The pouring rain stopped briefly in Wenchuan yesterday morning to allow a Black Hawk military helicopter, struggling with wind shear, poor visibility and rough terrain, to touch down on a narrow valley platform and unload rescuers and provisions. Chen Renliang, deputy director of the National Key Laboratory of Rotorcraft Aeromechanics, described it as a 'a suicide mission'. The crew was the first rescue team to land at the quake epicentre following many unsuccessful attempts. Failed attempts in the more than 40 hours after the earthquake raised doubts and criticism in internet chat rooms about the People's Liberation Army's ability to manoeuvre. 'I can't believe that the PLA pilots are such cowards,' Lin Dan, an IT engineer in Beijing, said. Basic knowledge of aeromechanics would help people to better understand the situation, Professor Chen said. He said the biggest problem was wind shear - differences in wind speed and direction. 'A helicopter can fly because it has a rotor that generates an aerodynamic force. The biggest enemy to a rotor is called wind shear, which could cause the entire helicopter to fly like a leaf behind your car. 'Wenchuan county is at the bottom of three deep valleys with an altitude of up to 2,400 metres. There is very little open space, and the terrain is extremely rough,' Professor Chen said. 'A helicopter that goes into such a region would encounter some of the most powerful and unpredictable wind shear. Even if the pilot only lost control for a few seconds, the helicopter could smash into a rock. 'Training and computers can be of some help but the only thing you can really count on is the pilot's experience. The military has done its best. Everyone on board is a hero.' More help may arrive by air. But Zhang Chenglin, former director of the rotorcraft laboratory, had concerns. 'We have learned bloody lessons before - sending troops down regardless of conditions. I hope people will not pressure our pilots too hard. They have lives, too.'