Accident Overview
On 13 August 2003, at approximately 15:15 local time, a Eurocopter EC 120 B helicopter, registered as HB-ZER, was destroyed in an accident at the Gebirgslandeplatz Hotel Steingletscher, a mountain landing site near Gadmen, Bern, Switzerland. The pilot and two passengers were on board. The pilot sustained minor injuries; the passengers were uninjured.
Flight History
The pilot, a Swiss citizen born in 1948, held a private pilot license for helicopters (PPL(H)) issued by the Federal Office of Civil Aviation. He had accumulated 232:56 total flight hours, including 21:30 hours on the EC 120 B type. On the day of the accident, the pilot refueled the helicopter with 220 kg of fuel and departed from Pfaffnau at 13:15 with two passengers for a sightseeing flight in the Bernese Oberland. After a flight, he approached the Steingletscher landing site from the west, landed, and subsequently performed five practice takeoffs and landings, all from west to east. Following the last landing at about 14:15, the pilot shut down the engine and systems, and the occupants left the helicopter for a break.
Accident Sequence
Approximately one hour later, the same occupants reboarded. The pilot started the engine and systems. The helicopter was parked approximately parallel to the pass road with its nose pointing east. The pilot noted a light tailwind. After lifting off, a brief hover check was performed. The pilot then intended to turn the helicopter to the right (into the wind) to depart down-valley. Two witnesses observed that after lift-off, the helicopter rose to about one meter above ground and began an increasing left yaw rotation while climbing. The pilot reported applying full right pedal without a marked decrease in rotation. He used cyclic control to stay over the landing site and decided to land by lowering the collective. During touchdown while still rotating left, the helicopter tipped onto its right side and came to rest above a steep embankment. A smoldering fire in the engine was extinguished by a bystander.
Meteorological Conditions
MeteoSwiss reported a weak high-pressure system over central Europe with northwesterly flow bringing moist air to the Alps. At the accident site, conditions included 1–2/8 cumulus clouds base 9000 ft AMSL, visibility about 20 km, wind from west-northwest at 5–8 kt with gusts to 12 kt, and temperature 22 °C. The weather forecast indicated possible isolated thunderstorms in the mountains and gusty valley winds. Eyewitnesses reported stronger wind from west-northwest, with the windsock at the landing site forming a 65–70° angle from vertical. Throughout the afternoon, alternating upslope and downslope winds were observed. The predominant wind direction was west-northwest (upslope), but intermittent cold air drainage from the Steingletscher caused temporary easterly winds (downslope).
Technical Investigation
Post-accident examination of HB-ZER, conducted with the manufacturer, found that all fractures and deformations resulted from impact forces during the rollover. No pre-existing defects or limitations affecting flight controls or engine performance were identified. The fenestron (ducted tail rotor) was capable of full effectiveness until ground contact. The Eurocopter EC 120 B is a five-seat light helicopter with a clockwise-rotating main rotor and a fenestron for torque compensation. The flight manual did not include sidewind or tailwind demonstrated limits or critical wind azimuth data, nor handling characteristics for helicopters equipped with a fenestron.
A manufacturer service letter (No. 1518-67-01, dated 26 April 2001) had been issued to customers addressing yaw control issues for helicopters with clockwise-rotating main rotors. It noted that under certain conditions (hover, very low speed with light wind), a left yaw departure can develop into a high rotation rate if the pilot does not quickly apply appropriate pedal. The pilot stated he was unaware of this information.
Cause
According to the investigation, the accident resulted from a collision with the ground after the pilot, following a tailwind takeoff, lost control of the helicopter in a yaw rotation while attempting to land.