On 22 August 2003, a Reims F406 aircraft, registered ZK-VAF, was being operated on a passenger charter flight from Darwin to Tindal, NT. At approximately 85-90 kts during the take-off roll, the nose landing gear (NLG) collapsed. The aircraft slid to a stop, the pilot shutdown the engines and all occupants evacuated the aircraft uninjured. Prior to this occurrence, on 2 and 19 June 2003, pilots reported difficulties obtaining a `down and locked' indication for the NLG. Maintenance actions rectified the problems at that time.
An examination of the aircraft following the NLG collapse revealed that no damage was evident to any NLG components, or the NLG attachment structure. The NLG rigging was checked and reported to be within tolerances. Damage to the aircraft included abrasion damage to the lower forward fuselage and NLG doors. Both propellers were substantially damaged from ground contact.
The NLG hydraulic actuator was removed from the aircraft for further examination by the ATSB and was taken to a specialist hydraulic facility for functional testing prior to disassembly. The actuator passed all required functional tests, however, it was noted that the integral microswitch had been incorrectly adjusted to the point that it did not obtain switchover during operation of the hydraulic actuator. The microswitch was effectively always providing a signal indicating that the actuator was `down and locked'. However, as the actuator microswitch was wired in series with the NLG overcentre microswitch on the aircraft, the landing gear indications would have appeared normal. The aircraft landing gear hydraulic system was powered during landing gear extension, however hydraulic power was removed once all three landing gear downlock microswitches were activated.
Disassembly of the NLG actuator revealed that all internal components were in good condition, with only minor wear evident. The actuator rod-end was noted to have an incorrect locking washer fitted. A detailed examination of the actuator components revealed that the installed locking washer did not conform to the part no. NAS 559-1 locking device specified by the actuator Original Equipment Manufacturer (OEM). Refer to Appendix A, Figure 1.
Comparison against Original Equipment Manufacturer component
An OEM locking device was obtained and compared against the installed washer. The OEM item was a key-like component and utilised a completely different mechanism for securing the assembly from the installed washer. The installed washer was placed between the rod-end and the lock nut and had a small tang that fitted into the rod-end shank keyway, but was not lockwired. The OEM item fitted into the keyway completely, lying underneath the lock nut and engaged with the slotted end of the actuator rod when the lock nut was tightened. The OEM item also provided for the installation of a locking wire between the drilled rod-end lock nut and the locking tab. Refer to Appendix A, Figure 2.
Damage to the installed washer
The washer that was installed to the rod-end assembly showed clear evidence of rotation against the underside of the lock nut and the actuator rod-end face. Two sides of the washer had been bent in opposing directions, against the respective flats of the nut and rod-end. The bent areas showed damage consistent with repeated manipulation and re-bending of the `tabs'. The small locking tang on the internal diameter of the washer had fractured, allowing the washer to freely rotate on the threaded rod-end shank. The fractured key tang was recovered from the rod-end keyway and cleaned to allow stereomicroscopic examination, which showed that the tang had broken away from the washer under sideways bending overload, such as would be produced by forces acting to twist the washer around the rod-end shank.