What happened
On May 27, 2008, a Hughes 269C helicopter, registration PH-RYF, was conducting a local training flight at Lelystad Airport. The flight, which included an instructor and a student pilot, was intended to practice hovering, taxiing, and takeoff and landing maneuvers. After an initial landing, the instructor directed the student to lift off again, rotate 90 degrees, and land once more.
Within seconds of the second touchdown, the aircraft began to shake violently. The instructor immediately took control of the aircraft and attempted to stabilize the flight by increasing engine power and adjusting the rotor blade pitch. Despite these efforts, the vibrations became uncontrollable. Within four seconds, the rotor blades became unbalanced, causing the helicopter to tilt sharply. The blades struck the ground, resulting in heavy damage to the airframe. Both occupants escaped the aircraft without injury.
The investigation
The investigation focused on the sudden onset of intense vibrations and the subsequent loss of control. The instructor identified the phenomenon as ground resonance, a condition where an imbalance in the rotor blades causes the center of gravity to shift away from the rotor axis, leading to severe oscillations.
To mitigate ground resonance, pilots typically attempt to fly the helicopter immediately off the ground. While the instructor attempted this maneuver, the intensity of the shaking prevented successful recovery. The investigation also examined the condition of the rotor dampers—rubber components designed to prevent blade displacement. While these dampers are subject to periodic visual inspections, the severe damage to the aircraft prevented a post-accident assessment of their functional state. The dampers on PH-RYF had been in service for less than 3,000 hours, well within their expected 6,000-hour lifespan.
Findings
- The primary cause of the instability was ground resonance caused by an imbalance in the rotor system.
- The severe vibrations led to the rotor blades striking the ground, causing significant structural damage.
- A failure in one of the rotor dampers likely allowed the blades to move out of position, triggering the resonance.