Federal regulators are investigating American Airlines over structural repairs to its aging fleet of MD-80 series aircraft.
A Federal Aviation Administration official said Friday that the investigation centered on 16 planes.
The Wall Street Journal reported FAA officials suspect American rushed to retire one of the planes to keep it from inspectors.
A spokesman for American declined to comment on the circumstances under which the one plane was retired but said mothballing the aircraft wouldn't let it escape scrutiny.
"The FAA has complete access to retired airplanes, and it exercises that access frequently," the spokesman, Tim Wagner, told The Associated Press. "All airlines have the authority to make decisions regarding the retirement of individual aircraft based on economic and competitive factors."
Wagner said American is responding to the FAA's investigation, and he declined to comment further.
FAA spokesman Lynn Lunsford said the investigation centered on repairs to the rear bulkhead of the MD-80 series aircraft. As of May, American had 270 MD-80 series jets, or 44 percent of its fleet, according to the company's Web site.
Airplanes expand and contract as the cabin is pressured for flight and then depressurized. That can lead to metal fatigue that requires close monitoring and sometimes repairs, especially around the rear bulkhead.
Improper rear bulkhead repairs were blamed for the 1985 crash of a Japan Airlines Boeing 747 that killed 520 people, still the worst single accident in aviation history.
The Journal reported that FAA inspectors believe at least 16 American jets may have flown for months or years with improper repairs to structural cracks.
Fort Worth-based American, a unit of AMR Corp., is slowly replacing the MD-80s with new, more fuel-efficient planes.
FAA investigations can lead to exoneration of the carrier or, as in recent cases involving American and Southwest Airlines Co., penalties that run into the millions of dollars.
Shares of American parent AMR Corp. rose 7 cents to $5.56 in midday trading Friday.