One of my coworkers, T.Scott, had his pen start leaking on him after a recent flight. And, in case it might help anyone else, I thought I’d write a short post about a strategy that has worked pretty well for me. In short, you can generally prevent pens from leaking if you keep them upright (tip pointed-up) during flight.
That way, if the air expands within the chamber (due to a change in air pressure), the air can harmlessly escape through the tip (rather than pushing the ink through it). Typically, I’ll use the pen’s clip to attach it to one of the in-flight safety cards in the seatback in front of me just before takeoff (while keeping the tip pointed up, of course).
PS Some pages that address this topic also recommend removing the cap during flight (while keeping them pen upright) to allow further airflow through the tip of the pen. I’ve had good results without having to remove the cap, myself, but that option is there if you had wanted to take extra precautions.