All files in the DG directory can be swapped around. Therefore you 
can fly an F-16 - frigate - truck etc. by typing:

REN AV-8B AV8B.BAK and then

COPY TRUCK AV-8B

Load the game as usual and take off. Now, when you switch to an 
external view you'll be flying a truck! 
Copy AV-8B.BAK to AV-8B to return to normal.