On Monday, 22 March 2004, the pilot in command of the Fairchild
Industries SA-227 aircraft, registered VH-HPE, operating a
scheduled Regular Public Transport flight, reported that excessive
forward control column force had been required 'to trim the
aircraft nose down' during departure from Sydney Airport. The pitch
trim selector was switched to the copilot position, control was
passed to the copilot, who was then able to trim the aircraft, and
the flight continued to Taree, NSW. After landing, an
examination by the crew revealed that the pilot in command's (left
side) control yoke pitch trim switch was operating in the reverse
sense from normal operation.
Discussions were held between the flight crew and the operator's
chief pilot and chief engineer and a decision was made to continue
with the following two scheduled flights before the aircraft
returned to a suitable maintenance facility for
rectification. A subsequent engineering examination revealed
that the pilot in command's pitch trim switch had been installed
upside-down and had to be removed and re-installed in the correct
orientation (refer figure 1). The pitch trim system was
checked for correct operation and the aircraft was returned to
service.
Figure 1: Left side pitch trim control
switch
In the days preceding the occurrence, the aircraft underwent
scheduled maintenance at a contractor maintenance facility.
During maintenance, there was a requirement to replace the left
side control column pivot bearings. 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. The trim switch was then re-installed into the
control yoke and the engineers reported that they conducted a full
installation and duplicate functional check of the pitch trim
system and completed the documentation in the aircraft maintenance
worksheets.
During the investigation, the aircraft maintenance engineers
responsible for the switch installation and functional check
indicated that they had completed the work and that the duplicate
functional check was conducted with no apparent
discrepancies. The aircraft was then handed over to other
maintenance engineers for the completion of further maintenance
tasks. The following day, the scheduled departure of the
aircraft was delayed due to on-going maintenance
rectifications. None of these further maintenance tasks
involved the aircraft pitch trim system. Following the delay,
the aircraft departed on the occurrence flight after the crew had
conducted pre-flight checks, including a check of the pitch trim
system cockpit indication for correct operation. The aircraft
maintenance engineers had been assigned the maintenance tasks away
from their normal location on a weekend and the aircraft was
required for scheduled operations on the Monday morning.
The maintenance contractor and the aircraft operator conducted
separate investigations into the trim switch misalignment and
concluded that the only plausible scenario leading to the
misalignment was that the engineers responsible for the pitch trim
switch installation had installed the switch incorrectly. The
discrepancy had not been detected during the installation and
duplicate functional checks or the flight crew's pre-flight
checks.
The type certificate data sheet holder for the aircraft type
reported that the aircraft Minimum Equipment List (MEL) provides no
relief for flight with one pitch trim system inoperative and so the
decision to continue the scheduled flights in this condition was
contrary to the requirements of the operator's flight operations
manual.
