Although thousands of passengers were inconvenienced over the past two days, analysts downplayed any notion that the cancellations would hurt the airlines financially. They said the costs would pale in comparison to high fuel prices.

Delta expected to cancel about 275 flights, or about 3 per cent of its schedule, before returning to normal operations early on Friday, said spokesman Chris Kelly.

American, the nation's largest airline, cancelled 141 flights by mid-afternoon Thursday, or about 6 per cent of its estimated 2,300 flights, officials said. The Fort Worth-based airline cancelled 318 flights on Wednesday.

At American, inspectors from the airline and the Federal Aviation Administration focused on fixing the spacing between cords used to secure bundles of wires in the auxiliary hydraulic systems of its MD-80 aircraft.

'In no way was safety compromised, but the (FAA) directive said 'Do it this way',' said American spokesman Tim Smith.

American completed inspections and in some cases alterations on 269 MD-80s by mid-afternoon, and another 21 were still undergoing work that was expected to be done Thursday, Mr Smith said. Nine planes remained to be inspected on Thursday night, he said.

He said American found seats for most passengers on other planes but also put some customers on other airlines' flights.

Delta expected heavy volumes on Thursday at its hub at Hartsfield-Jackson Atlanta International Airport, Ms Kelly said. Both Delta and the Transportation Security Administration were bringing in extra staff to handle the crowd of travellers, she said.

The inspections came almost three weeks after the FAA ordered a check of maintenance records at all US airlines following criticism of the agency's handling of missed fuselage inspections at Dallas-based Southwest Airlines.

The FAA imposed a US$10.2 million (S$14 million) civil penalty on Southwest this month for missing the inspections and then continuing to fly the planes with passengers on board even after realising the mistake. Southwest officials have said they repaired small cracks in the fuselages of six planes last year and four this month.

The FAA said it would check compliance with at least 10 safety orders at every US airline by Friday. A more complete audit is due by the end of June.

The airlines declined to detail how much they were losing on cancelled flights and extra inspections and maintenance. -- AP