Accident Overview
On 30 June 1987 at approximately 1402 local time, an SA315B "Lama" helicopter, registration HB-XPJ, operated by Rhein-Helikopter AG, was involved in an accident at Balzers, Liechtenstein. The helicopter was performing a commercial material transport flight to a hunting lodge at an elevation of 1500 m on the wooded western slope of Mittagsspitze. The pilot, a Liechtenstein national born 1948, held a professional pilot license and had accumulated 2,224 total flight hours, with 71 hours in the last 90 days. On board were the pilot, a flight assistant, and another client.
Sequence of Events
Before departure, the pilot had arranged with the client that upon arrival, the client would unhook the 400 kg load suspended from a 25 m steel cable and replace it with a transport net filled with empty containers for the return flight. The flight departed Balzers shortly before 1400. Upon reaching the destination, the pilot set the load down in a hover. After confirming via the control mirror that the client had executed the exchange, the pilot climbed vertically out of the forest, turned away from the slope, and began to accelerate. At approximately 70 kt indicated airspeed, the load suddenly disappeared from the mirror. The pilot felt a jolt on the tail rotor and immediately released the cable. The helicopter began to yaw to the left. The pilot entered autorotation and managed to stop the rotation. He found that the now heavily nose-down helicopter could be best controlled at about 50 kt in a left-hand circular flight path. After descending from roughly 900 m, he reduced power at about 100 m above ground and landed on a meadow. Although all damping struts buckled, the helicopter came to rest on its skids. No occupants were injured.
A witness, a police officer, observed the net suddenly inflating like a parachute and rising to the height of the tail rotor. He then heard a bang, saw something separate and drift downward like a parachute, and watched the helicopter descend nose-first in circles until it disappeared from view.
Findings
Examination determined that the steel cable of the transport net had become entangled in the tail rotor, tearing off two blades. The resulting imbalance caused the complete separation of the tail rotor and its gearbox from the lattice frame. The skin of the right half of the stabilizer was also damaged. The net contained five plastic crates and a plastic sheet measuring 7 by 3 meters. The pilot was properly qualified, and the helicopter had a valid certificate with no pre-existing defects.
Weather Conditions
The weather was characterized by a flat high-pressure system. The sky was slightly cloudy with a cloud base around 2400 m. Wind was light and variable, visibility greater than 10 km, temperature 26°C, and QNH 1018 hPa.
Cause
The accident was caused by damage to the tail rotor by the transport net due to improper stowage of a plastic sheet. When the pilot transitioned to translational flight, the improperly packed plastic sheet caught air, inflated, and created drag sufficient to cause the relatively light load to rise rearward rapidly, bringing the cable into the tail rotor arc.