What happened
On the morning of May 1, 1973, a pilot operating a Hughes 269 A, registration HB-XCZ, departed Locarno for Buttwil. Following a brief stop, the pilot commenced a private sightseeing flight with one passenger, traveling toward Lake Ägeri. During a climb to clear the Zugerberg ridge, the aircraft began oscillating around its vertical axis, accompanied by intense vibrations felt through the control stick.
Initially believing the instability was caused by wind, the pilot soon became convinced that the vibrations indicated a mechanical failure. Seeking to avoid further risk, the pilot decided to perform a precautionary landing on a nearby marshy meadow. During the approach, the pilot attempted to maintain a hover near the ground, but the aircraft began to sink rapidly. Despite applying full power, the pilot could not maintain control of the rotor RPM. The helicopter began to rotate rightward and eventually struck the ground on its rear right skid. During the impact, the main rotor struck the terrain. The occupants of the Hughes 269 A were uninjured, though the aircraft was heavily damaged.
The investigation
Investigators examined the flight conditions and the pilot's preparation. While the pilot had reviewed meteorological data earlier in the day, the specific warnings regarding strong Föhn turbulence were not adequately considered during the flight. The investigation also reviewed the aircraft's technical status, finding the Hughes 269 A to be airworthy with no pre-existing mechanical defects or weight and balance issues.
Findings
- The pilot flew into a zone of severe turbulence on the lee side of the Rossberg mountain.
- Inadequate flight preparation and failure to account for Föhn wind conditions led to the pilot's misinterpretation of the aircraft's behavior.
- The pilot misidentified atmospheric turbulence as a mechanical malfunction, prompting the unnecessary precautionary landing.
- The landing occurred in a downdraft zone, which prevented the pilot from maintaining a stable hover.
- The sudden rightward rotation of the aircraft was likely caused by the pilot increasing the rotor blade pitch to arrest a high sink rate, creating torque that the tail rotor could not compensate for, potentially exacerbated by insufficient rotor RPM.