Event Details
On 4 December 2020, at approximately 12:15 UTC, an Air Tractor AT-502-B, registration LV-GUQ, experienced a runway excursion during landing at a private airstrip near Berón de Astrada, Corrientes, Argentina. The aircraft had departed at 12:00 UTC from the same airstrip (La Carolina estate) for an aerial application flight. After 15 minutes of flight and completing the work, the pilot returned for landing. During the deceleration phase with all three wheels on the ground, the aircraft veered uncontrollably to the right, crossed the lateral limits of the runway, and came to a stop in a drainage ditch parallel to the runway. The accident occurred in daylight with good weather conditions. The pilot, the sole occupant, sustained no injuries.
Investigation Findings
The runway was grass, oriented 09/27, approximately 1500 meters long and 60 meters wide, with optimal dimensions for the aircraft type. The runway was crowned and had a drainage ditch on the north side. The grass was cut and in good maintenance condition. However, the night before the accident, heavy rain had fallen. The runway's permeability became saturated, leading to water accumulation in puddles and slow drainage.
Prior to the accident landing, the pilot had performed five landings on the same runway. He noted that the ground was soft and the wheels left ruts, so he did not overlap landing tracks to preserve the runway surface. During those landings, he had no difficulty controlling the aircraft. The wind was from the south at approximately 13 knots, as obtained from an internet application, and did not pose a problem. The landing direction was west.
The aircraft sustained minor damage to the propeller from impact with the ground. The investigation analyzed the phenomenon of hydroplaning. Specifically, viscous hydroplaning occurred at relatively low speeds, where the viscosity and inertia of water prevent it from being expelled from the tire-pavement contact area, causing the tire to skid. In this case, the runway contamination consisted of a mixture of grass, mud, and water. The formula for critical hydroplaning speed is Vc = 9√p (where p is tire pressure in psi). Hydroplaning can occur with as little as 2-3 mm of water on a smooth tire, but with tread, it requires 5-6 mm. The aircraft's tires had tread. The investigation noted that hydroplaning is difficult to predict.
No probable cause was stated in the provided excerpts.