NTSB Factual Report

On September 17, 2009, a Boeing 767-400 experienced a fracture of the left main landing gear (MLG) truck beam while the aircraft was stopped on a taxiway D with parking brake set prior to takeoff at Newark Liberty International Airport (EWR). The aircraft was being operated by Continental Airlines as a passenger flight from Newark, New Jersey to Frankfurt, Germany. None of the occupants were injured and the airplane sustained minor damage. The airplane had accumulated 6,062 cycles and 40,581 hours in about 9 years of service.

The 767 MLG is a conventional, four-wheel, dual-tandem landing gear that has a metering pin orifice shock strut. The gear has four support points: the forward trunnion, the aft trunnion, the drag brace, and the side strut. The shock strut outer cylinder of the MLG assembly transfers operational loads from the truck assembly to the four support points. The assembly consists of a truck beam, axles, wheels and tires, brake rods, and a protective shield. The truck beam is the primary supporting member of the truck assembly. It connects the forward and aft wheel sets on the primary landing gear bogie and pivots on the lower end of the shock strut outer cylinder.

The truck beam assembly was examined by the NTSB Materials Laboratory. The assembly had two circumferential fractures that fractured the assembly into three pieces. The primary fracture was aft of the pivot bushing and a secondary fracture was forward of the pivot bushing. Examination of the fracture surface aft of the pivot bushing revealed intergranular stress corrosion cracking (SCC) emanating from a corrosion pit on the lower inside diameter of the truck beam. The fracture forward of the pivot bushing was consistent with overstress. The truck beam is made from 4340M steel. The interior of the truck beam was coated with cadmium-titanium, epoxy primer, and a corrosion inhibitor compound (CIC). Primer was missing on the truck beam 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. In adjacent regions the primer was peeling or blistering. There were localized regions where the primer was intact but CIC was missing. Corrosion on the steel was observed in areas where the primer was missing. The aft drain hole was plugged by an unidentified substance. Samples of the deteriorated coating from several locations and the substance plugging the drain hole were taken for chemical analysis. After the exam, the truck beam pieces and coating samples were sent to Boeing Research and Technology (BR&T) for further analysis.

Observations by BR&T were consistent with the findings of the Materials Laboratory. Several areas on the inner surfaces that exhibited deterioration of the protective finishes were examined. Damage ranged from paint blisters, to exposed metal substrate, to significant corrosion and pitting. Sections of the primer in the forward section of the truck beam had a soft wrinkled appearance and had delaminated from the substrate.

Adhesion tests of the primer on the inner truck beam surface were performed with and without scribing the coating. Without scribing, little to no primer was removed. Examination of the coating backside indicated delamination occurred between the cadmium plate and the steel substrate.

Eighteen samples of finishes and lubricants were removed from the fractured truck beam and analyzed for chemical composition by Boeing Research & Technology. The samples included: pivot bushing grease, debris from the aft drain hole, solid debris from the forward drain hole, and paint chips, among others. Organics residues consistent with BMS 3-11 hydraulic fluid were found in 17 of the 18 samples, and components of runway de-icing fluids were found in 7 of the 18 samples. The sample taken from the plugged aft drain hole was consistent with a mixture of metal corrosion products, grease, primer, hydraulic fluid, and CIC.

DISCUSSIONS:

The failure of the truck beam occurred due to the presence of hydraulic fluid and deicing fluid in the truck beam interior. Corrosion protection of the truck beam interior surface consists of a three part system: cadmium plating, primer, and CIC. Without corrosion protection, water that enters the interior of the truck beam will corrode the steel substrate. The long term exposure to hydraulic fluid and deicing fluid led to delamination of the CIC and primer from the interior surface leading to corrosion of the cadmium plating. Once the cadmium plating was consumed, corrosion of the steel followed. Liberated corrosion and delamination products plugged the aft drain hole trapping more water inside the truck beam that lead to pitting corrosion on the lower inside diameter of the truck beam aft of the pivot bushing. The pitting corrosion was located in a region of tensile stress. The tensile stress and the presence of water led to intergranular SCC. Once the crack grew to a critical size, a circumferential fracture occurred and separated the aft portion of the truck beam.

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. Examination of samples taken from around the interior of the truck beam exhibited evidence of hydraulic fluid and deicing fluid contamination. The absorption of hydraulic fluid within the epoxy primer and CIC compromised the corrosion protection system. The debris created by this breakdown clogged the drainage holes and allowed condensation to build inside the truck beam. This moisture ultimately led to the corrosion and failure of the truck beam.

NTSB Probable Cause

Failure of Main Landing Gear (MLG) truck beam was due to contamination of the inner surface corrosion protective layers from hydraulic fluid and deice fluid that resulted in corrosion and intergranular stress corrosion cracking.