Northwest Airlines Flight 08
On June 23, 2009, at 0301 UTC, an Airbus A330-323 (N805NW) operated by Northwest Airlines as flight 08 between Hong Kong and Tokyo experienced airspeed and other flight deck anomalies while in cruise flight at FL390, about 50 miles southwest of Kagoshima Airport, Japan. The aircraft was in level flight on autopilot at Mach 0.81, heading 065 magnetic along RNAV route M750. The crew was aware of convective weather and adjusting weather radar. Just prior to the event, the aircraft entered cirrus clouds with light turbulence, moderate rain, a brief period of intense rain, and hail. The crew received a master warning and caution, and the autopilot, autothrust, and flight directors disengaged. Airspeed fluctuations were reported on the Captain’s, First Officer’s, and standby indicators. A stall warning occurred, flight law switched to Alternate Law, and messages including NAV ADR DISAGREE and NAV IAS DISCREPANCY were displayed. The fluctuations lasted about one minute; the crew manually controlled the aircraft using pitch and power. After ATC clearance to turn away from the weather, the fluctuations recurred for about two minutes. The crew then re-engaged autopilot and completed the flight in Alternate Law. No injuries or damage occurred. Post-flight maintenance checks revealed no airspeed system discrepancies.
TAM Airlines Flight 8091
On May 21, 2009, at 2147 EDT, an Airbus A330-233 (PT-MVB) operated by TAM Airlines as flight 8091 from Miami to São Paulo experienced a loss of primary speed and altitude information while cruising at FL370 over international waters south of Haiti. The crew noted an abrupt drop in outside air temperature and St. Elmo’s Fire, followed by loss of Air Data Reference System, autopilot and autothrust disconnections, and loss of primary airspeed and altitude. The crew used backup instruments; primary data was restored after a short time. The aircraft remained in Alternate Law with a rudder travel limit flag. The flight landed at São Paulo with no injuries or damage. Post-flight maintenance checks revealed no airspeed system discrepancies.
Aircraft Systems
Both aircraft were Airbus A330s equipped with Pratt & Whitney 4168 engines. N805NW was a -300 variant with optional software that displays a NAV IAS DISCREPANCY caution when a 16-knot difference exists. PT-MVB was a -200 variant. Pitot probes on N805NW and the No.1 and No.2 probes on PT-MVB were Thales Aerospace part number C16195AA; PT-MVB’s No.3 probe was a Thales C16195BA. The probes were subject to service bulletins and airworthiness directives following earlier issues. The A330 has three pitot probes and six static ports; pitot measurements are processed by Air Data Reference units.
Meteorological Conditions
Near the Northwest event, satellite images showed cloud tops near 47,000 feet with a temperature of -64.27°C, indicating cumulonimbus clouds. The local sounding showed a conditionally unstable atmosphere. For the TAM event, cloud top temperatures were approximately -83°C, corresponding to cloud tops near 54,000 feet; the aircraft passed downwind of convective activity through anvil blow-off.
Investigation Findings
Examination of flight data from both events showed initial airspeed fluctuations on the No.1 ADR, followed by autopilot and autothrust disconnections. In the Northwest event, the initial fluctuation was between 245 and 110 knots; in the TAM event, the No.1 ADR airspeed dropped to about 60 knots. Both events involved altitude changes: about 300 feet for TAM, less for Northwest. The Airbus examination concluded that the behavior of both aircraft was consistent with partially and intermittently obstructed pitot probes, and that the crew responses and autoflight/EFCS reactions were as expected. The investigation also noted that similar events had occurred previously.