On March 12, 1994, at 2320 mountain standard time, a Rockwell 681, N78CH, sustained substantial structural damage during landing with the main gear partially extended at Phoenix, Arizona. The aircraft was operated by American Corporate Aviation, Inc., of Houston, Texas, and was engaged in air ambulance flight operations under the provisions of 14 CFR 135. The aircraft incurred substantial damage. The certificated airline transport pilot, the two flight nurses, and the two passengers were not injured. The flight originated at Show Low, Arizona, on the day of the mishap at 2230 hours as an aeromedical evacuation flight to Phoenix. According to the pilot's statement, while inbound to Phoenix he moved the landing gear selector switch to the down position; however, the landing gear did not extend. The pilot used the emergency gear down system, which consists of using pressurized nitrogen to force the landing gear down. The pilot reported that the nose gear extended; however, both main landing gear extended only partially and the main wheels failed to rotate to the streamlined position. Subsequent attempts to fully extend the main gear were unsuccessful. The aircraft landed and the main gear collapsed, resulting in damage to the lower fuselage bulkheads and gear attach structure. A Federal Aviation Administration (FAA) airworthiness inspector from the Scottsdale, Arizona, Flight Standards District Office examined the aircraft. He found no hydraulic fluid in the system lines, and no evidence of hydraulic fluid sprayed around the landing gear wells. The nitrogen system pressure was measured at 50 psi; according to the maintenance manual for the aircraft, the normal system charge pressure is 425 to 525 psi. Detailed examination of the hydraulic system lines revealed a 1/4-inch metal line ruptured at a point under the rear bench cabin seat. Hydraulic fluid saturated the immediate area. Corrosion pits were observed on the line in many places. The inspector reported that the maintenance records indicated the ruptured line was an original equipment item installed at aircraft manufacture on or before January 26, 1970. Examination of the nitrogen system lines revealed a leak in the wing center section crossover line. Corrosion pitting was observed in several places on the line.