What happened
On November 4, 1984, a student pilot departed from Brig early in the morning to conduct flight training at Sion aerodrome alongside an instructor and another student. The morning session focused on precision landing exercises without engine power to prepare for an intermediate flight exam. After completing five supervised flights that included simulated engine failure procedures, the student was cleared to fly solo at 11:41 local time.
The instructor provided specific instructions for three 600-meter touch-and-go exercises, emphasizing attention to radio commands and maintaining an approach speed above 70 knots. The pilot successfully completed the first exercise. During the second approach, the student announced final approach but misunderstood a subsequent air traffic control instruction intended for another aircraft. Despite this confusion, he proceeded with the landing.
The aircraft touched down within the first 150 meters of the runway. Immediately after touchdown, the pilot retracted the flaps one notch and applied full power to initiate the takeoff roll. Shortly thereafter, the Robin DR 400-120D began to oscillate laterally on the paved surface. Approximately 450 meters past the threshold, the aircraft abruptly veered left at a 30-degree angle, leaving the runway and crossing a grassy area toward the taxiway.
As the aircraft approached a large perimeter gate approximately 50 meters away, the pilot attempted to pull up to clear it. The nose gear struck the metal gate structure at a height of 1.65 meters, causing the pylon to bend. The aircraft slid along the fence line before coming to rest. The left wing subsequently struck a service road vehicle parked behind the fence. The engine block, firewall, and instrument panel detached from the airframe but landed near the car without causing a fire. The pilot sustained only minor facial injuries.
The investigation
SUST investigators examined the aircraft, the flight records, and the physical evidence at the scene. The Robin DR 400-120D (HB-EQX) was found to be airworthy with valid documentation and maintenance records. Weight and balance were within limits. No mechanical failures were identified in the landing gear, braking system, or flight controls prior to the impact.
Post-accident analysis of the propeller and gate pylon indicated that the engine was producing significant power at the moment of collision. Tire pressure was found to be slightly below prescribed limits but was deemed unlikely to have contributed to the loss of control. Weather conditions were favorable, with light winds from the southeast and good visibility.
The instructor, positioned behind parked helicopters, had an obstructed view of the critical takeoff phase and could not intervene effectively. The student pilot reported that the radio confusion did not influence his flying actions during the roll-out.
Findings
The primary cause of the accident was a loss of control during the ground roll of a touch-and-go maneuver, leading to a collision with a perimeter obstacle. The investigation identified several contributing factors:
- Shimmy Oscillation: The Robin DR 400 exhibited a tendency for nose gear shimmy (lateral oscillation) on paved surfaces, particularly when the nose wheel was unloaded during takeoff rotation.
- Braking Response: The aircraft's toe-brake system, which operates at the end of the rudder pedal travel, amplified the oscillation through abrupt directional changes as the pilot attempted to correct the path.
- Pilot Reaction: The sudden lateral acceleration likely impaired the student's ability to manipulate the controls and throttle effectively. Instead of cutting power and applying emergency braking, the pilot attempted to fly out of the situation, resulting in the impact with the fence and gate.
Safety action
The report highlighted the specific handling characteristics of the Robin DR 400 regarding nose gear shimmy during touch-and-go operations. It noted that while shimmy is not inherently dangerous for experienced pilots who can modulate rudder inputs finely, it poses a significant challenge for student pilots who may react with abrupt control inputs. No further safety recommendations were issued in this specific final report beyond the factual findings.