Engine quit on final approach while returning from dropping parachutists. A forced landing was made in a field and the aircraft collided with a ditch on rollout. Post crash examination of the aircraft by faa inspectors indicated the left fuel tank contained one pint of fuel and the right fuel tank contained 1 gallon of fuel. Aircraft information indicated 1.5 gallons of fuel is unusable in each tank. The engine operated normally after the accident and there was no pre or post crash fuel leakage from the aircraft.