The control tower supervisor who failed to respond to two planes trying to land at Reagan National Airport Wednesday has been suspended while the incident is investigated, the nation’s top aviation safety official said.

FAA administrator Randy Babbitt, himself a former airline pilot, said he was “personally outraged that this controller did not meet his responsibility to land these two airplanes.”

The planes — an American Airlines Boeing 737 flying in from Miami with 97 people on board, and a United Airlines Airbus 320 flying in from Chicago with 68 people on board — landed safely within minutes of each other, just after midnight.

The controller — the lone person on duty in the tower — did not respond to pilot requests for landing assistance or to phone calls from controllers elsewhere in the region. According to internal records, the pilots also used a “shout line,” which pipes into a loudspeaker in the tower, to try and reach the controller.

The tower normally is staffed by one air-traffic controller from midnight to 6 a.m. The Associated Press, citing an aviation official who spoke on condition of anonymity because of the ongoing investigation, said the supervisor who was on duty simply fell asleep.

After learning of the incident, Transportation Secretary Ray LaHood ordered a second air-traffic controller to be on duty overnight at the airport. LaHood also instructed the FAA to examine staffing levels at other airports around the country.

“It is not acceptable to have just one controller in the tower managing air traffic in this critical airspace,” LaHood said.

Because the controller was not available, the planes’ pilots took matters into their own hands, broadcasting their progress as they approached and landed. They also were communicating with controllers at a separate facility in the region that does not handle landings.

Babbitt, in a statement issued Thursday, said he was “determined to get to the bottom of this situation for the safety of the traveling public.”

“Fortunately, at no point was either plane out of radar contact, and our back-up system kicked in to ensure the safe landing of both airplanes.”

The incident, which the National Transportation Safety Board also is reviewing, is the second time in as many years that the tower at National has gone silent, said a source familiar with tower operations who spoke on the condition of anonymity because he is not authorized to speak for the FAA.

The previous time, the lone controller on duty left his swipe-card pass key behind when he stepped outside the tower’s secure door and was unable to get back in, the source said. A controller at another facility mentioned that incident to a pilot as he was trying to land early Wednesday.

A missed handoff

The nation’s air traffic control system has many layers, with a network of en-route controllers directing planes when they are at or near cruising altitude. The airspace beneath that is controlled by Terminal Radar Approach Control facilities known as TRACONs. Takeoffs and the final miles of runway approach are handled by controllers in airport towers.

After midnight, when traffic eases, only one person until now has been scheduled for duty at the National Airport tower. The shift has been reserved for a supervisor rather than a regular controller.

The planes that landed without tower help were two of the last three inbound commercial flights expected at the airport until 5 a.m., the source said.

A few minutes after midnight, radio recordings show, the TRACON controller handling the flight from Miami made a routine verbal handoff, telling the pilot to contact the National tower.

Unable to reach anyone at there, the pilot aborted the approach, circled the airport and radioed the Potomac TRACON controller for help in aligning the plane for landing. A few minutes later, when the United plane approached for landing, the TRACON controller told him that the tower was not staffed.

The TRACON controller had a similar conversation with a second American plane.

“So you’re aware,” the controller said, “the tower is apparently not manned. We’ve made a few phone calls. Two airplanes went in the past 10 to 15 minutes, so you can expect to go into an uncontrolled airport.”

“Is there a reason it’s not manned?” the American pilot asked.

“Well, I’m going to take a guess,” the controller replied, “and say that the controller got locked out. I’ve heard of it happening before.”

“That’s the first time I’ve heard of it,” the pilot said.

“Fortunately, it’s not very often,” the controller said. “It happened about a year ago. I’m not sure that’s what happened now, but there’s nobody in the tower.”

Finding their way

The first two planes landed and used information from their airlines to find the correct gates. By the time the third plane touched down, after about half an hour of silence, communication from the tower had been restored.

The greatest risk posed by silence from the tower was on the ground rather than in the air. Planes routinely land in smaller airports without guidance from a tower.

In a circumstance like the one that occurred at National, pilots get on the control tower radio frequency and relay their position, speed and distance to other pilots as they approach and land.

“So, other airplanes would know, ‘Okay, he’s clear of the runway, so I’m good to go,’ ” said the source familiar with tower operations.

On the ground, however, the slow nighttime hours are when maintenance crews crisscross the runway — sometimes towing planes — as they prepare for the next morning.

“There are people in the control tower for a reason,” the source said. “There’s a whole lot of activity going on during the night.”

Those maintenance workers contact the tower on a special frequency to get clearance before crossing a runway. Inbound pilots contact the tower on a different frequency.

At airports where the tower shuts down for the night, ground crews and incoming pilots are required to use the same radio frequency to coordinate their actions.

Air traffic controllers who direct more than 1.5 million flights annually in the Washington region made a record number of mistakes last year. Dozens of the errors triggered cockpit collision warning systems.

Nationwide, errors by air traffic controllers increased by 51 percent last year. The record number of errors — locally and nationally — reflects a majority of cases in which planes came too close and some in which a potentially fatal outcome was narrowly averted.

In January, an American Airlines plane carrying 259 people almost collided with a pair of 200-ton military cargo jets after departing New York’s John F. Kennedy International Airport. Official records show that a distracted controller did not respond to a warning from a colleague that the planes were on a converging course.