American said an undetermined number of flights would also be canceled Saturday, but it hoped to complete safety inspections of all its mid-range MD-80 aircraft by Saturday night.
The financial toll and loss of goodwill will likely grow as well, as the inspection-related mess spread further to other carriers and hurt an industry already bleeding cash thanks to high fuel costs.
American said it was cancelling 570 more flights on Friday - the fourth straight day of large-scale cancellations since the nation's largest airline learned that the wiring on 300 planes needed to be re-inspected.
Industry experts estimate American's revenue losses will range between $40 million to $50 million, reports CBS News correspondent Nancy Cordes.
Lawmakers were asking questions and some fed-up air travelers headed for trains. Others gave the airlines a pass, saying the companies were doing the best they could.
"If somebody's got a choice between being in a plane crash and being late, is there a choice?" Jane Bernard, a writer from New York who was delayed by at least three hours en route from LaGuardia Airport to Miami, said Thursday.
Friday's cancellations hit hardest at Dallas-Fort Worth International and Chicago O'Hare airports, hubs through which American funnels many of its passengers.
Lines at DFW appeared back to normal Thursday, as more passengers learned of the cancelations before driving to the airport.
"There were a thousand people in here on Tuesday," said Theresa Williamson, a nurse from Tucson, Ariz., who had her first flight home canceled Tuesday but was back at Terminal A on Thursday waiting in line to check her bags. "I felt bad for the employees, because there were only two or three of them."
Just ahead of her in line was Veronica Johnson, a Washington-area resident who was in Dallas for a business meeting and finally found a flight home after the frustrations of being booked on four flights, only to have each one canceled.
"My employer was panicking, my kids were panicking, I was panicking," Johnson said. "I just think they ought to be more conscious of how (the grounding of planes) affects their customers."
American's top executives said they understand customers' anger, and they continued to offer mea culpas.
Chairman and Chief Executive Gerard Arpey said he took responsibility and that neither American's mechanics nor the FAA were to blame.
Arpey said the cost would run into the tens of millions of dollars, but said gave no precise figure.
American had initially expected to cancel 570 Friday flights, but on Friday morning the airline raised the number to 595 - more than a quarter of its schedule.
That pushed the four-day total to around 3,100 canceled flights.
Alaska Airlines, Midwest Airlines and Atlanta-based Delta Air Lines Inc. joined the wave, each canceling a small number of flights on MD-80 aircraft Thursday.
At least 250,000 passengers have been affected by the American cancellations this week alone.
Other carriers like Continental Airlines Inc., JetBlue Airways Corp., AirTran Airways and Northwest Airlines Corp. said they passed the first round of FAA audits with a clean slate and did not expect extra maintenance work or flight delays. It was impossible to say whether that could change since the FAA is conducting another round of safety audits.
The cancellations come at a time of high fuel prices and mixed success among the major air carriers at getting domestic fare increases to stick. The fact that airplanes are flying very full is making it difficult for airlines that cancel flights to find empty seats on other carriers to rebook their passengers.
"This disruption is severe," said Webster O'Brien, an industry expert with aviation consulting firm Simat, Helliesen & Eichner. "People are going to be unhappy. There isn't going to be an easy way to walk everybody out of it."