Brought down by carb ice?

A

Aquila2009

Guest
I was flying my Rv-8 with a lycoming 320 in deteriorating conditions, low clouds, rain, almost freezing... and finally climbed confessed and picked up an IFR clearance. I was in IMC, with the temperature at about 30-32 degrees and worried about picking up structural ice.

Next thing I know my low egt alarms are going off, I’m losing power and I can’t get it back. I’ve got full throttle, full rich, and the carb heat has been on. I eventually land safely in a field by the grace of God.

I suspect carb icing but I don’t know what I did wrong. I had the carb heat on almost the entire last 20 min of the flight. I don’t remember it exactly but the carb inlet temp was well above freezing.

I’m nervous now to fly IMC that the engines going to ice over. What did I do wrong or what can I do in the future to minimize that risk?
 
Were you able to run the engine after this incident?
And there was no structural ice (or on the air inlet)?
 
Let'sgoflying! said:
Were you able to run the engine after this incident?
And there was no structural ice (or on the air inlet)?
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It was inspected because the prop hit a fence post. Nothing abnormal noted. Engine runs great now. No structural ice. Not sure about the inlet. I didn’t check have the presence of mind to check.
 
Good to hear you made a successful off-airport landing starting in IMC. Well done!

The RVs are great airplanes, and the -8 is my personal favorite of the series (mostly because I like the cockpit room and the centreline seating). But is an RV-8 really the airplane you want to be flying in IMC in icing or near icing conditions? I don't think anything about the airplane, including its induction system, was intended for those sort of conditions.
 
Any chance that leaving the carb heat full on caused icing deeper in the induction system that the carb heat could never touch?
 
Being an Amatuer built aircraft, the builder could have done whatever they wanted reguarding the pre heat . Alot of the Vans I've seen they just pull heat off a very small part of one of the exhaust tubes. Perhaps it was just not enough heat? Or the carb box/ inlet iced over before the carb.
 
I have Cub friends who’ve lost their planes due to carb ice in benign conditions (0-320 power). Improv your carb heat source. Mine is taken directly off the muffler shroud. That’s as effective as it gets. It needs to be effective when you need it.
 
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Perhaps the fuel tank vent iced over?

Knock on wood, I don't think my o-320 has ever needed carb heat.
 
Does the carb heat bypass the air filter / cold air inlet completely?
How does the size of the heat muff compare with, say, the 172 in the next hangar?
 
GRG55 said:
Good to hear you made a successful off-airport landing starting in IMC. Well done!

The RVs are great airplanes, and the -8 is my personal favorite of the series (mostly because I like the cockpit room and the centreline seating). But is an RV-8 really the airplane you want to be flying in IMC in icing or near icing conditions? I don't think anything about the airplane, including its induction system, was intended for those sort of conditions.
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Completely agree. I didn’t want to be there and shouldn’t have pressed in deteriorating Vmc conditions. Hopefully never make that mistaken again.

What would you define as near icing. Is say 50 deg F a good limit?
 
aquila2009 said:
What would you define as near icing. Is say 50 deg F a good limit?
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It’s not a straight temp, it’s a combination of temp and humidity.

You can get carb ice as high as 70F if the humidity is high enough.

I got carb ice in the T6 in 60F flying near precipitation.
 
Fearless Tower said:
It’s not a straight temp, it’s a combination of temp and humidity.

You can get carb ice as high as 70F if the humidity is high enough.

I got carb ice in the T6 in 60F flying near precipitation.
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Yah, I remember getting carb ice as a student in a C-152. As I remember it was a warm spring day.
 
Carb inlet temp? Temp out of the venturi is what matters. The temp drops significantly there. That's what carb ice is all about.
 
carb-ice-potential-chart.jpg
 
Stewartb said:
Carb inlet temp? Temp out of the venturi is what matters. The temp drops significantly there. That's what carb ice is all about.
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Yup. That right there.

Most Lycomings get away with little carb heat. The 172's muff is a ridiculouly small thing, maybe four inches long and mostly open, and clamped to an exhaust riser. In the OP's flight conditions, with the temperature low and water absolutely everywhere, a small muff isn't going to cut it. The engine sump--shoot, the whole engine--is going to get pretty cool and the carb won't get much heat off the sump. If his carb heat muff isn't up to the task, carb ice would be likely.

The other comment about the fuel vent icing over is valid, too.

Not likely ice farther up in the induction system. That occurs at much lower temperatures when there are ice crystals in the air, and the pilot applies carb heat which melts those crystals and they refreeze farther up the induction piping instead of just passing on through.
 
G-force said:
Being an Amatuer built aircraft, the builder could have done whatever they wanted reguarding the pre heat . Alot of the Vans I've seen they just pull heat off a very small part of one of the exhaust tubes. Perhaps it was just not enough heat? Or the carb box/ inlet iced over before the carb.
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Have you looked at the heat source in the Piper warriors and archers?
 
Dan Thomas said:
Most Lycomings get away with little carb heat. The 172's muff is a ridiculouly small thing, maybe four inches long and mostly open, and clamped to an exhaust riser. In the OP's flight conditions, with the temperature low and water absolutely everywhere, a small muff isn't going to cut it.
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The STC’d Lycoming O-360-A4M in my 172N loves to make ice on a cool (<70F) humid day.
 
Carb ice is most prevalent in low power descents. Sadly carb heat is least effective in low power descents. Finding a more effective heat source should be a priority for guys who've had carb ice.
 
