consider the effects of carb-icing on his/her specific aircraft on the specific day and act accordingly. This may include scrapping the lesson and grabbing a coffee.
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Rotor kop » Tue Nov 12, 2019 6:44 am

In helicopter i always pull carbheat on a decent, especially one from high and that is shallow angle (time during decent). As i am about to start using power again to arrest the decent and land, i then push in carb heat all the way. Aircraft may be different.
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Chalkie » Tue Nov 12, 2019 7:58 am

The application of carb heat, however it is taught, is a one-size-fits-all approach. No it is not perfect and yes it does depend on the airframe and engine installation. The one-size-fits-all approach, errs on the safe side. For example: any aircraft fitted with a Conti O-200 / C65 / C85 etc. has the carburettor hanging below the engine on the air riser tubes. In this installation if the air filter is a loose fit in the cowl then carb icing is a real problem it OAT below 12C and high humidity.

Now look at a Lycoming installation and here I can recount actual experience in my RV-9A with an O-320 Lycoming. As R ET said above, evaporation causes cooling. In my RV-9A the carburettor had a temperature probe in the venturi and as the carburettor is bolted to the sump the carb body was heated by the sump which contains hot engine oil. (80 to 90C)

At full power on the first takeoff with cool oil, the carb temp at the coast, never approached the icing range. During cruise, the carb temp sat at about 35C and the EFIS amber warning was set at 5C and again at 50C with green in-between.

Downwind an RV-9 is difficult to slow down with a fixed pitch prop, as a result the throttle would stay closed until stable on finals and slowed down to 55 to 60 KIAS. Closing the throttle results in only idle fuel flow so less evaporation and less cooling as a result the carb temperature would race up above 55C and stay there.

So on a Lycoming with the carb bolted directly to the sump (some installations use an insulating phenolic spacer) the chances of carb ice on final are minimal, but carb ice in the cruise and during takeoff with low OAT, low oil temp and high humidity could be a possibility. In a cold humid environment, carb heat before takeoff would be advised. Off for takeoff, naturally.
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Dragon » Tue Nov 12, 2019 9:34 am

While I agree with Chalkie and Romeo ET that knowledge and understanding is key and every aircraft and situation is different and requires thought, I do not believe it is in the best interests of most of us to be left with an open agenda on the carb icing. For the vast majority of us muscle memory is a far better idea all be it on the conservative side.

Now I don’t have a huge amount of piston time but I was taught by two of the gurus, and they said for landing circuit you apply it start descending when you turn base and you put it away at 300ft on final. I am pretty sure this will cover most Lycoming and Continental engines. If your aircraft needs something else then try to have a fixed checklist of your own and use it religiously as a minimum. Nothing to stop you using it at other times when necessary as well though.
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by southside » Tue Nov 12, 2019 10:07 am

Out of the green arc, carb heat on.
Short finals-CUFF checks.
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by TikkaT3 » Tue Nov 12, 2019 11:06 am

In a 182 with a Continental 0-470 which is susceptible to carb-ice I was taught to apply carb-heat on base and remove on final.

Base leg = C.C.P.P.F.T. (Carb-heat on, Cowls closed, Pitch full fine, Power 15 inches, Flap 30, Trim for 75 and 500fpm decent).

Finals = R.P.U.F.F.C. (Radio call final approach, Pitch full fine, Undercarriage down and locked 3 green, Fuel on both, Flap as required, Carb heat off).
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by jimdavis » Tue Nov 12, 2019 12:11 pm

Carb heat and mixture seem to be the least understood parts of an aeroplane - and they are both vital to the engine performing at its best when you need it.

The right way to deal with them is to RTFM (Read The Flight Manual). It gives me the squits when folks say "I was taught..." The implication is "My instructor knows EVERYTHING." Unfortunately that's seldom true, so it's only sensible to check what the POH says.

The answer is to do what the manual says for that particular aircraft. You can't necessarily carry one bunch of checks and procedures from one aircraft to the next and from one environment to the next. There is no rule that says how much flap you must select on which leg of the circuit. You should use flaps as required. There is not even a rule that says you should make sure the gear is down and locked before landing - it would be a big mistake to do that in a float plane. Similarly with fuel - you select the tanks intelligently - not according to a checklist. And carb heat calls for intelligent use based on an understanding of that particular aircraft's requirements.

You are not going to use the same checklist in a Cub and a Twin Comanche - you do what that aeoplane requires under specific circumstances.

It's all part of situational awareness.

At the moment I am working on an accident report about and Bonanza that lost power after takeoff from Kimberley, killing both occupants. The pilot switched the fuel pump on to HI instead of retracting the undercarriage. How does that happen? Three things - first, that model of Bonanza has the pump switch right next to the gear indicator lights and the gear selector. Second, the action is identical for both the pump and the gear lever - you have to pull it out before you can move it up. Third, the pilot's situational awareness had gone AWOL.

When you do things in a cockpit - they should not be unthinking, rote actions. You don't just lean down and flip that switch. You have to say to yourself "...I want the landing light on. This is the landing light switch and I am going to move it up, and watch the ammeter for a reaction."

Equally, "I am flying a Cherokee on a warm dry day - I will not use carb heat." Or "I'm flying a Piper Cub which ALWAYS needs carb heat before throttling back." Or "I'm flying a 172 at the coast, and even though it's a warm day - this aircraft is prone to icing. Today's high humidity makes it particularly vulnerable - I am going to keep my finger right out." Or "I am flying a fuel-injected Arrow I do not need to worry about carb icing."

For all carburettor aircraft I make a habit of pushing the carb heat in as I cross the fence, in case I need full power.

