On Friday, the union cited "serious maintenance-related issues" that pilots have reported in the last several days, including a hydraulic leak from the main landing gear, overheating warnings and fuel seeping on to runways.
A spokesman for American, Bruce Hicks, said such reports were not unusual and didn't explain the high number of maintenance write-ups that have caused the recent spike in delays and canceled flights.
Hicks called the union's claims "an outrageous and disappointing attempt to divert attention from the real issues of the operational disruption caused by some pilots' illegal job action."
The union also seized on an email from a Federal Aviation Administration official who advised a pilot to tell the FAA if he felt pressured to compromise safety, and adding that American was under special scrutiny. American noted that the FAA official said in the same email, "This surveillance is focused on frivolous maintenance discrepancy reports" and other actions of pilots.
FAA officials tried to avoid taking sides in the dispute. They said the agency always increases monitoring of airlines that fly under bankruptcy protection to make sure financial problems don't compromise safety. They said that nothing in their monitoring of American has changed since the airline and parent AMR Corp. filed for bankruptcy protection in November.
On Thursday, American canceled 100 flights, or 5.4 percent of its schedule, and only 49.9 percent arrived on time, according to FlightStats.com. Regional carrier SkyWest Airlines was second in cancelations at 42, or 2.2 percent of its flights.
By late Friday afternoon, American had scrubbed 39 flights, or 2 percent of its schedule an improvement over Thursday, but still more cancelations than United, Delta, Southwest and US Airways combined. About 62 percent of American's flights had arrived on time, compared with 81 percent to 93 percent at the other biggest airlines.