Crowds of passengers wait in line at O'Hare International Airport in Chicago, 09 Apr 2008

American Airlines is cancelling nearly 600 flights Friday, as at least one-fifth of its jet fleet remains grounded for ongoing safety inspections.

Including Thursday's 570 cancellations, the largest U.S. airline has scrubbed more than 3,000 flights since Tuesday, causing chaos at airports nationwide and leaving travelers frustrated.

American's chief executive, Gerard Arpey apologized Thursday for the cancellations, which are costing the company tens of millions of dollars. The airline was forced to ground about half its fleet - more than 300 twin-engine, medium-range jets commonly known as MD-80s - to conduct federally mandated safety inspections of the electrical wiring.

The government's Federal Aviation Administration recently increased its scrutiny of airline safety after inspection and maintenance lapses were discovered at another carrier, Southwest Airlines.

Other U.S. airlines - Delta, Alaska and Midwest Airlines, among others - have grounded some aircraft this week as a result of the government audit, but the problems were most severe at American. The airline has paid for travelers' hotel stays and offered all delayed passengers a refund or a $500 coupon for future travel.

The U.S. airline industry is already suffering from high fuel prices and growing customer dissatisfaction over delays, flight cancellations and lost luggage.

Some information for this report was provided by AP, Bloomberg  and Reuters.