Do fountain pens leak on planes?

They can — but only if you give physics an opening. Cabin pressure drops as you climb; air trapped in a half-empty reservoir expands and pushes ink toward the feed.

The rule is full-or-empty: fly with the pen completely filled (no air to expand) or completely clean. Carry it nib-up in a shirt pocket or case during ascent. Do that and thousands of writers fly weekly without incident.