What happened
On April 12, 2016, an experimental unmanned helicopter, a RUAS SD-150 HERO with registration I-UASC, was conducting a flight test at the Siena Ampugnano airport in Italy. The flight was part of an experimental program to verify controllability characteristics for future certification.
While the aircraft was in a hover at approximately 35 meters AGL, the engine experienced a sudden drop in RPM. This loss of power caused the aircraft to descend rapidly, resulting in a high-energy impact with the ground near the runway. The aircraft sustained significant damage to the main rotor, tail rotor, fuselage, and landing skids. There were no injuries to the crew, who were operating the aircraft from a ground control station and via visual line of sight.
The investigation
The ANSV investigation focused on the engine's performance and the aircraft's flight control systems. Analysis of the Engine Control Unit (ECU) data revealed a significant performance degradation trend in the flights leading up to the accident. During the accident flight, the engine's RPM and throttle settings dropped drastically from target levels (6500 RPM and 100% throttle) to approximately 4500 RPM and 70% throttle for about two seconds.
Investigators examined the engine, a Göbler Hirthmotoren S1220/01M, in Germany. The inspection of the reed valves—components that regulate airflow into the combustion chambers—revealed that a carbon fiber lamella in the valve for cylinder No. 2 had been partially destroyed, with about 1 cm² of material missing. Furthermore, the mounting screws for these valves were found to be improperly secured, which likely allowed excessive movement and led to the progressive damage of the lamella.
While some debris and mold were found in the fuel tank, tests confirmed that the fuel filters were not obstructed and that the fuel and oil quality met all required specifications. The investigation also ruled out any malfunction in the Flight Control Computers (FCC) as the cause of the power loss.
Findings
- The primary cause of the engine failure was the damage to a reed valve lamella, which was induced by improper assembly of the component.
- The loose mounting screws on the reed valve assembly allowed for excessive movement, leading to the structural failure of the carbon fiber lamella.
- The aircraft lacked an automatic emergency recovery capability, such as an automated autorotation or a system to limit descent rate, which would have mitigated the impact energy during an engine failure.
Safety action
- The ANSV issued a safety recommendation (SRGC) to EASA, suggesting that unmanned helicopters should be evaluated for mandatory equipment of automatic emergency recovery devices to limit vertical descent rates following engine failures.