Eurocopter AS 355F1 undergoes forced landing following engine vibration issues

Casualties unknown • circa 750 Meter nordnordöstlich von Flaurling, Gemeinde Flaurling, Tirol, AT

A maintenance test flight of an AS 355F1 helicopter ended in a dynamic rollover near Flaurling, Austria, after mechanical issues were detected during a power check.

What happened

On December 3, 2002, at approximately 14:35 UTC, a Eurocopter AS 355F1 helicopter, operated by a flight rescue organization, departed from Innsbruck Airport (LOWI) for a mandatory maintenance test flight. The aircraft was undergoing a 500-hour inspection, which had recently included the replacement of the combiner gearbox and both freewheel units.

On board were the pilot and a first-class maintenance technician who had participated in the recent overhaul. Due to a low, thick cloud ceiling, the flight was conducted at low altitude. After successfully performing a hover test at Innsbruck and proceeding toward Telfs, the crew began a power check of the engines. While performing the power check on engine number 1, the aircraft suddenly experienced intense vibrations, a loud hammering noise, and a brief yawing motion around the vertical axis.

In response to the instability, the pilot initiated an autorotative landing on a meadow that sloped slightly in the direction of flight. The landing area was bordered by power lines and railway tracks. Because the aircraft was equipped with snow skids, the skids dug into the soft ground immediately upon touchdown. This caused the nose of the helicopter to strike the ground and a rotor blade to impact the earth, leading the aircraft to capsize onto its left side.

The investigation

The investigation focused on the mechanical state of the drivetrain following recent maintenance work. Investigators examined the newly installed combiner gearbox and the freewheel units to determine why the power check triggered such severe vibrations.

Findings

  • The investigation identified slippage of the left freewheel clutch under load as the primary cause of the vibrations.
  • Evidence of similar, though less pronounced, slippage was also found on the right clutch.
  • The damage was attributed to multiple engagement and disengagement cycles occurring during the recent maintenance and testing phases.
  • The aircraft sustained a total loss due to the impact.
  • There were no injuries to the two occupants.

Probable cause

The sudden onset of severe vibrations and yawing during the engine power check was caused by the left freewheel clutch slipping under load, a condition resulting from repeated engagement and disengagement cycles during recent maintenance work.

Frequently asked questions

What happened in the 2002-12-03 Eurocopter AS 355F1 accident near circa 750 Meter nordnordöstlich von Flaurling, Gemeinde Flaurling, Tirol, AT?

A maintenance test flight of an AS 355F1 helicopter ended in a dynamic rollover near Flaurling, Austria, after mechanical issues were detected during a power check.

What aircraft was involved and where did it happen?

The accident on 2002-12-03 involved a Eurocopter AS 355F1, at circa 750 Meter nordnordöstlich von Flaurling, Gemeinde Flaurling, Tirol, AT.

What was the probable cause of the accident?

The sudden onset of severe vibrations and yawing during the engine power check was caused by the left freewheel clutch slipping under load, a condition resulting from repeated engagement and disengagement cycles during recent maintenance work.

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