Austral Líneas Aéreas Flight 2553

Austral Líneas Aéreas Flight 2553 was an Argentinian domestic scheduled passenger flight from Posadas to Buenos Aires. On October 10, 1997, the McDonnell Douglas DC-9-32 that was operating the flight crashed on the lands of Estancia Magallanes, Nuevo Berlín, 32 km away from Fray Bentos, Uruguay. All 74 passengers and crew died upon impact. The accident remains the deadliest in Uruguayan history.

Aircraft and crew
The aircraft involved in the accident was a McDonnell Douglas DC-9-32, registration LV-WEG. It had its maiden flight in 1969, and as of the time of the accident had flown a total of 56,854 hours and 54,800 takeoff and landing cycles.

The captain was 40-year-old Jorge Cécere, who had been with the airline since 1989 and logged 9,238 hours, including 223 hours on the DC-9. The first officer was Horacio Núñez, who was also 40. He had been with the airline since 1993 and had 2,910 flight hours. He was more experienced on the DC-9 than captain Cécere, with 1,384 hours on that aircraft.

Accident
The aircraft, which left from Posadas and was due to land in Aeroparque Jorge Newbery, Buenos Aires, was forced to divert towards Fray Bentos to avoid a storm. Examination of the aircraft's flight data recorder (FDR) revealed that shortly after the diversion occurred, the aircraft airspeed indicator began to fall to an alarmingly low indicated airspeed. Unknown to the pilots, this was caused not directly, by a loss of power, but by ice formed inside the pitot tube, which reads the airspeed for the indicator by measuring the pressure of inflow air. The ice obstructing the pitot tube reduced the air inflow, thus giving an erroneously low indicated airspeed.

In response to what they interpreted as a loss of engine power, the pilots disconnected the autopilot and gradually increased power from the engines in order to maintain airspeed. Seeing no improvement, they contacted the control tower in Ezeiza Airport and requested clearance to descend to a lower altitude. After receiving no response, the Captain decided to descend to a lower altitude to increase speed even with no clearance received from the Air Traffic Control. While descending from their assigned altitude of 35,000 ft and reaching 31,700 ft, the Captain identified the faulty airspeed indication and ordered the First Officer to stop descending and to reduce speed, because the readings were unreliable. However, the First Officer disregarded the Captain's commands and deployed the wings' slats to maintain their altitude and lower the plane's stall speed. Consequently, at this point the airplane was actually flying at a higher speed than normal; it was descending, which further increased airspeed to a point dangerously near to VNE, the "never exceed speed", above which structural damage to the aircraft might occur.

With the slats extended at a speed beyond their operational limits, one of them was torn from the aircraft, causing catastrophic asymmetry in the airflow over the wings. The aircraft immediately became uncontrollable and crashed.

According to an investigation by both the Argentine and Uruguayan Air Forces, the pitot tube—the primary instrument for measuring aircraft airspeeds—froze when the aircraft passed through a 15000 m high cumulonimbus cloud, blocking the instrument and causing it to give a false reading. Compounding this problem was the absence of the alarm designed to report such a malfunction (raising serious questions about inspection irregularities by the Argentine Air Force).

During the descent, the FDR recorded an increase in the airspeed from 300 km/h to 800 km/h in three seconds, which could only signify the sudden unfreezing of the pitot tube. Specialists estimated that the aircraft crashed at a 70-degree nose down attitude, at a speed of 1200 km/h. Depending upon the source, the crater left by the crash was 6 m deep and 30 m wide, 25 ft deep and 30 ft wide, or 25 ft deep and 80 ft wide.