guy norris / los angeles
Two more failure scenarios are to be checked before US accident report is published
Boeing is negotiating the lease of a 767 as a testbed for two final mechanical failure scenarios that the US National Transportation Safety Board (NTSB) requires to be evaluated before completing its report on the crash of Egyptair flight 990 in the Atlantic on 31 October, 1999.
Boeing says that no firm dates for the tests have been fixed because "we are still in the process of locating an aircraft for the systems group to perform the test on". The extended timescale, which is already expected to cause the final report date to slip beyond the original 1 September target, is also linked to the definition of test procedures by the NTSB, US Federal Aviation Administration and the Boeing and Egyptair systems team.
The systems team has investigated 18 failure scenarios that could potentially have caused the 767's elevators to deflect trailing edge down without input from the flightdeck. Boeing says that none of them reproduced the final flight profile indicated by analysis of the onboard data recorder or shore-based radar. Amid mounting tension between Boeing and Egyptair, which reportedly revolves around Egyptian accusations of the manufacturer's slowness to respond to requests for technical information, the US company reported that two further compound potential failure scenarios could also exist.
The second scenario involves testing a potential interaction between a pogo spring (which links the 767 elevator control system's bellcranks to a valve on the power control actuator) and a valve that is jammed in a specific position. Initial studies showed this could force the elevators down.
The tests will form the NTSB's final detailed technical analysis of the investigation. Unless the tests reveal surprising results, the final accident report is expected to point to human, rather than mechanical, causes.