What happened
On August 8, 2018, at approximately 19:40 local time, a FC-4 gyrocopter was conducting flight tests at Modlin Airport (EPMO) as part of an approved ground and flight test program. The specific mission involved performing low-level flybys and shallow turns relative to the runway axis.
During the takeoff roll, immediately after the rotor pre-rotation had reached approximately 200 RPM, the rotor speed dropped to about 150 RPM. This drop in RPM was likely caused by an overly aggressive acceleration attempt by the pilot, intended to shorten the takeoff distance. During this maneuver, the rotor head tilt was deflected more than 10 degrees toward the rear. Because the cockpit lacks a dedicated rotor head tilt indicator, the pilot relied on the tactile feel of the control stick to determine the tilt angle.
This configuration led to the onset of rotor blade flapping. The resulting excessive oscillations caused the lower rotor blade to make contact with the composite tip of the left vertical stabilizer, resulting in structural damage. The pilot managed to stabilize the aircraft, completed the flight, and landed safely. Upon taxiing to the hangar area, damage to the lower rotor blade and the left vertical stabilizer was identified. Debris from the stabilizer was also found on the runway by airport personnel.
The investigation
The user-led investigation established that the aircraft sustained a dent and several scratches on one of the lower rotor blades, as well as damage to the left vertical stabilizer. The investigation focused on the sequence of the takeoff roll and the pilot's method of monitoring rotor head tilt.
Findings
- The primary cause of the incident was a drop in rotor RPM resulting from an aggressive takeoff attempt.
- The rotor head tilt exceeded the 10-degree limit prescribed in the instructions.
- The pilot lacked a cockpit instrument to monitor the rotor head tilt angle, relying instead on manual sensation.
- The excessive tilt and RPM drop triggered rotor blade flapping, leading to the physical contact between the blade and the stabilizer.
Safety action
Following the incident, the left vertical stabilizer was repaired and the aircraft underwent a control weighing. The operator implemented a safety recommendation to perform a periodic inspection of the damaged rotor blade every 25 flight hours.