Hi Harry.
I have seen that phrase from several people in these forums. Would you say this is a common practice to always fill every tank at every opportunity? Is it an old practice going back to the 30's as such? e hypothesis?
Running out of fuel in flight is a bigger deal than running out while driving your car. Because of this, pilots are trained to be conservative about their fuel supply. There is another reason to fill the tanks up that Harry hasn't mentioned. Fuel gauges in airplanes are notoriously inaccurate today and they were no better 74 years ago. The only way you can be certain of how much fuel you have is when the tanks are full. Federal regulations (posted below) requires that the fuel gauge only be accurate at one point, it must read "zero"at the level of "unusable fuel" which is fuel remaining in the tanks that might not be usable in extreme flight attitudes. Most of the time you can use the "unusable fuel" in normal flight attitudes. Another way to look at the regulation is that the engine can not run out of fuel prior to the gauge indicating "zero" but the engine may continue to run after that point in normal flight attitudes
Some aircraft fuel gauges are marked simply "0" which also has a "RED LINE"; 1/4; 1/2; 3/4 and "F". Others have markings in gallons. Pilots know to take these markings with a grain of salt since there is no regulation setting out a required accuracy for these marks, except for the RED LINE, and they are not very accurate.
It is difficult to make accurate fuel gauges for airplanes due to the shape of the fuel tanks in the wings, they are long and wide but very shallow, limited by the thickness of the wing. This means that the float on the sending unit has a very limited range of movement that is picked off by a rheostat which is coarse and the wiper may stick between windings. Also, most wings have dihedral meaning that the wing tips are above the wing roots making accurate readout of the fuel even more difficult. In turbulence and when the airplane is maneuvering the fuel can slosh back and forth in the long shallow tanks making the fuel gauges move up and down as the fuel moves around.
Pilots know that they can't rely on their fuel gauges so the most accurate and reliable way to determine the fuel on board is to fill up the fuel tank. Unless there is a major concern with exceeding the maximum gross weight limit, pilots will fill their tanks completely before takeoff. Because of the many separate fuel tanks in the fuselage Earhart could be very confident with their knowledge of the fuel remaining as each tank was drained one by one. After all the fuselage fuel was consumed they only had the inaccurate fuel gauges to rely on in measuring the fuel remaining in the wing tanks.
Since pilots can't rely on their fuel gauges they rely on their clocks. The operating manual tells you what your rate of consumption is at various power setting and altitudes and you divide the amount of fuel in the full tank by the rate of consumption to compute your endurance. If you have multiple tanks you can time how long each lasts and compute your actual rate of consumption which should be a bit more accurate than relying on the aircraft manual and I would expect that Earhart or Noonan did this as each of the fuselage tanks were emptied. But this consumption rate is only valid if the power setting and altitude and temperature remain constant and it is likely that Earhart reduced power as the plane got lighter so their calculated fuel consumption rate could have been inaccurate towards the end.
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Federal Aviation Regulations, 14 CFR -
§ 23.1553 Fuel quantity indicator.
A red radial line must be marked on each indicator at the calibrated zero reading, as specified in §23.1337(b)(1).
§ 23.1337 Powerplant instruments installation.
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(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:
(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under §23.959(a);
§23.959 Unusable fuel supply.
(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.
Regulations in 1936 were either identical or very similar.
see:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr;sid=3b903689cc55804124f8563b0ed49d04;rgn=div5;view=text;node=14%3A1.0.1.3.10;idno=14;cc=ecfrgl