The day before the accident flight, 12 gallons of fuel was added to the fuel tanks and the airplane was flown three times. The accident flight departed with an unknown quantity of fuel and while maneuvering, the engine quit due to fuel exhaustion. During the emergency descent, one of the two occupants applied full up elevator control at an airspeed above maneuvering speed (VA) which caused both forward wing spars to fail in the positive direction. The airplane descended near vertical and impacted the ground in a nose-low attitude. Examination of the airplane at the accident site revealed no evidence of flight control preimpact failure or malfunction. Examination of the fuel system revealed no evidence of fuel. Metallurgical examination of the fracture surfaces of the wing spars revealed no evidence of preexisting cracks or corrosion. The airplane had been operated for approximately 30 minutes since departure. The student pilot in the left seat was the only occupant who was authorized to fly the airplane. No determination could be made as to why the private pilot rated passenger was in the airplane.