A Plane Letter is a letter that many teens write to each other. You can only open them when you get on the plane (or car) to your vacation destination. These may contain inside jokes between the two of you, or games (e.g. wordsearches etc.) made by the person to occupy you during the journey. The world goes crazy on the last day of school when everybody is making plane letters for each other.
May: "Can you write me a plane letter? I'm going to China on Sunday."
-On the plane-
Have an amazing time in China, I'm going to miss you so much!! Remember the time we attempted at making those cookies - what a disaster!! I know it's going to be a long flight, so I made you a wordsearch and loads of other games!
See you at school, Dean xxx