NTSB Identification: SEA04LA050.
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Accident occurred Sunday, February 29, 2004 in Herron, WA
Probable Cause Approval Date: 07/07/2005
Aircraft: Cessna 170, registration: N2534V
Injuries: 2 Fatal.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The airplane was en route when according to the last nine minutes of radar data, the airplane descended from 3,400 to 300 feet. Sixteen days after the aircraft was reported missing Search and Rescue personnel located the airplane in a water inlet at a depth of 132 feet; 19 days later the aircraft was recovered from the accident site. A postaccident examination of the engine and the airframe revealed that the left exhaust muffler had separated at the forward seam, with evidence of sooting noted on the scat hoses surrounding the area. It was also observed that the forward end cap and exhaust riser were separated from the main body of the muffler. Magnified examinations found that almost all of the fractures were blackened and severely oxidized. The failure of the muffler was the result of undetected oxidation and cracking resulting from normal thermal degradation. The cracking would have allowed exhaust gases direct access to cabin air through the heater system. Toxicological testing of the pilot and his passenger revealed carbon monoxide levels of 45 percent and 46 percent, respectively.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The failure of the left muffler while maneuvering, resulting in a carbon monoxide leak into the cabin rendering the pilot incapacitated, and the subsequent uncontrolled descent and impact with water.

Full narrative available

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