The Silent Killer
The NTSB’s latest safety recommendation targets the dangers of carbon monoxide leaks caused by defective exhaust systems
The accident pilot held a private certificate and was instrument-rated. His logbook was incomplete, but investigators determined that at the time of his last medical examination, about 18 months before the accident, he had a total of 700 flight hours. The passenger held a private certificate. Her current flight times weren’t determined.
All major components of the airplane were accounted for at the accident scene. The rear, fresh-air vents were in the closed position. One front vent was closed while the other was partially opened. The overhead vent knob and the overhead vent were in the open position, and the panel-mounted fan switch was in the high position. The heater control box remained attached to the fuselage and was in the open position. The two forward-floor heat vents were also in the open position.
The engine was removed from the wooded area and examined in a hangar at the airport in Wolfeboro, N.H., under the supervision of the NTSB. When the muffler shroud was removed, investigators saw a large hole in the muffler that measured approximately 4 ½ inches long and one inch wide.
The NTSB’s metallurgist found that the inlet end of the muffler case on the left side was crushed and contained a large crack that extended approximately 2⁄3 of the muffler circumference. Most portions of the crack were darkly discolored, as if they had been exposed to exhaust gases for a period of time. Other areas were shiny, as if they were recently created.
The toxicological testing report from the FAA Toxicology Accident Research Laboratory in Oklahoma City showed that the pilot’s lung fluid had a CO saturation of 43%, while the passenger’s blood was at 69%. To give you an idea of how much CO saturation is too much, generally, a person breathing air with a 0.5% concentration of CO for 30 minutes will obtain a blood-saturation level of 45%.
The maintenance manual published by the airplane’s manufacturer stated, in part, that “the entire exhaust system, including the heat-exchange shroud, muffler, muffler baffles, stacks and all exhaust connections must be rigidly inspected at each annual or 100-hour inspection. The possibility of exhaust system failure increases with use. The system also must be checked carefully before winter operation when the cabin heater will be in use.” The manufacturer explained that if any component can’t be inspected visually, a pressure check or a ground check using a carbon-monoxide detector should be conducted. This involves running the engine at full-static RPM with the cabin heat valves open and taking air samples at several locations within the cabin. The manufacturer says that if the detected CO concentration exceeds 0.005%, the muffler must be replaced. The NTSB determined that the probable cause of the accident was an exhaust gas leak because of inadequate maintenance, which resulted in carbon-monoxide poisoning, and the incapacitation of the pilot.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.