What happened
On August 11, 2001, at approximately 09:00 UTC, a Fokker F100, registration CS-TPB, operated by Portugália, was undergoing pushback from stand A23 at Lisbon Airport. During the maneuver, the towbar disconnected from the nose gear axle, likely due to an incomplete locking of the towbar claw.
Upon being alerted by ground personnel, the pilot applied the aircraft brakes to bring the plane to a complete stop and subsequently engaged the parking brake. During this braking process, the inner cylinder of the left main landing gear fractured, causing the gear to collapse. The aircraft tilted to the left, resting on the shock absorber body, which caused the left wingtip to momentarily contact the ground. There were no fatalities among the 88 people on board, though 3 individuals sustained minor injuries. Passengers were evacuated from the aircraft without further incident.
The investigation
The GPIAAF investigation focused on determining the cause of the fracture in the landing gear's inner cylinder and shock absorber body. The investigation involved technical cooperation with representatives from Fokker Services B.V., Messier Dowty, and the Dutch Transport Safety Board.
Investigators examined maintenance records, including recent 'D' checks and overhaul history, and analyzed flight data recorder parameters. The analysis of acceleration parameters during previous landings, takeoffs, and taxiing showed no evidence of abnormal loads prior to the accident. Physical examinations of the fractured components were conducted to identify the failure mechanism.
Findings
Preliminary analysis of the fracture surfaces suggested that a fatigue fracture initiated at the radial transition section of the inner cylinder. The crack propagated at an angle of approximately 45 degrees through about 60% of the tubular section, followed by a static rupture of the remaining section.
The rotation of the fractured inner cylinder within the shock absorber body caused an explosive failure of the gland nut and the subsequent fracture of the shock absorber body. This was further exacerbated by asymmetric loads applied by the torque link. While a manufacturing defect was not confirmed, the investigation identified a potential stress concentration at the discontinuity of the chrome surface. It was concluded that the heavy braking loads applied during the emergency stop following the towbar disconnection likely introduced adverse stresses into an already pre-fractured inner cylinder.