Good to know, like I said before, some guys in Oshkosh are restoring DC 3s to make them modern again.
Maybe they won't be used for passenger flights, but they are still used for other professional uses.
I don't think DC3s are pressurized, so it's impressive that a pen won't leak with all of this pressure.
As ambient pressure decreases the air in the pen expands. Ink, a water based fluid, does not expand. If the pen is nib up the expanding air simply flows out through the feed to equalize the pressure. If the pen is nib down, the air will expand and push the ink out through the feed, creating a nasty, but colorful, mess.
And even modern jetliners cruise at a cabin altitude of about 6,000 feet as I remember (somebody who knows exactly can chime in) so there is always some air expansion. You can see this if you try to write during the descent from altitude. Now the ambient pressure is higher than the internal pressure so air flows in and even your perfectly tuned, usually wet writing pen will skip and feel dry. If you try writing on the climbout you will make a nasty mess and earn ugly looks from the flight attendants and your seatmates.
I flew helicopters for fifteen years and for the last eight of them I always carried an Elysee fountain pen. Granted, helos don't usually go that high (it makes it hard to read the highway signs that tell us where we are) but even when we cruised at several thousand feet my trusty pen never leaked.