Why would a plane need to do a couple of 360’ turns on the taxi way/run-up area. The crew were given clearance to taxi and hold. They told the tower they needed to turn the plane 360’ and were cleared. When they asked for a second 360 they said they were trying to resolve a fuel imbalance issue. I think this was a Gulfstream. What was going on with this.
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1$\begingroup$ I think it's more likely that the airplane was flying and did a 360 to buy time to balance their fuel, I can't see any reason why they'd do it on the ground. $\endgroup$– GdDCommented Nov 22, 2019 at 11:52
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$\begingroup$ Just curious, what clues in the question lead you to believe the airplane was actually flying? $\endgroup$– Michael HallCommented Nov 22, 2019 at 18:48
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1$\begingroup$ @GdD You won't transfer fuel by gravity in the air by a coordinated turn. You would have to slip or skid the turn to create a "downhill slope" for the fuel, in which case there's no point in turning anyway. You just impart a bit of skid with rudder while flying along your normal track. In any case, if there's an imbalance in flight and the power transfer system is inop, you just open the gravity transfer valve and it will equalize itself without doing anything if the ball/brick is centered. Skidding would speed it up a bit, but it isn't necessary. $\endgroup$– John KCommented Nov 22, 2019 at 18:55
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$\begingroup$ I wasn't suggesting that @JohnK, they'd pump fuel, the reason for a 360 might be to hold a position above the ground. $\endgroup$– GdDCommented Nov 22, 2019 at 19:13
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$\begingroup$ Oh Ok that makes sense. $\endgroup$– John KCommented Nov 22, 2019 at 19:32
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1 Answer
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They probably had a fuel imbalance that was marginally outside the limit for takeoff, and perhaps the powered fuel transfer system was unserviceable and was deferred. Typically there is a gravity transfer capability by just opening a transfer valve and letting fuel run across on its own with the imbalance present, or by skidding slightly.
If they were say 50 or 100lbs above the max lateral imbalance for departure, a spin around on the ramp a couple times with the gravity transfer valve open so that some fuel "sloshes" through the gravity transfer line to the opposite tank, due to the centrifugal force of the turn, may have been enough to get the value in limits.
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3$\begingroup$ As to how such a fuel imbalance could arise...one candidate would be if they were sitting in a ground hold for a while with the APU running, as it will only draw fuel from one side of the plane. $\endgroup$ Commented Nov 22, 2019 at 23:26
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1$\begingroup$ Another candidate is the increasingly popular practice of taxiing with only one engine running to save fuel. If the fuel feed for each engine only draws from one wing tank it'll only run down the fuel on one side of the plane. $\endgroup$ Commented Nov 23, 2019 at 2:04