Plane mad means that someone really loves airplanes and everything about them. Like how little kids really love their favorite toys, some people love airplanes that much. They might think airplanes are really cool because they can fly through the air and take people to faraway places.
People who are plane mad might spend lots of time reading about planes, watching videos of them, and going to air shows and aviation museums to see them up close. They might even collect models of planes that they can play with and show off to their friends.
So basically, being plane mad just means that someone thinks airplanes are super cool and loves learning about them and playing with them.