NTSB Identification: DCA02MA042.
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Nonscheduled 14 CFR operation of FEDERAL EXPRESS CORP
Accident occurred Sunday, June 02, 2002 in Subic Bay, Philippines
Probable Cause Approval Date: 12/20/2005
Aircraft: McDonnell Douglas MD-11F, registration: N601FE
Injuries: 2 Uninjured.
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft accident report.
As the MD-11F airplane was on final approach to landing, the flight crew selected flaps 50 (full extension) and continued to approach the runway. About one minute later, the flight crew heard a loud bang and felt the airplane shudder. The airplane then began to roll to the left, and nearly full right control wheel input was required to counter the left roll. The flight crew performed a missed approach, declared an emergency, and diverted to another airport. During the landing, the airplane experienced a tailstrike. The flight control malfunction was initiated by the failure of the bolt assemblies securing the left inboard flap outboard hinge to the wing trailing edge. The lower outboard nut failed first due to stress corrosion cracking. The lower inboard bolt failure was initiated by fatigue and was completed by overload following the failure of the nut on the lower outboard bolt. The two upper bolts failed by overload following the failure of both lower bolt assemblies. The failure of all four bolt assemblies allowed the flap hinge to pull free from the wing trailing edge, drop and jam the left, inboard flap at the fully extended position. The separating flap hinge pulled the components of the left hand spoiler system from their original position resulting in the deployment of spoilers L1, L4, and L5. The deployment of three of the five left-hand spoilers caused the uncommanded left roll of the aircraft. Hydraulic system no. 3 was also damaged when the flap hinge separated allowing the hydraulic fluid to drain out of the system. Once the hydraulic quantity is depleted, the number three spoilers on each wing will not operate. According to Boeing engineers, the loss of the R3 spoiler would affect the ability of the right wing flight controls to counter the left roll of the aircraft. The airplane was originally used as a manufacturer flight test airplane. The impact of the flight-testing of the airplane to the overall fatigue life of the flap hinge bolts could not be determined. Prior to the accident, there were no requirements for the inspection of the flap hinge bolt assemblies on MD-11 aircraft. The manufacturer issued a service bulletin after the accident recommending inspection and/or replacement of the flap hinge, lower bolt assemblies. The bulletin recommends changing from alloy steel bolts and nuts to Inconel. Inconel is a nickel-based alloy that is highly resistant to general corrosion and stress corrosion cracking, which can occur in low alloy, high strength steel. The FAA issued an Airworthiness Directive mandating the Service Bulletin.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The failure of the lower outboard nut and bolt assembly of the left inboard flap's outboard hinge that resulted from stress corrosion cracking and fatigue. Factors contributing to the accident were the inadequate materials specification of the bolt, and inadequate inspection requirements for the assembly. Full narrative available
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