Flight History
On 18 May 2011, flight OSL5428 departed Rosario International Airport (ROS) at 20:35 with intermediate stops at Córdoba (COR), Mendoza (MDZ), and Neuquén (NQN) before the final destination Comodoro Rivadavia International Airport (CRD). The aircraft, a Saab 340A registered LV-CEJ, was operated by a pilot in command, copilot, and cabin crew. After completing the first two stopovers, the crew and 19 passengers (18 adults and one minor) boarded at Neuquén for the final leg, taking off at 23:05.
Accident Sequence
The aircraft climbed along airway T 105 to reach flight level (FL) 190 per the flight plan. After 24 minutes of flight, the pilot leveled off at 17,800 feet for approximately 9 minutes. Due to icing at that level, the crew descended to FL 140, a maneuver that took five minutes. During the descent, icing conditions worsened. At FL 140, severe ice accumulation occurred during about two minutes of straight and level flight. The aircraft then experienced a complete loss of lift, leading to loss of control and an abnormal attitude. It plunged to the ground, impacting with a post-crash fire. The accident occurred at night under instrument meteorological conditions (IMC).
Investigation Findings
The official investigation determined that the loss of control resulted from severe ice formation. Factors included inadequate monitoring of icing cues (temperature, cloudiness, precipitation, and ice accumulation; airspeed and angle of attack); a forecast of slight icing that did not match the severe conditions encountered; insufficient risk evaluation by the crew, leading to inadequate briefings on de-icing systems, power use, autopilot, and diversion strategies; increased stress as control inputs failed to produce expected effects; icing conditions exceeding the aircraft's ice protection systems (certified under FAR 25 Appendix C); maintaining speed near stall speed; not selecting IAS mode on the autopilot; partial execution of flight manual procedures; confusion of pre-stall buffet with propeller ice vibrations; stick shaker activation at a lower speed than expected; use of a stall recovery technique that prioritized angle of attack reduction at the expense of altitude; and unusual aileron control response likely due to ice accumulation on control surfaces.
Probable Cause
The official probable cause states that the crew lost control of the aircraft during cruise, which led to an uncontrolled impact with the ground due to severe ice formation. This resulted from entering icing conditions without adequate monitoring of external and internal warning signals, receiving a forecast for slight icing when severe icing occurred, inadequate risk evaluation, increased stress levels, icing beyond the aircraft's certified protection, inappropriate speed and autopilot use, partial adherence to procedures, delayed recognition of a stall, and a stall recovery technique that was inappropriate for the conditions.