![WZ-9 helicopters fly at night(4/6)](http://www.ecns.cn/hd/2017/04/19/a0d854a6356042cf8fd4b5612298e24b.jpg)
Cadets from a helicopter group of the PLA Air Force Xi'an Flight College perform pre-flight checks on a WZ-9 helicopter prior to a night flight training exercise at a military training base in southwest China's Sichuan Province in mid-March, 2017. (Photo/81.cn)