American Airlines and Delta Air Lines cancelled dozens of flights Thursday as they continue their inspections of wiring bundles on some of their planes.

American, the largest airline in the U.S., cancelled 132 flights of its estimated 2,300 flights scheduled for Thursday, spokesman Tim Wagner said. That was about six per cent of American's Thursday schedule after the Fort Worth, Texas-based airline cancelled 325 flights on Wednesday.

Delta has not specified the number of flights it was cancelling Thursday. However, the Atlanta-based carrier said it began voluntarily reinspecting wiring on 117 MD-88 aircraft on Wednesday night.

American said it began its inspections after an audit of the carrier by a joint team of inspectors from the Federal Aviation Administration and the airline.

The inspections focus on proper spacing between two bundles of wires in the plane's auxiliary hydraulic system, and those wires must be installed exactly according to an FAA directive, American said.

American has completed inspections on 243 MD-80 aircraft, and about 45 planes "are still being worked on," Wagner said Thursday.

"We expect all but a few to be back in service by tomorrow," he said.

Heavy traffic expected at Atlanta airport

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

Kelly said she didn't yet have estimates on how many passengers were affected by the flight cancellations. Delta said its inspections are expected to be completed by Saturday.

The inspections come almost three weeks after the FAA ordered a check of all U.S. airlines' maintenance records. That was after controversy erupted over its handling of missed safety inspections at Dallas-based Southwest Airlines.

Southwest penalized for missing inspections

The FAA hit Southwest this month with a $10.2-million US civil penalty for missing the inspections and then continuing to fly the planes with passengers on board even after realizing the mistake. Southwest has said it will appeal the penalty.

The FAA said then that it would check compliance with at least 10 safety orders, called airworthiness directives, at every airline by March 28. The agency said a full audit covering at least 10 per cent of all safety directives will be finished by June 30.

Southwest said it reported the missed inspections itself, and that manufacturer Boeing agreed that keeping the planes in operation until they could be re-examined within 10 days didn't pose a safety hazard.

Six of the jets required repairs for small cracks. Those repairs have been completed and the planes returned to service, Southwest spokeswoman Marilee McKinnis said Thursday.