What happened
On February 20, 2025, at approximately 12:57 UTC, an H 36 Dimona motorized glider, registration I-ILMA, crashed into Lake Varese, Italy. The aircraft was performing a recreational flight, which was the pilot's fourth flight of the day following three short training sessions with an instructor.
During the flight, the aircraft experienced a sudden and significant change in engine noise. Shortly thereafter, the aircraft entered a steep left turn, followed by a period of altitude oscillations and a loss of airspeed. The aircraft eventually entered a high-angle-of-attack state and a violent right-hand spin at a very low altitude. Due to the low height—estimated at less than 60 meters AGL—the pilot was unable to execute the necessary recovery maneuvers. The aircraft struck the surface of the lake, resulting in the destruction of the aircraft and one fatality (the pilot, who sustained serious injuries).
The investigation
The ANSV investigation focused on the mechanical state of the engine controls and the flight dynamics leading to the impact. Investigators examined the wreckage and found that the engine's throttle cable had snapped inside its sheath.
Analysis of the engine's audio and video surveillance confirmed that at the moment of the failure, the engine RPM surged from approximately 1500 RPM to 3400 RPM (full power). This surge was caused by internal return springs on the carburetors, which automatically pull the engine to maximum power when the cable breaks. While this feature is common in CS-23 certified aircraft, it is not a requirement for the JAR-22 certification base of the H 36 Dimona, and no specific procedures for such an event were documented in the Pilot's Operating Handbook (POH).
Fractographic analysis of the broken cable revealed significant oxidation and evidence of stress corrosion cracking. The investigation also noted that the pilot, while highly experienced in gliders, had limited recent experience with motorized aircraft.
Findings
- The primary cause of the accident was a loss of control resulting from the sudden surge to full engine power.
- The throttle cable failure was triggered by progressive stress corrosion cracking within the cable sheath.
- The sudden increase in power, combined with the pilot's attempt to manage the aircraft through high-pitch attitudes, led to a stall and a subsequent violent spin.
- The low altitude at the time of the engine surge made a successful recovery maneuver impossible, as the required recovery height exceeds the altitude available.
- The lack of specific emergency procedures in the POH for a throttle cable breakage contributed to the pilot's inability to manage the sudden power surge.