Air Canada A340 Centerline Gear Separation at Frankfurt

Incident Overview and Immediate Response On a flight from Frankfurt to Vancouver, an Air Canada Airbus A340-313 experienced a loud bang in the cabin during rotation. Passengers reported the cabin floor rising, and the flight crew discovered that the landing gear would not retract normally. After a subsequent cycling attempt, a gear position disagreement was indicated. Departure control informed the crew that pieces of the centerline landing gear had separated from the aircraft. The captain ordered the cabin crew to prepare for an emergency landing. The crew used manual gear extension procedures, circled for two hours to burn off and jettison fuel, and landed at Frankfurt without further incident. No injuries were reported, and no additional damage was sustained by the aircraft. ## Technical Description of the Centerline Gear The aircraft was equipped with a centerline landing gear to support an increased maximum certified operating weight of 271,000 kilograms. The gear is a dual-wheel, forward-retracting assembly positioned under the fuselage centerline. It consists of a shock strut, a drag stay with a downlock, and a retraction actuator. The shock strut is a two-stage oleo/pneumatic absorber. Its Stage 1 low-pressure chamber is normally charged to about 700 psig with a hydraulic oil/nitrogen mixture, while the Stage 2 high-pressure chamber is charged to about 2,000 psig with nitrogen. Damping rings control fluid flow, and a floating piston separates the chambers. ## Maintenance and Servicing Procedures The order of servicing the two chambers is critical. The high-pressure chamber must be charged first. If the low-pressure chamber is charged first, both chambers can end up at approximately 2,000 psig, making the strut excessively stiff. Nine days before the incident, the aircraft had a hydraulic failure in Vancouver that required an emergency gear extension. The centerline gear was manually extended on the ground, and its chambers were discharged and recharged. The servicing crew followed the example on the gear leg placard, which listed the charging steps in the wrong order. This placard example invited a reversal of the proper sequence, resulting in an overcharged Stage 1 chamber. ## Contributing Factors and Findings Flight data from the two preceding flights indicated that the centerline gear strut did not compress sufficiently on landing. During the incident, the low-pressure gauge pointer marked 1,700 psig, and the calculated Stage 1 pressure during the shock load exceeded 2,000 psig. Stress analysis showed that the nominal 700 psig pressure was insufficient to fail the gland nut, indicating that the overcharged condition was a key factor. The gland nut itself had a manufacturing defect: an extra groove had been cut into the thread flank due to improper lathe technique, reducing its retention strength. The Daily Gas Pressure Monitoring check only verified that pressures were above minimum values and did not alert technicians to overcharged conditions that remained below the gauge red line. ## Probable Cause The centerline gear separated from the aircraft during takeoff when the weakened gland nut was forced out by excessive gas pressure. Contributing factors included the erroneous example in the placarded servicing instructions, which led to overcharging of the strut, and the manufacturing defect that reduced the gland nut's strength.