Carb Heat

[futurepilot1]

I was getting ready for my lesson yesterday, when my CFI and another student got into a discussion about Carb Heat. The student had had a partial engine failure due to carb ice. He was doing a touch and go. He had applied full carb heat during the descent. Once on the ground, he removed carb heat, full power, flaps up, just like the checklist said. At about 300ft AGL he experienced a partial engine failure. It didn't die completely, just had a huge power loss and sputtered. He was able to get enough power to turn around and use the perpendicular RWy for a landing. Now the curious part. My CFI said it sounded like he did everything correct, but the student said his CFI told him that, due to the cold conditions he should have kept carb heat on the whole time. My CFI said that was incorrect. Comments?? Thoughts?? I was under the impression that, when the engine is running, outside air temperature has little to do with Carb icing.:dunno:

 

[moxiepilot]

when the engine is running, outside air temperature has little to do with Carb icing.:dunno:

it actually has a lot to do with it. there should be a diagram in the airplane flying handbook that shows relative humidity and temperature's relationship to the potential for carb ice. i would look for it, but i haven't had enough coffee yet. sorry.

 

[Old Pete]

Sounds like what happened to my student. It very well could be the same incident. If I recall, when this happened, Dewpoint was about 9F temp 25F. If you look at the charts, with those conditions, it's not in the high probability of carb ice section of the chart. I'm not entirely convinced that it wasn't something else that caused the loss of engine power. The day before he flew it, the aircraft had a magneto replaced and the day after he flew that aircraft the plane went back into maintenance due to a rough running engine.
As far as the use of carb heat, I recommend following the POH. It has always served me well. I have never seen a checklist for that aircraft that specifies take off with the carb heat applied.

 

[shdw]

I didn't hear one very important symptom: When carb heat was applied did he experience a further loss in RPM before an increase back to normal power? Without that I would be skeptical to him even having carb ice. The few times I picked some up there was always a noticeable loss of power from the water entering the engine when carb heat was applied.

 

[ppragman]

If he had carb ice, but was just mindless moving the carb heat level to comply with the checklist, then the only diagnosis I could offer would be that the carb heat wasn't effective enough prevent ice from forming, or formed prior to application then didn't melt fully, and when he took off, the increased engine temps caused some of the ice build up to melt and to be ingested, but that's just a wild ass guess.

 

[drunkenbeagle]

If he had carb ice, but was just mindless moving the carb heat level to comply with the checklist, then the only diagnosis I could offer would be that the carb heat wasn't effective enough prevent

You can't take out the carb heat when it is needed. Just makes things worse. If you get carb icing, the heat needs to stay on for the remainder of the flight.

 

[Douglas]

I didn't hear one very important symptom: When carb heat was applied did he experience a further loss in RPM before an increase back to normal power? .

The original post didn't specify if the student turned on the carb heat before he 180 back to the runway.

You can't take out the carb heat when it is needed. Just makes things worse. If you get carb icing, the heat needs to stay on for the remainder of the flight.

That an opinion?

 

[ppragman]

You can't take out the carb heat when it is needed. Just makes things worse. If you get carb icing, the heat needs to stay on for the remainder of the flight.

Really, is that a fact? Ridiculous, so if I get carb ice while descending with partial power in a 172, then clear it with carb heat, then go back cruise power when I level off that I should leave the carb heat on for the rest of the 300NM cross country? No thanks, I'll use it to clear the ice, then turn it off.

Bottom line, once the ice is gone, and power is restored, there's no need to have the lever pulled if your generating enough power to keep the temps up. You'll notice that you're not going to get carb ice at power settings roughly inside the green arc on the tach (at least that's the case on any 150/152/172/182 or otherwise light carb'd bird that I've flown). Don't use partial carb heat, because its possible if its really cold to warm the air into the critical region for ice to form.

Also, think about where your carb heat comes from, that air isn't necessarily filtered depending on the model your flying, so you might not want to leave it on for long periods of time if there's particulate in the air.

 

[nosehair]

On the touch and go, the student probably put the carb heat in, raised the flaps, and applied power for the take-off, which is a normal routine, however...

When it is really cold like this, I leave the carb heat in until developing full power and running smoothly (except for the normal roughness of having full power with carb heat on) then slowly take the carb heat off listening for the normal engine rpm increase to a full power sound.

During these times when carb icing is rampant, sloppy inattention to the application of carb heat is much more likely to cause the engine problems that we read about, but rarely experience.

If you have the habit of pulling the power back, and then applying carb heat, you may just get a visit from the carb ice gremlin. In the right temp/humidity/dewpoint conditions, ice will form in the carb as quickly as the fog that covers a mirror when you breath across it - instant ice.

