Strong winds ripped off a large section of roof at Beijing International Airport Terminal 3 on Tuesday night, scattering debris across the airport and frightening passengers. But airport authorities said the incident posed no danger to flights. Wind speeds on the runway began to reach 86.4 km/h at around 7pm, triggering a yellow alert - and an hour later, the gust tore a piece of metal covering from the roof of the terminal's Section D and blew lightweight insulation material across the airfield. 'The incident has not caused any impact on the airport's operational safety,' the authorities said on a microblog yesterday. They said maintenance workers cleaned up the debris and repaired the roof, but it took 18 hours to do so. The incident - the second time the terminal's roof had 'gone with the wind' in less than a year - prompted questions about the structural integrity of construction materials and raised concerns from some experts about the design of the world's second-largest airport. Professor Li Zhanhua, a researcher at the Chinese Academy of Sciences' Institute of Mechanics and an expert in fluid dynamics, said she was not surprised by the episode. The terminal, a landmark building erected ahead of the Olympic Games in Beijing in 2008, stands alone on flat land and is exposed to the full force of the wind, according to Li. It has a massive roof with a streamlined shape resembling a plane. 'The faster air moves over a surface, the lower pressure it creates. So when wind blows over T3's roof at high speed, it sucks it upward, like lifting a jumbo jet during take-off,' she said. 'Given the size of the roof, the up-lift force may add up to a considerable amount, exceeding the estimates of designers.' More than 200 flights were delayed in December when winds tore a large hole in the roof. Professor Hu Weiping, an expert on metal fatigue at Beihang University, said the repeated structural failure required a thorough investigation of the roof's safety by a panel of scientists and engineers from different disciplines. The panel could install sensors and run computer simulations on the wind's impact on different parts of the roof to determine whether the quality of the materials used, such as the strength of bolts, could withstand strains in extreme conditions, he said. 'It shouldn't be treated as an isolated accident,' Hu said. 'Such mishaps must be stopped before someone gets hurt.'