CHICAGO – Travel nightmares continued Thursday as safety checks grounded thousands of flights in the wake of a scandal over skipped aircraft inspections.

American Airlines has cancelled more than 2,400 flights since Tuesday as it tests the wiring on its entire fleet of MD-80 airplanes.

Safety inspections have also grounded scores of United, Southwest and Delta flights in recent weeks. Alaska Airlines and Midwest Airlines also grounded MD-80 planes on Thursday.

Last week, FAA whistleblowers denounced conduct by higher-ranking officials that permitted Southwest Airlines to skip aircraft inspections, allowing some to fly with cracks in their fuselages.

The FAA conducted an audit of all domestic airlines' maintenance paperwork after Southwest was fined 10.2 million dollars for flying dozens of planes without undergoing fuselage inspections.

US aviation safety chief Robert Sturgell said last week that a two-week audit of all domestic airlines, prompted by incidents at Southwest, shows 99 percent compliance with safety directives.

The safety scandal comes after one of the worst years on record for flight delays, cancellations, and long, on-board delays.

Some 29 percent of US flights last year were delayed or cancelled, affecting about 163 million passengers, according to a report released Wednesday.

More than 88,234 flights were stuck on the tarmac for one hour to five hours or longer, affecting nearly 5.9 million passengers.