How can I use a fountain pen while onboard an airplane?
There are things you can do to reduce the risks, as described in the other answers, but I would be wary of using any kind of pen containing liquid ink on a plane (although a &quot;felt or fiber tipped&quot; pen may be OK; never investigated that).  I've had a Pilot Precise technical-style pen that uses disposable refills (the entire guts of the pen), leak while sealed in the tip-up position.
If you need to write on a plane, pens with gel ink (which also includes most ballpoint pens), are much less likely to leak.  The best type would be a Fisher &quot;Space Pen&quot; (various other pens can also use those refills).  These are manufactured pressurized and are designed to use that pressure to feed a very thick ink without leaking (they can write in the vacuum of space without leaking).  If writing with a fountain pen is important, I would just wait until you land to do it.