A cluster of Mi-171 transport helicopters attached to an army aviation brigade under the PLA Xizang Military Command lift off from their parking aprons for a round-the-clock high-altitude flight training exercise in plateau area of southwest China's Xizang Autonomous Region on November 28, 2019. Photo:China Military

A Mi-171 transport helicopter attached to an army aviation brigade under the PLA Xizang Military Command prepares to carry out night landing during a round-the-clock high-altitude flight training exercise in plateau area of southwest China's Xizang Autonomous Region on November 28, 2019. Photo:China Military

A Mi-171 transport helicopter attached to an army aviation brigade under the PLA Xizang Military Command prepares to carry out night landing during a round-the-clock high-altitude flight training exercise in plateau area of southwest China's Xizang Autonomous Region on November 28, 2019. Photo:China Military

A cluster of Mi-171 transport helicopters attached to an army aviation brigade under the PLA Xizang Military Command sit abreast with each other while awaiting approvals to lift off for a round-the-clock high-altitude flight training exercise in plateau area of southwest China's Xizang Autonomous Region on November 28, 2019. Photo:China Military