Accident Summary
On the morning of 8 May 1987, a Hughes 369D helicopter (registration HB-XEX) operated by Fuchs Helikopter was conducting external load flights for the installation of high-voltage power line pylons near Filzbach, Switzerland. The loading area was located on municipal land at Brämboden on the Kerenzerstrasse, about 1.5 km west of Filzbach. After completing 21 rotations, transporting concrete to mast construction sites above and below the loading point, the pilot returned for one more load.
Pilot and Aircraft Information
The pilot was a Swiss citizen born in 1947, holding a professional helicopter pilot license. He had a total flight experience of 883 hours 45 minutes, of which 767 hours 8 minutes were on the accident type. In the preceding 90 days, he had flown 35 hours 46 minutes total and 30 hours 35 minutes on type. The helicopter was certified for flight and, according to the pilot, in airworthy condition. No other persons were on board.
Sequence of Events
While hovering at approximately 5 meters above ground, the pilot set down an empty concrete bucket. He then gestured to the driver of the concrete mixer truck, who was acting as a ground assistant, to indicate that the next bucket would be the last. However, the driver misinterpreted this gesture as meaning no more concrete was needed. Realizing the misunderstanding, the pilot decided to land. During the landing maneuver, he did not notice that a truck parked behind him had been moved forward by about one vehicle length since his previous landing. The helicopter's tail struck the roof of the truck's cab, causing the tail rotor to detach. The helicopter began to rotate around its vertical axis. The pilot managed to bring the aircraft to the ground, but the main rotor blades struck a construction crane parked at the edge of the landing area. The crane's control box was destroyed and parts of its lattice mast were damaged. No persons were injured. The helicopter was destroyed.
Investigation Findings
The investigation determined that the pilot was properly licensed and qualified for the flight. The helicopter was in a flightworthy condition. The concrete mixer driver was not a trained flight assistant; he had been helping hook and unhook concrete buckets, first under the direction of the client and then alone. There was no radio communication between the pilot and the ground assistant. Analysis of the truck's tachograph showed that the vehicle had been moved at 1000 local time, shortly after the helicopter's previous landing at the loading area. Weather conditions were characterized by high pressure: clear sky, light and variable wind, visibility over 10 km, temperature 10 °C, dew point -7.6 °C, QNH 1022 hPa, sun azimuth 116°, elevation 43°.
Cause
The accident was primarily due to the pilot not recognizing the changed conditions at the landing area. Contributing factors were the lack of air-ground radio communication on the loading site and the use of a ground assistant unfamiliar with helicopter operations.