Long mnemonics and checklists on base and final approach should be avoided - you are meant to be flying the aircraft - not messing around with your head in the cockpit.

Sorry, I got into instructor mode there. :oops: What I say is not gospel - it's just how I see things.

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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Cherokee6 » Tue Nov 12, 2019 12:15 pm

TikkaT3 wrote:
Tue Nov 12, 2019 11:06 am

Base leg = C.C.P.P.F.T. (Carb-heat on, Cowls closed, Pitch full fine, Power 15 inches, Flap 30, Trim for 75 and 500fpm decent).

Finals = R.P.U.F.F.C. (Radio call final approach, Pitch full fine, Undercarriage down and locked 3 green, Fuel on both, Flap as required, Carb heat off).
:shock: That seems like an awful lot of acronym-flying to me. And very prescriptive - for example, 15 inches of manifold pressure on base? What if your downwind is a bit wider, or closer, necessitating a shallower or steeper descent on base, just as one example? I don’t know what my MP is on downwind, base or final - I know my prop is fine of course and mixture rich but power is applied by feel in order to maintain the correct sight picture. Sometimes I want a steep approach in order to dive through turbulence, in which case my base and final are adjusted accordingly. But I never even look at my MP gauge at those times. I also have no idea what my VSI is showing. Again, all I’m doing is looking at the sight picture and keeping it the way I want it, with my airspeed as desired.

Am I alone in this? Am I being sloppy? What do the experts say?
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by jimdavis » Tue Nov 12, 2019 12:32 pm

Cherokee6 wrote:
Tue Nov 12, 2019 12:15 pm
TikkaT3 wrote:
Tue Nov 12, 2019 11:06 am

Base leg = C.C.P.P.F.T. (Carb-heat on, Cowls closed, Pitch full fine, Power 15 inches, Flap 30, Trim for 75 and 500fpm decent).

Finals = R.P.U.F.F.C. (Radio call final approach, Pitch full fine, Undercarriage down and locked 3 green, Fuel on both, Flap as required, Carb heat off).
:shock: That seems like an awful lot of acronym-flying to me. And very prescriptive - for example, 15 inches of manifold pressure on base? What if your downwind is a bit wider, or closer, necessitating a shallower or steeper descent on base, just as one example? I don’t know what my MP is on downwind, base or final - I know my prop is fine of course and mixture rich but power is applied by feel in order to maintain the correct sight picture. Sometimes I want a steep approach in order to dive through turbulence, in which case my base and final are adjusted accordingly. But I never even look at my MP gauge at those times. I also have no idea what my VSI is showing. Again, all I’m doing is looking at the sight picture and keeping it the way I want it, with my airspeed as desired.

Am I alone in this? Am I being sloppy? What do the experts say?
Ha ha ha, Cherokee 6. No you are certainly not alone. I guess that was a double transmission because I think I said exactly the same in the post before yours. :lol: :lol: :lol:

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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Cherokee6 » Tue Nov 12, 2019 12:57 pm

jimdavis wrote:
Tue Nov 12, 2019 12:32 pm

Ha ha ha, Cherokee 6. No you are certainly not alone. I guess that was a double transmission because I think I said exactly the same in the post before yours. :lol: :lol: :lol:

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Thank god for that, Jim! I‘m always amazed when people say things like “I turn final at 350 AGL, with 15 inches of manifold pressure and 400fpm on the VSI”. I wonder how they know all that stuff because I genuinely have no idea what my typical approach looks like in those terms. All I can say in response is that I follow a sight picture that “feels right”. It’s a bit embarrassing to not know all those clever things, so I just shut up rather. :lol:
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Wayne Boonzaier » Tue Nov 12, 2019 1:15 pm

jimdavis wrote:
Tue Nov 12, 2019 12:11 pm


For all carburettor aircraft I make a habit of pushing the carb heat in as I cross the fence, in case I need full power.

I will go with Jim on this one, pushing carb heat in over the fence makes the most sense as it eliminates the possibility of icing on the final approach and prepares the engine for full power, now to make sure that it becomes a habit
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Re: WHEN SHOULD CARB HEAT BE TURNED OFF IN THE LANDING SEQUENCE ?

Unread post by Chalkie » Tue Nov 12, 2019 4:37 pm

TikkaT3 wrote:
Tue Nov 12, 2019 11:06 am
In a 182 with a Continental 0-470 which is susceptible to carb-ice I was taught to apply carb-heat on base and remove on final.
The reason that engine is susceptible to carb ice is because the carb is attached to the inlet air ducts and not the sump, so as Jim says RTFM and do what Mr Cessna says. Then as Jim says, on short finals, carb heat off.
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Re: When should carb heat be turned off in the landing sequence?

Unread post by Romeo E.T. » Tue Nov 12, 2019 5:09 pm

OK...so my version (all types/all conditions)

1) Apply carb heat, whilst still producing power (take advantage of the greater heat exchange)
2) Check to see a noticeable power drop (carb heat "is" working, due to a higher temp/less dense air being inducted)
3) If no RPM drop - - - be suspicious, very suspicious, if the engine starts to run rough, splutters etc, maybe there was carb icing already
4) close throttle to commence the descend and approach
5) Somewhere about the halfway mark, consider a "confidence building" small warm up, but not so much as to "mess-up" your approach profile, to verify power availability, for later on when on short final, to the flare
6) close the carb heat on final approach, possibly later, but still early enough to not "interfere" with the flare and hold off manouvre
7) as a final double check, as Jim says "For all carburettor aircraft I make a habit of pushing the carb heat in as I cross the fence, in case I need full power"
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