Austral Líneas Aéreas Flight 2553

Austral Líneas Aéreas Flight 2553 was an Argentine 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.

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.