Investigation Summary
The accident originated from a fatigue failure of the master connecting rod in the front bank of cylinders of the Pratt & Whitney R-2800-CB3 engine. Metallurgical analysis identified corrosion pits on the rod head as the origin of the fatigue cracks. When the rod failed, it compromised the engine crankcase, leading to an immediate and severe fire. Cockpit Voice Recorder (CVR) data revealed that the crew identified the fire at 14:55:13 but reversed the critical steps of the emergency checklist. Instead of feathering the propeller first to stop engine rotation and preserve governor oil pressure, they pulled the firewall selector (fire handle) and discharged the extinguishing agent while the propeller was still windmilling. This action cut off the oil required to move the propeller blades into the feathered position. The resulting high-RPM windmilling propeller and continued rotation fed the fire, which eventually burned through the wing spar and skin. Witnesses observed the right wing fold upward as the aircraft attempted a turn from base to final, leading to a total loss of control.
Final Conclusions
The National Transportation Safety Board determined the probable causes were the fatigue failure of the master connecting rod due to corrosion pitting, which compromised the engine crankcase and started a fire, and the flight crew's failure to follow emergency procedures by pulling the fire handle before feathering the propeller, which diminished the effectiveness of the fire suppression system. The investigation was concluded with the issuance of the Final Report on October 14, 1997.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Held DC-6 and DC-7 type ratings. Required to wear corrective lenses.
First Officer
Held a commercial pilot certificate and flight engineer certificate; not type rated in the DC-6.
Flight Engineer
Held a commercial pilot certificate, flight engineer certificate, and A&P mechanic certificate. Required to wear corrective lenses.
Systems & Failure Modes
Master Connecting Rod
A central component in radial engines that connects the pistons to the crankshaft; its failure in this case punctured the crankcase.
Firewall Selector (Fire Handle)
A control that shuts off fuel, hydraulic fluid, and oil to the engine at the firewall to isolate a fire.
Propeller Feathering System
A system that turns propeller blades edge-on to the airflow to stop rotation; it requires engine oil pressure to function correctly.
Interesting Facts
- 01The number 3 engine master rod failed due to fatigue cracking originating from corrosion pits.
- 02The crew reversed the emergency checklist by pulling the fire handle before feathering the propeller.
- 03Pulling the fire handle cuts off the oil supply to the propeller governor, making feathering difficult or impossible at high RPM.
- 04The aircraft was approximately 3/4 of a mile short of runway 35 when the right wing buckled upward.
- 05The wreckage was located on a small island in the Yukon River.
- 06The fourth occupant on board was a jump seat rider who was a bush pilot for Grant Aviation.
Safety Actions & Advisories
Emergency Checklist Compliance
The investigation emphasized the critical importance of following the established sequence in emergency checklists. In the DC-6, feathering the propeller must occur before pulling the fire handle to ensure oil pressure is available to move the blades, a procedure the crew reversed, leading to the uncontrollable fire.
Location
NO VERIFIED COORDINATES · Coordinates are the estimate published in the ntsb final report. No map is shown, because a marker here would imply a precision the record does not have.
Reported location: Russian Mission, Alaska, United States