Fairchild Industries Inc SA227-DC, VH-HWR

SIGNIFICANT FACTORS

1. The wiring for the pilot’s control wheel horizontal stabiliser trim
   switches was reassembled in the reverse sense.
2. 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.
3. 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.
4. 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.
5. Pre-flight inspection by the flight crew did not detect the reversed trim
   motion.
6. 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.

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.

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.

1. 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.