What happened
On November 25, 2009, an AT-402A aircraft, registration PT-WQV, operated by Americasul Aeroagrícola Ltda., was performing agricultural spraying operations in Barra do Bugres, Mato Grosso. The flight departed from the Aero Agrícola Rondon aerodrome and proceeded to a landing strip located at Fazenda São Francisco.
During the landing phase on the grass runway, the pilot maintained a stabilized approach on runway 09. However, as the nose wheel made contact with the ground, the aircraft began to veer toward the right. The pilot attempted to correct the deviation using differential braking, but the maneuver was unsuccessful. In an attempt to stop the aircraft, the pilot applied reverse thrust. The aircraft subsequently drifted off the runway, striking a fence post on the right side. The impact caused the aircraft to cross the runway and strike additional fence posts on the opposite side, eventually coming to rest at a 120-degree angle from the runway centerline. The aircraft sustained severe damage to the propeller blades, the right wing leading edge, the right horizontal stabilizer, and the right flap. The pilot was uninjured.
The investigation
CENIPA investigators examined the runway conditions and the aircraft's mechanical state. The investigation established that the grass surface was wet due to overnight dew, though no standing water or puddles were present. The investigation focused on the loss of directional control and the effectiveness of the pilot's corrective actions. Analysts specifically looked into the phenomenon of viscous hydroplaning, where a thin film of moisture acts as a lubricant between the tire and the surface.
Findings
- The primary cause of the excursion was viscous hydroplaning, which significantly reduced the friction between the tires and the wet grass, rendering differential braking ineffective.
- The pilot's decision to attempt directional control via braking rather than performing a go-around (arremetida no solo) was identified as a non-conservative judgment.
- The presence of a protective fence immediately adjacent to the runway perimeter acted as a significant obstacle, exacerbating the damage to the aircraft.
- The pilot was fully qualified, with valid commercial and agricultural licenses, and the aircraft was within weight and balance limits with a valid airworthiness certificate.