Investigation Summary
The investigation into Flight 55 revealed a critical intersection between environmental hazards and maintenance oversight. While the Fairchild F-27 was designed to withstand significant loads, the aircraft encountered localized 'mountain wave' turbulence so severe that a search pilot later described it as having 'no positive control whatsoever.' This turbulence acted upon a wing structure already compromised at Wing Station (WS) 197. Post-accident review of the airline's X-ray radiographs showed that fatigue cracks were visible in the structural components near the No. 1 access doors prior to the accident, but maintenance personnel had failed to detect or report them. The sudden structural failure caused the right outer wing and empennage to separate, resulting in a fireball (likely from fuel ignition during breakup) and a steep dive into Foxies Lake.
Final Conclusions
The NTSB determined the probable cause was an in-flight structural failure caused by severe to extreme turbulence that was not forecast. The failure was localized to a section of the right wing weakened by pre-existing fatigue cracks. A contributing factor was the failure of maintenance personnel to detect these cracks during previous radiographic inspections.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Described as an excellent pilot with seven years at Northern Consolidated prior to the merger.
First Officer
Jerry Svengard
Systems & Failure Modes
Right Wing (WS 197)
The primary site of structural failure where fatigue cracks led to the separation of the outer wing panel.
Empennage
The tail section separated from the aircraft during the high-stress breakup sequence.
Radiographic Inspection (X-ray)
Maintenance system used to detect internal cracks; the interpretation of these X-rays failed to identify existing fatigue.
Interesting Facts
- 01The aircraft crashed into Foxies Lake, a frozen marshland near Pedro Bay, Alaska.
- 02Search and recovery efforts were hampered by temperatures of -11°F and winds gusting to 55 mph.
- 03Post-crash X-ray analysis revealed fatigue cracks were present in both wings near Wing Station 197 before the final flight.
- 04The turbulence was caused by high winds 'burbling' over the top of Pedro Mountain, creating a localized mountain wave effect.
- 05The aircraft had been in service since 1959 and had accumulated 17,194 airframe hours.
- 06The flight was a multi-stop service from Anchorage to Dillingham, with scheduled stops at Iliamna, Big Mountain, and King Salmon.
Safety Actions & Advisories
Fleet-wide Fatigue Inspections
The NTSB recommended that all F-27 aircraft with more than 5,000 service hours undergo immediate wing fatigue inspections. This resulted in the discovery of 13 cracks across eight other aircraft.