What happened
On 2 February 2013, an Etihad Airways Airbus A340-600, registration A6-EHF, was performing a scheduled passenger service from Abu Dhabi to Melbourne. While cruising at flight level 350, transitioning from the Colombo FIR to the Melbourne FIR, the aircraft encountered moderate to heavy turbulence. During this period, the aircraft experienced significant oscillations in airspeed readings on both the captain's and the standby indicators.
These fluctuations caused the flight control system to transition from "Normal" to "Alternate" Law, resulting in the loss of several flight envelope protections. While the 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. Simultaneously, the crew noted high N1 vibration on the No. 2 engine. Due to the loss of RVSM capability, the crew diverted the flight to Singapore Changi International Airport, performing a necessary fuel dump to reach a safe landing weight. There were no injuries among the 295 persons on board.
The investigation
The GCAA AAIS investigation focused on the technical failures regarding the unreliable airspeed indications and the engine vibration. Investigators analyzed the flight data recorder and cockpit voice recorder to reconstruct the sequence of system rejections. The team examined the aircraft's air data modules and the impact of environmental conditions on the pitot probes. Additionally, the investigation looked into the mechanical cause of the N1 vibration on the No. 2 engine and the role of the weather radar settings during the encounter with convective activity.
Findings
- The primary cause of the unreliable airspeed indications was the intermittent obstruction of the left-side pitot probes, likely caused by the accumulation of ice crystals which created temporary blockages.
- The high N1 vibration on the No. 2 engine was caused by water ingress through a gap following the disbanding of an Omega Seal; this water froze and accumulated under the annulus fillers.
- A contributing factor was an incorrect radar tilt setting, which prevented the flight crew from identifying an area of embedded cumulonimbus clouds ahead.
- The aircraft was operating at an altitude and ambient temperature that fell outside the certified icing envelope specified by manufacturer requirements and JAR standards.