Ramon lopez/WASHINGTON DC GuyNorris/LOS ANGELES
US and Egyptian aviation officials are in conflict over the cause of the 31 October 1999 EgyptAir Boeing 767-300ER crash off the US east coast.
While the US National Transportation Safety Board (NTSB) insists that the cause is not yet determined, Egyptian investigators claim there is evidence that there may have been a mechanical control fault of a type which has happened before. Egyptian claims of a defect come as Boeing admits finding a fault in its 767 elevator control tests.
The NTSB presented the facts gathered to date in the accident investigation on 11 August, but drew no conclusions. Chief Egyptian investigator Capt Mohser el-Missiry, however, maintains that a mechanical problem may have caused the crash, discounting speculation of a suicide dive initiated by a flight deck crew member.
EgyptAir says there is nothing on the cockpit voice recorder or the flight data recorder to indicate that EgyptAir Flight 990 was intentionally crashed into the ocean.
Furthermore, "metallurgical analysis...showed that the rivets on two of the three bellcranks in the right elevator were sheared in a direction that would force the elevator down. The rivets on the remaining bellcrank on the right elevator were sheared in the opposite direction. This pattern...is consistent with the possible failure or jam of two power control units that would cause the aircraft to pitch down," says the airline. It says similar problems were discovered on 11 other Boeing 767s.
Meanwhile, Boeing is expected to issue a service bulletin (SB) calling for a one-time special inspection of elevator bellcranks on 767s. Boeing says the inspection, which is likely to be mandated by a US Federal Aviation Administration airworthiness directive "within days", is related to a "ground event and has nothing to do with Egyptair".
Boeing laboratory tests late last month unexpectedly revealed a potentially undetectable failure scenario in which a failed bellcrank remained rigidly in place, instead of rotating freely as designed. "This would mean that, using the inspection procedure, a failure would be not be detected," says Boeing.
Boeing says the SB, and the tests, follow a fleet-wide survey for failed bellcranks which was originally triggered by the discovery of two failed units, believed to have been found during a pre-flight walkaround earlier this year on an Aeromexico aircraft.
Details of the Aeromexico incident came to light in a 4 June letter from the Egyptian Civil Aviation Authority (CAA) to FAA administrator Jane Garvey in which it urged further investigation into the 767's elevator control system. In the letter, the CAA says Egyptian and EgyptAir officials working with the NTSB were "...made aware of several reports of Boeing 767 elevator control discrepancies that happened both before and after the EgyptAir accident".
Source: Flight International