Actually, carb ice is most likely to happen when the pressure drop across the Venturi is the greatest -- that is, when the air is moving the fastest. What that means is that carb ice is most likely to happen at full power. Hippocrates allegedly said "experience is spurious" (no way he actually said that, but stick with me), but my single experience with real carb ice was in a climb.

BUT, none of this is likely relevant, as the carb temp was "well above freezing."

Maybe it was some kind of induction ice (air filter, for example), maybe it wasn't. But you're talking about a homebuilt; there's no way anyone can say without knowing how the induction system is put together in your particular airplane.
 
OneCharlieTango said:
Actually, carb ice is most likely to happen when the pressure drop across the Venturi is the greatest -- that is, when the air is moving the fastest. What that means is that carb ice is most likely to happen at full power. Hippocrates allegedly said "experience is spurious" (no way he actually said that, but stick with me), but my single experience with real carb ice was in a climb.

BUT, none of this is likely relevant, as the carb temp was "well above freezing."

Maybe it was some kind of induction ice (air filter, for example), maybe it wasn't. But you're talking about a homebuilt; there's no way anyone can say without knowing how the induction system is put together in your particular airplane.
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Carb ice is most likely at low power because the throttle plate is almost closed, and the air is squeezing past its edges at terrific velocity and therefore its temperature is dropping enormously. The fuel is also in the airstream at that point and is evaporating and absorbing heat, cooling the air further. It's the reason why the engine might not want to idle soon after startup on a humid morning with the throttle fully closed. Ice is closing off the small gap between the throttle plate and carb bore wall. And it's why, on the chart posted earlier, the icing risk at low power is the largest sector of the whole graph.

f0133-02.jpg
 
I think low power probably didn't give you enough heat to deal with the icing conditions. Bad stuff, avoid ice.
 
Even with the plot above it can be worse.

Margy was flying and we had gone down to 2W5 to do some pattern work. She flew back to VKX (where we were based) at low power (no sense in burning a lot of fuel if we'd just have to shed it off to get to approach speed). It was a fairly warm but humid day. As she reaches the pattern environment, the engine sputters. She decides to head straight for the numbers (good idea) and we land. After rolling out, I ask her to give the throttle a little gas while I pull carb heat. Sure enough our "relatively immune to carb ice" pressure carburetor had iced up.
 
Dan Thomas said:
Yup. That right there.

Most Lycomings get away with little carb heat. The 172's muff is a ridiculouly small thing, maybe four inches long and mostly open, and clamped to an exhaust riser. In the OP's flight conditions, with the temperature low and water absolutely everywhere, a small muff isn't going to cut it. The engine sump--shoot, the whole engine--is going to get pretty cool and the carb won't get much heat off the sump. If his carb heat muff isn't up to the task, carb ice would be likely.

The other comment about the fuel vent icing over is valid, too.

Not likely ice farther up in the induction system. That occurs at much lower temperatures when there are ice crystals in the air, and the pilot applies carb heat which melts those crystals and they refreeze farther up the induction piping instead of just passing on through.
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Can you tell me a little more about the fuel tank vent icing. The one symptom I noted looking at the engine data afterward we’re pretty erratic fuel flows that start about 20 min prior and leading up to the engine quitting they dropped dramatically. The mechanics said they found fuel in the carb but maybe the pump wasn’t able to get enough at that point.
 
If the fuel tank is not allowed to vent to atmosphere, a suction will be created in the tank. This suction pressure will increase as the fuel level decreases.

So in a gravity system the suction pressure will reduce fuel flow.

Do you have an electric fuel pump? If so was it on when things were going South?
 
AKBill said:
If the fuel tank is not allowed to vent to atmosphere, a suction will be created in the tank. This suction pressure will increase as the fuel level decreases.

So in a gravity system the suction pressure will reduce fuel flow.

Do you have an electric fuel pump? If so was it on when things were going South?
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No I barely maintained control in IMC. I didn’t turn it on or switch tanks.
 
Vents should be located to prevent icing. Cessna vents hide behind lift struts and the fuel caps are also vented so I have redundancy. I uses the same concept for my Cub. A properly built airplane should have very low risk of fuel vent fouling.
 
OneCharlieTango said:
Actually, carb ice is most likely to happen when the pressure drop across the Venturi is the greatest -- that is, when the air is moving the fastest. What that means is that carb ice is most likely to happen at full power.................
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Bad advice, NOT TRUE!!!
 
Great job in handling the situation and not becoming another VMC TO IMC statistic. I have suspected ice a few times in our club 182 with a 470. It’s a known ice maker. It has and easy me vigilant on possible icy conditions as well as the early signs im making it. This hazy cooler weather we’ve had mid Atlantic seems particularly icy.
I like to lock my MP once at altitude and if I see the needle drop more then a needle width I apply carb heat.
 
Stewartb said:
Vents should be located to prevent icing. Cessna vents hide behind lift struts and the fuel caps are also vented so I have redundancy. I uses the same concept for my Cub. A properly built airplane should have very low risk of fuel vent fouling.
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They are right behind the exhaust on the belly of the -8 so theoretically pretty warm. I wonder if flying through rain prior entering the clouds caused moisture to get in the line somewhere and it then froze. I’ve read some owners have added their own back ups
 
Never really had carb icing with the turbocharger - but had to watch carb temp to avoid over heating in the summer. Air filter ice was a different matter....
 
My Lancair has a built in back system. The header tank (which feeds the motor) vent is in the same location as on the -8. If somehow it froze or was clogged by bugs, there is a transfer line between header and a wing tank. Thus, if the header tank vent was blocked it could vent from the wing tanks vent.
 
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