Accident
The accident occurred ten hours into the regular service between Taipei and Los Angeles. The Boeing 747SP-09 was 350 miles (550 km) northwest of San Francisco, cruising at an altitude of 41,000 ft (12,500 m). The flight crew consisted of captain Min-Yuan Ho, first officer Ju Yu Chang, flight engineer Kuo-Win Pei, relief captain Chien-Yuan Liao, and relief flight engineer Shih Lung Suwere. The sequence began when the No. 4 starboard outermost engine stalled at a low thrust setting and flamed out. That engine had failed twice during previous flights (both while cruising at FL 410 & 430, respectively). In each of those cases, the engine was restarted, after descending to a lower altitude. The maintenance response to the logbook entries that noted the problems included engine inspection, fuel filter drainage and replacement, vane controller inspection and replacement, water drainage from Mach probes, insufficient modules, and other filter replacements. None of those acts fixed the recurrent stalling and flameout problem of the No. 4 engine.
After the flameout, the captain instructed the flight engineer to attempt to restart the engine, while the plane remained at FL 410 (41,000 feet), with the autopilot still engaged and the Bleed air on. That was contrary to the flight manual procedure, which required the plane to be below 30,000 feet (9,100 m), before attempting to restart a flamed out engine. The restart attempt failed.
The airspeed continued to decrease, while the autopilot rolled the control wheel to the maximum left limit of 23 degrees. As the speed decreased even further, the plane began to roll to the right, even though the AP was maintaining the maximum left roll limit of 23 degrees. By the time the captain disconnected the autopilot, the plane had rolled over 60 degrees to the right and the nose had begun to drop. Aileron control was the only means available to the autopilot to keep the wings level as the autopilot does not connect to the rudder during normal cruise flight. To counteract the asymmetrical forces created by the loss of thrust from the No. 4 engine, it was essential for the pilot to manually push on the left rudder. However, the captain failed to use any rudder inputs at all, before or after disconnecting the AP. The resulting uncontrolled flight path is depicted in the pictured diagram.
As the plane descended through clouds, the captain's attention was drawn to the artificial horizon which displayed excessive bank and pitch. Because such an attitude is highly irregular, the crew incorrectly assumed the indicators to be faulty. Without any visual references (due to the clouds around the aircraft) and having discarded the information from the ADIs, the crew became spatially disoriented. The plane entered a steep dive at a high bank angle. Altitude decreased 10,000 ft (3,000 m) within only 20 seconds, a vertical descent averaging 30,000 feet per minute (150 m/s). The crew and passengers experienced g-forces reaching as much as 5g.
Only after breaking through the bottom of the clouds at 11,000 feet (3,400 m) could the captain orient himself and bring the plane under control, leveling out at 9,600 feet (2,900 m). They had descended 30,000 ft (9,100 m) in under two and a half minutes. The flight crew were under the impression that all four engines had flamed out, but the National Transportation Safety Board believes only engine No. 4 had failed. After leveling out, it was found that the three remaining engines (1, 2 & 3 as numbered from left to right looking forward from the cockpit) were supplying normal thrust. A restart attempt brought No. 4 back into use. They began climbing and reported to air traffic control "condition normal now" and they were continuing on to Los Angeles. They then noticed that the inboard main landing gear were down and one of the plane's hydraulic systems was empty. Because they didn't have sufficient fuel to reach Los Angeles with the drag added by the landing gear, they diverted to San Francisco. Learning there were injured people on board, an emergency was declared and they flew straight in to the airport and landed without further incident.
Read more about this topic: China Airlines Flight 006
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