The head of the Air Traffic Organization at the Federal Aviation Administration resigned Thursday morning amid recent reports of several controllers sleeping on the job.

Hank Krakowski submitted his resignation Thursday morning to FAA Administrator Randy Babbitt, who said he accepted it, federal officials said.

Krawkoski joined the FAA in 2007. Prior to that he spent about 30 years at United Air Lines in senior management positions, including as vice president of flight operations.

“Hank is a dedicated aviation professional and I thank him for his service,” Babbitt said in a statement. “Starting today, I have asked David Grizzle, FAA's chief counsel, to assume the role of acting ATO chief operating officer while we conduct a nationwide search to permanently fill the position.”

Babbitt said recent reports of “unprofessional conduct on the part of a few individuals have rightly caused the traveling public to question our ability to ensure their safety.”

On Wednesday federal officials ended the practice of leaving one controller on duty in airport towers during overnight shifts.

The FAA also revealed that a Nevada air traffic controller allegedly fell asleep Wednesday morning as a medical flight carrying a patient tried to land.The plane landed safely at Reno-Tahoe International Airport with the help of a radar controller based in California, the FAA said. The controller was suspended and the incident is under investigation.

However, the incident Wednesday was the fifth time this year that a controller apparently slept while on duty, including at Reagan National Airport, where a controller supervisor was suspended last month after he admitted to napping in the tower .

The FAA plans to conduct a “top to bottom review” of the nation’s air traffic control system, Babbitt said.

Babbitt announced last month that he was revamping air traffic control guidelines. He ordered radar controllers who guide planes as they descend from cruising altitude to confirm that controllers in airport towers are prepared to handle incoming flights before handing them off.

Babbitt also said he would instruct controllers to offer the pilots an option to land elsewhere if a control tower is unresponsive for any reason.

“We are all responsible and accountable for safety–from senior FAA leadership to the controller in the tower,” he said. “Employees at the FAA work diligently every day to run the safest air transportation system in the world. But I will continue to make whatever changes are necessary to ensure we concentrate on keeping the traveling public safe.”

U.S. Sen. John D. Rockefeller IV (D-W.Va.), who chairs the Senate transportation committee, said he contacted Babbitt on Wednesday about the emerging problem.

“Yesterday I told Administrator Babbitt he needed to do whatever it takes to keep our aviation system safe,” Rockefeller said in a statement. “ I’m pleased he’s taken this message seriously. Accountability starts at the top – and a change in leadership offers an opportunity to change the way business is done. We simply can’t have an aviation system where some of the people responsible for safety are literally asleep at the switch.”

The NTSB is investigating the incident at National Airport and says fatigue is one of the issues under review.

The recent events involving controllers sleeping on the job and a near-collision in January involving an American Airlines jumbo jet carrying 259 people and a pair of 200-ton military cargo jets over New York have increased concern about air safety.

Recorded errors by air traffic controllers increased last year by 51 percent nationwide.

[This story has been updated]

From the archives:

FAA ends single-controller overnight shifts

At National Airport, aborted landings are not uncommon

FAA to revise U.S. air traffic control rules after National Airport incident

Reagan National controller drug tested, suspended after sleeping on the job

FAA moves to fire Knoxville controller it says was napping