Tail boom failure on Hughes 369E following rotor strike by loose sack

Casualties unknown • FI

A Hughes 369E helicopter experienced a catastrophic tail boom failure during a forestry operation in Siilinjärvi when a large empty sack was blown into the main rotor.

What happened

On July 11, 1997, a Hughes 369E helicopter, registration OH-HWA, was conducting forestry operations near Siilinjärvi, Finland. The crew was engaged in a fertilization mission, utilizing a site near the ruins of a barn to swap empty fertilizer spreaders for filled ones.

During the 14th flight of the morning, the pilot positioned the aircraft over a filled spreader. As an assistant was attaching the spreader to the hook, a large, empty 600 kg fertilizer sack was blown from a nearby storage area into the helicopter's main rotor. The impact caused the aircraft to vibrate violently and pitch forward by approximately 20 to 30 degrees. In an attempt to correct the pitch, the pilot pulled back on the cyclic; however, this maneuver caused the main rotor blades to strike the tail boom, severing it completely.

The loss of the tail rotor caused the helicopter to spin violently to the right. The pilot responded by reducing power to idle and performing an autorotation, bringing the aircraft to a controlled landing on the ground. The severed tail boom was thrown from the aircraft, striking a truck and hitting an assistant on the ground, causing bruises to his shoulder and hip. The aircraft sustained significant damage.

The investigation

The investigation examined the storage of materials at the landing site and the aerodynamic conditions during the approach. Investigators found that empty sacks were being stored in a corner of a ruined building. At the time of the accident, the pile of sacks had reached the level of a window opening in the structure.

Findings

  • The primary cause of the accident was a large empty sack blowing out of a storage area and into the main rotor.
  • The airflow generated by the helicopter as it approached the loading area created positive pressure within the windowless room. This pressure forced the sack out through an open doorway.
  • The accumulation of empty sacks near the window opening increased the likelihood of the sack being caught in the rotor airflow.
  • The pilot's corrective action (pulling back on the cyclic) directly contributed to the rotor blades striking the tail boom after the initial impact with the sack.

Probable cause

The accident was caused by an empty fertilizer sack being blown from a storage area into the main rotor due to aerodynamic pressure changes, which subsequently led to a tail boom failure during the pilot's attempt to correct the aircraft's pitch.

Frequently asked questions

What happened in the 1997-07-11 aircraft accident near FI?

A Hughes 369E helicopter experienced a catastrophic tail boom failure during a forestry operation in Siilinjärvi when a large empty sack was blown into the main rotor.

What aircraft was involved and where did it happen?

The accident on 1997-07-11 involved a aircraft, registration OH-HWA, at FI.

What was the probable cause of the accident?

The accident was caused by an empty fertilizer sack being blown from a storage area into the main rotor due to aerodynamic pressure changes, which subsequently led to a tail boom failure during the pilot's attempt to correct the aircraft's pitch.

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