Where do RF common-mode interference currents flow?
Everything is differential mode if you look at a big enough picture — there are no circuits mysteriously lacking a return path here. The incoming radio wave causes a potential — a voltage difference — between two separated conductors. Which two conductors? Whatever ones are available!

When you are concerned about shunting RFI to earth ground, it is because there is a potential difference between earth and that part of your circuit. But this is not a special property of earth — it's just playing the role of one of the conductors. In an aircraft, the potential difference must be between two different parts of the aircraft. Any two conductors — or parts of the same conductor — having some significant physical separation can function as a receiving antenna in this way.

The goal of your shunt is not to send the interference specifically to earth in a somehow one-wire fashion; it is to give the interference arriving on some two conductors a short circuit between them — one shorter than the path it could instead take through your sensitive electronics if the filter were absent.