Airbus A330-341, PK-GPC

History of the flight

The Airbus A330-341 aircraft was operating a scheduled
international fare-paying passenger service from Adelaide to
Melbourne and the pilot in command was the handling pilot for the
flight. During the initial descent into Melbourne, the crew
configured the auto flight system to the approach mode. That action
armed the auto flight system localiser and glideslope modes for the
runway 16 instrument landing system (ILS), and permitted the crew
to engage the second autopilot for the approach. As the aircraft
descended through 2,500 ft, the crew placed the ground spoiler
handle to the armed position. Shortly after, the radio altimeter
indications disappeared from both pilots' electronic flight
instrument displays. Both autopilots then disengaged. About 20
seconds later, both flight directors disengaged from the localiser
and glideslope modes but re-engaged in the basic modes of current
vertical speed and heading.

The pilot in command elected to continue the approach and to
manually fly the aircraft, because he considered that he would be
able to control the aircraft without auto flight system approach
commands or radio altimeter information. The autothrust was
unaffected by the disengagement of the autopilots, and remained
engaged.

At the completion of the landing approach, the pilot in command
flared the aircraft for the landing, and retarded both thrust
levers, which disengaged the autothrust system. The aircraft landed
on the left and right main landing gears, bounced, and became
airborne for four and a half seconds before touching down again on
both main landing gears. The aircraft bounced again, became
airborne for one second, and then touched down for a third time on
both main landing gears. The right main landing gear then lifted
off the runway for about one second, after which the aircraft
settled onto both main landing gears. Two seconds later, the thrust
levers were advanced to go-around power, and after a further five
seconds, the aircraft became airborne again. The nose landing gear
remained airborne throughout this sequence. Additionally, the
ground spoilers did not deploy, and the thrust reversers did not
activate.

The pilot in command repositioned the aircraft for another
approach onto runway 16. During the second landing, the aircraft
again bounced following the touchdown, then settled onto the
runway. Four seconds later, the ground spoilers deployed; however,
the thrust reversers did not activate when selected by the crew.
The landing rollout was completed without further incident, and the
aircraft was taxied to the terminal.

There were no injuries to any persons on board the aircraft.

Aircraft information

The aircraft was a fly-by-wire type. Three flight control
primary computers and two flight control secondary computers
controlled the flight control system. The computers processed crew
and autopilot inputs to provide appropriate electrical output
signals to the hydraulically powered flight control surfaces.

Crew input to the flight control computers was made via
electrical signals from either of the two side stick controllers,
and autopilot input was made via an interface with the aircraft's
Flight Management and Guidance System.

The inputs to the flight control computers were processed in
accordance with respective flight control 'laws'. Regardless of the
pilot's inputs, the control computers will prevent excessive
manoeuvres and/or exceedance of the safe flight envelope. Those
laws were dependent on whether the aircraft was in the ground,
flight or flare mode of flight. In the ground mode, there was a
direct relationship between sidestick deflection and the flight
control surfaces. In the flight mode, deflection of the flight
control surfaces was governed to achieve a load factor proportional
to sidestick deflection, independent of speed. Flight mode provided
3-axis control of the aircraft, and provided flight envelope
protection and manoeuvre load alleviation.

In the flight mode, the normal laws were:

• Nz law for pitch control, including load factor protection. (Nz
  law is vertical acceleration in the normal axis of the
  aircraft);
• lateral normal law for lateral control (roll and yaw),
  including bank angle protection; and
• protection against high speed (VMO), pitch angle (theta), and
  stall (angle of attack).

In flare mode, the normal laws were:

• flare law in place of Nz law for pitch control to allow for
  conventional flare;
• lateral normal law for lateral control (roll and yaw) including
  bank angle protection; and
• protection against stall.

Flare mode permitted crews to use the same landing technique as
for non-fly-by-wire aircraft. Transition from flight mode to flare
mode occurred when the aircraft's radio altimeters sensed that the
aircraft altitude was less than 100 ft above ground level.

If faults were detected in both radio altimeters, switching from
flight mode to flare mode would occur when the landing gear was
extended, provided the autopilot was off. If the autopilot was
engaged, switching from flight mode to flare mode would occur when
the autopilot was disengaged, provided the landing gear was
extended.

The manufacturer reported that flight tests for the A330 type
included landing in flight mode. ie without transition to flare
mode. Landing in that condition was not considered difficult,
however, it required a different handling technique than would
otherwise apply for non-fly-by-wire aircraft. In such
circumstances, a pilot would need to apply back pressure on the
sidestick to initiate the landing flare, then release that back
pressure to maintain the desired pitch attitude until
touchdown.

The aircraft was equipped with two radio altimeter systems that
provided information about the aircraft height above ground level.
Data from the radio altimeters was also used by many of the
aircraft systems' logic sequences to determine whether certain
operating parameters had been met to permit operation of a
particular system. The radio altimeter antennas were located along
the keel of the aft fuselage of the aircraft, and were connected to
the aircraft electronic system by coaxial cables. Inspection of the
radio altimeter system antennas subsequent to the occurrence
revealed that they had sustained water ingress at the antenna
coaxial cables. The water ingress into the radio altimeter antennas
resulted in the radio altimeter signals being interpreted as out of
range signals, rather than as a failure of the radio
altimeters.

During the period 11 June 2001 to the date of the occurrence,
there were 19 entries in the aircraft's maintenance log reporting
problems with the radio altimeters fitted to the aircraft. Repairs
had been carried out on the radio altimeters, including replacement
of a transceiver unit and cleaning of components due to water
ingress.

The aircraft was equipped with autoflight and flight director
systems. Radio altitude signals from the aircraft radio altimeters
were used to engage the autoflight system into the LAND mode when
the aircraft altitude was 400 ft above ground level. The loss of
valid radio altimeter signals in LAND mode would result in the loss
of both autopilots and the flight directors reverting to the basic
modes of vertical speed and heading. The autopilot also used radio
altitude signals to adapt the autopilot gains during an ILS
approach, with the required gain being dependent upon the distance
of the aircraft from the runway threshold. Any involuntary
disconnection of the autopilot triggered an AP OFF INVOLUNTARY
warning message to the crew.

The aircraft was equipped with wing mounted ground spoilers. The
ground spoilers would arm when the crew placed the speed brake
control lever to the armed position, and would activate after
landing provided certain parameters had been met. Those parameters
included both main landing gears transitioning from flight to
ground ('weight on wheels'), and a radio altitude of less than 6 ft
or a wheel speed higher than 72 kts on the front and rear wheels of
the main landing gears.

The aircraft's engines were equipped with thrust reversers.
Deployment of the thrust reversers would not occur unless the
aircraft was on the ground with the ground spoilers extended, radio
altitude less than 6 ft, and the engine thrust levers in the
reverse position.

The aircraft was equipped with an Allied Signal solid state
digital flight data recorder. The recorded data was examined and
revealed that each of the flight control primary and secondary
computers had operated normally throughout the flight. The recorded
data revealed that during both approaches, the autopilots
oscillated in the lateral and longitudinal axes.

Both autopilots disconnected simultaneously, but an AP OFF
INVOLUTARY warning did not accompany the disconnection. The LAND
mode engaged at 400 ft radio altitude. One second later, both
flight directors disengaged from the localiser and glideslope
modes, then re-engaged in the basic modes of current vertical speed
and heading. The recorded data also revealed that the signals from
both radio altimeters were invalid throughout most of both approach
sequences into Melbourne.

The investigation was unable to determine the relevant
experience and training of the crew.