In response to the lapses, the Federal Aviation Administration announced it was immediately putting a second controller on the midnight shift at 26 airports and a radar facility around the country that currently have only one overnight person.
"I am totally outraged by these incidents. This is absolutely unacceptable," Transportation Secretary Ray LaHood said in a statement. "The American public trusts us to run a safe system. Safety is our No. 1 priority and I am committed to working 24/7 until these problems are corrected."
The controller at Reno-Tahoe International Airport was out of communication for about 16 minutes when the aircraft carrying at least three people was landing about 2 a.m. Wednesday, the FAA said. No injuries were reported.
Still, the head of the airport called for increased staffing.
"The flying public needs an assurance from the FAA that this situation will be addressed at all airports," airport chief Krys Bart said early in the day.
She said the pilot of the medical flight and airport staff had tried to contact the controller multiple times without success. The FAA said the pilot was in contact with regional radar controllers in northern California during the landing.
The FAA last month put two controllers on duty during the midnight shift at the Reno-Tahoe airport but went back to one controller several days later after implementing new procedures, FAA spokesman Ian Gregor said.
Elsewhere, two jetliners landed at Washington's Reagan National Airport last month without tower assistance after the lone air traffic supervisor fell asleep.
An investigation by the FAA uncovered a second incident of an air traffic controller sleeping on the job in February during the midnight shift at McGhee Tyson Airport in Knoxville, Tenn.
The agency also said a controller in Seattle had been suspended for falling asleep during a morning shift on Monday. The controller already faced punishment for falling asleep on two other occasions during an evening shift in January, the FAA said.
Tin addition, two controllers in Lubbock, Texas, were suspended after two failed handoffs two weeks ago.
LaHood previously ordered an examination of controller staffing at airports across the nation and directed that two controllers staff the midnight shift in Washington.
A separate investigation was ordered by the House Transportation and Infrastructure Committee.
U.S. Sen. Harry Reid of Nevada said the Reno incident was unacceptable and his office was asking the FAA that a minimum of two air traffic control personnel work in the tower at all times.
The Piper Cheyenne plane involved in the Nevada incident is a twin engine turboprop with seating for five.
"The pilot evaluated the airfield. The weather was clear. The aircraft did land without incident," Bart said.
It was not immediately clear where the flight was coming from.
Bart said the airport, which serves the Reno, Carson City and Lake Tahoe areas, opened a new air traffic control tower in October 2010.
She said the airport has three runways, including two with modern instrument landing and lighting systems.
The incidents come nearly five years after a fatal crash in Kentucky in which a controller was working alone. Investigators said the controller in Kentucky was most likely suffering from fatigue, although they placed responsibility for the crash that took 49 lives on the pilots.
The National Air Traffic Controllers Association has warned against putting controllers alone on shifts and assigning tiring work schedules.
At most airport towers, there's no bathroom in the cab - the room on the top of the tower. With only one controller on duty, the position has to go unattended at times if the controller needs to use a bathroom. It's common for the nearest bathroom to be located down a flight of stairs from the cab.
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Associated Press writers Oskar Garcia and Ken Ritter in Las Vegas, and Joan Lowy in Washington contributed to this report.
