What happened
On 29 April 2014, a Boeing 737-476(SF), registration EI-STD, was performing a commercial cargo flight arriving at East Midlands Airport. The flight, which had originated in Athens and included a stop in Paris, proceeded with a manual landing following an improvement in weather conditions.
During the landing rollout, as the aircraft' and speed decreased through approximately 60 kt, the crew transitioned from autobrake to manual braking. As the commander applied the brake pedal to disengage the autobrake system, the aircraft experienced a sudden shudder and a leftward roll. This was accompanied by a loud bang as the lower portion of the left main landing gear detached from the aircraft. The left main wheels, brakes, and axle assembly separated from the strut, causing the aircraft to veer off the runway centerline. The aircraft eventually came to a halt resting on its right main landing gear, the remains of the left gear, and the lower cowl of the left engine. There were no injuries to the two crew members on board.
The investigation
The AAIB examined the wreckage and the detached landing gear components. Metallurgical analysis of the left main landing gear inner cylinder revealed that the fracture occurred approximately 75 mm above the axle. High-magnification examinations of the chrome plating showed extensive "chicken wire" cracking across much of the surface.
Investigators identified three distinct failure mechanisms at the fracture site: stress corrosion, fatigue, and ductile overload. Further analysis of the steel substrate revealed a specific area of localized heat damage, characterized by the presence of overtempered and undertempered martensite. This thermal damage had compromised the chrome plating, creating a site for corrosion to initiate.
Findings
- The primary cause of the failure was stress corrosion cracking and fatigue propagation within the high-strength steel substrate of the left main landing gear inner cylinder.
- This degradation was initiated by a localized area of heat damage to the chrome plating, which exposed the underlying steel to a corrosive environment.
- The detachment of the axle, wheels, and brakes was triggered by a momentary increase in bending load on the weakened structure during the transition from autobrake to manual braking.
- While the chrome plating exhibited widespread crazing, the specific heat damage that compromised the steel's metallurgy could not be definitively traced to a specific event, though it may have occurred during the overhaul or refinishing process.
Safety action
- The landing gear overhaul company has conducted a comprehensive review of its manufacturing and refinishing processes.
- The airport operator issued a safety bulletin to air traffic controllers regarding emergency communication protocols and initiated a review of internal communication procedures and equipment investment.