What happened
On November 16, 1997, a privately operated Pulsar XP, registration HB-YHN, departed from runway 29 at Ambri for a VFR flight toward Wangen-Lachen. The aircraft was occupied by two individuals: the pilot and a passenger. Following a normal takeoff and initial climb, the aircraft proceeded toward Airolo through the Stalvedro gorge.
Witnesses observed the aircraft pitching unexpectedly on its transverse axis before deviating slightly to the right. Further observations by a witness near Airolo described the aircraft performing an increasingly steep left turn, oscillating heavily on both longitudinal and vertical axes. The aircraft then entered a constant, regular vertical descent. The Pulsar XP struck the ground at an angle of approximately 20 degrees while in a rotating state. Both occupants sustained fatal injuries, and the aircraft was destroyed.
The investigation
Investigators examined the aircraft and the environmental conditions at the time of the accident. Technical analysis by EMPA confirmed that there were no mechanical failures or engine issues contributing to the crash; the propeller was likely still rotating upon impact. While a slight difference in the incidence angles between the left and right wings was noted in the kit construction, it was not the primary cause. The aircraft's weight was at the maximum allowable limit.
Meteorological data from the Piotta station indicated winds from the north at 12 knots, with gusts up and to 22 knots. The investigation focused on the complex aerodynamic environment of the Alpine valley, where north winds descending from the St. Gotthard Pass create significant turbulence and downdrafts, particularly near the junctions of lateral valleys and the narrow Stalvedro gorge.
Findings
- The primary cause of the accident was the loss of control due to extreme meteorological conditions created by strong north winds.
- The pilot likely underestimated the severity of the turbulence within the Stalvedro gorge while attempting to maintain a direct route toward Airolo.
- The aircraft likely experienced two stalls at an altitude of approximately 400 meters, leading into a flat spin.
- The specific geometry of the cockpit controls—where the throttle is only accessible to the left seat—meant that the controls were effectively unrecoverable once the aircraft entered the spin.
Safety action
To improve safety during north wind conditions, it is recommended that the Ambri VFR chart advise single-engine aircraft to utilize runway 11 for takeoffs when north winds reach up to 20 km/h, despite the resulting tailwind, to mitigate the risks of the difficult terrain and downdrafts.