Suspected flight control problem, Boeing Co 737-838, VH-VXN

On 17 February 2005, a Boeing Company 737-838 aircraft,
registered VH-VXN, with seven crew and 150 passengers, was being
operated on a scheduled passenger flight from Adelaide, SA to
Sydney, NSW. The crew reported that, as the aircraft was climbing
through flight level (FL) 180 (18,000 ft), they noticed the
stabiliser trim wheel moving opposite to the direction of the
control column (elevator) movement.

The pilot in command was the handling pilot for the sector and
was manually flying the aircraft when the movement was observed.
The crew considered that the trim movement was uncommanded and
consequently completed the non-normal procedure for a runaway
stabiliser. As the non-normal checklist did not contain the words
'Plan to land at the nearest available airport', the crew levelled
the aircraft at FL270 and continued the flight to Sydney.

Following the occurrence, a built in test equipment check was
carried out on the flight control system and no faults were found.
The two flight control computers were subsequently removed from the
aircraft and tested at the operator's avionics workshop with no
faults being found in either unit.

Previous flight

A different flight crew operated the aircraft on the preceding
flight to Adelaide. That crew reported uncommanded stabiliser trim
wheel movement while the aircraft was being taxied to the terminal,
after landing at Adelaide. The crew also reported that when the
aircraft was shutdown, the autopilot warning horn sounded. They did
not notice any autoflight flight director system status
annunciations on their respective Electronic Attitude Director
Indicator (EADI), nor did they observe the illumination of the
autopilot disengage indicator lights on the pilot and copilot
instrument panels when the warning horn sounded. The operator's
engineering personnel were advised about the apparent uncommanded
movement of trim. A built in test equipment check was subsequently
carried out prior to the flight to Sydney but no fault was detected
in the flight control system.

Stabiliser trim

The horizontal stabiliser is positioned by the main electric
trim motor and is controlled through either of the stabiliser trim
switches on each pilot's control column, or by the autopilot trim
servo motor. The stabiliser may also be positioned by manually
rotating the stabiliser trim wheels located on the control stand
between the two pilots.

Pitch control of the aircraft includes a speed trim system. This
system is used to improve flight handling characteristics during
operations with low gross weight, rearward centre of gravity and
high thrust when the autopilot is not engaged. The system provides
positive speed stability characteristics to the pilot by adjusting
the control column force so that the pilot must provide a
significant amount of 'pull' force to reduce airspeed, or a
significant amount of 'push' force to increase airspeed. The system
trims the stabiliser in the direction calculated to provide the
pilot positive speed stability characteristics. Since pilots
typically attempt to trim control column force to zero and the
speed trim system attempts to trim to positive stick force, the
speed trim system operation may be opposite to the direction the
pilot is trimming.

Autopilot flight director system

The autopilot flight director system is a dual system consisting
of two individual flight control computers and a single mode
control panel. The two flight control computers are identified as
'A' and 'B'. For autopilot operation, the computers send control
commands to their respective pitch and roll hydraulic servos, which
operate the flight controls through two separate hydraulic systems.
For flight director operation, each computer positions the flight
director command bars on the respective EADI located on the
instrument panel for each pilot.

Either autopilot can be engaged in command mode or control wheel
steering mode by pushing the appropriate engage switch on the mode
control panel, which is located on the glare shield in front of
each pilot (Figure 1).

If the autopilot flight director system is engaged in control
wheel steering mode, the following system status annunciations will
appear above the attitude indications on each pilot's EADI:

Flight data recorder information

Following the occurrence, data from the aircraft's flight data
recorder (FDR) was downloaded and analysed by the aircraft
manufacturer and the Australian Transport Safety Bureau (ATSB). The
data indicated that during the previous flight the autopilot was
engaged after landing at Adelaide. The autopilot was engaged by
pushing the control wheel steering mode autopilot engage switch for
the 'B' autopilot flight director system. The autopilot engagement
occurred when the 'B' system flight director switch was selected to
the OFF position while the aircraft was taxiing to the terminal.
The flight director switches are usually selected to the OFF
position by the crew while carrying out the 'taxi in' normal
procedures when the aircraft has vacated the runway after
landing.

The 'B' system flight director switch and the control wheel
steering engage switch are in close proximity to each other on the
glare shield mode control panel, above the centre instrument panel
(Figure 1).

The data showed that various up and down trim movements were
commanded by the autoflight flight director system following
engagement of the control wheel steering mode after landing at
Adelaide. The data also showed that the 'B' autopilot flight
director system remained engaged when the FDR recording ended for
that flight. The aircraft manufacturer advised that 'it is expected
that it [the autopilot] disengaged later on (with warning horn)
when additional power switching or configuration changes occurred.'
The aircraft operator advised that the autopilot would have
disengaged when the engines were shutdown and the electrical power
source to the autoflight flight director system transferred from
the engine driven generators to the auxiliary power unit driven
generator.

The aircraft manufacturer also advised that, from their review
of the FDR data, 'no trim anomalies could be seen on the following
climb out [the occurrence flight]'.