KATHMANDU: Unfamiliar terrain and human error caused the fatal crash of US Marine helicopter in Dolakha district, according to an accident investigation conducted by a 15-member team that comprises eight US Army investigators.

The UH-1 Huey, carrying four crew and nine passengers, lost radio communication on May 12 and the wreckage was found after three days of intensive air and ground searches in the area. Six US Marines, two NA soldiers and five locals were killed after the ill-fated chopper crashed while returning from a relief mission in earthquake-devastated Dolakha. The aircraft was part of Marine Light Attack Helicopter Squadron 469 from the 3rd Marine Air Wing based at Camp Pendleton.

The aircrew encountered greater than anticipated mountain wind condition and cloud formation after crossing a saddle into a channelised terrain, on the western site of the ridge line of the Kalinchowk in Dolakha where the accident occurred at a height of 11,300ft above mean sea level, the report that was submitted to the Ministry of Culture, Tourism and Civil Aviation stated.

The controlled flight into terrain (CFIT) had occurred with the UH-1 Huey chopper when an airworthy aircraft under the complete control of the pilot was inadvertently flown into terrain, according to an investigation that blames human error as the most probable cause of the fatal crash.

“The off-blasting and rotating building clouds in the Valley resulted in an inadvertent flight into instrument meteorological conditions, quickly followed by CFIT,” the report said, adding that lack of familiarity of the specific weather pattern in the Kalinchowk mountain range had led the aircrew for CFIT. “The Huey crew were flying without knowing the location they were headed for.”

The panel recommended the aviation regulatory body to ensure prompt and efficient air traffic control service at Tribhuvan International Airport. “Radar coverage for the air traffic under visual flight rules should be strengthened,” the panel said.

TIA authorities were not only unaware of the number of persons on the chopper but also failed to establish communication with the low-flying helicopter, adds the report, noting that there was a need to provide instant weather updates to the crew and make approach frequency reliable, as well as powerful. “Specific routing system is recommended to notice the choppers flying into non-radar areas.”

The NA’s Mid Air Base has also been asked to improve its liaisoning practice while dealing with international military choppers. “There should be single aviation command and control entity to handle military choppers,” the panel said in its report.

Team leader Brig Gen Sudhir Shrestha presented the findings before the ministry this afternoon, while Minister Kripasur Sherpa, MoCTCA Secretary Sureshman Shrestha, CAAN’s Director General Sanjiv Gautam and representatives from the ministry and Nepali Army were present on the occasion.