Air temp too cold?

FredFenster

FredFenster

Pre-takeoff checklist
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Apr 18, 2013
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Greg L
2 weeks ago the air temp on the field was -10F, altimeter setting was 30.59. Spectacular density altitude :wink2: Aircraft was a Cherokee 140, had a preheater going all night so the oil temp needle was at least off of the bottom and the rest of the engine was warm to the touch. Did a run up, everything seemed normal. Got on the runway, went to full power and it started running rough. Did another run up, seemed OK, full power again and more rough running. I went back to the hangar and parked it.

Last weekend, temps were in the 20's, flew it for 3 hours and never had a problem. Is it possible to be too cold or the air too dense for a carb'd engine to run properly? That's all I can come up with as to why it wouldn't run right that day.
 
Was the oil cooler cover installed? Piper recommends (requires?) is when ambient temps are below 50F. I've noticed even at 30F, without the cover, the engine operates differently (180 hp cherokee).

Or maybe it jus didn't want to get out in the bitter cold. I have that problem with my dog. He'll stand in the doggie door with his head outside, and you can see the wheels turning....do I really need to go out in this? It's so nice and warm inside....
 
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I would think it more likely that you had a fuel system icing issue. Although it is possible that the density altitude was low enough that your carb needs to be adjusted richer than the control was allowing.
 
murphey said:
Was the oil cooler cover installed? Piper recommends (requires?) is when ambient temps are below 50F. I've noticed even at 30F, without the cover, the engine operates differently (180 hp cherokee).

Or maybe it jus didn't want to get out in the bitter cold. I have that problem with my dog. He'll stand in the doggie door with his head outside, and you can see the wheels turning....do I really need to go out in this? It's so nice and war inside....
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Yes, the oil cooler cover was installed. I was doing the same thing as your dog while trying to preflight and add a little air pressure to the tires. Even the air compressor didn't want to work right, the pressure relief valve kept opening as soon as the tank got up around 40psi!
 
FredFenster said:
2 weeks ago the air temp on the field was -10F, altimeter setting was 30.59. Spectacular density altitude :wink2: Aircraft was a Cherokee 140, had a preheater going all night so the oil temp needle was at least off of the bottom and the rest of the engine was warm to the touch. Did a run up, everything seemed normal. Got on the runway, went to full power and it started running rough. Did another run up, seemed OK, full power again and more rough running. I went back to the hangar and parked it.

Last weekend, temps were in the 20's, flew it for 3 hours and never had a problem. Is it possible to be too cold or the air too dense for a carb'd engine to run properly? That's all I can come up with as to why it wouldn't run right that day.
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You were probably too lean. You have a couple of options depending on what your airplane allows. Some planes use induction air restrictors for cold ops. Literally you cover part of your air intake, in my case at the air filter on the nose. That limits intake air and corrects for the unusually high air density. A simpler solution is to utilize carb heat. Some airplane instructions prescribe doing it as necessary to smooth engine operation. Essentially it enriches your mixture by warming the air to decrease it's density. The latter is more common than the former. See what your airframe instructions suggest or talk to some old dudes with experience in type to see what they've used.
 
Aha, never thought of using the carb heat to see if it would have run better. I'll have to look at the POH tonight and see what it says. And I suppose if there's ice in the fuel system, sumping the tanks won't do anything to get that out.
 
You can't drain ice through a sump drain. Ice in the fuel system has never caused any fuel supply issues for myself or anyone I know. A few drops of water are normally present in my fuel tanks. This drops are no threat when frozen. You can read your POH regarding using Isopropyl. Most airframes allow using 1% isopropyl in the fuel to eliminate moisture (ice).
 
FredFenster said:
Aha, never thought of using the carb heat to see if it would have run better. I'll have to look at the POH tonight and see what it says. And I suppose if there's ice in the fuel system, sumping the tanks won't do anything to get that out.
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No, but if the plane lives in a hangar, and you preheated the engine warm to the touch, it shouldn't be an issue. While people are accustomed to performance degradations of high density altitude, dealing with extremely low density altitude produces its own issues. Reality is when it was running rough, you probably could have pulled back the throttle until it ran smooth, and still been making more than rated power. This is where EFI systems really shine, they fine tune for the exact conditions on a continuous, real time, basis.
 
Whoa! Pre-heating has nothing to do with cold weather air density causing excessively lean mixture. If you don't need to lean for normal ops at normal temps there's no evidence that your carb is supplying adequate fuel for normal ops in sub-zero temps. Limiting throttle is horrible advice. Every pilot knows that full throttle supplies extra fuel. Employing the economizer during take-off (by not using full throttle) would be the worst action the pilot could take if the engine was already running lean.
 
Stewartb said:
Whoa! Pre-heating has nothing to do with cold weather air density causing excessively lean mixture. If you don't need to lean for normal ops at normal temps there's no evidence that your carb is supplying adequate fuel for normal ops in sub-zero temps. Limiting throttle is horrible advice. Every pilot knows that full throttle supplies extra fuel. Employing the economizer during take-off (by not using full throttle) would be the worst action the pilot could take if the engine was already running lean.
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Exactly, preheating reduces the likelihood that there is an icing issue in the fuel system. The only times I've seen it is when there are cold soaked ice droplets in the carb bowl. Never saw that in a preheated engine. Much more likely he was 75°LOP at full power-full rich.

Limiting throttle will also limit airflow, since we're working on the 'back side' of the mixture curve, the reduction of airflow will have a greater effect than the reduction in the power valve. As long as the mixture is set to full rich, and the engine is running smooth, you are highly unlikely to get into destructive operating regimes, especially in that weather.

Do I think there are better solutions? Yes I do, Carb Heat should deal with it adequately. Failing that, faced with using smooth reduced throttle for take off or rough full throttle, I will take reduced throttle and a smooth engine.
 
I think you make too much stuff up. This is a great example, at least from my perspective as a cold weather airplane operator.
 
Try some carb heat. I fly an old Lycoming O-145 sometimes and when its really cold (10f)it performs much better if i put the carb heat on and leave it on until im wide open on the Take off roll
 
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