N365CPCessna 172S · 2001

The pilot of the low-level pipeline patrol flight reported that after about one hour into the flight, a “very loud pop” noise emitted from behind the instrument panel followed by an unidentified high-pressure air/gas release into the cabin. After not seeing any flames, the pilot opened his window to vent the cabin and decided to conduct an off-airport emergency landing on an empty multi-lane highway. During the landing rollout, the left wing impacted a metal road sign, and the airplane came to a stop on a grass median. The airplane sustained substantial damage to the left wing. A postaccident examination of the airplane revealed that a high-pressure air conditioning hose failed and separated at a metal crimp connection at the firewall pass-through. The hose ends at the separation showed signs of age deterioration. Maintenance records revealed that the hose had been in service for about 20 years, accumulating 12,512.5 total hours. According to the air conditioning system maintenance manual, the system’s components are to be inspected every 600 hours or every annual inspection, whichever comes first, and are to be cleaned or replaced as necessary.

The commercial pilot and passenger were returning to the airport after completing the pipeline patrol. As the airplane approached the airport, the pilot listened to the recorded weather and contacted the air traffic control tower. The reported wind was steady at 12 to 15 kts from 170°. The pilot was cleared to land on runway 18, and the pilot stated that he "set up for a short field landing in order to make the first taxiway on the right." He said that just before getting to the threshold of the runway, the wind "either quit or abruptly reduced" and the plane started to "sink" rapidly. The pilot thought he had cleared the runway threshold lights and added "a little" back pressure on the flight control yolk. The airplane's left horizontal stabilizer struck a runway threshold light resulting in substantial damage. The pilot indicated that there were no airplane mechanical malfunctions with the airplane during the flight.