FACTUAL INFORMATION
History of the flight
At 1100 Western Standard Time on 2 August 2004, a Fairchild Industries Inc. Metro 23 aircraft, registered VH-HWR, departed Perth on a scheduled passenger service to Kalbarri, WA. with two crew and nineteen passengers. Normal trim inputs were made by the pilot in command (PIC) during the departure and initial climb. He reported that at about the time the flaps were retracted, the control forces increased nose upward in the pitch axis.
The PIC reported that he looked at the horizontal-trim indicator and noticed a large deflection, but did not initially relate this to the control problem or identify the indicator deflection as abnormal. Rather, the PIC assumed that the problem related to the flap retraction, and he instructed the copilot to reselect the flaps to the take-off position, but this appeared to have no effect. The PIC did not attempt to switch electrical control of the aircraft's pitch trim system to the co-pilot's control using the pedestal mounted selector switch. He reported that the control forces required to maintain straight and level flight were very high and fatiguing, and he elected to fly the aircraft in this configuration back to Perth Airport.
A subsequent engineering examination revealed that the pilot in command's (left side) control yoke pitch trim switch had been wired incorrectly and that the left side pitch trim system was operating in the reverse sense from normal operation.
Flight data recorder information
The aircraft was fitted with a solid-state flight data recorder (SSFDR). The parameters recorded by the SSFDR included pitch and roll attitude angles, indicated airspeed, pressure altitude, magnetic heading and stabiliser position. Control column position was not recorded on the SSFDR.
This data was compared with the data readouts from the previous flight, and also to the flight following the incident flight. This comparison showed that stabiliser movement during the incident flight differed from that observed during the comparison flights. During the incident flight, following rotation, the stabiliser moved in an aircraft nose-up direction only. In the comparison flights, following climb out, the stabiliser moved in the opposite sense or a nose-down direction.
Aircraft maintenance
Prior to the incident, the aircraft had undergone maintenance for the flight controls being heavy in the roll (aileron) axis. The problem was traced to a binding bearing in the left side control yoke. To access the bearings, it was necessary to remove the control yoke and the control yoke pitch trim switch by de-soldering the switch wiring and removing the switch from the yoke housing. After the control column bearings were replaced, the control yoke was re-installed, and the trim switch wiring was re-soldered to the respective terminals. During this task, the wiring labelling was misread and the trim switch wires were inadvertently transposed, which would result in the trim switch operating in the reverse sense when activated. The trim switch was then re-installed into the control yoke. There were no markings or labels on the control yoke or the trim switch to indicate trim up or down.
During the aircraft maintenance activity, there were a number of different maintenance engineers involved over several shifts. The handover between the shifts was completed through the use of a shift handover book and details of the aircraft's pitch trim system wiring information was not referred to the incoming shift engineers through the handover book.
Aircraft maintenance manual and post maintenance trim switch functional test
The aircraft operator's maintenance worksheets recorded that the task to remove and replace the control yoke bearing was accomplished in accordance with the Fairchild Aircraft Maintenance Manual (FAMM) Section 27-10-10. This section contained maintenance steps to be followed in relation to the removal and refitting of the control yoke and control yoke switches. However, it contained no reference to a following section, 27-40-01, that detailed the removal and installation procedures for the pitch trim control switches. That procedure included the following note in relation to the operational check of the trim switch:
Pushing switch UP moves horizontal stabilizer toward NOSE DOWN direction;
pushing switch DOWN moves stabilizer towards NOSE UP
Civil Aviation Safety Authority requirements
The Australian Civil Aviation Safety Authority (CASA) promulgated specific inspection requirements for flight controls in Civil Aviation Regulation 42G. Those requirements are for the inspection and functional checks of any part of an aircraft flight control system that is assembled, adjusted, repaired, modified or replaced in the course of carrying out maintenance on an aircraft. In these cases, the flight control system must be inspected by the person who carried out the work and additionally by an independent person.
During the maintenance activities to the aircraft prior to the incident, several tasks were performed that required a duplicate inspection in accordance with the CASA requirements. An examination of the aircraft maintenance records indicated that two duplicate inspections were omitted, including one for the left side control yoke wiring reconnection. A review of the aircraft operator's maintenance control and engineering procedures manual indicated that this requirement was not clearly defined. In addition, in this occurrence, engineers reported that they were unsure of when such a procedure was to be employed.
Pre-flight actions by the flight crew
The PIC stated that he had performed the pre-flight cockpit checks while the copilot conducted the aircraft external checks. He stated that he had performed a daily trim check in accordance with the approved flight manual, during which he said he noticed something was 'not quite right'. He stated that one pilot's trim switch activated the sonalert1 aural warning system, while the other remained silent. The aural warning system in this aircraft was known to have activation characteristics that were different from the rest of the operator's aircraft fleet, and this was in his mind when he discussed the issue with the copilot. However, he was then distracted by a baggage loading issue and did not return to the perceived discrepancy prior to take-off.
Previous occurrences
The ATSB investigated a similar previous incident that occurred on 22 March 2004, involving a different operator (see ATSB report BO/200400998) in which the pitch trim switch had been incorrectly re-installed into the control yoke of a Fairchild Industries Inc. Metro 23 aircraft, resulting in the operation of the pitch trim switch in the reverse sense. As a result of that and other similar occurrences, CASA advised the US Federal Administration of the occurrences and published an article titled Nose up, nose down regarding trim switches in the November/December 2004 issue of Flight Safety Australia magazine. The article analysed the cause of those failures and highlighted the importance of maintaining switches and following correct procedures to prevent similar occurrences.