During the run-up carb heat check, pay close attention to the sound and specific rpm drop and look for that sound and specific rpm drop later when you apply carb heat prior to a reduction in power. On cold mornings, I recommend taxiing out with carb heat on to eliminate the subtle build-up of ice that may cause a mis-read on the rpm run-up stuff. Take the heat off as you get up to run-up rpm so you know it is not already suffering a power loss, then check carb heat performance loss. Use this standard throughout the flight when icing conditions are severe.

Keep the carb heat on throughout the rest of your run-up and until starting the take-off roll and developing full power, then take off the carb heat.

 

[tgrayson]

You'll notice that you're not going to get carb ice at power settings roughly inside the green arc on the tach

The NTSB reports have many cases of carb icing and suspected carb icing at cruise and even takeoff power. A good chunk of the above chart shows the non-zero probability of icing at cruise power, which I would interpret to be green arc on the Cessnas.

The Cessnas also have procedures for maintaining partial carb heat at cruise if conditions warrant.

 

[shdw]

The original post didn't specify if the student turned on the carb heat before he 180 back to the runway.

I took this, "The student had had a partial engine failure due to carb ice. He was doing a touch and go. He had applied full carb heat during the descent." to mean he had carb ice and turned on the carb heat. Then he got it again on takeoff. Maybe I read it wrong.

But this doesn't make sense either, "Once on the ground, he removed carb heat, full power, flaps up, just like the checklist said. At about 300ft AGL he experienced a partial engine failure." Carb ice doesn't develop easily at full power and it is even less likely that it developed in a matter of 10 seconds at full power.

I also question what a student feels power loss is? Carb ice doesn't just snap appear, it is a steadily increasing process. Not just a sudden "huge power loss and sputter." I would be more apt to attribute such an experience to some water in the gas tank than carb ice, given the circumstances.

 

[ppragman]

The NTSB reports have many cases of carb icing and suspected carb icing at cruise and even takeoff power. A good chunk of the above chart shows the non-zero probability of icing at cruise power, which I would interpret to be green arc on the Cessnas.

The Cessnas also have procedures for maintaining partial carb heat at cruise if conditions warrant.

Then pull carb heat, warm up the carb, and make it go away. However, I'd prefer not to run around with partial carb heat without a carb temp gauge.

 

[Douglas]

I guess I don't know about the first part, post touch down.

Carb ice doesn't just snap appear, it is a steadily increasing process.

Trust me it can.
Snapped twice, as if somebody was chucking ice cubes at us.



The C-150 is an amazing aircraft.
The C-150 carburetor likes its drinks with ice.

 

[shdw]

The C-150 is an amazing aircraft.
The C-150 carburetor likes its drinks with ice.

I don't know about them, only have an hour in one. My carb time is in 152/172/seminole/grob 109 glider and I never saw carb ice develop like that. It didn't even develop when I cruised through light icing conditions in the seminole. I was over square though, that could be why.

 

[PGT]

FWIW - I flew a 250nm xc each way in a 152, both times I suffered carb heat about 1.5 hours into the leg. I had to leave the carb heat on for a few min (what seemed like) for it to go away.

 

[mshunter]

During the run-up carb heat check, pay close attention to the sound and specific rpm drop and look for that sound and specific rpm drop later when you apply carb heat prior to a reduction in power. On cold mornings, I recommend taxiing out with carb heat on to eliminate the subtle build-up of ice that may cause a mis-read on the rpm run-up stuff. Take the heat off as you get up to run-up rpm so you know it is not already suffering a power loss, then check carb heat performance loss. Use this standard throughout the flight when icing conditions are severe.

Well thats a good way to injest something into the engine.

 

[Douglas]

FWIW - I flew a 250nm xc each way in a 152, both times I suffered carb heat about 1.5 hours into the leg. I had to leave the carb heat on for a few min (what seemed like) for it to go away.

And the worst part of carb ice this time of year and where we llve, it gets cold in that airplane quick with the carb heat on.

 

[wjmiller3]

I don't know about them, only have an hour in one. My carb time is in 152/172/seminole/grob 109 glider and I never saw carb ice develop like that. It didn't even develop when I cruised through light icing conditions in the seminole. I was over square though, that could be why.

The Seminole is less likely to get carb ice anyway cause the intake is behind and below the engine, so its air is heated a little to begin with. And the one time I ever did get carb ice in a seminole was at cruise power. And as for running over square...... Your dangerous!!

 

[nosehair]

Well thats a good way to injest something into the engine.

Yes, it is. Mainly warm air. :insane:
But you are right about being careful of dusty polluted air since the warmed air is bypassing the air filter. Normally, on the cold below freezing mornings when this is advisable, the air is very cold and clear.

 

[shdw]

Yes, it is. Mainly warm air. :insane:
But you are right about being careful of dusty polluted air since the warmed air is bypassing the air filter. Normally, on the cold below freezing mornings when this is advisable, the air is very cold and clear.

Who sweeps your taxiways and ramps? I have heard that some planes run good on rocks though, but it is just a rumor.