What happened
On September 16, 1971, a Piper PA-31 Navajo, registration HB-LFR, was conducting a transition training flight at Geneva-Cointrin Airport. The flight involved a student pilot and an instructor, with two additional pilots on board as passengers. During a series of touch-and-go maneuvers, the instructor initiated a simulated engine failure at a very low altitude, approximately 1 to 5 meters above the ground.
To simulate the failure, the instructor manipulated the mixture control of the right engine, causing it to lose power and creating significant drag. As the student attempted to maintain control, the aircraft began to yaw to the right. In an attempt to abort the failed exercise, the instructor abruptly moved the mixture control back to the full rich position while the throttles remained at full power. This sudden surge of power from the right engine created an intense asymmetric thrust, inducing a sharp roll and yaw to the left. The aircraft entered a steep bank and a nose-down attitude, leading to an immediate stall and impact with the ground near taxiway 3. The aircraft was destroyed, the two pilots sustained severe injuries, and the two passengers suffered light injuries.
The investigation
SUST examined the flight sequence, the mechanical state of the aircraft, and the instructional methods used during the training. The investigation confirmed that the aircraft was properly maintained and that the pilots were medically fit. Investigators analyzed the aerodynamic effects of the instructor's engine manipulation, specifically how the sudden application of power on one side created the uncontrollable bank. The investigation also reviewed the training curriculum and the instructor's level of supervision by the flight school.
Findings
- The primary cause of the accident was the aircraft falling below its minimum controllable airspeed at low altitude, triggered by improper engine manipulation.
- The instructor's method of simulating an engine failure by using the mixture control rather than the throttle created excessive drag and a significant yawing moment.
- The sudden restoration of power to the right engine caused an uncommanded steep left turn that the student could not recover from due to the low altitude.
- The instructor's decision to introduce such a complex and difficult maneuver during the early stages of the transition training was pedagogically unsound.
- There were indications of inadequate instruction and oversight of the instructor by the flight school.
Safety action
- The investigation noted that the flight school's supervision and the instructor's training may have been insufficient.
- It was observed that critical new regulatory instructions regarding transition training had not been effectively communicated to the pilot.