Vacuum fill fountain pens can be safely carried in airplanes, like the Pilot Custom 823 (which is good but somewhat expensive for a travel pen) or the TWISBI Va

Vacuum fill fountain pens can be safely carried in airplanes, like the Pilot Custom 823 (which is good but somewhat expensive for a travel pen) or the TWISBI Vac700R and Vac Mini. My favourite ink is the Pilot Iroshizuku take-sumi, but it's not tamper proof if it matters.