What happened
On January 30, 2022, a commercial pilot was performing aerial firefighting duties near Panguilemo Aerodrome (SCTL) in the Maule Region of Chile. After completing mission objectives, the pilot began the return flight to the base. During the approach, the Thrush Aircraft Inc. S2/R-T660 experienced a partial loss of engine power. Shortly thereafter, the engine ceased functioning entirely, necessitating an emergency landing in an agricultural field approximately 13 km south of the aerodrome. The pilot escaped the incident without injury, and the aircraft sustained no damage.
The investigation
DGAC Chile investigators examined the aircraft's maintenance records, fuel levels, and digital engine monitoring data. The investigation focused on the fuel system, specifically the interaction between the wing tanks and the header tank. While the pilot believed fuel levels were relatively balanced, data from the MVP-50 engine monitor revealed a significant imbalance. The investigation also reviewed the operator's manual and flight maneuvers performed during the firefighting mission.
Findings
- The engine shutdown was caused by an interruption of fuel flow to the engine.
- Digital monitoring showed that while the left tank held 109 liters, the right tank had dropped to 15 liters or less, reaching an unusable level.
- The fuel imbalance was likely caused by continuous left-hand turns and uncoordinated flight maneuvers during aerial work, which caused fuel to migrate from the right wing.
- As the right tank depleted, the header tank was unable to maintain sufficient pressure, allowing air to enter the fuel lines.
- The pilot did not perform a continuous cross-check of the digital fuel indicators to monitor the imbalance.
- The aircraft's design lacks a check valve or non-return valve, allowing fuel to shift between tanks during maneuvers.
Safety action
- The investigation noted that the operator's manual lacked specific procedures for fuel management and did not authorize "Fire Extinction" operations, only "Forest Patrol."
- Recommendations were made for operators to establish strict fuel management procedures, including mandatory cross-checks and minimum fuel requirements.
- The investigation suggested that airworthiness authorities evaluate the feasibility of recommending the installation of check valves or low-level warning systems for the header tank in similar aircraft types.