WASHINGTON - Publicly fuming, the Federal Aviation Administration chief collected the resignation of the head of the US air traffic system, doubled controller staffing at more than two dozen airports and ordered a sweeping review of the entire system.

The actions Thursday were aimed at ensuring planes fly safely as the government sought to reassure the public that air travel is safe despite at least four instances of controllers sleeping on the job.

Related readings:  Air controller asleep as plane landed in US

But present and former controllers told The Associated Press that grueling work schedules and the design of the job itself - sitting in a dark room at night waiting for pilots to call - have made taking naps on the job necessary, even if unauthorized by the FAA. One whistle-blower complained to the Transportation Department that cots can be found in one radar center, most often with controllers asleep in them.

The National Transportation Safety Board warned FAA after a deadly 2006 air crash that controllers' schedules were creating unsafe situations in which they were going into work after only a few hours of sleep. But little had changed until this week when Federal Aviation Administrator Randy Babbitt said he was immediately adding a second controller on overnight shifts at 26 airports and a radar facility that had been staffed with a lone controller. Presumably the second controller provides a margin of safety if the first falls asleep.

Babbitt's order came hours after the pilot of a plane transporting a critically ill passenger was unable to raise the single controller working at 2 am Wednesday in the tower of the Reno-Tahoe International Airport in Nevada.

The FAA said the controller, who was out of communication for 16 minutes, was sleeping. Controllers at a regional radar facility in California assisted the plane, which landed safely.

Hank Krakowski, the head of the agency's Air Traffic Organization, resigned Thursday and a replacement search was under way, Babbitt said.

"Over the last few weeks we have seen examples of unprofessional conduct on the part of a few individuals that have rightly caused the traveling public to question our ability to ensure their safety," Babbitt said in a statement Thursday. "This conduct must stop immediately."

Babbitt and National Air Traffic Controllers Association President Paul Rinaldi met privately Thursday with lawmakers on Capitol Hill to assure them that FAA is on top of the problem.

"We take our responsibilities very seriously and believe staffing levels and fatigue are at the root of the problem," Rinaldi said in a statement. "We will continue to work with the FAA and through our professional standards workgroup to provide the highest level of professionalism and safety."

The first disclosure that a controller had fallen asleep when he was supposed to be directing air traffic was on March 23. That was when two airliners landed at Washington's Reagan National Airport without assistance from the tower after pilots' repeated attempts to reach the lone air traffic supervisor on duty failed. The supervisor later acknowledged to investigators that he had fallen asleep.