What happened
On the morning of January 10, 1998, a Grumman G-164A, registration CS-APW, operated by Agroar – Trabalhos Aéreos Lda., departed from an agricultural runway at Álamo, near S. Miguel de Machelar, to perform aerial chemical spraying operations. The aircraft was carrying approximately 500 liters of chemical payload.
Approximately two minutes after takeoff, the engine experienced a partial loss of power accompanied by significant fluctuations in RPM. Unable to maintain altitude, the pilot elected to jettison the entire chemical load and perform an emergency landing on an unprepared, slightly uphill field. During the final approach, the pilot noted an excessive sink rate, which he attributed to the upward slope of the terrain. The aircraft struck the ground first with the tailwheel and then with the main landing gear, eventually coming to rest in an inverted position. The pilot, the sole occupant, escaped the wreckage uninjured.
The investigation
The investigation examined the aircraft's maintenance records, the engine's ignition system, and the fuel system. Investigators noted that the aircraft had been refueled the previous day using gasoline from drums. While the electrical system, magnetos, and spark plugs showed no anomalies, the investigation focused on the possibility of fuel contamination. The investigation also noted that the carburetor float bowl was difficult to inspect in the field, as such checks were typically reserved for maintenance intervals.
Findings
- The engine experienced a partial loss of power shortly after takeoff.
- No mechanical failures were found within the engine or its accessories.
- The propeller blades were deformed backward, indicating the engine was at low power or stopped at the moment of impact.
- The pilot's approach to the landing site involved an increased angle of attack due to the upward gradient of the terrain, which led to a significant decrease in airspeed toward the stall speed, causing the aircraft to sink into the ground.
- The most probable cause of the power loss was fuel contamination, likely caused by water ingress from condensation in the fuel tanks or from the drums used for refueling, which may not have been fully removed during the fuel draining process.