THE AIRCRAFT WAS RETURNING TO DROP A BANNER DURING A BANNER TOW OPERATION. THE BANNER CONTACTED TREES DURING THE MANEUVER, THE AIRCRAFT THEN STALLED AND COLLIDED WITH THE TERRAIN. A POST CRASH FIRE CONSUMED MOST OF THE AIRCRAFT. THE INVESTIGATION REVEALED THAT THE RELEASE MECHANISM FOR THE BANNER HAD NOT BEEN PULLED. OPERATION OF THE RELEASE MECHANISM SHOWED IT TO BE OPERATIONAL. THE INSPECTOR STATED THAT THE BANNER BEING TOWED AT THE TIME OF THE ACCIDENT WAS 'HUGE'.