Stress and nervousness can cause cats to go to the bathroom more often. You can consider taking with you a portable, collapsible litter box to help make your cat comfortable when doing its business, but it should be small enough to be able to fit in their carrier. You may also want to consider getting diapers specifically made for cats. This is great because we can never guess when cats want to do their business and wearing diapers would help both of you to have more relief when on the plane.