What happened
On the evening of January 21, 2020, a flight training mission involving an instructor and a student was underway in an Aero AT-3R100, registration SP-RWC. The flight, conducted under Visual Flight Rules (VFR) at night, began after the crew had refueled the aircraft in low-light conditions. During the flight, the crew observed a rapid decrease in the fuel level of the left tank while the right tank level appeared stable. Initially, the crew misidentified this discrepancy as a faulty fuel gauge.
Approximately 37 minutes into the flight, the crew realized that the fuel filler cap for the right wing tank was missing. Because the fuel valves were in the open position, the aircraft had essentially become a connected system where fuel was being siphoned from the left tank into the right tank and then out of the open right wing filler due to aerodynamic suction. This continuous loss of fuel led to the engine failing after 51 minutes of flight. Despite attempts to restart the engine in flight, the power loss persisted. The crew performed an emergency landing in a wooded area, resulting in the total destruction of the aircraft. Both occupants survived the crash with no serious injuries.
The investigation
The PKBWL investigation examined the aircraft's maintenance records, the flight history, and the physical state of the aircraft following the impact. Investigators analyzed the fuel system configuration and the aerodynamic effects of the wing profile on the filler cap. The investigation also reviewed the operator's Flight Manual (AFM) and the procedures used during the pre-flight inspection and refueling process.
Findings
- The primary cause of the accident was the improperly secured fuel filler cap in the right wing, which allowed it to be sucked out by aerodynamic forces during the early phase of flight.
- The loss of the cap created a continuous flow of fuel from the left tank to the right tank, where it was subsequently expelled from the open filler.
- The crew's decision to treat the fuel level discrepancy as a gauge malfunction prevented timely intervention.
- Pre-flight procedures were compromised because the aircraft was refueled after the initial inspection was completed.
- Nighttime refueling under poor lighting conditions contributed to the error.
- The existing Flight Manual contained outdated instructions that did not specifically require checking the wing tank caps for this specific aircraft variant.
- A probable malfunction of the right tank's fuel gauge prevented the crew from knowing the true fuel state.