What happened
On January 18, 2016, an AT-402B agricultural aircraft, registration PR-CVL, was returning to its base in Querência do Norte, Brazil, after completing an agricultural spraying mission. During the return flight, the engine experienced a sudden loss of power, with the engine speed dropping and remaining stuck at an idle setting (Ng at 48%).
The pilot attempted to restore power by advancing the power, propeller, and mixture levers and activating the fuel pump, but the engine failed to respond. Realizing the aircraft could not reach the intended runway, the pilot executed the emergency landing procedures and performed a precautionary landing in a cattle pasture. The aircraft came to a stop after traveling 1 and 35 meters, causing damage to a wire fence, but the aircraft sustained no structural damage and the pilot was uninjured.
The investigation
CENIPA investigators focused on the engine's Fuel Control Unit (FCU) as the suspected cause of the power loss. The FCU was removed for laboratory testing by the Institute of Aeronautics and Space (IAE). Initial inspections showed no external physical damage such as cracks or dents, but bench tests revealed that the fuel flow did not correspond to the position of the FCU lever. Specifically, investigators identified that the pressure in the aneroid bellows was inoperative.
Subsequent analysis by Pratt & Whitney Canada (P&WC) in Brazil confirmed the presence of micro-cracks in the aneroid bellows and identified a leak in the acceleration area of the component. The investigation noted that the bellows was in contact with the internal wall of its vacuum chamber, suggesting that vibrations might have caused the cracks, though the exact source of the vibration could not be determined. The investigation also noted that the FCU had undergone an overhaul and returned to service shortly before the incident, though no specific deficiencies in the overhaul process could be objectively confirmed.
Findings
- The inoperative aneroid bellows within the engine's Fuel Control Unit caused a fuel flow blockage, preventing the engine from producing power above idle.
- The pilot's decision to perform a precautionary landing in a pasture prevented further injury or damage.
- The source of the vibration that potentially led to the micro-cracks in the bellows remained undetermined.