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 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.