What aircraft was involved and where did it happen?

The accident on 2019-07-14 involved a Sokol W-3A, at am Bürglkopf, Gemeinde Fieberbrunn, Tirol, AT.

What was the probable cause of the accident?

The sudden loss of engine power was caused by corrosion found in the compressor vane opening holes, which led to an inability to maintain flight altitude and necessitated an emergency landing.

Helicopter Power Loss Leads to Emergency Landing in Tyrol

A Sokol W-3A helicopter experienced a sudden loss of power during a power line inspection, resulting in an emergency landing and rotor damage near Fieberbrunn.

What happened

On January 14, 2019, at approximately 11:34 UTC, a Sokol W-3A helicopter was performing a specialized inspection of high-voltage power lines near Fieberbrunn, Austria. The mission, conducted for a power grid operator, aimed to inspect lines and clear snow from adjacent trees following heavy snowfall.

During the flight, while the aircraft was traversing the area near Almgasthof, the pilot experienced a sudden and abrupt loss of engine power. Unable to maintain altitude, the pilot attempted to maneuver the aircraft toward a suitable landing site on the western side of the Almgasthof. The helicopter descended at a high rate of sink.

Upon touchdown, the main rotor blades struck a birch tree located to the left of the aircraft's longitudinal axis. This impact caused severe damage to the main rotor blades, some of which were severed. Fragments from the main rotor blades subsequently struck the tail rotor and a nearby building's facade, causing additional damage. The pilot successfully brought the aircraft to a halt by cutting fuel supply to the engines. There were two fatalities among the crew and one injury sustained by a passenger, though the aircraft was destroyed.

The investigation

The Austrian Federal Safety Investigation Board (SUB) examined the aircraft's maintenance history, flight documents, and the engine's technical condition. The investigation included an audio spectrum analysis of the cockpit voice recorder and video recordings, as well as a detailed examination of the engine components. Investigators specifically focused on the engine's compressor stages and the condition of the anti-ice systems. The investigation also reviewed the flight preparation, including the pilot's weather briefing and the aircraft's weight and balance calculations for the mission.

Findings

The primary cause of the power loss was identified as corrosion in the opening holes of the compressor vanes within the engine.
The aircraft was operating in a mission profile involving low-level flight (approximately 15–20 meters above treetops) to inspect power lines.
The pilot had activated the engine and EAPS anti-ice systems and the rotor blade de-icing systems in manual mode prior to takeoff.
The impact with the tree during the emergency landing was the direct cause of the structural failure of the main and tail rotor blades.