de Havilland Canada DHC-8-201, VH-TQG

Local safety action

As a result of this occurrence, on 2 February 2002, the operator
issued Flight Operations Standing Order 132/01 requiring flight
crews to conduct the ORIGINATING CHECKLIST prior to flight
following any engineering actions.

Maintenance personnel did not ensure the return to service of
the main landing gear system due to task interruption.
The flight crew did not confirm the main landing gear inhibit
switch position to prepare the aircraft for flight following
maintenance.

While maintenance personnel were completing their checks of the
aircraft following maintenance, the flight crew interrupted the
task in order to expedite the flight. That resulted in the position
of the main landing gear inhibit switch not being verified by
maintenance personnel.

When the flight crew prepared the aircraft for flight, they did
not confirm the position of the main landing gear inhibit
switch.

When the flight crew selected the landing gear to the down
position (extended), the landing gear inhibit switch was in the
INHIBIT position, thereby preventing normal extension. No caution
advisories were illuminated. Had they been illuminated, the crew
would have been directed to the ALTERNATE LANDING GEAR EXTENSION/
LANDING GEAR MALFUNCTION checklist and that would have led them to
check the inhibit switch for position.

The crew was aware that on the previous flight, the aircraft had
sustained a low-pressure indication of the number-2 hydraulic
system. As the extension of the landing gear was dependent on the
number-2 hydraulic system being operational, the flight crew
possibly considered the gear problem was related to the previous
number-2 hydraulic system pressure anomaly and more serious in
nature. Upon arriving overhead Lord Howe Island, the flight crew
had limited time to troubleshoot the landing gear problem as the
aircraft fuel reserves would have been minimal, and any delays in
landing would have compromised fuel reserves required for a safe
return to the mainland.

The DHC-8-201 (Dash 8) aircraft was being operated on a
scheduled passenger service from Sydney to Lord Howe Island.
Overhead Lord Howe Island, when the flight crew was preparing for
landing, the main landing gear failed to extend following normal
selection. The crew then broadcast a PAN (radio code indicating
uncertainty or alert), notified air traffic services (ATS) of a
main landing gear problem, and requested a diversion to Port
Macquarie. Radio transmissions between the aircraft and ATS were
intermittent and radio relays from other aircraft in the area were
employed. The request for diversion was granted and the aircraft
tracked, first for Coffs Harbour, then Port Macquarie while the
crew reassessed their fuel reserves. Enroute to Port Macquarie, the
flight crew estimated that there was sufficient fuel on board for a
diversion to Sydney, so they requested and received a clearance to
track direct to Sydney.

Enroute to Sydney, when within very high frequency radio range,
the flight crew contacted the operator on the company radio
frequency and attempted to troubleshoot the main landing gear
anomaly. Maintenance personnel suggested a check of the position of
the landing gear inhibit switch. The switch was found to be in the
INHIBIT position, rendering the gear unable to extend. The flight
crew repositioned the switch to the NORMAL position and normal
landing gear operation resumed. The aircraft continued to Sydney
and completed an uneventful landing.

Prior to the flight, maintenance personnel replaced contact pins
on the main hydraulic pressure transmitter connector to resolve a
number-2 hydraulic system low pressure indication. During that
maintenance, the landing gear inhibit switch had been placed in the
INHIBIT position. Following maintenance, the maintenance engineers
completed a hydraulic system operational check to return the
aircraft to flight status. The check did not include a cycling of
the main landing gear system, nor was there a requirement to do so.
The number-2 hydraulic system retracted and extended the main
landing gear.

Landing gear inhibit switch

The two-position (NORMAL and INHIBIT) landing gear inhibit
switch was guarded (plastic cover to confirm position) to the
NORMAL (OFF) position. The INHIBIT position provided an open
electrical circuit to the landing gear down solenoid of the gear
selector valve, preventing normal operation of the gear and also
preventing illumination of the LDG GEAR INOP caution advisory
light. Selecting the landing gear inhibit switch to the INHIBIT
position idled the normal landing gear extension system actuators
to ensure unhindered operation during alternate extension.
Alternate extension of the landing gear used the freefall
characteristics of the landing gear, and was used for emergency
extension of the gear. The landing gear inhibit switch was also
selected in flight crew training to provide the crew with realistic
practice in using the alternate landing gear extension system.

Aircraft return to service and flight crew
checklists

The operator reported that the maintenance procedures for
returning the aircraft to service after maintenance included a
requirement to check the landing gear inhibit switch position. They
reported that the post-maintenance checks were not thoroughly
completed because the maintenance personnel were interrupted by the
flight crew during the task.

The operator checklists for the aircraft type were required to
be conducted using the challenge and response method. The aircraft
manufacturer's NORMAL PROCEDURES checklist, required that the
landing gear inhibit switch was checked for selection to the NORMAL
position. The operator's Civil Aviation Safety Authority approved
ORIGINATING BEFORE START/ BEFORE START and NORMAL (originating)
checklists included a check to confirm that the landing gear
inhibit switch was in the NORMAL position. The BEFORE START
checklist used by the crew, did not have such a requirement. The
operator required that the ORIGINATING BEFORE START/ BEFORE START
checklist be actioned following maintenance, other than ramp
activity, of the aircraft. Under existing requirements, the
ORIGINATING BEFORE START/ BEFORE START checklist was not required
to be completed by the flight crew prior to takeoff.

The manufacturers ALTERNATE LANDING GEAR EXTENSION/ LANDING GEAR
MALFUNCTION (with illumination of landing gear inoperative caution
light or loss of number-2 hydraulic system pressure) checklist
included a check of the landing gear inhibit switch for the INHIBIT
position. That checklist was not actioned as the landing gear
inoperative caution light did not illuminate, nor was there a loss
of number-2 hydraulic system pressure indicated during the flight.
The operator reported that the crew decision to not use the
alternate landing gear extension system once overhead Lord Howe
Island was based on operational considerations.