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 (see Figure 1). 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 (see Figure 2).
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 takeoff.
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.
- Sonalert – When pitch trim actuation is detected
a tone generator emits an audible low frequency sound in the cockpit to alert
the crew when the stabiliser trim is in motion.
ANALYSIS
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.
SAFETY ACTION
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.
RECOMMENDATION
As a result of this and previous occurrences, the Australian Transport Safety
Bureau issues the following safety recommendation:
R20040078
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.
