Hank Krakowski, the head of air traffic organization at the Federal Aviation Administration, has resigned after Ray LaHood, the United States transport secretary, said he was "totally outraged" as it emerged five air traffic controllers have been suspended this year for falling asleep while on duty. In one incident, two passenger jets landed without any guidance from the control tower at Ronald Reagan National Airport in Washington D.C. after the lone controller fell asleep. "I am totally outraged by these incidents," LaHood said. "This is absolutely unacceptable. The American public trusts us to run a safe system," he added, vowing to do anything necessary to resolve the issue. His comments came as the FAA announced new plans to tackle fatigue in controllers, saying they were taking immediate steps to employ additional staff at 27 airports across the U.S. The FAA said it would "not tolerate" controllers falling asleep on duty. "This type of unprofessional behavior does not meet our high safety standards," a spokesperson said. Krakowski's resignation will likely be seen as an indication that the crisis in control towers across the U.S. is growing. Randy Babbitt, the administrator of the FAA, confirmed Krakowski had submitted his resignation, which had accepted. "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," he said. "This conduct must stop immediately. I am committed to maintaining the highest level of public confidence and that begins with strong leadership." Babbitt said the FAA was conducting a "top to bottom review" air traffic control procedures. A fifth controller was suspended this week after falling asleep while on duty at Reno-Tahoe International Airport in Nevada in the early hours of this morning, forcing an air ambulance carrying a patient to land without directions from the tower. The FAA said the flight crew made contact with a controller at a different facility and made a safe landing. "The controller, who was out of communication for approximately 16 minutes, has been suspended while the FAA investigates," a spokesperson said. The National Air Traffic Controllers Association, a labor union representing controllers in the U.S., said safety was their members' "number one priority" and they were "fully committed to providing the safest and most professional services possible." The addition of new staff at control towers, they said, was "a strong first step that will go a long way to ensuring these isolated cases of fatigue never happen again." Paul Rinaldi, the president of the NATCA, said: "We take our responsibilities very seriously and believe staffing levels and fatigue are at the root of the problem. We will continue to work with the FAA and through our professional standards workgroup to provide the highest level of professionalism and safety."