I haven't flown with a vacumatic but my aerometric 51 has not given me issues in my carry on luggage or in use in the cabin on the handful of occasions I've flown with it.
Nevertheless, I place my pens in zip lock plastic bags just in case. Maybe with some paper towel or facial tissue. And I almost always fill them or empty them before flying. Belt and suspenders—a quick and easy precaution to prevent unlikely annoyance.
I've also taken various fountain pens on road trips from 5280' ASL (1610 m) in Denver to 11,158' ASL (3400 m) at Eisenhower Tunnel on I70 and back down, also with no issues.
"It might take an average airliner about 20 minutes to reach a cruise altitude of, say, 35,000 feet (11,000 m), at which point the pressurization system might maintain the cabin at the pressure you’d experience at 7,000 feet (2100 m): about 11 pounds per square inch (0.758 bar, 75.8 kPa). Your ears may pop, but the effect is mild because the [change in pressure is equivalent to a climb rate of] only 350 feet (110m) per minute. [That's like climbing a 6% grade at 65 mph (104 km/h)]. When the airplane descends, the pilot sets the system controller to the altitude of the destination airport, and the process works in reverse."
Bookmarks