question for CFIs - carb heat on landing

[secretapproach]

The landing procedure I was taught calls for carb heat when crossing the runway threshold on downwind but putting carb heat back to cold on final approach. This is in the POH of the plane I learned in (Robin). I know that when I did a couple discovery rides in a 152 at another school before starting my training they leave carb heat pulled until landing. The reason given for putting it back to cold is for full available power in case of a go-around. That's one less thing to think about if a dog runs across the runway on final. The instructors at my school say even if the Cessna POH says to leave carb heat on until touchdown they would put it to cold before landing. My feeling says they are right - for go-arounds.

I'm probably going to do a checkout in a 152 this weekend (in Portugal!!) so I started wondering about this. Does anyone care to share an opinion?

 

[carlos]

I was taught to leave it on until touchdown. I was also taught to push the throttle and carb heat in with a single motion on go-arounds (just push both in with one hand at the same time). So if you need to reapply power to go around, it's not really two separate actions. Seems to me that if you put it back to cold on final, depending on how long your final turns out to be, you run the risk of developing some carb ice, which would reduce your power on go-around anyway. But that's just my opinion...

(I just realized this question is in CFI Corner. I'm not a CFI, so treat my suggestion accordingly

)

 

[EatSleepFly]

I teach my students to leave the carb. heat on till touchdown. If a student can't handle pushing in the carb. heat knob after applying full power for a go-around, then I would recommend they don't ever fly anything bigger than that 172, because theres a more serious problem- lack of ability to multitask...

...That being said, I havent had anyone yet who's had a problem taking out the carb. heat.

 

[mikek123]

I always left it on until landing and if a go-around was needed just push both carb heat and throttle in together. Even if you were to forget to put the carb heat off on a go-around the plane will still climb very nicely, you don't lose that much power. You wouldn't put the flaps up on short final in case of a go-around so why mess with the carb heat.

 

[secretapproach]

[ QUOTE ]
I always left it on until landing and if a go-around was needed just push both carb heat and throttle in together. Even if you were to forget to put the carb heat off on a go-around the plane will still climb very nicely, you don't lose that much power. You wouldn't put the flaps up on short final in case of a go-around so why mess with the carb heat.

[/ QUOTE ]

A couple times on my touch and gos with an instructor I was relatively late getting off the runway and both of us had overlooked that carb heat was still on and we only noticed it when the trees past the runway were not disappearing below the windshield fast enough. Pushed it in and she took off like a rocket (or at least it felt like it in that situation). You mentioned the flaps. When I was doing several touch and gos I would leave the flaps at a medium take-off setting so I didn't have to mess with them on the ground. I would put them out full on my final landing. I came to appreciate mechanical flap control compared to electrical - mechanical is much faster!

Is the carb heat right under the throttle on 172s and 152s?

Thanks everyone for your opinions - keep 'em coming!

 

[carlos]

The carb heat is next to the throttle, as seen in this picture of a C-152 panel.

 

[E_Dawg]

The problem with doing it how you described is that you are pushing in the carb heat at a time when you are very busy. At least when you make the decision to go around, you push throttle full in and have time to look down at the airspeed and the carb heat to verify what you are doing.

If you pull the carb heat out every time you're over the theshold what happens on that one bad day when you retract the flaps, or worse, the gear instead of the carb heat?

You still run the risk of doing the wrong thing on a go around but the fact is that go arounds are much less common than landings, and you'll also have to retract the flaps and the gear (if retractable), almost guarenteeing that you'll at least have to glance down to verify what you are doing.

Just MHO

 

[secretapproach]

[ QUOTE ]

If you pull the carb heat out every time you're over the theshold what happens on that one bad day when you retract the flaps, or worse, the gear instead of the carb heat?
Just MHO

[/ QUOTE ]

In the Robin I kill the carb heat right after turning to final so it's not the touchdown phase. The conditions for carb ice would have to be really bad to build ice just on final, wouldn't they. Am I being too optimistic?

Here's a picture of the plane I had my first solo in - I know that Robins are not very well known in the US.

 

[davetheflyer]

Follow the POH.

I always taught my students to keep the carb heat on until clear the runway or initiating a go around. If going around, have them push the carb heat and throttle forward at the same time in one smooth motion (throttle in hand and thumb on carb heat).

Otherwise, you run the possibility of having the carb ice up while the engine is at low power on the approach. Then you don't have the power when you need it.

 

[sbav8r]

A carburetor draws air through a constriction, called a venturi, which causes a drop in air pressure. The carburetor uses this drop to draw fuel into the airflow and mix it. The velocity of the airflow through this restriction determines the amount of fuel to be drawn into the carburetor and subsequently the engine.

Remember that when there is a pressure drop a temperature drop will be present as well. Therefore the temperature will drop farther at high power settings such as cruise power settings or take-off. In relation to your question this would be during go around.

Carburater icing is commonly missunderstood and it is believed that low power settings result in carb ice. On the contrary, moving from low to high power settings is when carb ice becomes most critical.

Using carb heat in the pattern has two purposes. The first purpose is to clear the carburetor of any ice which may be developing from cruise, before you pull the power back. At this time more heat is available than at the lower power setting.

If ice has built up during cruise or approach and power is applied, the engine may shut down, because a quick application of power would only increase the formation of ice.
This is why many manufacturers suggest leaving the carb heat on throughout the landing. By leaving the carb heat on as the power is applied you are more likely to melt any ice present and avoid creating more ice.

Many people have stated that you should turn it off before applying full power inorder to acheive the maximum amount of power for go around. Just remember that it may be too late to apply heat if the engine begins to run rough, the engine could shut down during this transition due to the quick decrease in pressure in the carb. It's far better to bring the power up and then shut off carburetor heat to avoid a loss power completely even if that means a 3 second temporary loss in RPM.

Sometimes CFI's teach things because that is the way they were taught. Sometimes you need to question why they recommend certain tactics because they may not always be correct.