Investigators have determined that an Airbus A330's weather radar capabilities had been degraded before the jet encountered severe turbulence last year, after failing to detect a region of convective crystallising ice.
Seven occupants of the A330-300, operated by Qantas Airways, were injured as it met turbulence at flight level 380 while approaching the northern coast of Malaysia. During the 20s disturbance the aircraft was subjected to vertical forces ranging from 1.59g to minus 0.48g, and strong changes in the wind speed and direction.
"The cloud associated with the convective activity consisted of ice crystals, a form of water that has minimal detectability by aircraft weather radar," says the Australian Transport Safety Bureau in its final report into the incident.
"Consequently the convective activity itself was not detectable by [the A330's] radar. As the event occurred at night, with no moon, there was little opportunity for the crew to see the weather."
While the twin-jet was fitted with a serviceable hybrid weather radar, including a Rockwell Collins WRT-2100 'MultiScan' transceiver, the investigators point out that the radar had several features disabled at the time.
The control panel, for example, did not allow selection of the MultiScan mode - an automatic scanning configuration designed to improve detection of convective turbulence - and this meant the crew had to adjust the antenna tilt manually.
Algorithms associated with the MultiScan function had been deactivated. Owing to "issues" with weather-detection algorithms, says the incident report, the carrier had also reverted to an earlier software release for the equipment.
The Qantas incident occurred on 22 June last year, just three weeks after the loss of another A330, operated by Air France, in a region of convective weather over the South Atlantic.
Investigators examining the destruction of the French A330, flight AF447, on 1 June have queried the reasons why the crew continued heading towards the weather cell while other aircraft on similar courses diverted around it.
On the Qantas jet the flying pilot was the second co-pilot, supported by the commander, while the first officer was in the crew rest area. The crew was unable to provide an early warning to passengers before the turbulence struck.
"The report by the crew that the radar did not detect any reflectivity in the cloud prior to the occurrence would suggest that the cloud did not extend into the lower levels where radar was able to detect the cloud at range," says the ATSB.
Although the ATSB says it "could not be determined" whether a fully-capable radar would have prevented the encounter, Qantas has since opted to modify radars of this type to operate in full MultiScan mode and incorporate the latest software.