What happened
On April 29, 1997, a Hughes 300 helicopter, registration HB-XNG, was conducting a training flight near Altenrhein, Switzerland. The flight, part of a professional pilot training program, involved a transition from navigation exercises to a low-level flight task requiring the crew to fly through confined spaces, specifically along the course of the Thur river.
As the helicopter flew approximately 4 to 5 meters above the water, the instructor identified a telephone cable crossing the river. At the same moment, the instructor noted a slight drop in engine RPM. To recover the engine speed, the instructor adjusted the collective pitch, initiating a right-hand turn to avoid the obstacle. During this maneuver, the aircraft descended toward the turbulent water surface. The right landing skid made contact with the river, causing the helicopter to plunge into the water. The aircraft was swept downstream by the current and destroyed. Both the instructor and the student successfully escaped the wreckage and reached the riverbank without injury.
The investigation
Investigators examined the aircraft's maintenance records, which showed the helicopter was properly maintained and airworthy. The investigation focused on the environmental conditions and the operational decisions made during the flight. Heavy rainfall in the Alpstein region had recently caused the Thur river to swell, creating a high-water state with significant spray and waves, effectively turning the river into a turbulent stream. This changed the riverbed topography and obscured the usual sandbanks that could have served as emergency landing sites.
Technical analysis of the engine was not possible due to the extensive destruction of the airframe. While the instructor suggested that water spray or high humidity might have caused a momentary power loss, the manufacturer stated that such power reductions are not expected during low-level water flights. Investigators also considered whether the pilot's coordination of the throttle during the collective pitch adjustment contributed to the loss of altitude.
Findings
- The primary cause of the accident was a loss of altitude during the low-level flight over the water for unknown reasons.
- The visibility of the telephone cable was severely compromised by the changed river conditions, the presence of trees, and water droplets on the windscreen.
- The high water level and turbulent conditions increased the difficulty of the maneuver and removed potential landing options.
- The instructor's familiarity with the area led to a level of risk-taking that allowed the aircraft to fly into a confined space with significant hazards.
Safety action
- The investigation identified that the high level of risk-taking during a training flight was a contributing factor to the accident.