As dragondog infers, on the earlier P-51's the normal procedure was to first burn fuel from the Left integral wing tank so to enable the return line from the engine to have a place to put the excess fuel going to the carb. Once you had created space in the Left integral wing tank, the next step in the procedure was to burn fuel from the aft tank to get the CG problem under control and then finally switch to the drop tanks. The alternative was the fuel going overboard. You could help the pilots out by making enough space in one wing tank so that the switching of tanks was unnecessary, or at least less critical. And of course if you could redesign the airplane to permanently fix the aft CG problem that would help a great deal.
A friend of mine was assigned the job of "rescuing" aircraft that were being ferried by dedicated "ferry only" pilots and was sent to Nashville because a ferry pilot followed his orders and refused to fly with any fuel in the aft tank. The aft tank kept filling up and that was because for some reason the return line had been routed to the aft tank.
One interesting aspect of the Mustang that people don't seem to comment on was that it was probably more "automatic" than any other aircraft of its time. In addition to an aneroid controlled high/low supercharger it also had automatic control of the radiator exhausts and a mike button built into the throttle. In contrast the P-47 pilot had to adjust his turbosupercharger settings and worry about the anti-compressibility "dive brakes" although in later models of the P-47N they did add a throttle mike button. The P-38 pilot had to make sure his turbos were both turning before he took off, had to turn the hydraulics on and off, had to turn the aileron boost on and off, had a "horn button" mike switch in the middle of the control wheel, had a complex series of fuel tanks to handle, and in later P-38L had to consider the cowl flaps that kept the intercooler from getting too cold, and had to find the drop tank jettison button in an awkward spot - and of course had two engines to manage. The F4F, F6F, F4U and P-61 had switches for supercharger speed and stage control, although the later F4U-4 and -5 went to more automatic controls. The Wildcat also had to crank his gear up and down by hand. No wonder they referred to the P-51 as the "Cadillac"; you just got in a drove.
