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.
The incorrect adjustment of the NLG actuator microswitch would not have been readily apparent to pilots of the aircraft, due to the arrangement of the microswitch in series with the NLG overcentre microswitch. As such, all landing gear operation and indications would have appeared normal.
The flat washer installed on the nose gear actuator rod end was significantly different from, and mechanically inferior to, the OEM design intended item. The use of a flat washer on the assembly meant that the locking tang was exposed to considerable sideways loads when the lock nut was being tightened or loosened, as a result of friction between the mating surfaces producing a tendency for the washer to rotate. It is probable that these loads led to the premature failure of the washer tang and the loss of security. The absence of a mechanism for securing locking wire between the lock nut and washer was also anomalous and was further evidence that the use of the installed lock washer was inappropriate.
The lack of adequate security of the lock nut on the actuator rod end would allow the rod end to turn within the hydraulic actuator rod and change the rigged position of the actuator. If the rigging changed to the point where the actuator's internal mechanical locking mechanism was prevented from engaging every time the NLG was lowered, then, combined with external dynamic loads, it would be possible for the NLG to collapse.
Advice from the aircraft manufacturer confirmed that the installation of the incorrect NLG actuator rod-end locking device, combined with a incorrectly adjusted NLG actuator microswitch, could lead to a NLG collapse if the dynamic loads experienced during take-off overcame the overcentre mechanism of the NLG drag brace assembly.
Alternatively, the hydraulic landing gear system power is removed once all three landing gear downlock microswitches are activated. It is also possible that, with the nose landing gear actuator microswitch incorrectly adjusted to the `down and locked' position, hydraulic power was removed from the landing gear system after activation of the two main landing gear down lock microswitches. If this occurred prior to the NLG actuator internal locking devices engaging, the NLG may have been held in the down position by the overcentre mechanism of the drag brace assembly. Had this occurred, then external dynamic loads would be able to collapse the NLG.
From the supplied aircraft documentation, the investigation was unable to determine when the incorrect NLG actuator locking washer was installed or the NLG actuator microswitch was incorrectly adjusted.
As a result of this occurrence, the aircraft operator has advised that they have conducted a fleet wide inspection of all landing gear actuator locking devices to ensure they conform to the actuator manufacturer's specifications.
The aircraft manufacturer has advised that it intends to issue a 'mandatory' Service Bulletin, SB F406-56, which will instruct that only the correct landing gear actuator locking devices are to be fitted. The Service Bulletin will also require that strict compliance with the actuator manufacturer's requirements. In addition, the Nose Landing Gear maintenance requirements will be amended in the aircraft maintenance manual to emphasise the requirements of the actuator and control indication check.
Direction Generale de l'aviation Civile of France
The French Direction Generale de l'aviation Civile (DGAC) have advised the aircraft manufacturer that they intend to mandate the requirements of the aircraft manufacturer's Service Bulletin through the issue of an Airworthiness Directive.
|Date:||22 August 2003||Investigation status:||Completed|
|Time:||0810 hours CST|
|State:||Northern Territory||Occurrence type:||Landing gear/indication|
|Release date:||24 June 2004||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||None|
|Aircraft manufacturer||Reims Aviation S.A.|
|Type of operation||Charter|
|Damage to aircraft||Substantial|
|Departure point||Darwin, NT|
|Departure time||0740 hours CST|
|Role||Class of licence||Hours on type||Hours total|