The northern runway at Chek Lap Kok was closed for two hours yesterday morning after a large crack was found in the tarmac. The Airport Authority said a 4.4cm-wide, 4cm-deep crack running about 1.5 metres along the runway was discovered during a regular inspection at 9am. The runway was closed while repair work was carried out. It was reopened just after 11am. The southern runway was used for takeoffs and landings and no flights were delayed. The authority last night refused to answer questions about the closure, the first time in six months the runway has been closed for repairs during operating hours. The crack had apparently appeared between the inspections at 6am and 9am. 'It is not unusual to find cracks on asphalt runways,' an authority spokeswoman said. 'This is part of a normal wear and tear, with impact and friction caused by landing and takeoff of flights.' The authority said the 3.8km-long runway was due to be resurfaced next month. The southern runway cracked soon after its construction because reclaimed ground it was built on was settling faster than expected. Former director of aviation Peter Lok Kung-nam said the problem was unusual. 'It's reclaimed land so there must be air-filled tubes inserted to prevent heat expansion and cold contraction,' he said. 'One possibility is that the tubes have been blocked.'