What happened
On May 11, 2021, an AT-502B aircraft, registration PR-ORS, was performing a local agricultural spraying mission over a cotton plantation at Fazenda Savana, in the municipality of Riachão das Neves, Bahia. Shortly after takeoff, the aircraft's engine lost power. The pilot, who was the sole occupant of the aircraft, attempted an emergency landing and touched down on a secondary dirt road approximately 1,200 meters beyond the end of the agricultural airstrip. The aircraft traveled about 157 meters along the ground before coming to a stop off the road. The aircraft sustained substantial damage, including damage to the left main landing gear strut, the left wing extralodos, and internal engine components. The pilot suffered no injuries.
The investigation
CENIPA's investigation focused on the powerplant failure. Technical analysis of the engine, conducted in collaboration with the manufacturer's laboratory in Canada, revealed that the engine failure was triggered by the rupture of the fourth blade of the compressor turbine rotor. This rupture was attributed to high cycle fatigue. The investigation found that the failure of this specific blade caused subsequent overloads that led to the rupture of other blades on the disk rotor. Investigators also examined the possibility of chemical corrosion, noting that the use of pesticides containing sulfur—common in cotton farming—could potentially contribute to sulphidation, a form of corrosion. While the aircraft's maintenance records, including the most recent 100-hour inspection, were up to date and compliant with the maintenance manual, the investigation could not rule out the possibility that exceeding temperature limits or chemical exposure contributed to the fatigue process.
Findings
- The engine failure was caused by the rupture of a compressor turbine blade due to high cycle fatigue.
- The rupture of the primary blade led to a chain reaction of overloads in other blades on the rotor disk.
- The use of sulfur-based pesticides in the area may have contributed to the initiation of the fatigue through sulphidation.
- A potential exceedance of Inter-Turbine Temperature (ITT) limits could not be ruled out as a contributing factor.