Boeing announced on Thursday that dozens of its popular 737NG planes had been taken out of service after cracks in them were detected, marking another setback for the crisis-stricken US aircraft maker.The new difficulties compound the troubles facing the US manufacturer, which has faced tumbling profits, federal scrutiny and calls for its CEO to resign after deadly crashes involving the 737 MAX, the successor aircraft for the 737NG.READ | Financial pressure mounts to fix Boeing's troubled jetlinerAustralian national carrier Qantas said it had found cracks in three of its 737NGs and removed them from service for repairs, after inspecting 33 jets which had flown more than 22 600 times - the threshold set by US regulators for such inspections.Nine of the planes were grounded in South Korea this month, including five operated by Korean Air, according to authorities in Seoul. US carrier Southwest Airlines has taken three planes out of service due to the problem.Several other leading carriers said inspections had not turned up cracks on their aircraft.InspectionsREAD | Boeing conducts final test flight of 737 MAX with software fixBoeing had previously reported a problem with the model's "pickle fork" - a part which helps bind the wing to the fuselage.US regulators earlier this month ordered inspections of older NG aircraft, directing that planes with more flying hours to be checked within seven days.A Federal Aviation Administration spokesperson said operators could not fly the planes until the issue was addressed.A Boeing spokesperson on Thursday said in Sydney that fewer than 5% of 1 000 planes had cracks detected and were grounded for repair.Boeing and Qantas stressed travellers should not be concerned."We would never fly an aircraft that wasn't safe," said Andrew David, the CEO of Qantas Domestic, adding that the airline considered the 737NGs to be "very reliable"."Even where these hairline cracks are present they're not an immediate risk, which is clear from the fact the checks were not required for at least seven months."David said the three grounded 737s had flown about 27 000 times and that other aircraft with the same number of flights showed no problems.Stephen Fankhauser, an aviation expert at Australia's Swinburne University of Technology, said that the parts were designed so the "structure can tolerate some level of damage or degradation"."The inspection period is set to ensure the cracks do not continue to grow to a dangerous length and then significantly compromise the strength of the airframe," he said.The FAA on October 3 ordered immediate checks of Boeing 737NG planes that had flown more than 30 000 times.Planes with at least 22 600 flight cycles should be inspected within the next 1 000 trips, the order said.