Originally Posted by
Bobster
It might not even be possible to fly without seats if there is a minimum weight that you can't go below. Basically there are handling problems when the airplane is too light; it responds differently to things such as turbulence, asymmetric thrust, and control surface movements. I found an NTSB report about a B707 that crashed partly because it had the seats removed and was too light.
Lightness is not an issue, initial prototypes of aircraft can have no interior fittings at all. What you need to do then is to recalculate the balance of the aircraft to find that it is still within acceptable range of being balanced fore-and-aft about its centre, as otherwise it will be beyond being able to be trimmed level with the trim controls. This is a standard pilot's calculation done before each flight. If really needed, there are fuel tanks fore and aft of the balance point that can be filled appropriately to bring it within range.
The issue with the aircraft with the interior removed was probably that it became beyond trim limits, and the trim calculations were incorrect.