Pilots of an Etihad Airways Airbus A340-600 diverted to Singapore after a sudden encounter with turbulent weather during cruise generated unreliable airspeed data and left the jet unable to maintain altitude separation requirements.

While en route to Melbourne at 35,000ft, and approaching the PIPOV waypoint over the Indian Ocean, the returns from the aircraft's weather radar - which had no auto-tilt function - suddenly intensified to indicate surrounding convective weather.

Airspeed on the captain's primary flight display rapidly dropped from 283kt to 77kt before fluctuating, and the standby instrument recorded a fall from 280kt to 142kt. The first officer's reading stayed stable.

United Arab Emirates investigators from the General Civil Aviation Authority determined that the autopilot and autothrottle, as well as the flight directors, disengaged and the A340 switched to alternate flight-control law - a mode in which angle-of-attack protection is lost.

The preliminary inquiry says that the aircraft had started to depart from its altitude after the autopilot disengaged, performing an "inadvertent climb" which took it 832ft above its assigned 35,000ft cruise level.

Within about 20s the airspeed indications recovered and the jet reverted to normal law. But about a minute after the initial disturbance began, the airspeed began fluctuating again. This second disturbance, lasting about 44s, again caused the A340 to drop into alternate law and disconnected the autothrust.

Since the first officer's instruments appeared to be functioning correctly, the captain designated him as the flying pilot. The first officer returned the aircraft to its assigned altitude.

Although the airspeed indications stabilised, and the autothrust was re-engaged, the crew could not bring the autopilot back online, and the first officer continued to fly the jet manually. The A340 remained in alternate law for the rest of the flight.

The crew transmitted that the aircraft (A6-EHF) could not maintain altitude owing to the jet's performance and the turbulence, and that it had lost the capability to operate in reduced vertical separation minima airspace.

It subsequently descended to conventional airspace at 29,000ft and diverted to Singapore. None of the 295 occupants was injured.

While the inquiry into the Etihad A340-600 incident highlights that icing is notably a cause of unreliable airspeed indications at high altitude, it has yet to establish conclusions about the event.

But the circumstances bear a similarity to those preceding the Air France flight AF447 accident in June 2009, when an A330 cruising at 35,000ft flew into a storm cell, suffering icing of its pitot system.

The General Civil Aviation Authority says that dispatch documentation provided to the Etihad crew included charts indicating an isolated embedded cumulonimbus cloud up to 45,000ft in the area of the incident.

Analysis showed that the A340's weather radar, set on manual tilt, showed "almost no" reflectivity before the turbulence started to increase. The radar returns then sharply intensified.

"An incorrect tilt may lead to only scan the upper, less reflective, part of a cell," the inquiry notes. "As a consequence, a cell may not be detected or may be underestimated."

Use of weather radar to avoid storm-cell penetration emerged as an issue in the AF447 investigation. Icing led to airspeed fluctuations and switching to alternate control law, and the crew's response resulted in an advertent climb and high-altitude stall.

Like the case of AF447, three pilots - one of whom had returned to the cockpit after a rest period - worked to resolve the Etihad situation. Despite resetting all the flight-control and flight-guidance computers, as well as other systems, by using quick-reference handbook procedures, the pilots could not re-engage either of the two autopilots.

Source: Air Transport Intelligence news