What happened
On July 10, 2004, an SA 315B helicopter, registration HB-XFX, was performing a passenger and material transport flight from Zermately to the Margherita Hut at an altitude of 4,554 meters. During the approach to the Monte Rosa massif, the aircraft was flying near its performance limits in challenging environmental conditions.
While maneuvering near the ridge, the helicopter experienced a sudden loss of altitude. Witnesses and passengers described the sensation of falling into a "void" or air pocket. The aircraft struck a mountain edge, and the impact was severe enough to cause the engine to lose all power. Following the initial collision, the helicopter drifted toward a glacier, where it made a second impact. The pilot and passengers sustained significant injuries, and the aircraft was heavily damaged.
The investigation
Investigators examined the meteorological conditions and the aircraft's flight path. At the time of the accident, winds of 25 to 30 knots were recorded in the region. The investigation focused on the aerodynamic effects of the terrain, specifically how the wind interacting with the Signalkuppe and the Seser Pass created complex air patterns.
Technical examinations of the SA 315B revealed no mechanical failures or pre-existing technical defects. The aircraft's weight and center of gravity were within legal limits, and the engine was functioning correctly until the physical impact with the terrain occurred. The investigation also looked into the lack of an onboard oxygen system, noting that while not required by regulations for this specific operation, the high altitude could have impacted mental performance due to hypoxia.
Findings
- The primary cause of the accident was a loss of control of the helicopter.
- The initial impact with the terrain caused such extensive damage that the engine could no longer provide power.
- The aircraft was operating at its performance limit within a difficult environment.
- Localized downdrafts and turbulence were created by "flow split" and "gap flow" effects, where wind moving through mountain passes and around peaks causes rapid changes in vertical air movement.
- High wind speeds (up to 60 km/h) and complex wind shear near the mountain ridge contributed to the unstable flight conditions.