Carb Icing - KABC 050555Z AUTO 02003KT 10SM CLR 02/00 A3044 RMK AO1

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TazzyTazzy

Pre-takeoff checklist
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Mitch
Happy new year everyone. I have a carb icing and safety of flight question in a 172N. I want to do this flight tonight, and based on weather from last night, tonight's weather should be about the same. As a newbie (100 hrs), I haven't flown in weather this cold yet. So, lets say I want to do a night VFR flight.

Assume that I take off in 45f degree weather at around sunset, with a dew point at 41 (86% relative humidity). Then, by the time I land or do some stop & go to keep night currency after my flight the metar is showing:

KABC 050555Z AUTO 02003KT 10SM CLR 02/00 A3044 RMK AO1

This puts the relative humidity at 91% with no forecast fog and typically doesn't get fog - I'd like to avoid visibility debate for this. Last night was clear in these conditions and forecast to be the same and clear tonight.

Looking up charts and such, it seems I'm getting into the severe carb icing range. Would this be considered an unsafe flight?

What procedures should I do that I wouldn't do otherwise in conditions not expected for carb ice. Do I use carb heat all the time? What about take off and forming carb ice on take off/climb out? Using carb heat will affect the performance of the engine, but it's pretty cold air and might still be colder than air on a hot 105f degree day (I guess).

On run-up, I pull carb heat and note the RPM drop. I'll still do that, but what if the engine chokes and cleans up after (I know that's a sign of melted ice that engine is sucking down), but then do I keep the carb heat on if there was ice? Do I leave it on until roll out?

I live in the Sacramento, it's not supposed to be this cold darn it. I never trained in these conditions, just covered some in flight school, but not experienced these temps while I trained so I didn't think about these details.

What other factors do I need to consider, other than a couple of blankets and hand warmers and such if needed in an emergency. I know I'll need to check for frost.

Thanks.
 
In that plane, just go fly it as normal, using carb heat per the POH -- essentially, any time you're operating below the tach green arc or when symptoms appear. If symptoms persist, turn it on and leave it on.
 
TazzyTazzy said:
Looking up charts and such, it seems I'm getting into the severe carb icing range. Would this be considered an unsafe flight?

What procedures should I do that I wouldn't do otherwise in conditions not expected for carb ice. Do I use carb heat all the time?
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Just do the flight and be prepared to pull the carb heat on if you have signs that you need it. DO NOT use it all of the time. For one, carb heat on will probably disable the filter on the ground by selecting the alternate air source, so you can ingest FOD. Two, you want to have the option of alternate air if you need it.

Also, when you do use it, just pull it all the way engage. No partially pulling the carb heat control. Also, pull it on whenever you reduce power below the green. Make it part of your prelanding check list-- carb heat on-- when you are descending to land at the airport. Leave it on until you are on the ground.

Remove carb heat when you go around, or want to take off.

Also, if you do find that you are in conditions where you have to leave carb heat on in cruise in order to prevent carb ice (rare, but it can happen), then you will need to lean some to bring the mixture back to where it should be because the carb heat caused the air to be less dense, leading to a richer (greater fuel to air ration) mixture.

I suggest you read the POH on carb heat in the plane you will be flying.
 
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TazzyTazzy said:
Assume that I take off in 45f degree weather at around sunset, with a dew point at 41 (86% relative humidity).

* * * *

What other factors do I need to consider, other than a couple of blankets and hand warmers and such if needed in an emergency. I know I'll need to check for frost.

Thanks.
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45 degrees, and you need hand warmers?!? :rofl: (Don't worry, that's just me being jealous.)
 
Outside the green arc, enable full carb heat just like you were trained.

The only real gotcha is that extended idling after run up prior to takeoff is not advisible. If you have to wait for takeoff, I'd suggest redoing the mag check and carb heat check as that will make it apparent if you have some ice.

A 2 deg dewpoint spread won't give you ground fog, but upslope fog may be an issue. Beware if it turns into 1 deg or 0 deg. Lack of forecast fog DOES NOT mean there will be no fog.

As for survival equipment, it really depends where you're going. Over the Sierra, you'll need a lot more than that. Down hwy 99, just be prepared to walk out of wherever you might land and spend a few hours in the weather. ALWAYS bring water. A first aid kit might be a good idea.
 
