Tuesday, April 22, 2008

NTSB cites metal fatigue in US Airways Boeing 757 wing panel incident

US Airways Boeing 757-200Remember the incident last month, in which a US Airways Boeing 757-200 aircraft lost a panel from the upper surface of its left wing while in flight? To briefly review, on March 22, 2008, a 4 foot by 5 foot wing panel separated from the aircraft during the cruise phase of a scheduled passenger flight between Orlando and Philadelphia (US Airways Flight USA1250). Upon separation, the wing panel struck and cracked the outer layer of a window in the passenger cabin of aircraft. No one was injured, and the damaged aircraft landed safely at its destination. Today the U.S. National Transportation Safety Board (NTSB), which has been investigating the incident, issued an update on the case, noting that several fasteners used to attach the panel to the wing had failed due to metal fatigue.

The NTSB update provides the following factual information:
Although the separated wing panel has not been located, investigators have been able to examine the remaining components of the assembly to determine why the panel fastening system failed. Initial examination of these components revealed that two of the three clips that secured the leading edge of the panel to the wing had failed due to metal fatigue sometime prior to the incident flight. The remaining clip failed during flight 1250, causing the panel to separate from the aircraft.

After problems with the 757 wing panel fastening system were identified in the late 1980s, the Federal Aviation Administration issued an Airworthiness Directive (AD)
requiring operators of 757s to install a redesigned fastening system. Eastern Airlines, which ceased operations in 1991, operated the 757 involved in this incident at the time the AD was issued in 1991, and had installed the redesigned system. It was these redesigned fasteners that failed on flight 1250.

Since the incident on March 22, US Airways reported to the Safety Board that it had inspected the wing panels on all of its 757s and found problems with wing panel fasteners on several other aircraft, which were since repaired and returned to service.
The NTSB states that it is continuing "to evaluate the design,installation, inspection and maintenance of the failed components to determine the cause of the failure and the impact on the 757 fleet."

Another interesting notation in the NTSB update says that the Board has reclassified this event from an accident to an incident after finding that there was no substantial change to the aircraft's handling characteristics after the panel separated from the wing. This finding was based on data extracted from the aircraft's flight data recorder (FDR), and statements from the flight crew.