Investigation Summary
The aircraft was climbing through 2,000 feet in an area known for severe turbulence generated by the nearby Alaskan Mountains. The flight was a wet-lease operation, where Evergreen International Airlines provided the aircraft and crew to operate on behalf of Japan Airlines. During the climb, the 747 experienced extreme pitch and roll oscillations, including an uncommanded 50-degree left bank. Airspeed fluctuated wildly by as much as 75 knots. During the failure sequence, the No. 2 throttle slammed to its aft stop, the No. 2 reverser indication showed deployment, and the No. 2 engine electrical bus failed. The intense lateral forces caused the number two engine pylon to fail. Investigation revealed that the pylon had a pre-existing two-inch fatigue crack in the forward firewall web, which had reduced its ultimate load-carrying capability by approximately 10 percent. When the turbulence-induced loads hit the weakened structure, the pylon failed, and the entire engine assembly detached from the wing. The crew declared an emergency and performed a wide-radius turn to return to the airport, landing safely approximately 20 minutes after departure without injury to the five occupants.
Final Conclusions
The NTSB determined the probable cause was the lateral separation of the No. 2 engine pylon due to severe or possibly extreme turbulence. This encounter resulted in dynamic multi-axis lateral loadings that exceeded the ultimate lateral load-carrying capability of the pylon. The failure was exacerbated by a pre-existing fatigue crack near the forward end of the pylon's forward firewall web. The NTSB adopted the final report on October 13, 1993.
Photographic Evidence (1)
Video Analysis
Engine Ripped Off Boeing 747 | Japan Airlines 46E — On March 31st, 1993, a Boeing 747 Freighter departed Anchorage, Alaska. But just moments after departure, it suffered a severe windshear and lost an engine.
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Airframe & Maintenance
Crew Experience
Captain
First Officer
Flight Engineer
Systems & Failure Modes
Engine Pylon
The structural component that attaches the engine to the wing. In this case, the pylon failed laterally under extreme turbulence-induced loads.
Forward Firewall Web
A structural part of the pylon where a pre-existing fatigue crack was discovered, weakening the assembly's resistance to lateral forces.
Interesting Facts
- 01The aircraft was a 23-year-old Boeing 747-121 previously operated by Pan Am and TWA.
- 02The number two engine separated completely from the aircraft in flight.
- 03A two-inch fatigue crack was found in the pylon's forward firewall web.
- 04The fatigue crack reduced the pylon's lateral load capability by about 10 percent.
- 05Another Evergreen 747 (Flight 42E) had experienced minor damage from turbulence shortly before Flight 46E took off.
Safety Actions & Advisories
Departure Route Modification
The NTSB recommended that the FAA modify departure routes at Anchorage during periods of moderate or severe turbulence to avoid mountain-generated wind fields.
Pylon Inspections
The NTSB recommended mandatory inspections of Boeing 747 engine pylons for fatigue cracks.
Meteorological Hazard Program
The NTSB urged the development of a meteorological aircraft hazard program for airports near mountainous terrain.