Synopsis
On 11 August 2001, at approximately 09:00 UTC, a Fokker F-100 aircraft, registration CS-TPB, operated by Portugália for flight NI 604, was being pushed back from parking position A23 at Lisbon Airport. After the aircraft had traveled about 30 meters, the towbar disconnected from the nose gear, presumably due to incomplete locking of the towbar head. Ground personnel alerted the captain, who applied the brakes until the aircraft came to a complete stop and then set the parking brake. Immediately afterward, the inner cylinder of the left main landing gear fractured, causing the gear to collapse. The left wing tip momentarily contacted the ground. The captain ordered the evacuation of the 82 passengers, which proceeded normally. No injuries were reported among the 6 crew and 82 passengers; all were uninjured.
Factual Information
The aircraft involved was a Fokker F-100, manufactured in 1989, with serial number 11262. It had accumulated 23,943 hours and 22,739 cycles. The left main landing gear, part number 201072011, serial number DRG 4272/88, had 22,247 cycles since new and 3,069 cycles since its last overhaul. Both main landing gears had approximately 3,000 cycles since overhaul and 30 cycles since the last D-check. All maintenance inspections complied with the approved maintenance program. The flight data recorder was read and showed normal acceleration parameters during landing, taxi, and takeoff phases, indicating no abnormal loads. The cockpit voice recorder was not considered relevant.
The towbar detachment was the initiating event. Examination of the fractured inner cylinder revealed a fatigue fracture that originated at the run-out of the radial transition section between the inner cylinder and the axle attachment base, propagating at approximately 45 degrees until it reached about 60% of the tubular section, after which static failure occurred. The subsequent rotation of the fractured inner cylinder within the shock strut caused explosive failure of the gland nut, fracture of the shock strut housing, and probable deformation of the remaining upper portion of the inner cylinder. The fracture of the shock strut housing in the area of the torque link attachment lugs was caused by asymmetric loading after the gear separated.
Analysis and Conclusions
The investigation was conducted by the Portuguese aviation authority (INAC) with cooperation from the Dutch Transport Safety Board, Fokker Services, and Messier-Dowty. A preliminary hypothesis of fatigue fracture was confirmed, but the exact cause could not be determined due to corrosion and smearing of the fracture surfaces, typical of compression fatigue. The number of cycles from pre-crack to catastrophic failure was not established; it was only noted that development took “some time” but not how long. The possibility of a manufacturing defect was excluded, and the investigation focused on fatigue concentration and the nature of the loads. It was admitted that braking loads following the towbar detachment could impose adverse stresses on already pre-cracked inner cylinders. However, the investigation remained inconclusive regarding the root cause of the inner cylinder fracture.
Recommendations
The investigation issued two final recommendations:
- Perform magnetic particle inspection of the inner cylinder according to Fokker F100-32-133 and Messier-Dowty F100-32-103 service bulletins and subsequent revisions.
- Modify the towbar locking system to prevent accidental release.
A preliminary recommendation to visually inspect inner cylinders of landing gears with cycles within 25% of the accident gear had already been carried out; no cracks were found.