Air traffic controllers made more mistakes recently at Minneapolis-St. Paul International Airport than at five of the 10 American airports that had more plane traffic. And MSP had almost as many errors as the Atlanta airport, which is the nation's busiest with nearly three times the passengers.

The reasons are unclear, but the numbers may reflect a physical reality. Unlike some larger and newer airports, MSP's layout and its use of two major runways can increase the complexity of controlling takeoffs and landings. Hemmed in by freeways, homes and a national cemetery, it lacks the space some other airports have for more runways.

"It's just a different steer to wrangle," said Sam Tomlin, a veteran air traffic controller at MSP. "There's a lot of stuff going on."

Controller mistakes typically involve planes getting too close to each other. Seventeen errors occurred at MSP from late 2010 to early 2012, the most recent period for which such data are available. They are included in reports, covering hundreds of airports, that were released by the Federal Aviation Administration in response to a Freedom of Information Act request by the Star Tribune. The FAA has since changed its method of classifying problems, but has not released those records.

The errors at MSP happened at a faster pace than previously. From late 2006 through late 2010, there were a total of 21 errors.

The FAA acknowledges that intersecting or converging runways at MSP can create complications and opportunities for errors.

"Every time you have an intersection, you have an opportunity for conflict," said FAA spokeswoman Laura Brown. "It doesn't mean that you can't operate safely." She said comparing traffic control at airports is difficult and that "each airport has its own set of challenges."

Nationwide, tower errors increased 82 percent over five years. While the FAA has attributed much of the spike to better reporting, the Office of the Inspector General for the U.S. Department of Transportation this year stressed problems.

"[W]e found that the increase in reported errors was linked, in part, to a rise in actual errors rather than increased reporting," the Inspector General's Office concluded. The mistakes are "a major air safety concern, particularly in light of dramatic increases."

Examples of errors

The government sets minimum distances that must be kept between planes to ensure safety. Tower errors typically involve controllers allowing planes to come closer than permitted — often within 3 miles.

One of the 17 errors at MSP ended up in an FAA summary of "untoward events." The Nov. 11, 2010, episode involved a Delta Air Lines Airbus and a Mesaba Airlines turboprop that were instructed to take off from parallel runways.

The FAA said a controller arranged to have the Mesaba flight follow the Delta one. The controller asked the Mesaba pilot if he "could accept an immediate takeoff and the pilot replied that they were ready." The controller then told the pilot to "line up … don't plan on stopping," and added, "I'll have a turn at the end."

After the Mesaba plane left the runway, the controller told its pilot to turn left "without ensuring adequate separation," according to the National Transportation Safety Board (NTSB). The plane came within 1.84 miles of the Delta Airbus instead of keeping the required 3-mile distance.

The error was one of five category "B" mistakes, the second most serious under the old classification system.

That episode was mentioned in an NTSB investigation of a Sept. 16, 2010, near-collision of a US Airways Airbus and a Bemidji Aviation cargo plane. The NTSB said that the two instances were evidence of a broader problem and that both mistakes involved "lack of awareness between the North and South [runway] local controllers." It noted that the FAA took steps to lessen the likelihood of a recurrence.

In the September 2010 episode, the Bemidji plane flew 50 to 100 feet below the US Airways jet. The category "A" error was included in an earlier round of mistakes and not with the 17.

The 17 errors included:

• A controller twice issued orders to misidentified planes, confusing a Mesaba flight with a Pinnacle plane and addressing another Mesaba plane by the wrong flight number. One of the misidentified Mesaba planes failed to turn in the desired direction after takeoff and didn't maintain adequate distance from another flight.

• A Pinnacle Airlines flight was taxiing and told to stop outside of a runway where a Delta jet had been cleared for takeoff, but the Pinnacle pilot "stopped just past the hold short line." With the departing Delta flight already rolling down the runway, "the controller determined it to be unsafe to cancel the takeoff." Meanwhile, an arriving Delta flight "was instructed to go around."

• A Delta plane was cleared to land ahead of an approaching Pinnacle Airlines flight that was due on a parallel runway. The control tower told the Pinnacle pilot to "reduce to their final approach speed as soon as practical." One minute and 36 seconds later, the tower told the Pinnacle pilot he "would need to be taken off the approach" and climb to 3,000 feet. But the plane still got too close to another.

'Luck lets you down'

While the recent errors were not exceptional, they serve as "warning signals" to the FAA, said Jim Swenberger, who worked for three decades as a controller, supervisor and air traffic control investigator for the agency.

"The rules are there for a purpose and we have to adhere to them," said Swenberger, now a consultant on air traffic control. "We have the safest system in the world … that wasn't arrived at by luck."

"I hate luck," he said. "The one time you really need it, luck lets you down."

MSP was the nation's 11th-busiest airport when the 17 errors occurred. That number exceeded the number of errors at five airports that have more plane traffic: Dallas/Fort Worth International (9), Denver International (1), Los Angeles International (14), Detroit Metropolitan Wayne County International (11) and Phoenix Sky Harbor International (4).

MSP also had the same number of errors from late 2010 to early 2012 as the combined total of three other major airports: John F. Kennedy International and LaGuardia in New York and San Francisco International.

The busier airports that had more errors were Hartsfield-Jackson in Atlanta (20), Chicago O'Hare (24), George Bush Intercontinental in Houston, (19), Charlotte-Douglas International in North Carolina (34) and Philadelphia International (19).

'Hitting the gap'

People in the air traffic control business say it's hard to know why MSP experienced a recent increase in errors and had more than some busier airports. But they say the number and layout of runways may be a factor.

Air traffic controller Tomlin said some newer airports give controllers more room to maneuver than MSP, which has four runways. Dallas/Fort Worth has seven runways; Denver has six. More of their runways are parallel, avoiding conflicts. "It offers you so much more flexibility than our geography does," he said.

MSP often must rely on using a runway for both arrivals and departures. That means controllers must time a departure to begin after an arriving plane crosses the end of a runway and before the descending plane behind it arrives.

"We in the business call it 'hitting the gap,' " Tomlin said. "It's one in, one out. One in, one out. One in, one out."

"Places like … Atlanta, Dallas-Fort Worth, they have so many runways and so many runway configurations, they can get a sole runway for the arrival runway."

Lack of space

At MSP, two parallel runways facing northwest and southeast take advantage of prevailing winds to handle nearly 80 percent of the traffic. They are crossed by a third runway, seldom used for takeoffs and landings. The fourth and newest runway facing north and south opened a few years ago and handles some traffic that otherwise would use the parallel runways and fly over south Minneapolis.

"It would have been more efficient to run three parallels than to run two parallels," Swenberger said. Referring to the fourth runway, he added, "The runway was built for noise abatement. It wasn't built for the efficient use of the air traffic system."

"In the long run, the procedures that are enacted are more cumbersome, more difficult to accomplish," he said of air traffic control.

But MSP spokesman Patrick Hogan said the location of the fourth runway wasn't related to concerns about noise, just space.

"While a third parallel runway might have been more efficient, it wasn't possible with the space available," Hogan said. "MSP is surrounded on all sides by highways, intersected by a national cemetery and in very close proximity to the largest shopping mall in the United States. There was no place to put another parallel runway."

Pat Doyle • 612-673-4504