Airbus A330-323 N805NW incident near Kagoshima, JP

Probable cause

Brief and temporary blockage of the pitot probes in cruise flight, most likely due to ice crystals aloft, leading to erroneous airspeed indications and airplane automation degradation as designed.

Contributing to the incidents were design features of the Thales AA probes which left them more susceptible to high altitude ice crystal icing than other approved pitot probe designs.

NTSB analysis

Both aircraft were in normal cruise flight at high altitude, in the vicinity of convective weather conditions, and both flight crews were aware of the weather. Crew statements and recorded data for both flights did not indicate any airplane anomalies prior to the events. Shortly before the unreliable airspeed events, each airplane encountered a visible effect of the weather conditions, the Northwest crew reported seeing moderate rain and hail, and the TAM crew reported an abrupt temperature change and St. Elmo’s Fire. Within seconds both airplane’s autopilot and autothrust disconnected, master warnings and cautions were indicated, primary airspeed display was lost or fluctuated, and the airplanes transitioned into Alternate Flight Law. According to Airbus documentation and analysis, if an airspeed discrepancy of more than 20kts or an altitude discrepancy of more than 400ft is detected between one ADR and the two others, the subject ADR is rejected. Then if a discrepancy occurs between the two remaining ADR, all auto flight functions are lost and autopilot, flight director, and autothrust disconnect. Applicable procedures for both airplanes call for autopilot and autothrust to be OFF, and for the pilot to pitch to 5 degrees and set thrust levers to the CLB detent. Both flight crews turned the airplanes to exit the weather area and the airspeed indications returned to normal within a short time.

The behavior of the airplane systems is consistent with, and supports a brief and temporary blockage of two or more pitot probes likely by an accretion of ice crystals aloft. Existing airplane flight manual/QRH procedures were sufficient to prevent a dangerous situation from occuring, however the airplane would, as designed, be operating in a lower level of automation. The autopilot disengaged automatically as intended due to its airspeed monitoring function. The flight crew’s attempt to reengage the autopilot during the unreliable airspeed event, and erroneous altitude reading, followed by pilot side stick inputs, exacerbated the altitude excursion experienced by the TAM flight, however the airplane was not actually in danger of a stall.

Text from the NTSB. Open the NTSB docket for DCA09IA064 for the full record.