WASHINGTON -- One day after Southwest Airlines grounded 128 planes for missing inspections, the Federal Aviation Administration cleared the planes to fly before the inspections are completed.

The planes were grounded on Tuesday after Southwest Airlines says it inadvertently missed the periodic deadline for inspecting the system that backs up the hydraulic rudder. The equipment allows pilots to steer if the main system fails.

Nearly 100 flights were cancelled - leaving some passengers angry.

"That's something that shouldn't be overlooked right," said one passenger. "It's just negligence."

But the FAA has now allowed the planes to fly, while inspections continue over the next five days.

In a statement the FAA said it had "evaluated the risk and agreed the airline could continue to operate the planes during the short interim."

It's not the first time Southwest has made headlines over safety and inspections.

Last summer the FAA proposed a $12 million fine for Southwest's alleged failure to comply with regulations on aircraft repairs; in 2011 the FAA ordered inspections of dozens of older Southwest planes after a 5-foot hole opened up on a plane mid-flight; and in 2009 Southwest was fined $7.5 million for failing to inspect planes for cracks in the fuselage.

In this photo provided by passenger Christine Ziegler, shows an apparent hole in the cabin on a Southwest Airlines aircraft Friday, April 1, 2011 in Yuma, Ariz. Authorities say the flight from Phoenix to Sacramento, Calif., was diverted to Yuma due to rapid decompression in the plane. FAA spokesman Ian Gregor says the cause of the decompression isn't immediately known. But passengers aboard the plane say there was a hole in the cabin and that forced an emergency landing. AP Photo/Christine Ziegler

"The safety of customers and employees remains our highest priority and we are working quickly to resolve the situation," Southwest said in a statement.

Southwest said late Wednesday that two-thirds of the 128 planes had now been inspected. During the company's 50 years of operation, only one person has died as a result of an accident.