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Aviation History
2002
2002 - 3665.PDF
AIR TRANSPORT INVESTIGATION AARON KARP / WASHINGTON DC & DAVID LEARMOUNT / LONDON NTSB divided over failsafe issue Contention surrounds a proposed redesign of an MD-83 component following the Alaska Airlines crash in January 2000 A split has developed inside the US National Transportation Safety Board (NTSB) over whether a com ponent with exceptional reliability needs to be failsafe. A crucial pitch-control mecha nism flying in more than 1,800 civil aircraft is not failsafe and should be redesigned, according to some senior NTSB officials conducting a final scrutiny of the draft report on the 31 January 2000 Alaska Airlines Boeing MD-83 crash. Opinion within the NTSB is split, however, on whether a redesign is necessary, and the recommendation from the final hearing is that, "if practical", Boeing should incorporate a failsafe device in the type's horizontal sta biliser pitch control assembly. The MD-83 component that failed is the screw-jack assembly controlling the horizontal stabiliser pitch angle. The autopilot uses the stabiliser to control pitch and pilots use it to trim the aircraft when fly ing manually. The screw-jack mech anism is the sole component that sets and limits stabiliser pitch, and NTSB director of aviation safety John Clark says: "If this component fails, it leads to catastrophic fail ure." The same basic mechanism is used in all McDonnell Douglas DC-9s, Boeing MD-80s and MD- 90s, and the DC-9-based 717. In the accident aircraft, says the NTSB, inadequate lubrication of the screw-jack had worn down and finally stripped the thread from the gimbal nut that travels up or down the screw-jack to alter stabiliser pitch. Aerodynamic forces then jammed the stabiliser fully leading- edge-up, pitching the aircraft nose- down so strongly that elevator forces could not counteract it. Boe ing says if the assembly jams at any point in the normal travel range the aircraft remains controllable, but in the accident it moved out side the normal limits. A factor in the lubrication failure was that the channels in the nut into which lubricating grease is pumped man ually at eight-monthly intervals had become blocked by dried grease, says Boeing. Other NTSB staff argue the com ponent is reliable. Boeing notes that this accident is the only time the assembly's gimbal nut thread has stripped - or that the basic assem bly has failed - causing a serious accident in "more than 100 million flying hours" since the DC-9 was introduced in 1965. "I'm pretty confident we've nailed [the cause] as a lubrication issue," explains NTSB systems group chairman Jeff Guzzetti. Clark, however, argues for a redesign: "If maintenance were perfect there would never be a problem. It's the unknown that's worrisome...if something is going to go wrong, can we mitigate it or prevent it happening?" Early in the investigation, after a maintenance audit, the Federal Aviation Administration fined the carrier $1 million as a result of the highly unsatisfactory findings. The NTSB has also castigated the FAA's maintenance oversight. ENGINE FAILURE GUY NORRIS / LOS ANGELES & ANDREW DOYLE / SINGAPORE Turbine fault suspected as cause of CF6 burst Brisbane, Australia. The crew ret urned to Brisbane immediately and none of the passengers or crew on board the 767-200ER was injured. Part of the left engine's cowling came off and debris damaged the pylon and leading edge of the wing. "Preliminary inspection of the dam aged engine by the manufacturer in Brisbane showed a section of a high- The uncontained failure of a General Electric CF6-80A2 powering an Air New Zealand (ANZ) Boeing 767 last week was the first suspected turbine disc burst on a CF6-80A. The failure of the left engine high-pressure turbine (HPT) occurred on 8 December, as the twinjet was climbing through 11,000ft shortly after take-off from ANZabandons GECAS A320 lease after IAE engine choice Air New Zealand (ANZ) has dropped plans to lease five Airbus A320s from GE Capital Aviation Services (GECAS) after selecting the International Aero Engines (IAE) V2500 turbofan to power its new Airbus fleet. The US lessor was apparently unwilling to supply aircraft with the V2500. The airline last week revealed its selection of the V2500 to power 15 A320-" family aircraft it is acquiring from next September, plus up to 20 more if it exercises purchase rights. Its choice of the engine is linked to an agreement by IAE shareholder Pratt & Whitney to invest in the expansion of the Christchurch Engine Centre - a joint venture between the US manufacturer and ANZ. ANZ has placed firm orders with Airbus for five A320s, and had planned to lease at least five from GECAS. However, due to the airline's desire to equip its fleet with a single engine type, it has switched to International Lease Finance (ILFC) and Lombard Aviation Capital. GECAS sister company GE Aircraft Engines is a 50% shareholder in CFM International, and its entire A320-family order backlog is CFM56-powered. ANZ will lease five from ILFC and five from Lombard. It has options to take 10 more from ILFC. pressure turbine came loose and fed back through the engine, penetrat ing the rear casing," says ANZ. The HPT, which is the focus of the investigation, has been sent for inspection by the Australian Trans port Safety Bureau (ATSB). The air craft is expected to be grounded for up to four weeks pending repairs. GE is developing an inspection programme for more than 420 CF6- 80As following the failure, and the US Federal Aviation Administration is expected to follow with an imme diate airworthiness directive man dating the action. A US Airways- operated CF6-80C2 suffered a similar failure in September 2000, which was the first turbine disc fail ure in the type's 16-year history. Although GE declines to specu late on the root cause it says "we have sent a metallurgist and a safety investigator to help the ATSB which has the suspect parts." The cause of the failure is, however, believed to be a fatigue crack in one of the high- pressure turbine discs. The suspect parts of the failed engine are thought to have been manufactured in the mid-1980s and have around 12,000 cycles. The 767-200ER engine casing and pylon were damaged by the turbine burst www.flightinternational.com FLIGHT INTERNATIONAL 17-30 DECEMBER 2002
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