What happened
On March 27, 1968, a pilot departed from Wangen-Lachen airfield in a Zlin Z-526, registration HB-URZ, to perform a series of aerobatic maneuvers. The flight was part of a training camp designed to prepare pilots for Swiss aerobatic championships. After successfully completing several maneuvers, the pilot initiated a negative looping maneuver. During this sequence, the aircraft gained excessive speed because the engine power was not sufficiently reduced.
While flying in an inverted position, the pilot attempted to recover by pulling the aircraft into a dive. However, due to the high airspeed and the significant control forces required by the Zlin Z-526 model, the pilot was unable to pull the aircraft upright. The aircraft eventually struck the surface of the upper Lake Zurich at a shallow angle, resulting in the total destruction of the aircraft and the death of the pilot.
The investigation
The investigation examined the aircraft's maintenance history, the pilot's flight experience, and the environmental conditions at the time of the accident. The Zlin Z-526 was found to be in an airworthy condition, with no mechanical failures identified in the engine or flight controls. Investigators also reviewed the pilot's recent training logs, noting that while he was an experienced aerobatic pilot, he had only recently transitioned to this specific aircraft type.
Physical evidence from the wreckage, recovered approximately 250 meters from the shore, showed the pilot remained strapped into his seat. An autopsy was performed, which revealed no signs of alcohol consumption, drug use, or underlying pathological conditions that would have prevented him from flying. However, investigators noted the pilot had previously expressed concerns regarding the physical strength required to manage the heavy control forces of the Z0-526 due to his stature.
Findings
- The primary cause of the accident was the pilot's failure to reduce engine power sufficiently before initiating the negative loop, which led to excessive airspeed.
- The high speed increased the aerodynamic loads on the elevators, making it physically difficult for the pilot to pull the aircraft out of the dive.
- The pilot's decision to end the inverted flight with a downward dive, rather than attempting a roll to level flight, placed the aircraft in an unrecoverable state.
- It is possible that the pilot experienced a physiological episode, such as a blackout or physical overexertion, due to the high G-forces experienced during the maneuver, which may have influenced his decision-making.