What happened
On May 25, 2011, a Schweizer S269C helicopter, registration I-PAZZ, was engaged in an aerial spraying mission to disperse fungicides over a vineyard near Farra di Soligo, Italy. While performing a left turn at approximately 30 feet AGL, the pilot experienced a loud noise from the engine, followed by a complete loss of power.
In an attempt to manage the emergency, the pilot successfully released the onboard pesticide payload. However, unable to return to the departure point, the pilot attempted an emergency landing, which resulted in a violent impact with the vineyard. The aircraft struck a row of vines, breaking a support post, and sustained heavy damage, including a separated tail boom and deformed landing skids. The pilot sustained serious injuries and was transported to a hospital in Trebbiuso.
The investigation
The ANSV investigation focused on the mechanical state of the Lycoming HIO360-D1A engine. While the engine had undergone a major overhaul approximately 539 flight hours prior to the accident, investigators discovered a structural failure in the connecting rod of the fourth piston.
Laboratory analysis of the broken connecting rod revealed evidence of fatigue failure originating from multiple points. Specifically, investigators identified areas of galling and fretting on the connecting rods. The investigation also noted that the engine's magneto timing was set at 25 degrees, exceeding the permitted 20 degrees, and found pitting on the valve spring retainers for all cylinders except the fourth.
Findings
- The primary cause of the accident was the structural failure of the number 4 piston connecting rod.
- The failure was driven by progressive fatigue, which was initiated by galling on the connecting rod.
- This galling was likely caused by periods of high functional engine fatigue, such as operating at excessive manifold air pressure (MAP) or high RPM, which are common during heavy-duty agricultural work.
- The use of an agricultural kit increased the maximum takeoff weight by 45 kg, contributing to the high-stress operating conditions.