What happened
While on final approach for landing, the crew of a McDonnell Douglas MD-11F selected flaps 50 (full extension). Approximately one minute later, the flight crew experienced a loud bang and a physical shudder throughout the aircraft. The airplane began an uncommanded roll to the left, necessitating nearly full right control wheel input to maintain control. The crew subsequently executed a missed approach, declared an emergency, and diverted to an alternate airport. During the subsequent landing, the aircraft sustained a tailstrike.
The investigation
The investigation determined that the flight control malfunction was caused by the failure of the bolt assemblies securing the left inboard flap outboard hinge to the wing trailing edge. The failure sequence began with the lower outboard nut failing due to stress corrosion cracking. This led to the failure of the lower inboard bolt, which was initiated by fatigue and completed by overload. Following the failure of both lower bolt assemblies, the two upper bolts failed due to overload.
This failure allowed the flap hinge to detach from the wing trailing edge and drop, jamming the left inboard flap in the fully extended position. The separation of the hinge pulled components of the left-hand spoiler system out of position, causing the deployment of spoilers L1, L4, and L5. This uncommanded deployment of three out of five left-hand spoilers caused the aircraft to roll to the left. Additionally, the separation of the hinge damaged hydraulic system no. 3, causing a loss of hydraulic fluid. The depletion of this fluid rendered the number three spoilers on each wing inoperable. Boeing engineers noted that the loss of the R3 spoiler compromised the ability of the right wing flight controls to counter the left roll.
Findings
- The failure was initiated by stress corrosion cracking of the lower outboard nut.
- Fatigue and overload contributed to the subsequent failure of the remaining bolt assemblies.
- The aircraft had previously been used as a manufacturer flight test airplane, though the impact of this usage on the fatigue life of the bolts could not be determined.
- Prior to this event, there were no specific requirements for inspecting these flap hinge bolt assemblies on MD-11 aircraft.