JDiver

Well, what I mean is the plane is pressurized, but to an altitude of under 8,000 feet of comparable elevation - if you have air in the bag, it's a balloon and it will inflate; if you have nothing or liquid, (or solids in an ostomy bag) the air pressure differential should have minimal effect on the cath bag. Air is pretty compressible, water and stuff that is mostly water is not very compressible.

(Nest time you fly, try this: take an empty plastic or similar water bottle on a flight and see what happens - as you ascend it will be come rigid and try to expand; if you drink the water from a bottle and leave it closed on descent, the bottle will collapse - only when there is air, as when there is liquid there's just not that much difference.)