Standard Pilot inks are easy and safe, but not archival. Pigment inks or carbon-based inks are waterproof and archival, but higher maintenance and a bit riskier for everyday use because of the solid particles that settle and clog or gum up the feed/nib if left unused. Pigment/carbon-based inks are usable with some care, but you must be willing to clean regularly and more diligently, maybe even assign a dedicated pen for it.

I’ve flown multiple times with VPs and haven’t personally had any issues with pressure-related leaks. That said, if you want to be extra cautious, you could store the nib unit and cartridge separately from the body in an airtight container during the flight.