Covering form of preliminary or final report

Date of event :	2021-05-14
Incident number :	CZ-21-0184
Report :	Final report
Place of event :	Římov valley dam reservoir on the eastern outskirts of Veleš
Registration mark :	Serious incident
Weight category MTOM: :	<2250 kg
Type of operation :	Recreational and sport aviation
Plane / SFM :	Sports flying machines
Type of plane / SFM :	CH 7 Kompress Charlie
Health effects of event :	Without injury
PDF document :

Description:

SUMMARY

Synopsis

On 15 May 2021, the AAII received a notification from the Police of the Czech Republic that on 14 May 2021 in the afternoon, near the emergency landing site of the UL helicopter CH 7 Kompress Charlie, one of the three high voltage wires was damaged and its ends were found on both banks of the Římov valley dam reservoir on the eastern outskirts of Velešín. The damage caused the activation of phase protectors in the power substation and a power outage. The police inspected the UL helicopter at the landing site and, in view of the apparent damage to both blades of the main rotor, it was decided to detain the UL helicopter for further examination. The AAII inspectors subsequently visited the UL helicopter landing site and, in cooperation with the Police of the Czech Republic, gathered information relevant for the professional investigation of the causes of the serious accident.

Factual Information

On 14 May 2021, in the afternoon, several small helicopters were seen by random witnesses flying unusually low over the water surface of the Římov dam reservoir. One of the witnesses who was at the critical time on the bridge over the valley reservoir at a place called Svatý Ján literally stated: “I was on the Velešín side of the bridge when I saw five small helicopters passing under the bridge low over the water. This was sometime in the afternoon.”

Subsequently, these helicopters were spotted by another witness after landing in a meadow adjacent to U Hřiště Street on the northeastern edge of Velešín. These were five UL helicopters. Four of the UL helicopters departed after 20:00 CEST and the fifth UL helicopter, Kompress Charlie, registration mark I-D407, remained at the site until the following day. In his testimony the witness said exactly: “At around 18:45 on 14 May 2021, there was a power outage and I called the EG.D. power-supplying company helpdesk to find out when the power supply would be back on. The operator told me that he didn’t know anything at the given moment and that he had to find out what had happened. I only informed him that he didn’t need to find out because I had just seen several small helicopters over the dam and the power had gone out just as they flew over. Moreover, the helicopters were flying right towards the power line and so it was most likely that one of those helicopters caused it. In any case, I didn’t see the helicopter colliding with the power lines. Subsequently, I also saw one of the helicopters below the building the college of technologies in Velešín, where several persons were running around it, arguing about something in German.”

The pilot was at the landing site in the morning of 15 May 2021, loading the UL helicopter onto a transport trailer with other persons. At the location, he introduced himself to the Czech Police patrol as the owner of the UL helicopter and provided the following information: “I made an emergency landing with this helicopter yesterday, i.e. 14 May 2021 at around 16:00 hours, because I had a technical problem with the controls and felt vibrations. Therefore, I decided to land so as not to crash. I flew to the Czech Republic with my friends and there were five of us in total. We flew from Geroldhausen in Bavaria to České Budějovice to see the Czech lakes.”

The Czech Police patrol asked the pilot about the apparent damage to the main rotor blades and the pilot said: “I forgot to secure the engine cove, which lifted when the engine was warming up and the propeller blades broke it. This caused the visible smudges on the blades, on the underside, and the blades are scuffed on the sides.”

Analyses

The pilot was competent for VFR flying. With regards to his very little flying experience, he was probably unaware of the real danger of flying low over water in a valley below the surrounding terrain with a single-engine UL helicopter. The pilot did not have the complete documentation necessary to conduct the flight.

The pilot probably reduced the flight altitude below the permitted 150 m AGL at the southern edge of the Římov reservoir and was therefore flying at an altitude where no civil flight operations are permitted already several kilometres ahead of the scene. He was forced to adapt his flight route to the ruggedness of the surrounding horizontal banks, which he was practically following. He probably maintained a constant flight altitude by peripherally monitoring the sloping banks and the water surface. The altitude of flight, estimated at about 20 m above the water surface, was practically the same as the distance of the lower conductor from the water surface at the moment of contact of the main rotor with the obstacle. This fact caused the leading edge of the main rotor blade to cut the bottom wire, which damaged both blades only at the point close to the attachment to the rotor head. After the main rotor contact with the wire, the UL helicopter began to vibrate and its controllability deteriorated. The pilot immediately climbed out of the valley and made an emergency landing with the damaged helicopter on the nearest suitable surface he could see in front of him.

The pilot had no idea of the existence of the obstacle in the flight route and could not see the oxidised wires against the dark background of the tree-covered sloping banks. It can be stated with certainty that in the case of a flight a few tens of centimetres higher, or if the wires were visible just before impact, the pilot, in an attempt to avoid the obstacle, would have made such a control intervention that the flight would most likely have ended in a fatal accident.

Although the Commission did not have the opportunity to review the relevant UL documentation for the helicopter, a thorough inspection revealed no airworthiness limiting defect. The forensic engineering examination clearly demonstrated that the main rotor blades were damaged in flight by contact with a wire that was also severed by the main rotor blade.

Conclusions/Causes

The cause of the serious accident was an incorrect decision to fly in the area above the Římov dam reservoir below the minimum permitted altitude for VFR flights, at an altitude above the water surface colliding with an obstacle (overhead power lines).

Safety Recommendations

Given the circumstances of the air accident, the AAII issues no safety recommendations.