Strictly speaking I’ve flown with fountain pens before, but this is the first time where I’ve been aware of the risk of ink burping out of the nib due to pressure changes in the cabin.  I have four pens with me, which are as follows:

• Onoto Magna Classic (medium gold nib) in a pen roll in my hand luggage – no problems.
• Pilot Custom 823 (medium gold nib) in a pen roll in my hand luggage – no problems.
• Kaweko Custom Sport ‘Orangina’ (medium steel nib) attached to my Midori Traveler Passport by a loop, and in my jacket pocket hung on a seat hook – no problems.
• Pilot Custom 823 (FA gold nib) clipped on the inner pocket of my jacket – some ink burped from the feed, but only a little – pen was recently refilled.

So overall the only slight problem was with one of two pens with the reservoir sealed to help prevent leaks ….