Question

why are fountain pens are not allowed in airplanes?

Answer 1

As you know with increasing attitude Atmospheric pressure decreases , so the pressure inside the pen is more compared to the its surrounding Are (i.e inside the Flight).When we are using the pen , the ink gets spilled due to varying pressure.Hence the caution.
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Answer 2

because they can leak due to change in cabin pressure occurring due to altitude change of flight.

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