What happened
On December 4, 2020, an Air Tractor AT-502-B, registration LV-GUQ, was performing aerial application duties near Berón de Astrada, Corrientes. After completing its flight mission, the pilot attempted to land on a grass runway at the La Carolina estate. During the deceleration phase, after the aircraft's wheels had made contact with the ground, the aircraft began an uncontrollable drift to the right. The aircraft eventually exited the lateral limits of the runway and came to a stop in a drainage ditch running parallel to the landing strip. The incident resulted in no injuries to the pilot, but the aircraft sustained damage to its propeller due to ground impact.
The investigation
The investigation examined the runway conditions and meteorological data at the time of the event. While the runway was well-maintained and the grass was properly mended, heavy rainfall the previous night had left the surface saturated. Although the runway featured a cambered design and a drainage canal to manage runoff, significant puddles had formed due to the high volume of precipitation and reduced soil permeability.
Investigators analyzed the aircraft's operational limits and the prevailing winds. The pilot reported a south wind of approximately 13 knots; however, data from the National Meteorological Service indicated a wind speed of 18 knots. The investigation focused on the interaction between the aircraft's tires and the contaminated surface, specifically looking at the mechanics of viscous hydroplaning and the impact of crosswind components on directional control.
Findings
- The primary cause of the excursion was the occurrence of viscous hydroplaning on the contaminated runway surface, where a mixture of mud, grass, and water prevented effective braking and steering.
- The aircraft experienced a significant crosswind component of 17 knots from the left, which exceeded the maximum allowable landing crosswind limit of 13.03 knots specified in the aircraft's operating manual.
- As the aircraft decelerated, the aerodynamic effectiveness of the rudder decreased, leaving the pilot reliant on braking and mechanical steering, both of which were compromised by the slippery surface.
- The asymmetric braking caused by one wheel encountering a puddle led to a sudden yaw to the right, a movement that the pilot could not correct due to the lack of effective directional control on the saturated ground.