The Allied Pilots Association denied that there was an organized job action, which would be illegal.
Since Sept. 16, 49.8 percent of American's flights have arrived on time nationwide, compared to 89.5 percent for US Airways Group Inc. and 83.7 percent to 88.4 percent for other large carriers, FlightStats.com said.
Between Sept. 16 and Tuesday, American canceled 648 flights and experienced 8,290 delays nationwide.
At Philadelphia International Airport, 49.9 percent of American's flights arrived on time. American and regional carrier American Eagle operate 15 daily flights from Philadelphia to Dallas-Fort Worth, Chicago, and Miami.
Several industry observers said the situation should not derail a possible merger. US Airways has been vocal about its desire to merge with American.
"It doesn't make any difference at all to any merger," said veteran analyst Bob McAdoo of Imperial Capital L.L.C.
Either American and US Airways will merge and the labor issues will be resolved, McAdoo said, or the pilots will work out a new contract with management "and magically, all the planes will be fine again."
"In the alternative, management will track down the instigators" of the slowdown and get a restraining order. "One way or another, it will get fixed," he said.
CrankyFlier.com author Brett Snyder wrote in a column Tuesday, "My guess is that this will get sorted out in the courts one way or another, but for now American is running an awful operation."
On Aug. 8, American pilots overwhelmingly rejected a concessionary contract agreement while other major unions ratified their agreements. The bankruptcy judge will not let American exit bankruptcy without collective-bargaining agreements.
"On the evening of Sept. 12, word started to leak out that American management had released the terms it planned to impose on the pilots," which were worse than what the pilots had voted down, Snyder wrote. Soon after, "only half the airline's flights went on time," and the cancellation rate "went up a lot as well."
Analyst Ray Neidl of Maxim Group L.L.C. wrote in a client note "that any job actions, organized or otherwise, hurt the company in the short term."
"We remain concerned that it could escalate in what is a weak economy and, if not resolved, could lead to a possible further weakening and eventual liquidation" of American, Neidl wrote. "US Airways and other airlines could then pick up parts of the company by buying various assets that had value, but not necessarily guaranteeing jobs for AMR employees."
American pilots blamed the mechanical delays on the airline, which "operates the oldest fleet of any major U.S. carrier," has furloughed many mechanics, and has closed a large maintenance facility. "Management also decided some time ago to reduce its inventory of spare parts" in a cost-cutting move, the union said.
American said it was ready to resume contract negotiations. The union said it wants the request in writing.
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