Why are ink pens not allowed in airplanes?

There's a popular misconception that fountain pens and aircraft simply do not get along.Typically, passenger airliners fly at an altitude approaching 40,000 feet. At this height the air is so severely lacking in oxygen that if you were to breathe it normally, you would have approximately 12 seconds until you start suffering from disorientation and losing consciousness which will ultimately result in death. In a pen's case, air will expand or contract depending on the change in pressure. The cabin pressure of an aircraft at this height is the equivalent of being at 7,500 feet which has quite a severe impact on the inside of your pen as it will still have the same pressure it did at ground level. The result? A large collection of ink will gather near nib/feed section of the pen whilst the expanding air will push the ink out.