DALLAS — Federal officials say a maintenance company hired by Southwest Airlines used unapproved parts for repairs on some jets.

The parts will have to be replaced, but as they are not considered an immediate safety threat regulators will let Southwest keep flying the planes for 10 days — until next Tuesday — while it decides how to fix the problem.

Southwest said Wednesday that the incident led it to ground 46 planes — nearly 9 percent of its fleet — for several hours last Saturday. That led to 15 canceled flights and widespread delays — Southwest said its on-time performance fell to 68 percent, down from 78 percent in June, the last month for which government statistics are available.

An investigator for the Federal Aviation Administration raised questions about the parts during an inspection Friday of a facility that maintains planes for Southwest. The parts, called exhaust gate assembly hinge fittings, are used in deflecting hot engine exhaust away from wing flaps. Southwest uses only Boeing 737 aircraft, which have an engine on each wing.

The maintenance company, which was not identified by Southwest or the FAA, used hinge assemblies made by a subcontractor who is not certified to make the parts, according to the agency.

That led to discussions Friday night between Southwest, Boeing Co. and the FAA about what to do next, but the airline really had no choice. Federal regulations prohibit knowingly operating a plane with unapproved parts, so Southwest grounded planes that had received the hinge fittings.

By late Saturday, engineers determined that the use of the parts didn't pose an immediate safety danger, so the FAA let Southwest use the planes temporarily.

"The parts have to come off the planes, it's just a matter of how quickly that has to be done," said FAA spokesman Lynn Lunsford. "Unapproved parts don't belong on airplanes."

Lunsford said late Wednesday that FAA officials were still talking with Southwest about the situation. He said it was too early to know whether Southwest would face any penalties.

Southwest spokeswoman Beth Harbin said the issue hinged on the documentation of repairs on the part by a vendor that does maintenance for the airline. Harbin said she didn't know of previous concerns about the repairs.

The FAA cracked down on use of unapproved parts in aircraft in the 1990s and reduced their use, said Thomas Anthony, who led FAA investigations into the practice and is now director of the aviation safety program at the University of Southern California.

Anthony said unapproved parts are a potential safety hazard because they haven't been tested to see if they will hold up when working alongside other components on the plane.

"A part is not just a part," he said.

No matter who made the hinge fittings, Anthony said, it's the airline's responsibility to ensure that only FAA-approved parts go on its planes.

In March, Dallas-based Southwest agreed to pay $7.5 million to settle FAA allegations that the airline made nearly 60,000 flights on planes that had missed required examinations for structural cracks and flew them 1,450 times even after being notified of the missed inspections.

Federal safety officials are also investigating an incident in June in which a foot-long hole opened in the top of a Southwest jet bound from Nashville to Baltimore, forcing the pilot to make an emergency landing in West Virginia. There were no injuries. Government records indicated that eight cracks in the frame required repairs in January.