On March 31, 1993, the No. 2 engine and engine pylon separated from Japan Airlines, Inc. flight 46E, a Boeing 747-121, that had been wet-leased from Evergreen International Airlines, Inc., shortly after departure from Anchorage International Airport, Anchorage, Alaska. The accident occurred about 1234 Alaska standard time. The flight was a scheduled cargo flight from Anchorage to Chicago-O'Hare International Airport, Chicago, Illinois. On board the airplane were the flightcrew, consisting of the captain, the first officer, and the second officer, and two nonrevenue company employees. The airplane was substantially damaged during the separation of the engine. No one on board the airplane or on the ground was injured.
Flight 46E departed Anchorage about 1224 local time. The flight releaselweather package provided to the pilots by Evergreen operations contained a forecast for severe turbulence and indicated that severe turbulence was reported by other large airplanes. As flight 46E taxied onto the runway to await its takeoff clearance, the local controller informed the flightcrew that the pilot of another Evergreen B-747 reported severe turbulence at 2,500 feet while climbing out from runway 6R.
After takeoff, at an altitude of about 2,000 feet, the airplane experienced an uncommanded left bank of approximately 50 degrees. While the desired air speed was 183 knots, the air speed fluctuated about 75 knots from a high of 245 knots to a low of 170 knots. Shortly thereafter, the flightcrew reported a "huge" yaw, the No. 2 throttle slammed to its aft stop, the No. 2 reverser indication showed thrust reverser deployment, and the No. 2 engine electrical bus failed.
Several witnesses on the ground reported that the airplane experienced several severe pitch and roll oscillations before the engine separated. Shortly after the engine separated from the airplane, the flightcrew declared an emergency, and the captain initiated a large radius turn to the left to return and land on runway 6R. The No. 1 engine was maintained at emergency/maximum power. While on the downwind portion of the landing pattern, bank angles momentarily exceeded 40 degrees, alternating with wings level. About 1245, flight 46E advised the tower that they were on the runway.
We determined that the probable cause of this accident was the lateral separation of the No. 2 engine pylon due to an encounter with severe or possibly extreme turbulence that resulted in dynamic multi- axis lateral loadings that exceeded the ultimate lateral load-carrying capability of the pylon, which was already reduced by the presence of the fatigue crack near the forward end of the pylon's forward firewall web.
As a result of this investigation, the National Transportation Safety Board makes the following new safety recommendation.
To the Federal Aviation Administration:
- Develop and implement a meteorological program to observe, document, and analyze potential meteorological aircraft hazards in the area of Anchorage International Airport, Anchorage, Alaska, with an emphasis on the approach and departure paths of the airport. (A-93-136)
- Amend the design load requirements of 14 CFR Part 25 to consider multiple axis loads encountered during severe turbulence. (A-93-137)
- Require the Boeing-proposed B-747 engine pylon structural modification program to include increasing the lateral-load capability of the pylon structure. (A-93- 138)
- Require any future structural modifications of existing engine pylons to consider multiple axis loading. (A-93- 1 39)
- Issue an Airworthiness Directive to require compliance with Boeing Service Bulletin 747-54-2160. (A-93- 140)
- Consider the necessity and feasibility of requiring the modification of the aircraft departure routes at Anchorage International Airport during periods of moderate or severe turbulence to minimize the potential of aircraft encountering mountain-induced low level turbulence. (A-93-141)
To the National Weather Service:
- Use the WSR-88D doppler weather radar system at Anchorage, Alaska, to document mountain-generated wind fields in the Anchorage area and to develop in greater detail low altitude turbulence forecasts. (A-93-142)
Previously Issued Recommendations Reiterated in This Report
To the Federal Aviation Administration:
Develop a broader meteorological aircraft hazard program to include other airports in or near mountainous terrain, based on the results obtained in the Colorado Springs, Colorado, area. (A-92-58
