What happened
On 3 February 2013, an Airbus A340-600, registration A6-EHF, operated by Etihad Airways, was performing a scheduled passenger flight from Abu Dhabi to Melbourne. The aircraft was carrying 295 people, including 278 passengers and 17 crew members. While cruising at 35,000 feet near the boundary of the Colombo and Melbourne Flight Information Regions, the aircraft encountered moderate to heavy turbulence.
During this period of turbulence, the aircraft's airspeed indications began to fluctuate erratically. Specifically, the captain's primary flight display showed a sudden drop from 283 knots to 77 knots within two seconds, followed by a brief recovery. Simultaneously, the standby instrument showed a decrease from 280 knots to 142 knots. These discrepancies triggered the automatic disconnection of the autopilot, autothrust, and flight directors.
As a result of these erratic airspeed readings, the aircraft's flight control system transitioned from Normal Law to Alternate Law. This change resulted in the loss of certain flight envelope protections. Although the flight crew managed to re-engage the autothrust and flight directors, the two autopilots could not be re-engaged, necessitating manual flight for the remainder of the journey. Due to the loss of RVSM capability, the crew diverted the flight to Singapore Changi International Airport, performing a fuel dump to reach a safe landing weight.
The investigation
The GCAA AAIS investigation examined the aircraft's flight data, cockpit voice recordings, and the technical performance of the airspeed measuring systems. The investigation focused on the sequence of events leading to the unreliable airspeed indications and the subsequent degradation of the flight control laws. Investigators also reviewed the aircraft's weather radar data, which showed increasing intensity of precipitation and turbulence immediately preceding the event, and analyzed the crew's response in accordance with the airline's training manuals and Quick Reference Handbook (QRH) procedures.