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 they are not considered an immediate safety threat and regulators will let Southwest keep flying the planes for 10 days while it decides how to fix the problem. Southwest said Wednesday that the incident led it to ground 46 planes last Saturday, causing flight delays throughout its network. 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 FAA officials would meet with Southwest later Wednesday to discuss follow-up actions. 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 airline pulled 46 planes — nearly 9% of its fleet — out of service for several hours Saturday. That led to widespread delays and a few cancelations "probably on the scope of a pretty good Texas thunderstorm," Harbin said. 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. Dallas-based Southwest Airlines Co. carries more than 100 million U.S. passengers a year, more than any other airline. Southwest shares fell 9 cents to $8.59 in afternoon trading. Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more