Boeing 767 N66051 incident near Newark, New Jersey

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.

NTSB analysis

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. Examination of the left MLG revealed two circumferential fractures that divided the assembly into three pieces. Metallurgical examination of the fracture surface revealed intergranular stress corrosion cracking (SCC) emanating from a corrosion pit on the lower inside diameter of the truck beam. Corrosion protection of the truck beam interior surface consists of a three part system: cadmium plating, primer, and CIC. Several areas on the inner surfaces exhibited deterioration of the protective finishes. Examination revealed 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. Several samples of finishes and lubricants were removed from the fractured truck beam and analyzed for chemical composition. Organics residues consistent with BMS 3-11 hydraulic fluid and components of runway de-icing fluids were found in the samples. The failure of the truck beam occurred due to the presence of hydraulic fluid and deicing fluid in the truck beam interior. 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 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.

Text from the NTSB. Open the NTSB docket for ENG09IA018 for the full record.