Washington (CNN) -- The Federal Aviation Administration official in charge of operating the air traffic control system has resigned amid revelations that several controllers have fallen asleep on the job this year, the FAA chief said Thursday.
Stepping down is Hank Krakowski, who has been the head of the FAA Air Traffic Organization. David Grizzle, the FAA's chief counsel, will be the acting chief of the unit during a search to fill the post, according to Randy Babbitt, the agency's administrator.
"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. This conduct must stop immediately," Babbitt said in a written statement.
"I am committed to maintaining the highest level of public confidence and that begins with strong leadership," he said.
The development came after another air traffic controller apparently fell asleep while on duty, the sixth such incident this year that the FAA has disclosed.
The latest incident occurred Wednesday morning at Reno-Tahoe International Airport in Nevada "when a controller fell asleep while a medical flight carrying an ill patient was trying to land," the FAA said.
U.S. Transportation Secretary Ray LaHood called the trend "absolutely unacceptable" and transportation officials immediately began making changes and reviews to address the problem.
For example, Babbitt and LaHood said the FAA will assign an extra air traffic controller on the midnight shift at 27 control towers that have been staffed with only one controller during those hours.
The FAA said Babbitt and National Air Traffic Controllers Association President Paul Rinaldi "are launching a nationwide 'Call to Action' on air traffic control safety and professionalism" and will visit air traffic facilities to underscore those points.
The move will include "an independent review of the FAA's air traffic control training curriculum and qualifications and the expansion of NATCA's Professional Standards committees." The FAA is also reviewing staffing and scheduling.
"We are conducting a top-to-bottom review of the way we operate our air traffic control system," Babbitt said "We are all responsible and accountable for safety -- from senior FAA leadership to the controller in the tower. 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."
Along with the Nevada incident, cases of sleeping controllers occurred at Ronald Reagan National Airport in Washington and the McGhee Tyson Airport in Knoxville, Tennessee. Three incidents involving the same person occurred at Boeing Field/King County International Airport in Seattle.
The FAA said it also suspended two controllers in Lubbock, Texas, for an incident in which they failed to hand off control of a departing flight to the Fort Worth Air Traffic Control Center, and responded only after several attempts by the same center to hand them control of an arriving flight. An FAA statement did not indicate whether the Lubbock controllers were thought to have been asleep.
Krakowski's departure was seen as ironic by some, who said he worked hard to improve the safety culture of the FAA.
Krakowski was the guiding force behind the FAA's Air Traffic Safety Action Program (ATSAP), which sought a nonpunitive environment to encourage FAA employees to report safety concerns.
CNN's Mike Ahlers contributed to this report