Southwest Airlines Co., the largest low-fare carrier, won U.S. approval to keep flying 39 Boeing Co. 737s with unauthorized parts as long as replacements are completed by Dec. 24.

Extra inspections also must be done, said the Federal Aviation Administration, which began an inquiry after finding the parts on Aug. 21. Of 82 jets fitted with the unapproved hinge brackets and actuator brackets, 43 have had new units installed, Beth Harbin, a Southwest spokeswoman, said yesterday.

The parts refocused attention on airline maintenance after Southwest agreed in March to a $7.5 million fine, the largest collected by the FAA, for flying jets without fuselage checks in 2006 and 2007. U.S. officials are investigating why a foot-wide hole opened in a fuselage in July, forcing an emergency landing.

“This situation is not about having unsafe parts on our airplanes,” Chief Operating Officer Mike Van de Ven said in a statement. “The parts have been inspected and the FAA agrees that they meet the requirements of the aircraft manufacturer.”

Southwest said it is conducting an “aggressive, yet safe” schedule to replace the parts. Flight schedules won’t be affected, the Dallas-based airline said. The planes involved in the FAA parts probe made up about 15 percent of Southwest’s fleet of about 544 planes, all Boeing 737s.

The brackets, which help deflect hot engine exhaust away from the wings, were made by a company lacking FAA authorization for the work. The FAA is continuing its investigation into Southwest’s use of the parts, which the airline has said were supplied since 2006.

Southwest must examine the parts “for wear and tear” every seven days until authorized replacements are installed, the FAA said. The airline also was ordered to locate and dispose of any other unapproved parts made by the same vendor, and to report daily on the additional inspections.

Southwest, the FAA and Chicago-based Boeing worked out the replacement plan in talks that began on Aug. 22. While the unapproved parts were deemed safe, U.S. regulations prohibit planes from being flown with components lacking federal certification.

D-Velco Aviation Services of Phoenix, a Southwest maintenance vendor that hired the subcontractor responsible for the parts, was suspended earlier by the airline. The partsmaker hasn’t been identified.

Southwest fell 15 cents, or 1.9 percent, to $8.02 in New York Stock Exchange composite trading yesterday before the parts agreement was announced. The shares have declined 7 percent this year.

(Source: Bloomberg.com)