What happened
On the night of 19 November 2009, a Schweizer 269C helicopter, registration HB-XYI, was conducting a night training flight near Unterbärgetal, Switzerland. The flight, involving a flight instructor and a trainee pilot, was intended to fulfill requirements for night flying training.
During the flight, the crew attempted a field landing at a location in the Unterbärgetal valley. As the aircraft progressed through the final approach, the crew relied on distant vehicle headlights for altitude references. However, as they descended, these references disappeared. The instructor decided to use a steep approach due to the lack of light. When the landing light was eventually switched on during the final phase, the high atmospheric humidity caused intense reflections, and the crew realized that ground-level fog had significantly reduced visibility.
In an attempt to avoid the blinding glare from the fog, the instructor turned the landing light off and initiated a go-around. During this maneuver, the lack of visual references led to a loss of control, causing the helicopter to strike the terrain. The aircraft bounced, moved uncontrollably across several axes, and eventually came to a heavy rest in a field. The impact caused severe damage to the aircraft, including a severed tail rotor drive shaft.
The investigation
The Swiss Accident Investigation Board (SAIB) examined the flight history, crew experience, and environmental conditions. The investigation reviewed the flight path, weather reports, and the technical state of the aircraft. Investigators also looked into the flight school's training manuals and the specific procedures used during the approach.
Findings
- The primary cause of the accident was the loss of control during a night field landing, which resulted in the collision with the ground.
- The crew had limited night flying experience, and the flight instructor was performing his first night training flight in an instructional capacity.
- The choice of a field landing site was inappropriate given the prevailing weather conditions, which included mist and patches of fog.
- The approach procedures used for the night landing were unsuitable.
- The helicopter's technical equipment was considered marginal for performing field landings at night, as it lacked an artificial horizon.
- The flight school's training manual lacked sufficient detail regarding the requirements and conditions for acting as a night flight instructor.
- The flight mission was deemed too demanding for the current training level of the crew.
Safety action
Following the accident, the flight training organization issued a safety bulletin to warn against the dangers of "black hole" approaches during dark nights and the risks of rapid fog formation in valleys. The organization also updated its operations manual to implement stricter requirements for night flight instructors, including minimum flight hour thresholds and specific prerequisites for conducting field landings at night.