Primary finding
Probable cause
The in-flight fatigue failure of an air-conditioning system hose which resulted in a precautionary landing and impact with a road sign.
Investigator assessment
Analysis narrative
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.