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Ron Levy said:
In that plane, just go fly it as normal, using carb heat per the POH -- essentially, any time you're operating below the tach green arc or when symptoms appear. If symptoms persist, turn it on and leave it on.
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Yep..
 
Ron Levy said:
In that plane, just go fly it as normal, using carb heat per the POH -- essentially, any time you're operating below the tach green arc or when symptoms appear. If symptoms persist, turn it on and leave it on.
Click to expand...

Thx. Yes, I use carb heat whenever its below the green while airborne. My CFI implanted a permanently into my brain.
 
PPC1052 said:
45 degrees, and you need hand warmers?!? :rofl: (Don't worry, that's just me being jealous.)
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Its getting down to the upper 30s here. This is about as cold as it gets for the most part. There will be a few days a year below freezing, but not many. I haven't used a hand warmer in years, but I figure it should probably in the flight bag this time of year for emergency. I also carry 3 little emergency blankets. I have a pretty small flight bag and these don't take up much room. I also have two glow sticks.

We don't wear gloves much here, well my other half does...but he's a princess. :)
 
MAKG1 said:
As for survival equipment, it really depends where you're going. Over the Sierra, you'll need a lot more than that. Down hwy 99, just be prepared to walk out of wherever you might land and spend a few hours in the weather. ALWAYS bring water. A first aid kit might be a good idea.
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Just flying I'm the valley for the most part. No sierra flying yet for me. For first aid, I have a little one person kit from REI.. :dunno:

Yes, I always bring a bottle of water.
 
Just a quick note that I didn't see in the other's comments. When checking carb heat operation during the run up, don't just simply turn on, see RPM's drop, and turn immediately off. Give it a few seconds on and see if the RPM's come back up (indicating you've cleared some ice). I see lots of pilots go "on" and "off" in 5 seconds or less...they see the R's drop, but they have no idea that if there was ice (from idling for a prolonged time...like MAKG said), they may take off with it still in the carb. That would be a "bad thing"....

Have fun !
 
Terry L in KY said:
Just a quick note that I didn't see in the other's comments. When checking carb heat operation during the run up, don't just simply turn on, see RPM's drop, and turn immediately off. Give it a few seconds on and see if the RPM's come back up (indicating you've cleared some ice).
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...and then when you deselect the carb heat, check to see if the RPM is higher than before you pulled it, thus confirming that carb ice was present but was melted out.
 
Good points. I do carb heat, note rpm drop, check vacuum, note rpm, carb off, note if higher than 1700. It takes me a more then 2 seconds as I'm going down the checklist. Cfi said to do it in this order to make sure there was no developed carb ice.
 
Anyone use a Carburetor Temperature Gauge? I hate guessing at anything, I purchased but have not yet installed a temperature gauge for my carb. Its a little digital unit that looks a lot like a Tiny Tach. It will be installed this winter during the condition inspection.

I fly a Homebuilt that I did not build. I spoke with the builder and he told me carb ice will never be a problem for the engine is cowled. I took this as a bold statement and pull carb heat when I lower RPM's.

How many have a carb heat gauge?

Tony
 
Start your takeoff roll with full carb heat. Nothing like a blast of full throttle to melt away that carb ice.
 
The notion that you need carb heat because you have cooler than normal temps is incorrect. Your instructor didn't do you any favors if that's what he taught you.

HAS, I have a carb heat instrument. Always have in my 180 as it was there when I bought it. Never needed it/don't look at it/don't believe in running partial carb heat. Mine is not an ice making engine. Others may have different experiences.
 
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Stewart, he said he was at risk for carb ice, which is true. He did not say this was the only tíme, or that cold temps alone required carb heat, both of which would be false.

My one direct carb ice encounter occurred in summer. One of the 182s also has a carb temp probe, and it's instructive to see just how cold it gets on a warm day.

But you also need high humidity to make ice, conditions the OP reported.
 
I also have a MP gauge. MP should drop as carb ice builds, correct?
 
