What happened
On September 24, 2022, a Robinson R44 Raven II, registration OK-STK, was performing a recreational flight near the former Lazy mine. Shortly after takeoff, the pilot reported feeling a sudden impact or jolt through the pedals, which they initially interpreted as a gust of wind or a collision with an unidentified object, such as a drone or model aircraft. The pilot continued the flight, performing maneuvers over the Těrlická and Žermanice reservoirs, before returning to the parking area.
Upon landing and conducting a post-flight inspection, the pilot discovered significant damage to one of the tail rotor blades, which appeared to have been sliced through approximately two-thirds of its metal structure. Because the pilot did not observe any flying objects in the vicinity at the time of the perceived impact, they initially suspected a collision with a UAV.
The investigation
The ÚZPLN investigation examined flight data from the SkyDemon application, analyzed the physical damage to the aircraft, and performed a chemical analysis of residues found on the damaged blade.
Investigators found that the pilot's flight path included a highly risky, extremely low-altitude pass over the water near a recreational area, which was in violation of VFR regulations. Furthermore, while the pilot claimed to have performed a control check following the impact, flight logs showed no heading changes that would indicate such a maneuver.
Technical analysis of the tail rotor blade ruled out a drone collision. There were no traces of debris from a UAV on the tail boom, nor were there any traces of common drone materials inside the blade cavity. Chemical analysis of the damage site revealed traces of phosphorus, oxygen, potassium, and zinc, which are inconsistent with the materials used in typical RC models or drones. Additionally, the lack of any biological matter or organic residue ruled out a bird strike.
Findings
- The damage pattern and mechanical abrasions on the blade are consistent with contact with the edge of a fixed metal obstacle.
- The incident occurred during the landing maneuver, as the pilot flew over steel beams at an extremely low altitude (approximately 2.5 meters AGL).
- The pilot's low-altitude flight over the reservoir was not for the stated purpose of "water scooping" practice, but was a high-risk maneuver that endangered people in the nearby recreational zone.
- The pilot likely underestimated the clearance required when maneuvering over the known obstacle during the final approach to the landing site.