Ice crystals blocked the pitot probes on two Airbus A330s that were cruising at high altitudes in the vicinity of convective weather activity, causing erroneous airspeed indications and reduced autoflight systems operation, according to a report issued in June by the U.S. National Transportation Safety Board (NTSB).
The incidents reported by NTSB involved an A330-200 of Brazilian registry that was en route with 176 people aboard from Miami to São Paulo, Brazil, on May 21, 2009, and an A330-300 of U.S. registry that was en route with 217 people from Hong Kong to Tokyo on June 23, 2009.
“Crew statements and recorded data for both flights did not indicate any airplane anomalies prior to the events,” the report said.
The Brazilian airplane was at Flight Level (FL) 370 (approximately 37,000 ft) over Haiti when the flight crew noticed an abrupt decrease in outside air temperature and observed St. Elmo’s fire, a coronal discharge of plasma that produces a faint flame-like glow on an aircraft flying through an electrically charged atmosphere. The airplane’s air data reference system ceased operating, primary displays of airspeed and altitude were lost, the autopilot and autothrottle disengaged, and the fly-by-wire system reverted from normal control law to alternate control law, which provides fewer protections against exceeding performance limitations.
“The flight crew continued using backup instruments,” the report said. “After approximately five minutes, primary data was restored. … The crew determined they could not restore normal law and continued the flight under the appropriate procedures.” The airplane was landed in São Paulo without further incident.
In the second incident, the crew of the U.S. airplane was using the on-board weather radar system to avoid thunderstorms while flying over Japan at FL 390. However, “just prior to the event, the airplane entered an area of cirrus clouds with light turbulence and moderate rain, with a brief period of intense rain and hail aloft,” the report said.
The autopilot and autothrottle disengaged, fluctuating airspeed indications were displayed, and a stall warning was generated. The crew “reported that the airspeed fluctuations and warnings lasted about one minute, and they controlled the airplane by pitch and power reference, per applicable checklist procedures, until normal airspeed indications returned,” the report said.
Airspeed fluctuations occurred again briefly as the crew turned the airplane farther away from the convective activity. After about two minutes, “the airspeed indicators returned to normal, and the crew re-engaged the autopilot and completed the flight in alternate [control] law,” the report said.
Investigators determined that the incidents were initiated when at least two of the three pitot probes on each airplane were blocked by an accretion of ice crystals.
