Switches Fitted In Reverse

After a Fairchild Metroliner III commuter airplane on a flight from Sydney to Taree, New South Wales, landed on March 22, 2004, an examination by the crew revealed that the pilot in command's left side control yoke pitch trim switch was operating in the reverse sense from normal operation, according to the Australian Transport Safety Bureau (ATSB) report. The case recalls that of the Airbus A320 with the cross-wired sidestick and the Beech 1900D with the misrigged elevator control cable (see ASW, March 15, 2004).

According to the ATSB's investigation report, released on March 17, the pilot in command felt that excessive forward control column force was required "to trim the aircraft nose down" during the departure and flight. As a consequence, control was passed to the copilot, whose pitch trim selector was functioning properly, and the flight proceeded to Taree. Two more flights were made in this condition to get the airplane to a suitable maintenance base to correct the problem.

From the report of investigation, these salient details emerge:

• In the days preceding the occurrence, the aircraft underwent scheduled maintenance at a contractor maintenance facility.
• After the control column bearings were replaced ... the trim switch was then reinstalled into the control yoke and the engineers reported that they conducted a full installation and duplicate functional check of the pitch trim system.
• The maintenance contractor and the aircraft operator conducted separate investigations into the trim switch misalignment and concluded that the only plausible scenario leading to the misalignment was that the engineers responsible for the pitch trim switch installation had installed the switch incorrectly.
• The discrepancy had not been detected during the installation and duplicate functional checks or the flight crew's pre-flight checks.
• The aircraft Minimum Equipment List (MEL) provides no relief for flight with one pitch trim system inoperative, and so the decision to continue the scheduled flights in this condition was contrary to the requirements of the operator's flight operations manual.

The maintenance provider has since re-emphasized the maintenance requirements, including the need for duplicate inspections. The operator published an alert to all company pilots reminding them of the need to confirm the correct sense of flight control systems prior to departure. In addition, the operator instigated a formal mechanism for crews to apply MEL conditions when operating from a remote airfield. (The full ATSB report may be viewed at http://www.atsb.gov.au/aviation/occurs/occurs_detail.cfm?ID=613)