Another mixture question: can excessive air density require more fuel than can be added?

Rosstafari

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To clarify what I’m trying to ask in the title, in an extremely high density altitude environment - caused by local pressure, temperature, whatever - could the air be so dense that the mixture control couldn’t be set rich enough to run the engine properly? Like my other questions right now, I’m specifically thinking single engine piston, though I’d be interested in hearing how it’d affect larger aircraft as well (like the C130’s servicing Amundsen Scott and McMurdo).
 
Yes, theoretically. However piston engines usually are set so that full rich is so excessively rich that it would never happen. Tutbines run differently. They run lean anyway, but power is controlled by fuel flow rather than airflow. Theoretically in this scenarii you would be limited in power by the physical stop on the power level, while ITT, and torque in the C130 would be below limits. However, again, Fuel Control Units are designed with a significant margin, and take off is usually accomplished well below the physical stop on the lever.
 
I’ve seen it in extremely cold air that the mixture full rich isn’t adjusted for. It wasn’t too far off, but the only way to tell was that as soon as you started leaning during the climbout, EGT would drop.
 
I’m sure it was just a typo, but you’d be looking at an extremely low density altitude situation, like @gotWXdagain (great user name, btw) described. Low temp creates more density in the air which is a low density altitude.
 
To clarify what I’m trying to ask in the title, in an extremely high density altitude environment - caused by local pressure, temperature, whatever - could the air be so dense that the mixture control couldn’t be set rich enough to run the engine properly?

Theoretically possible. But it would be so cold that you wouldn't be flying for other reasons (the oil would be too thick to turn the engine over, and the battery won't produce enough current to do so either). One of the principal reasons for having such a rich mixture at takeoff isn't power, it is cooling. But when it is that cold, the colder air will more than make up for the higher combustion temperatures from the leaner mixture.
 
I’m sure it was just a typo, but you’d be looking at an extremely low density altitude situation, like @gotWXdagain (great user name, btw) described. Low temp creates more density in the air which is a low density altitude.

Argh, yes. Low. Thanks.

Theoretically possible. But it would be so cold that you wouldn't be flying for other reasons (the oil would be too thick to turn the engine over, and the battery won't produce enough current to do so either). One of the principal reasons for having such a rich mixture at takeoff isn't power, it is cooling. But when it is that cold, the colder air will more than make up for the higher combustion temperatures from the leaner mixture.

Makes sense. I suspected it was one of those "Yeah, except this'd never happen because _____" questions.
 
I have experience fuel flow limiting, but the environmental conditions for such are nearly unheard of (extreme cold well below 0).

It’s become recognized as so rare that while it was always 1 of 3 power limiting conditions taught when I started flying now it’s mentioned in the operators manual along with a blurb that basically says “will be almost impossible to experience, worry more about Turbine Gas Temp limiting because that you will see a lot.”
 
I’ve seen it happen.


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Seriously?!? How cold was it? Kinda seems like something in that plane mightn't have been calibrated quite right to begin with (baseline mixture setting, mixture screw pitch, ... something.)
 
Seriously?!? How cold was it? Kinda seems like something in that plane mightn't have been calibrated quite right to begin with (baseline mixture setting, mixture screw pitch, ... something.)
Ya, something was wrong mx wise if you see this happen. For reference it won't happen at sea level and -40 if things are adjusted properly.

Yes, theoretically. However piston engines usually are set so that full rich is so excessively rich that it would never happen. Tutbines run differently. They run lean anyway, but power is controlled by fuel flow rather than airflow. Theoretically in this scenarii you would be limited in power by the physical stop on the power level, while ITT, and torque in the C130 would be below limits. However, again, Fuel Control Units are designed with a significant margin, and take off is usually accomplished well below the physical stop on the lever.
While this could yet again be a case of bad rigging, in the 99 at some of the high mountain west airports, like PUC for example, in the middle of summer, you'd just run out of travel on the power levers before exceeding anything.
 
Ya, something was wrong mx wise if you see this happen. For reference it won't happen at sea level and -40 if things are adjusted properly.


While this could yet again be a case of bad rigging, in the 99 at some of the high mountain west airports, like PUC for example, in the middle of summer, you'd just run out of travel on the power levers before exceeding anything.

I've seen it so cold (with like -4500' DENALT) that EGT would drop in the Navajo during climbout once you got into warmer air? Theoretically I could surmise that this is what he's talking about. Those kind of temperatures mean you don't see oil pressure in the Jo on the ground on the right engine - presumably because the oil line congeals in the right wing.

The 99 would run out of power lever like every day on a high time engine...
 
It was the first truly cold day in GFK one winter (like -25F cold) and leaning the mixture only produced decreases in ITT.

But yeah, I liken it to a mx calibration/rigging issue like maybe there’s a mixture full rich adjustment that hadn’t been done yet on that bird.
 
Ya, something was wrong mx wise if you see this happen. For reference it won't happen at sea level and -40 if things are adjusted properly.


While this could yet again be a case of bad rigging, in the 99 at some of the high mountain west airports, like PUC for example, in the middle of summer, you'd just run out of travel on the power levers before exceeding anything.

This would indeed be bad rigging. At hot and high airports a PT-6 should still reach either a temp or torque limit before a physical limitation. The hotter or higher you are, the greater the chance of reaching an ITT limit before a torque limit. With the weedier PT-6s, torque can be significantly reduced by temping out...
 
I've seen it so cold (with like -4500' DENALT) that EGT would drop in the Navajo during climbout once you got into warmer air? Theoretically I could surmise that this is what he's talking about. Those kind of temperatures mean you don't see oil pressure in the Jo on the ground on the right engine - presumably because the oil line congeals in the right wing.
I think the fuel metering section of the FI system wasn't rigged properly then.
 
I think it might relate to the density controllers. That one was rigged really well.
Ya, so even more so on the PA31. The density controllers keep that engine needing the same fuel almost entirely independent of the density altitude. You would have seen less inches on takeoff that day, on your gauge, but the volume of O2 going into the cylinders would be identical as if you were at a 5000Ft 100F airport. Same exact fuel should be required.
In fact the only time you'd see that change is if you were taking off above the critical altitude.
 
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