Event Description
On September 17, 2009, a Boeing 767-400 operated by Continental Airlines was stopped on taxiway D at Newark Liberty International Airport (EWR) with the parking brake set, prior to takeoff. The left main landing gear (MLG) truck beam fractured into three pieces. The aircraft was a passenger flight scheduled from Newark, New Jersey, to Frankfurt, Germany. None of the occupants were injured, and the airplane sustained minor damage.
Aircraft Information
The airplane had accumulated 6,062 cycles and 40,581 hours in approximately 9 years of service. The MLG is a conventional four-wheel, dual-tandem landing gear with a metering pin orifice shock strut. The truck beam is the primary supporting member of the truck assembly, connecting forward and aft wheel sets on the primary landing gear bogie and pivoting on the lower end of the shock strut outer cylinder.
Examination Findings
The truck beam assembly was examined by the NTSB Materials Laboratory. Two circumferential fractures were found, one aft of the pivot bushing (primary) and one forward (secondary). The primary fracture showed intergranular stress corrosion cracking (SCC) emanating from a corrosion pit on the lower inside diameter of the truck beam. The secondary fracture was consistent with overstress. The truck beam is made of 4340M steel and was coated with cadmium-titanium, epoxy primer, and a corrosion inhibitor compound (CIC).
Examination revealed that primer was missing on the inner surface along the bottom and up the sides aft of the pivot bushing, and along the top forward of the pivot bushing, exposing bare metal. Adjacent areas showed peeling or blistering primer. Localized regions had intact primer but missing CIC. Corrosion was observed where primer was missing. The aft drain hole was plugged by an unidentified substance.
Samples were sent to Boeing Research and Technology (BR&T) for further analysis. Observations were consistent with the Materials Laboratory findings. Damage ranged from paint blisters to exposed metal substrate to significant corrosion and pitting. Adhesion tests showed delamination between the cadmium plate and steel substrate.
Chemical analysis of 18 samples from the truck beam found organic residues consistent with BMS 3-11 hydraulic fluid in 17 samples and components of runway de-icing fluids in 7 samples. The substance plugging the aft drain hole was a mixture of metal corrosion products, grease, primer, hydraulic fluid, and CIC.
Conclusions
The left main landing gear truck beam failed as a result of exposure to hydraulic fluid and/or deicing fluid on the truck beam interior surface. The absorption of hydraulic fluid within the epoxy primer and CIC compromised the corrosion protection system. Debris from the breakdown clogged drainage holes, allowing moisture to accumulate inside the truck beam, leading to corrosion and fracture.