What happened
During the takeoff roll from Frankfurt, Germany, an Air Canada Airbus A340-313 experienced a loud bang in the cabin as it rotated for lift-off. Passengers reported a sensation of the cabin floor rising. Shortly after reaching 90 feet above the ground, the centerline landing gear assembly—comprising the ram piston, lower torque link, axle, wheels, and tires—separated from the aircraft. The detached components, weighing approximately 630 kilograms, traveled several thousand feet across the airport grounds without causing damage to the facility.
Following the separation, the flight crew attempted to retract the remaining landing gear, but encountered a position disagreement indication. After being notified by departure control that gear components had fallen from the aircraft, the captain initiated emergency procedures. The crew performed a manual gear extension and circled for two hours to jettison and burn off fuel before successfully landing the aircraft in Frankfurt. There were no injuries and no additional damage to the aircraft was reported.
The investigation
The investigation focused on the mechanical failure of the centerline gear and the pressures within the shock strut. Investigators examined the shock strut, a two-stage oleo/pneumatic system designed to absorb energy through the movement of hydraulic fluid and nitrogen gas. The investigation reviewed maintenance records from nine days prior, when a hydraulic failure had required the gear to be discharged and recharged.
Technical analysis of the digital flight data recorder revealed that the gear strut had not compressed sufficiently during previous landings, suggesting an abnormal internal state. Investigators also examined the manufacturing process of the gland nut, which holds the seal carrier in place. Furthermore, the investigation scrutinized the aircraft maintenance manual (AMM) and the specific instructions provided on the gear leg placards used by servicing technicians.
Findings
- The primary cause of the separation was the failure of the gland nut, which was forced out by excessive internal gas pressure.
- The gland nut possessed a manufacturing defect; an improper lathe thread-cutting technique had created an extra groove in the thread flank, significantly reducing its retention strength.
- The shock strut was overcharged because the servicing instructions on the gear leg contained an error in their example, which suggested a reversed charging order for the high- and low-pressure chambers.
- Reversing the charging order caused the Stage 1 low-pressure chamber to reach approximately 2,000 psig, far exceeding its nominal 700 psig requirement.
- The existing Daily Gas Pressure Monitoring check was insufficient, as it only required technicians to ensure pressures were above a minimum threshold and did not alert them to an over-pressure condition.
- The scaling of the low-pressure gauge was misleading, as the needle position for an overcharged state did not appear suspicious to the crew.
Safety action
- Messier-Dowty inspected all 59 installed gland nuts and implemented 100 percent inspection of thread profiles for all future production.
- The Aircraft Maintenance Manual procedures for centerline gear servicing were revised for clarity.
- The Daily Gas Pressure Monitoring Check was modified to include an over-pressure check.
- New gauges and placards were developed to prevent confusion regarding pressure readings.
- A design modification was implemented to manufacture the gland nut from steel rather than aluminum to increase strength.