More than 10,000 rescue workers have not been able to trace any of the 19 people who crashed Saturday in a helicopter in Sichuan province even after 48 hours.

    The copter was carrying five crew and 14 injured quake survivors when it encountered strong turbulence and thick fog and crashed on the densely forested Zhaogong Mountain, military sources said. The spot is near Xuankou township between Dujiangyan and Wenchuan, two of the worst hit places by the quake.

    The Mi-171 helicopter, under Chengdu Military Command, was returning after carrying a team of epidemic prevention experts to Lixian county and picking up the injured from there.

    Another 6,000 rescue workers were added to the 4,000-strong search team yesterday.

    Many helicopters, too, were employed in the search operation, rear admiral Zhu Xinmin, of the military command, said in Chengdu. But the high mountains, narrow canyons and dense forests were hindering the search.

    Rescue and relief flights have been hampered by harsh geographical conditions.

    "Some of the canyons are 200-300 m wide and more than 100 m deep and are like wells," said Cheng Qingxin, a pilot with the People's Liberation Army's General Staff Headquarters (GSH).

    Cloudy and misty weather, too, have thwarted pilots' life-saving missions. "How can you fly when you can't see the next mountain while flying over one?," a senior colonel with the GSH said.

    The crashed chopper's five crew, led by Qiu Guanghua, has completed 63 flights, carrying 25.8 tons of relief materials and 234 quake survivors.

    His regiment was the first to reach food, medicine and other relief materials to Wenchuan, Maoxian, Yingxiu and Beichuan, and carry the injured to safer places.

    Qiu has about 5,800 hours of flying experience and is among the country's top pilots. The 51-year-old is a member of the Qiang community and part of the first batch of ethnic minority pilots selected by late premier Zhou Enlai in 1974.

    A native of Maoxian, he flew past the badly-hit county several times after the quake but didn't get time to visit home even once. He is scheduled to retire in 10 months.

    The other four with Qiu are pilot Li Yue, mechanical officers Wang Huaiyuan and Chen Lin, and sergeant Zhang Peng.