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Old 16th Dec 2017, 18:27   #1 (permalink)
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Asymmetric fuel loads...

Most, if not all, aircraft have a limitation on asymmetric fuel loads in wing tanks. So how is this limitation determined? Is it through a design calculation, then verified empirically? Or is is it determined through performance limitations?
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Old 16th Dec 2017, 22:28   #2 (permalink)
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SIAS..(suck it and see..).Computer,simulator,the fly it...progressively..
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Old 17th Dec 2017, 12:26   #3 (permalink)
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The limitation is set by aileron trim per 25.161(b) and verified by flight test.

Transport Canada has a short AC on the equivalent 525.161(b) that provides some context: Flight Characteristics With Lateral Centre Of Gravity

Recently I worked on an external stores modification for a bizjet and found that the manufacturer had provided ample margin on this limitation.
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Old 17th Dec 2017, 13:12   #4 (permalink)
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Typically the 161 trim requirement isn't the thing that determines the max asymmetric fuel load. Landing gear design load cases are more common. (There's also consideration for asymmetric fuel load, and indeed asymmetric/offset CG in general, in 149 (VMC) but again, it's rarely the limiting factor.)

On the a/c I'm familiar with, the fuel limit is of the order of 800lbs, but we could trim well beyond that. Perhaps more than twice before running out of trim.

In connection with this, consider what most wing fuel leak procedures have you do - get as much as you can into the good tank (by transferring) and as much as you can out of the bad tank through the engine (by asymmetric power). In that circumstance you ignore the nominal fuel asymmetry limit, and keep transferring/burning until it becomes a handling/trim issue, and THEN you stop. That's a pretty good confirmation that the limit itself isn't coming from a handling concern.
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Old 26th Dec 2017, 03:14   #5 (permalink)
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Well, I've had some experience with what does not work... Many decades ago, I was right seat in a corporate Piper Cheyenne II. That aircraft had four tanks, wings and tips. The quantity indicator would indicate the tanks selected, and a momentary contact switch would allow you to see the quantity of the non selected tanks (so it was not displayed, unless desired). If I recall, the tips would carry 400 pounds of fuel each.

Following fueling on a very cold and blowy night, I did the walkaround, checked that the caps were on properly, and signed for the fuel. We got in, buttoned up, and taxied for departure. For whatever reason, pretakeoff checks did not include checking the quantity in the non selected tanks (it did the next day!). Upon rotation, my fellow pilot instructed me, with some concern in his voice: "help me fly.". Sure enough, it took two of us to apply enough aileron to hold it level. Then, about simultaneously, it occurred to both of us, that I check the non selected - tip - tanks. One full, the other empty. A 400 pound imbalance at the tip. Maximum permitted 200 pounds if memory serves me.

The failing of Piper was there the flight manual did not contain a procedure to run both engines from fuel of one side. In the case of a failed engine, you could crossfeed, but this procedure was not a part of twin engined flying. My fellow pilot briefly thought to pull the firewall shutoff, to enable the crossfeed, but I suggested, and he concurred, that this would make matters much worse.

We moved passengers, and disposable load as best as possible, but in the cabin, it had little effect. Once at altitude, with things more under control, we phoned Piper on the flight phone. A wise Piper tech rep relayed an unpublished procedure to run both engines from one side. After half an hour we were within trim capability, and muscle was no longer required. We added to our company procedures for the Cheyenne after that.

This taught me the benefits of the Cessna system, where the tip tanks are the main tanks, so the pilot's attention is normally drawn to their contents, and perhaps an imbalance. The Cessna system will allow the careless pilot to pump fuel overboard with a wrong tank selection.

While flight testing the DA-42-L360 for hot fuel testing, we go into an imbalance situation, which was the result of only having the heated test fuel in the left wing tank. We ended up exceeding the 5 gallon imbalance limit, to more than ten gallon imbalance, but it had little noticeable effect on how the plane flew.
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Old 28th Dec 2017, 12:13   #6 (permalink)
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One aspect that no-one has mentioned yet is the effect of a lateral mass asymmetry on crosswind landing limits. If an aircraft has stabilising dihedral effect (left roll from right sideslip and vice versa) then a crosswind from the 'heavy wing' side will not be a problem. However, a crosswind from the 'light wing' side will have less lateral control authority available for maintaining wings level during a kick-off-drift crosswind landing than if there was no lateral mass asymmetry, thereby reducing the maximum crosswind component that can be handles. Therefore, if confronted with a crosswind always try to land with the heavy wing into wind.

Note that these considerations apply not only to a lateral fuel asymmetry but also to a mass asymmetry resulting from external stores configurations with a lateral asymmetry.
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Old 28th Dec 2017, 12:47   #7 (permalink)
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Must admit cross wind etc wasn’t considered for this, but the a/c had so many other limits for the ferry flight this might not have been encountered.
Flight and trim limits were explored with manned simulation; actual handling slightly better than predicted - pre set quarter turn aileron trim ( probably not required ).
There was no choice of pylon; no consideration of unbalanced fuel distribution before flight because it was difficult to arrange.

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Old 28th Dec 2017, 13:33   #8 (permalink)
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Ah - so that's the origin of the fabled snag report "No 3 engine missing"...

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Old 30th Dec 2017, 06:27   #9 (permalink)
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Originally Posted by PDR1 View Post
Ah - so that's the origin of the fabled snag report "No 3 engine missing"...

I remember hearing about one of Connie's 747s "losing" an engine in flight. Word is, they never did find it. Guess it's still somewhere on the bottom of one of the Great Lakes? Oh, the airplane landed safely. They were just missing something!

Anyway, since the advent of pylon mounted jet engines, there have been a number of engines "lost" during flight. Most were eventually found and some not. Obviously, some of the airplanes that "lost" an engine during flight enjoyed more favorable outcomes than others as well...

Last edited by westhawk; 30th Dec 2017 at 06:37.
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