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This report on dodbuzz mentions a 'fuel tank float valve'. Neither google nor StackExchange seem to know what it is. What is it, why is it necessary?

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I don't know about the fuel float valve of the F35A. A float valve is basically the principle used in toilet flush: A device that:

  • senses liquid level in a tank, and
  • open/close a valve according to this level.

The device may rely on a float acting on a horizontal lever, or it may be a float sliding vertically along a rod.

There are several float valves on this schematics of the Boeing 737 fuel system:

enter image description here
Source

If this doesn't work, the tank may be reported as always full, as for the F35A of your report.

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  • $\begingroup$ Thanks for your answer, I'll wait a little longer, before accepting... $\endgroup$
    – mike
    Apr 22, 2017 at 13:17
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    $\begingroup$ While floats (though not float valves) are used in many light aircraft to sense fuel levels, most larger or turbine aircraft use capacitance level sensors with temperature compensation. Some aircraft also use floats for tank level sensing for fuel transfer purposes. $\endgroup$
    – J W
    Apr 22, 2017 at 18:25
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Going to take a bit of a stab in the dark on this one... I'm presuming the float valve mentioned is the one connected to a fuel measuring stick. In the middle of a magnetic level indicator there is a float valve external of the stick housing which contains a magnet. The indicator rod also has a magnet at the upper end I.e. The end closest the fuel. When the rod is lowered out of the bottom of the fuel tank a magnetic coupling occurs between rod and float and the level of fuel in the tank may be read off from the calibration intervals on the rod.

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An aircraft fuel system uses float values in the fuel vent system. This is to ensure the fuel is vented regardless of the attitude of the aircraft or the quantity of fuel on board. Especially on larger aircraft and in larger tanks, there will be multiple vent locations in the same tank.

From AMT Airframe Handbook (FAA-H-8083-31) Chapter 14 Aircraft Fuel Systems

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