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.
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 takeoff.
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.
The control difficulties experienced by the crew shortly after departure could be attributed directly to the horizontal trim system operating in the reverse travel sense to that commanded by the pilot in command’s (PIC) inputs to the horizontal stabilizer trim switches.
The investigation determined that systemic failures present during maintenance allowed the aircraft to be returned to service with a horizontal stabiliser trim system that operated in the reverse sense. Further, this incorrect flight control function was not detected during the pre-flight inspection by the flight crew.
A task that was maintenance intensive and/or extended over several shift periods involving numerous personnel required careful management in the co-ordination of effort to ensure every requirement was addressed to safely return the aircraft to an airworthy condition. In this incident there was a breakdown in this defence through absent or poorly defined handover procedures, documentation and co-ordination of the maintenance.
Disturbance of a flight control system during maintenance triggers the requirement for an additional layer of defence in the duplicate inspection procedure. In this incident the engineers were unsure of when the procedure was to be employed and this lead to a breakdown of the defence. A clearly defined procedure in the company maintenance control manual for invoking the duplicate inspection would have ensured a duplicate inspection was prescribed, which in turn should have identified the trim reversal prior to the aircraft’s release to service.
The aircraft provided the crew with an aural alert system with a known difference from its fleet siblings. The perception by the pilot in command (PIC) of the aircraft being inherently different, combined with a loading distraction at the critical trim function check time in the pre-flight sequence, probably led to a misinterpretation by the PIC of his response to the anomaly.
Once airborne and with the emergency in progress, the PIC established that he had control, but neglected to consider selection of the trim system control to the copilot’s control wheel as an option. This may have been as a result of his decision not to manipulate the trim system any further due to possible mechanical failure.
- The wiring for the pilot’s control wheel horizontal stabiliser trim switches was reassembled in the reverse sense.
- The maintenance manual post-maintenance functional test requirements for the horizontal stabiliser trim switches, which would have identified the reversed trim were not clearly noted by the manufacturer in the chapter that was referred to by the engineers for this task.
- The post-maintenance duplicate inspection requirements in accordance with Civil Aviation Regulation (CAR) 42G were not included in the maintenance worksheets for the horizontal stabiliser trim system prior to release of the aircraft for flight, and therefore were not performed or certified for.
- The post-maintenance functional test that was performed by the engineers did not meet the intent of CAR 42G duplicate inspection criteria, in that the functioning of horizontal stabiliser trim switches did not include correlation of the horizontal stabiliser surface motion to trim switch movement.
- Pre-flight inspection by the flight crew did not detect the reversed trim motion.
- Having identified a problem with the pitch trim, the flight crew did not select stabiliser trim control from the pilot to the co-pilot control wheel during the occurrence flight.
As a result of this occurrence, the aircraft operator took immediate action and issued a company memorandum to all engineering staff clarifying the requirements of Civil Aviation Regulation 42G in regard to flight control system maintenance and inspections requirements. This memorandum reiterated the requirement for duplicate inspections inclusive of all trim systems.
The aircraft operator also advised the ATSB that the maintenance control and engineering procedures manuals had been revised and that the following corrective action had been taken:
- a new engineering procedure has been introduced, which addresses hand over of maintenance co-ordination of tasks between shifts that involve multiple personnel across those shifts, and hand over procedures for maintenance tasks between engineers completing separate portions of the one task.
- The Engineering Procedures Manual (EPM) has been revised to include procedures for certification of stages of maintenance within a task and now incorporates a procedure to identify when duplicate inspections are required, and ensures the incorporation of duplicate inspection entries in maintenance documentation for those tasks deemed by civil aviation legislation and company policy to require them. This EPM Section also identifies personnel responsible for ensuring that duplicate inspection requirements are invoked when maintenance activities require them.
- All maintenance documentation has been reviewed and the layouts amended to address the appropriateness of maintenance log sheets, work cards and other such documents to facilitate these procedural changes.
The operator also advised that they had reviewed the company induction and training program for maintenance engineers and now emphasise the sections of the Fairchild Aircraft Maintenance Manual that relate to the pitch trim and control maintenance practices. The sonalert unit in the aircraft was replaced after the incident. This aircraft’s sonalert operation now conforms to that of the operator’s fleet.
As a result of this and previous occurrences, the Australian Transport Safety Bureau issues the following safety recommendation:
The Australian Transport Safety Bureau recommends that M7 Aerospace Pty Ltd review and amend its Fairchild SA-227 series maintenance manual to ensure that notes on operational tests, with regard to horizontal stabiliser movement versus trim switch position referred to in Section 27-40-10 for removal of the pitch trim switch, are included in Section 27-10-10 for related maintenance activities, or references to them are clearly noted in that part.
|Date:||02 August 2004||Investigation status:||Completed|
|State:||Western Australia||Occurrence type:||Control issues|
|Release date:||03 January 2006||Occurrence category:||Incident|
|Report status:||Final||Highest injury level:||None|
|Aircraft manufacturer||Fairchild Industries Inc|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Nil|
|Departure point||Perth WA|
|Departure time||0342 WST|
|Role||Class of licence||Hours on type||Hours total|