What happened
On June 5, 1960, a Piper PA-22 aircraft, registration HB-OPA, was participating in an aerial competition near Lake Maggiore. The flight, which began at Locarno airport, proceeded normally through the first stages of the competition. While returning toward Locarno, the pilot attempted to execute a right-hand turn at an altitude of approximately 600 meters to prepare for landing.
Instead of turning right, the aircraft entered a descending leftward spiral that tightened as it progressed. The flight path took the aircraft over the Ascona aerodrome. During the descent, the aircraft's engine experienced momentary interruptions, though witnesses noted no abnormal engine noise. The aircraft eventually struck the ground 120 meters east of the Ascona airfield, impacting trees and resulting in the total destruction of the aircraft. The pilot and passenger sustained minor injuries.
The investigation
An intensive investigation was conducted to determine the cause of the unusual flight path. Technical examinations of the wreckage, including the engine and flight controls, revealed no mechanical deficiencies or failures in the control surfaces. While investigators could not entirely rule out a potential issue with the rudder, the aircraft's behavior during the final moments suggested the controls were functioning normally.
The investigation focused heavily on the pilot's physical condition and actions. It was established that the pilot suffered from a pre-existing ankylosis (stiffening) of the left knee joint, which limited the leg's ability to extend. Although the pilot had been medically cleared to fly provided an instructor could verify his ability to perform rudder maneuvers, the investigation scrutinized how this physical limitation interacted with the aircraft's controls during the critical turn.
Findings
The investigation concluded that the primary cause of the accident was the pilot's loss of control of the aircraft. It is determined that due to the unintentional and simultaneous application of the rudder and ailerons caused by the pilot's physical impairment, the aircraft responded with a leftward skid rather than the intended right turn. Because the pilot was unaware that he had inadvertently applied the rudder, he failed to correct the resulting flight attitude, leading to the spiral descent.