the only specially modified coppers can fly in high attitude. i think only two: one of special model of mi-8 family, actually specifically designed for china. another is special model of "black hawk" family, which is also specially designed for china. both mainly used in high tibetan plateau.
but as my understanding, those models are not all weather.
beichuan still is not accessible due the blocked road. it had one highway passed through that city. now the road has been totally destroyed by mudslides and landslides. so far the troops and supplies got in on foot.
some of villages around beichuan were 100% lost. just disappeared under landslides and mudslides.