David Learmount/LONDON Guy Norris/Los Angeles
Operators of the Boeing MD-80 series, MD-90s and 717-200s and the McDonnell Douglas (MDC) DC-9 series are making urgent examinations of stabiliser jackscrews and other elements of the pitch-control system.
The checks follow preliminary inspections of the wreckage of the Alaska Airlines MD-83, which crashed into the Pacific on 31 January, that revealed pre-crash damage to the unit.
A section of the Alaska MD-83's stabiliser jackscrew has been recovered from the sea, stripped of thread. If the thread was stripped before impact with the water, says US National Transportation Safety Board (NTSB)chairman Jim Hall, "it is very significant in determining what caused the crash".
The stabiliser system inspections were initially recommended by Boeing on 9 February and almost immediately resulted in the discovery of metal shavings thought to be related to parts of jackscrew assemblies on two MD-80 series aircraft operated by Alaska Airlines.
After the discovery, the US Federal Aviation Administration issued an airworthiness directive (AD), making the inspection compulsory.
The AD, affecting around 1,900 MDC-designed twinjets worldwide - of which 1,100 are in the USA - calls for inspection of the jackscrew, gimbal and gimbal nut used in the pitch-trim actuation system, and instructs operators to report findings immediately.
As Flight International closed for press, about 200 aircraft had been inspected, of which only the two Alaska Airlines aircraft had revealed any concern. Both aircraft were manufactured in 1977 and each showed bronze-like metal shavings. One showed traces in the lubricant, while the other had spiral shavings on the jackscrew. The FAA says both sets of shavings were "consistent with a bronze alloy used for the gimbal nut".
American Airlines is the series' largest operator, with 284 MD-80/90s. It says that it will take around seven days to complete inspections.
The NTSB has released more evidence of the circumstances leading up to the crash, showing that, when the crew lost control of the Alaska Airlines MD-83, the aircraft pitched from 2.5° nose up to 59° nose down in 3s, exerting a negative 3g acceleration.
A full nose-down pitch runaway by the aircraft's horizontal stabiliser was confirmed as the accident's primary cause, although, at first, it had been controllable with opposite elevator, the NTSB confirms.
From 17,900ft (5,500m), after the final rapid pitch-over, the aircraft dived into the sea off Port Hueneme, California, in "just over a minute", nose down 70°, inverted and spiralling, says Hall.
About 12min before the crash, the aircraft was cruising straight and level at 31,000ft, with the autopilot engaged. On disengagement, says Hall, the stabiliser trim moved to full nose-down pitch in 6s and the aircraft began to descend at 7,000ft/min (35.6m/s). By 24,000ft, however, "the crew had regained what could be characterised as controlled flight", reports Hall.
During the next 9 min, the aircraft was in a controlled descent to about 18,000ft, where the crew deployed the leading-edge slats, then the flaps, for "about 30s" to test controllability. Control was retained and slats and flaps retracted. The stabiliser was still fully pitch-down, counteracted by 12° pitch-up elevator, which, Hall reports, is only 50% of the available elevator travel.
"At this time, the flaps began to extend to 11° and, 3s after the start of the flap movement, the slats began to deploy," says Hall.
He says the flight data recorder shows the aircraft pitching down at a maximum rate of 26°/s as it entered its final dive into the Pacific "about 4s after the beginning of the flap/slat deployment".
Source: Flight International
