WASHINGTON  For all the headaches of flying in the United States, the domestic airlines were until recently considered a logistical marvel, moving two million people a day with remarkably few accidents.

Now they are in chaos, with airlines grounding more than 500 planes and thousands of flights so far because they may not meet safety requirements. Travelers have seen this before but only rarely, when all planes were grounded after the Sept. 11 attacks and when the government grounded all DC-10s after an engine fell off one of them in 1979, killing 273 people.

But there is a big difference this time: there has been no crash.

What happened?

One answer is that some whistle-blower inspectors for the Federal Aviation Administration disclosed that they had been discouraged from cracking down on Southwest Airlines for maintenance problems, and they found a sympathetic audience with some Washington lawmakers.

That prodded the F.A.A. to order a national audit to check whether airlines were in compliance  and to propose a record penalty of $10.2 million against Southwest.