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.
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.
During the first landing, the nosewheel remained airborne throughout the landing sequence, indicating that de-rotation did not occur. Consequently, the front wheels of the main landing gears probably did not contact the ground for a sufficient period to allow them to accelerate to the required wheel speed condition. That resulted in the logic conditions for ground spoiler deployment not being met. Those required compression of both left and right main landing gears ('weight on wheels'), and a radio altitude of less than 6 ft or a wheel speed greater than 72 kts on the front and aft wheels of the main landing gears. Without a valid radio altitude signal of less than 6 ft, and without ground spoilers deployed, the logic conditions for reverse thrust were also not met, and it too was unavailable. The absence of an AP OFF INVOLUNTARY WARNING indicated that the crew had intentionally disconnected the autopilot during the approach.
The loss of valid radio altimeter signals did not result in the automatic switching from flight mode to flare mode when the autopilots disengaged. That was due to the water ingress into the radio altimeter antennas, and which resulted in the radio altimeter signals being interpreted as out of range signals, rather than as a failure of the radio altimeters.
Water ingress into the aircraft radio altimeter antenna coaxial cables led to the loss of normal aircraft handling characteristics.
|Date:||27 August 2001||Investigation status:||Completed|
|Time:||0942 hours EST|
|State:||Victoria||Occurrence type:||Control - Other|
|Release date:||14 May 2003||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Airbus Industrie|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Adelaide, SA|