What happened
On May 14, 2021, at 09:32 UTC, a Pilatus PC-6/B2-H4, registration I-HSKC, crashed near the Ravenna airport in Italy. The aircraft was conducting a visual flight training mission, specifically a class rating renewal flight, with two pilots on board: an instructor and a student pilot.
After completing several landing circuits, the aircraft was positioned in the downwind leg of the fourth circuit when it suddenly entered an extreme nose-down attitude. The aircraft plummeted toward the ground, impacting a field and subsequently catching fire. The impact also caused the aircraft to strike a perimeter fence and come to rest near a local building. Both pilots sustained fatal injuries and died upon impact.
The investigation
The ANSV investigation focused on the mechanical state of the aircraft' and the flight dynamics leading to the loss of control. Investigators examined the wreckage, specifically focusing on the horizontal stabilizer trim system. The aircraft was equipped with an electric pitch trim system powered by a dual-motor actuator using two relays.
Technical analysis of the relays recovered from the wreckage revealed significant irregularities and localized melting on the power contact surfaces. This physical evidence suggested that the relays had failed in a closed position, which allowed continuous electrical current to flow to the actuator.
Findings
- The primary cause of the accident was a pitch trim runaway, where the horizontal stabilizer moved to its full nose-down limit without pilot input.
- The failure was triggered by a mechanical or electrical malfunction in the trim system relays, which became stuck in the closed position.
- The rapid movement of the stabilizer (occurring within 6 to 10 seconds) created massive control forces that were difficult to counteract, especially at low altitudes.
- The student pilot's relative lack of experience in recognizing and responding to such a critical emergency may have hindered a timely recovery.
- The instructor pilot may not have immediately realized the severity of the situation due to the lack of an in-cockpit warning for this specific failure.
Safety action
Following the findings, the ANSV issued two safety recommendations to EASA:
- A recommendation to evaluate the implementation of a life limit for the electric pitch trim relays on the PC-6T fleet to prevent wear-related failures.
- A recommendation to reconsider the Aircraft Level Functional Hazard Assessment (FHA) regarding the horizontal trimmable stabilizer, specifically focusing on risk mitigation strategies for pitch trim runaway scenarios.