Flying is, without question, a modern marvel. The science, the engineering, and the service that goes into making your flight safe and special deserve all the praise in the world. However, two things can be true at once: flying can be fantastic and it can also be gross. That said, it may surprise you to learn that one of the grossest things you can do on a plane is wash your hands.  It's well-documented that planes can be pretty dirty. As several flight attendants previously told Travel + Leisure , places like tray tables, overhead bins, and instruction cards can be rather dirty.…