The wheels were down and the plane’s nose was pointed to the waiting runway at Reagan National Airport. The lights of the Kennedy Center and the Washington Monument flashed by outside the window, and Sharon Cooley braced herself for the thud of tire on tarmac.

Instead, with a mighty roar of engines, the plane rocketed into a steep climb.

The gasps on board were mirrored by the reaction in the airport waiting area as the underbelly of the climbing plane seemed to skim just overhead.

The remarkable thing about the aborted landing of Alaska Airlines Flight 6 from Los Angeles last week was that it wasn’t remarkable at all; it happens regularly at National. Another plane was sitting on the runway, and air-traffic controllers gave the pilot prudent instructions to keep everyone safe.

National’s control tower hit the news two days after Cooley’s scary experience when the midnight-shift controller supervisor fell asleep, forcing pilots of two airliners carrying a total of 165 passengers and crew members to land on their own early Wednesday. The Federal Aviation Administration suspended the supervisor, the National Transportation Safety Board is investigating, and Transportation Secretary Ray LaHood ordered that a second controller be added to the midnight shift.

Cooley’s experience, which left people on board shaken and caused a woman in the next row to say her “life flashed before her,” happens an estimated 10 to 20 times a month at National, according to controllers and FAA records.

Ten days earlier, Tamar Gutner was headed into National on a US Airways flight from Orlando.

“I could see the Potomac, the landing gear was down, and suddenly we’re going back up, sharply,” Gutner said. “The pilot didn’t say anything for several minutes, and I’m sure I wasn’t the only person worried about terrorism. The pilot finally came on and said there was an aircraft in the runway that prevented us from landing.”

Unnerving though the experience might be for passengers such as Cooley and Gutner, controllers see what they call a “go-around” as a fairly routine safety procedure that keeps planes out of greater trouble.

“Go-arounds, although not frequent, are a safety valve for the system,” said an FAA official who once worked the Reagan tower and who asked not to be named because he’s not authorized to speak for the agency. “Each controller tries to time their instructions perfectly, but it doesn’t always work because pilots and aircraft respond at different rates. So, you do the best you can, and if it doesn’t work, you send them around.”

The FAA had no immediate comment on go-arounds at National.

Last-second orders to abort a landing are not unique to National, but the heavy traffic load into a compact airport means that air-traffic controllers have to choreograph with particular skill during the busy hours of the day, cutting their calculations as finely as safety will allow. Now Congress, eager to make the most of the airport at its doorstep, wants to add flights at National. The Senate has proposed adding 10 round trips daily, while the House is working on a decision about how many more it finds appropriate.

“Reagan is basically a one-runway operation,” said the FAA official. “They run squeeze plays all the time, because they have to in order to move traffic. The old saying was ‘tighten them up until you get a go-around and then back off a bit and you will have perfect spacing on final’ ” approach.

National averaged 726 daily commercial flights last year, and they arrive and depart in a steady stream during the peak morning and evening hours. Pinned in by the Potomac River on one side and the George Washington Memorial Parkway on the other, the airport has nowhere to grow, so jetliners have one long primary runway to use.

By contrast, Dulles International Airport, with an average of 922 daily flights, can use three runways at the same time and has a fourth at an angle for use when conditions allow. Baltimore-Washington International Marshall Airport, with about 700 commercial flights on an average day, normally uses two runways, with a third reserved mostly for smaller planes.

There are a number of reasons that a pilot on final approach to land might be ordered to abort the attempt. Sometimes a plane may be approaching too fast or too high for safety. Other times the weather plays a factor.

FAA records for August 2010, for example, show at least seven instances in which planes on final approach to National were ordered to go around “due to traffic on the runway,” the same gut-wrenching experience Cooley and Gutner had this month.

That traffic on the runway could be there for a couple of reasons: A plane that just landed hasn’t moved out of the way quickly enough or, more likely, a plane cleared for takeoff has dawdled on the runway past the time when the controller expected it to be gone.

A third possibility: The arriving plane approached more quickly than anticipated, throwing off the tower’s timing.

Dulles has about as many go-arounds as National, according to one vet­eran controller who keeps track of such incidents.

The controller, who asked not be named because he’s not authorized to speak for the FAA, said that’s because Dulles often uses the same runway to handle both arrivals and departures. BWI usually uses one runway for arriving flights and a second for departures. In October, National recorded 39 go-arounds, Dulles had 20 and BWI had 3, according to an internal FAA document.

The controller said that although the plane on the runway and the plane that gets waved off from landing may merge into a single blip on the radar screen, “from the tower they don’t look that close.”

“Sometimes they look close, but the alternative would be a lot worse,” he said. “They’d be smashed up on the runway.”

Cooley said her husband saw her plane shoot back skyward from the terminal waiting area, where a gasp rippled through the crowd.

“We were back in the air for another 10 minutes,” she said. “Finally, the flight attendant came and said there had been another plane on the runway. I was so nervous. I just wanted to get my feet on the ground.”