What happened
On April 1, 2021, an Air Tractor AT-401B, registration PT-WQY, was performing a private flight from Tupaciguara, MG, to Barretos, SP. During the en-route phase, the pilot experienced an engine malfunction characterized by a gradual loss of power.
In an attempt to reach an unregistered airstrip in Planura, MG, the pilot performed an emergency landing in an unprepared field. During the maneuver, the aircraft struck shrubs near the threshold of the field, causing a loss of control. The aircraft subsequently collided with a medium-sized tree, which resulted in the structural separation of the airframe into two sections. The forward section of the aircraft struck the ground and caught fire. The pilot was uninjured, but the aircraft sustained substantial damage.
The investigation
CENIPA investigators examined the engine and fuel system of the PT-WQY to determine the cause of the power loss. While the aircraft held a valid Certificate of Airworthiness, investigators discovered that the engine was not equipped with the manufacturer-specified float-type carburetor. Instead, a device consisting of various additional parts and components had been installed at the front of the engine to modify the air-fuel mixture supply.
The investigation found that there was no approved technical data or manufacturer-specified requirements to support this modification. Furthermore, the airframe and engine logbooks lacked the necessary entries to document the approval for return to service following such a major alteration. The investigation could not determine exactly when these unauthorized changes were implemented, though they may have been present during the last scheduled inspection.
Findings
- The engine failure was linked to an unapproved major alteration to the aircraft's fuel supply system.
- The modified fuel system configuration was not provided for in the engine maintenance manual or the aircraft's type design specification.
- The lack of adherence to regulatory standards and approved technical data meant there was no demonstration that the fuel system could reliably supply adequate pressure and flow rate under all normal operating conditions.
- The aircraft's maintenance records for the airframe, engine, and propeller were not up to date.