Carburetor icing in Cessnas

[billvon 2,991]

From a question in the Incidents forum:

Carb icing is the result of ice forming in the throat of the carburetor. Many of the engines used by skydiving operations (Continental O470 for example) are susceptible to it. It can result in slow loss of power (if ice builds up slowly) or rapid loss of power (if ice restricts movement of throttle plate or significantly changes the mixture.)

In a carburetor, air is sucked past a throttle plate into a lower-pressure manifold. As the pressure drops, water can condense out - and per Boyle's law, the air also gets colder. This effect alone can cause water to condense out of air even in clear skies if conditions are correct (high humidity mainly) and can often be cold enough to freeze.

Another factor is fuel flow. As fuel is sprayed in near the venturi of the carburetor, some evaporates. This cools the mixture further and can aid in ice formation.

Of course, when you are in clouds, you already have water droplets swirling around, so the problem becomes that much worse.

Fortunately there is a way to counteract this - carb heat. This routes some warm air from the exhaust through the carburetor to increase temperatures and hopefully melt any ice (and prevent new ice from forming.) It is not used all the time since it reduces engine power.

Other aircraft deal with it in other ways. Some mount their carburetors in areas of the engine that stay warm. Some have fuel injection which goes a long way towards dealing with the problem. A few use diesels or turbines which do not have an icing issue.

In the case of the Danielson crash, carb icing may have played a role. The classic case involves a full power climb (lots of heat, wide open throttle so not much pressure change to cause temperature drop) followed by a rapid descent (throttle almost all the way closed, so maximum pressure and temp drop.) The engine may start to sputter, but at low power levels it's not that noticeable. Or it may run fine, with the ice still allowing enough mixture through to run the engine at low power.

Then the pilot enters the pattern and brings power up. The engine cannot draw enough air through the throttle, and so loses power. The mixture gets all wonky since the ice is interfering with the venturi, and the throttle may not respond well due to the ice near the throttle plate.

The pilot immediately realizes the problem and adds carb heat. But with the throttle body almost completely full of ice, not much hot air can get through. And with the exhaust cool from the long low-power descent, there's not much heat available anyway. If he's lucky, the ice starts to melt, and the engine runs even worse for a few seconds as it passes the ice and water through the pistons. Then he gets power back, hopefully before he gets to the ground.

Or he may not have enough time for all that to happen. At that point a dead-stick approach may be his only option.

Because of this scenario, many manufacturers recommend carb heat on during descents as a matter of course.

[DougH 270]

Thank you for taking the time to post that Bill.

"The restraining order says you're only allowed to touch me in freefall"
=P

[akjmpplt 0]

Quote

The classic case involves a full power climb (lots of heat, wide open throttle so not much pressure change to cause temperature drop) followed by a rapid descent (throttle almost all the way closed, so maximum pressure and temp drop.) The engine may start to sputter, but at low power levels it's not that noticeable. Or it may run fine, with the ice still allowing enough mixture through to run the engine at low power.

Then the pilot enters the pattern and brings power up. The engine cannot draw enough air through the throttle, and so loses power. The mixture gets all wonky since the ice is interfering with the venturi, and the throttle may not respond well due to the ice near the throttle plate.

The pilot immediately realizes the problem and adds carb heat. But with the throttle body almost completely full of ice, not much hot air can get through. And with the exhaust cool from the long low-power descent, there's not much heat available anyway. If he's lucky, the ice starts to melt, and the engine runs even worse for a few seconds as it passes the ice and water through the pistons. Then he gets power back, hopefully before he gets to the ground.

Or he may not have enough time for all that to happen. At that point a dead-stick approach may be his only option.

Because of this scenario, many manufacturers recommend carb heat on during descents as a matter of course.

I've got quite a bit of time flying 182s hauling jumpers. The Cont O-470 will make ice faster than a Frigidaire in the right conditions. What I found, much to my surprise, is that I didn't get carb ice in what is traditionally thought to be the "normal" time to get it. I several times have had carb ice on takeoff while at full power (one of which resulted in a dead stick landing back to the airport). I have never had carb icing during a descent, and all of my descents were made with NO carb heat (I did however pull heat on every few moments to check for ice). I had a carb air temp probe installed which allowed me to keep a close eye on things. I found that during low power descents the carb air temp was very near ambient temps...well above freezing...but during high power or cruise power operation is when the carb air temp would drop to the freezing level or below. Folks operating O-470s would be wise to invest in a carb air temp gauge IMO.

SmugMug

[pilotdave 0]

Just out of curiosity, why no carb heat on descent? I've never flown a 182, but I always used carb heat when operating below the green arc in the 172s I used to fly to help prevent shock cooling. Different with a constant speed prop?

