It was about 9:30 in the morning of 31 May last year. At 3,250m up on Bukur Danda on the northwestern reaches of the Arun Valley, the leader of an ornithological expedition and his porters were climbing to photograph a rare bird nest.

They heard a helicopter about 1 km away. "I remember wondering how come it was able to fly so low since it was overcast," recalls the researcher. The weather was changing fast, and he says the helicopter sounded lost because it made a 180 degree turn and headed northeast again the way it had come. "After hearing the rotors for a bit longer, as if it was circling, the sound abruptly stopped."

This is an account of the disappearance of 9N-ACU ('Charlie Uniform'), an Asian Airlines Mi-17, during a flight from Makalu Base Camp to Lukla with eight porters of a French expedition and two crew, including one Russian. The mountaineering season had just ended, an early monsoon was bringing masses of clouds, and climbers were in a hurry to get off the mountain.

The Mi-17 had been in contact with a Simrik Air helicopter also flying in the vicinity, and had reported no trouble. When the chopper failed to arrive in Lukla by 10:30, an immediate search and rescue was launched. Hampered by weather and terrain, the wreckage was never found-leading to speculation at the time that it may have been hijacked by Maoists. A Rs 50,000 reward for information on the wreckage produced no results.

The naturalist did not want his name used, but Nepali Times has verified his authenticity and cross-checked the account. The people at the research camp had also heard the helicopter and everyone agreed that it was flying dangerously low in cloud.

Nobody heard the sound of a crash, so it may have been quite a distance away from Bukur Danda when it went down.

The expedition sent word to the military search and rescue team at Chitre that the search should be concentrated in the Sankhuwa and Apsuwa area. But the weather deteriorated as the monsoon made an early start in eastern Nepal and it was called off.

"We carried out an air and ground search all over the area and resumed after the monsoon, but just couldn't find the helicopter anywhere," says an Asian Airlines representative, who says rescue teams looked at where the naturalist said he heard the craft disappear in the upper reaches of Sankhuwa Khola, or the middle part of Apsuwa Khola.

There is some extremely steep terrain in this area and what could have happened is that the pilot, hopelessly lost, tried to fly out of the valley but went into a side valley and crashed into a mountainside. Still, with such an extensive search, the wreckage should have been found. "It is really very rugged out there," the Asian Airlines official told us, "The only possibility now is that the helicopter triggered an avalanche on impact and got buried under ice and rocks."