What happened
On 13 July 2005, a PZL Swidnik W-3AS Sokol, registration SP-SUB, was performing aerial firefighting duties near Tineo, Asturias. The aircraft was returning to its base to refuel after conducting water drops to combat a fire in Candamo.
During the return flight, the crew began experiencing intermittent low-pressure indications in the second hydraulic system (HS2). Shortly thereafter, the pilot observed hydraulic fluid leaking from the overhead and right sections of the rear passenger cabin. Approximately two minutes after the initial HS2 fault, the first hydraulic system (HS1) also experienced a sustained low-pressure indication.
Following the loss of both hydraulic systems, the crew lost the ability to control the aircraft. The helicopter impacted the terrain approximately 2 km from the Tineo base. The impact caused the aircraft to overturn and catch fire, resulting in the complete destruction of the helicopter. The copilot escaped uninjured, but the pilot sustained serious burns and cuts during the evacuation.
The investigation
The CIAIAC investigation examined the aircraft's flight data recorder (FDR) and the hydraulic system design. Investigators analyzed the sequence of pressure drops and the physical damage to the aircraft. The investigation also reviewed the crew's experience, noting that while the pilots had extensive experience on the standard W-3 model, they had significantly fewer hours on the specific W-3AS variant.
Technical analysis focused on the hydraulic ground circuit, which is used during maintenance but remains pressurized during flight. Investigators also looked into the differences in emergency procedures between the W-3 and the W-3AS models, specifically regarding the use of the cockpit hydraulic selector switch.