TazzyTazzy said:
Good points. I do carb heat, note rpm drop, check vacuum, note rpm, carb off, note if higher than 1700. It takes me a more then 2 seconds as I'm going down the checklist. Cfi said to do it in this order to make sure there was no developed carb ice.
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Wait! From a couple of comments it seems I've been doing the carb heat check wrong.
The way my instructor taught me (Cessna 172 in case it matters), is throttle to 1700, carb heat on, check for drop, throttle to idle, check that it runs fine like this and doesn't die or anything carb heat off, check for rpm increase.
Of course bunch of other checks, like suction, in there, but that's not relevant.

The reason he gave me is that carb heat on and throttle idle is how we land, and you want to make sure the engine runs fine in that condition. Made sense to me.
 
I'd expect an iced carb to work just fine at idle if it works at 1700. You need around 1/3 as much air.

The carb heat check you're describing tests that the carb heat works, not whether you have ice.
 
Question I've never been able to answer:

On the Skyhawks with a Lycoming O-320, the green arc on the tachometer goes to 2000 RPM, which makes carb heat usage fairly common.

On the Warriors with the same engine (Lyc O-320), the green arc goes down to 500 RPM. As a result, carb heat is virtually never used as part of a normal operational procedure.

I never understood why the limits were so different on very similar installations.
 
hook_dupin said:
Question I've never been able to answer:

On the Skyhawks with a Lycoming O-320, the green arc on the tachometer goes to 2000 RPM, which makes carb heat usage fairly common.

On the Warriors with the same engine (Lyc O-320), the green arc goes down to 500 RPM. As a result, carb heat is virtually never used as part of a normal operational procedure.

I never understood why the limits were so different on very similar installations.
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I suspect the green arc on the tachometer is related to the propellor installation rather than carb ice issues.
 
MAKG1 said:
I'd expect an iced carb to work just fine at idle if it works at 1700. You need around 1/3 as much air.

The carb heat check you're describing tests that the carb heat works, not whether you have ice.
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The check is not to see if it can run at idle iced (since that would assume I was iced during the test).
I think his logic was that the engine might not run with the carb heat on at idle.
It does idle really low and it doesn't sound like it's super happy. Maybe he heard an OWT that there's a chance it might die, or maybe there was something to it.
I don't think I still have the original checklist for the plane, but it actually might have been described like this in it.
 
hook_dupin said:
Seems legit.

I still can't figure out why Cessna wants precautionary carb heat in the pattern and Piper doesn't.
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It isn't.

They are the same size, category, and manufacturer. They are not the same engine.

What difference would prop installation make for carb ice, and what does that mean anyway?. Both are fixed pitch two blade props 74 inches long and can have a few different pitches.
 
The reason is that in pipers the carburetor is installed close/under the oil sump and therefore remains hotter than in cessnas. Same engine but different carb placement.

hook_dupin said:
Question I've never been able to answer:

On the Skyhawks with a Lycoming O-320, the green arc on the tachometer goes to 2000 RPM, which makes carb heat usage fairly common.

On the Warriors with the same engine (Lyc O-320), the green arc goes down to 500 RPM. As a result, carb heat is virtually never used as part of a normal operational procedure.

I never understood why the limits were so different on very similar installations.
Click to expand...
 
Johann said:
Wait! From a couple of comments it seems I've been doing the carb heat check wrong.
The way my instructor taught me (Cessna 172 in case it matters), is throttle to 1700, carb heat on, check for drop, throttle to idle, check that it runs fine like this and doesn't die or anything carb heat off, check for rpm increase.
Of course bunch of other checks, like suction, in there, but that's not relevant.

The reason he gave me is that carb heat on and throttle idle is how we land, and you want to make sure the engine runs fine in that condition. Made sense to me.
Click to expand...

I do the same thing as the last part of my run up. You're not doing anything wrong. Putting the carb heat on verifies the carb heat is working. Putting it to idle verifies the engine will still run at idle with heat, which is how you land.

  1. Mixture rich (I would be leaned for taxi)
  2. Throttle 1700rpm
  3. Instrument, gauge, & gyro checks
  4. Mag checks, verify drop
  5. Carb heat on, verify drop
  6. Throttle to idle, verify idles ok
  7. Carb heat off
  8. Throttle to 1000rpm, lean mixture again until takeoff.
 
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