Reims F406, ZK-VAF

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.

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.