What happened
A scheduled flight departed Posadas-Libertador General José de San Martín Airport at 21:18 local time, bound for Buenos Aires. The DC-9 was carrying 69 passengers and five crew members. During the cruise phase at 35,000 feet, the aircraft encountered extreme weather, including heavy turbulence, storm activity, and icing conditions with temperatures reaching -59° C. While navigating through these conditions, the flight entered the Montevideo FIR without notifying the relevant air traffic control center.
At approximately 2/2:04 local time, the aircraft moved into an area of intense icing. Shortly after, the pilot-in-command began a descent without authorization. While the captain sought permission from Ezeiza Control, they were informed that the flight was within Uruguayan airspace and required coordination with Montevideo ATC. Permission to descend to FL257 was granted at 22:09:17, and the aircraft reached this altitude by 22:10:25.
During the descent, the first officer noted irregularities with the airspeed indicator. As the aircraft passed through FL300, the first officer deployed the slats. However, the aircraft was traveling at a speed exceeding the structural design limits for this configuration, leading to an asymmetric deployment. This triggered an uncontrolled, near-vertical descent into an open field approximately 21 km east of Nuevo Berlin. The impact caused the aircraft to disintegrate, creating a crater 10 meters deep and 30 meters wide. There were 74 fatalities and no survivors.
Findings
Investigations revealed that the airspeed indicator likely provided erroneous low-speed readings because the Pitot tubes had become blocked by ice. This led the crew to believe they needed to extend the slats, a maneuver typically reserved for much lower altitudes during stall recovery. The extension of the slats at such high speeds caused structural asymmetry and an unrecoverable loss of control.
Several contributing factors were identified, including:
- The absence of a Mach number indication on the speed indicators when flying below 250 KIAS.
- Inadequate crew training regarding flight instrument failures and upset recovery procedures.
- Insufficient training for recovering from stalls in a clean configuration.
- Deficiencies in crew resource management (CRM) and operational procedures.
- The lack of a warning light to indicate if the Pitot/Stall-Heater was turned off.