The Grey Box Aircraft, or Block Plane, is a client based phenomenon that occurs in Multiplayer Mode . The block plane is an automatic substitute if a client does not have the aircraft that someone else is flying. Given that the game uses visual files located on the client's computer, if another player starts in an aircraft that the client doesn't have, the game has no way of showing its visual model, thus it is replaced by this "Grey Box Aircraft". The block plane particular model are located inside the excecutable and are therefore impossible to not have.
There is the noted absence of the block plane in situations where an aircraft's .dat file includes the line SUBSTNAM "Name of substitute aircraft". All clients who lack this plane will then see the plane refered to in this line. If clients have neither aircraft, their YSFlight excecutable may crash.
There are three models, based on the aircraft or engine type of the lacking aircraft, jet powered, propeller driven, and helicopter.