What happened
On July 21, 2011, an Air Tractor AT-401, registration EC-EIZ, was conducting aerial spraying operations in the Ebro Delta region. The aircraft was operating from a temporary dirt strip at Playa Serrallo. During the seventh flight of the day, approximately 13 minutes after takeoff, the pilot noticed that movement of the throttle lever was not being transmitted to the engine, resulting in a significant loss of power.
With the engine unable to reach normal manifold pressure or RPM, the pilot performed an emergency landing on a nearby dirt path. While the pilot successfully landed on the path, the aircraft drifted during the final phase of the landing roll and entered an adjacent irrigation ditch. The aircraft sustained damage to the wingtip, propeller, and left aileron, and the engine was partially submerged in the water. The pilot, who was uninjured, was able to exit the aircraft on his own.
The investigation
The investigation focused on the mechanical failure of the throttle control system. Post-accident inspections of the carburetor assembly revealed that the nut connecting the throttle linkage to the carburetor was loose and unscrewed. Crucially, the investigation found that the safety pin required to secure the nut was missing.
The investigation examined the maintenance history of the aircraft, noting that the engine had been installed approximately 149 flight hours prior to the incident. While the aircraft had undergone two scheduled inspections (a 100-hour and an annual inspection) that specifically included checking the throttle linkage and safety pin—the most recent being only 35 hours before the accident—the missing pin was not detected. Investigators also looked into whether undocumented maintenance had occurred, as the pilot recalled recent work on the magnetos, though no written records supported any recent work on the throttle system.
Findings
- The primary cause of the incident was the failure of the throttle movement to reach the carburetor because the connecting nut had unscrewed.
- The nut became loose due to the absence of the safety pin.
- The investigation could not definitively determine if the safety pin was never installed during the engine installation two years prior or if it had been lost during subsequent operations.
- The high-vibration nature of agricultural spraying operations, involving frequent takeoffs, landings, and throttle adjustments, likely contributed to the nut loosening once the pin was absent.
- Maintenance records indicated that the safety pin should have been identified as missing during the two recent inspections, yet it went undetected.