Dave

[Beerlight 0]

Quote

Just out of curiosity, why no carb heat on descent? I've never flown a 182, but I always used carb heat when operating below the green arc in the 172s I used to fly to help prevent shock cooling. Different with a constant speed prop?

Dave

Shock cooling prevention with carb heat?? I've been around a while, never heard of that one before.

[pilotdave 0]

Thats the reason I was taught to put it on anyway (aside from the obvious reason)... Could be a myth... I never really thought about it. The reason didn't matter since it's on the checklist.

Dave

[diverdriver 5]

Quote

Thats the reason I was taught to put it on anyway... Could be a myth.

Dave

Myth. The little heat (comparitively) that you take into the carb from the carb heat does not compare to the heat change needed to cause "shock cooling". And the amount of heat drawn in is directly related to how much heat is being put out by the engine so low power (rpm in your case) would mean little heat drawn in for your stated purpose.

Carb heat is to prevent carb ice. That's all.

Chris Schindler
www.diverdriver.com
ATP/D-19012
FB #4125

[JSBIRD 1]

A jump pilot who reduces power below the green arc for decent, and then has to add power in the pattern, wouldn't be working for me very long.

Without some more training on the proper operation of a (jump) plane, this pilot will wind up bending an aircraft...and sit there wondering "Why?"

BASE359

"Now I've settled down,
in a quiet little town,
and forgot about everything"

[akjmpplt 0]

Quote

Just out of curiosity, why no carb heat on descent? I've never flown a 182, but I always used carb heat when operating below the green arc in the 172s I used to fly to help prevent shock cooling. Different with a constant speed prop?

Carb heat wasn' needed, so I didn't use it.

SmugMug

[fred 0]

Quote

A jump pilot who reduces power below the green arc for decent, and then has to add power in the pattern, wouldn't be working for me very long.

Can somebody explain the 'green arc'?

[DBCOOPER 5]

The green arc is the normal operating range. In my 182 the green arc for rpm is 2100 to 2600 and for manifold pressure 15 t0 23 in. hg.

There is a device out there to detect carb ice. Its called "Iceman". It works by using an led as a light source and a photo cell as a trigger for the alarm circuit.It screws into the side of the carb into the throat where the ice forms Once the ice starts forming and the photo cell does not see as much light it triggers the alarm (audible and visual).
There has been a bunch of times where it has alarmed when I would not think ice was forming (FULL POWER CLIMB) and the application of carb heat cleared the alarm. Its pretty cheap as far as airplane stuff goes ($365 plus install) and works extreamly well.

Replying to: Re: Stall On Jump Run Emergency Procedure? by billvon

If the plane is unrecoverable then exiting is a very very good idea.

[popsjumper 2]

Good stuff. An educational thread.
Brings home to me how much more I could learn about aircraft.
Reading logs and such is one thing but there's soooo much more. I'm off on a learning safari now.
Thanks guys.

Newbie-type question:
So, wouldn't an optimized temperature control on the intake be a good idea? Why just ON/OFF?
Expense?

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[DougH 270]

KISS?

"The restraining order says you're only allowed to touch me in freefall"
=P

[diablopilot 2]

The carburetor heat can be used in a graduated manner, proper use varies from aircraft to aircraft, and consulting the POH is the proper thing to do.

When encountering Carb ice in a 182 pulling the carb heat full on is usually the first fix, but once cleared, if carb icing conditions continue, then use of just enough heat to prevent the forming of ice is recommended.

Carb heat does reduce engine performance, as warm air is less dense and both the air fuel mixture to the engine and the volume supplied to the engine are reduced.

----------------------------------------------
You're not as good as you think you are. Seriously.

[kallend 2,027]

Quote

From a question in the Incidents forum:

Carb icing is the result of ice forming in the throat of the carburetor. Many of the engines used by skydiving operations (Continental O470 for example) are susceptible to it.

All carbureted engines are susceptible, but some are much more susceptible than others due to the design of the intake system and location of the carb.

Quote

In a carburetor, air is sucked past a throttle plate into a lower-pressure manifold. As the pressure drops, water can condense out - and per Boyle's law, the air also gets colder. .

Not Boyle's law, Bill, Joule-Thomson effect.

...

The only sure way to survive a canopy collision is not to have one.

[scdrnr 0]

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A jump pilot who reduces power below the green arc for decent, and then has to add power in the pattern, wouldn't be working for me very long.

While its obviously optimal to make a continous poweroff descent to landing, its not possible at all airports. A lot DZ's exist on busy airports or have noise abatement procedures that preclude doing the normal jump plane landing.