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

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 take-off.

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.

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.

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.

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.