On Sept. 22, 2017, Air Traffic Control at Boston Logan Airport was navigating a tricky situation, guiding a support vehicle as it conducted an inspection on one of Logan's runways.

The vehicle lost contact with controllers. It rolled onto an intersecting runway where a midsize business jet was about to touch down. Realizing its mistake, it reversed course as the plane was about 3,000 feet away. But after that plane landed and exited the runway, the support vehicle crossed the intersection once more without clearance as another plane was approaching Logan.

Eventually, controllers reconnected with the support vehicle. It had been having radio problems that kept it from connecting with the tower.

This event was classified by the Federal Aviation Administration (FAA) as a  runway incursion — that’s when there’s something on the "protected area of a surface designated for the landing and take off of aircraft" that shouldn’t be there. It could be an airplane that hasn’t been authorized to cross a runway, a support vehicle, or perhaps a person in the wrong spot.

In 2017, the National Transportation Safety Board analyzed six years of runway incursion incident reports from the nation’s airports. Boston's Logan Airport had the seventh highest number in the country.

In 2017, the NTSB analyzed six years of runway incursion incident reports.
NTSB Runway Incursion Forum, Office of Aviation Safety, NASA

Though other cities ranked higher, Boston presents a number of challenges, according to aviation experts. 

“If they were going to give a prize for having complexity, Boston would get it," said Frank Ketcham, an airline pilot and a commercial aviation specialist at UC Berkeley. "It's a very, very challenging airport that draws on the cognitive abilities and the experience of the pilots. That’s to say that there’s several airports throughout the nation that have special issues or threats that pilots have to contend with, but Boston really has several.”

Pilots landing in Boston have to make additional calculations when there are ships in the harbor. In some cases, they don't get to use the whole runway, a practice called "land and hold short operations." Boston has inclement weather and high demand. Radio frequencies can be congested.

Logan also has mixed operations that include small aircraft and helicopters. CapeAir, a local regional carrier, uses a special, small runway that is sometimes mistaken for a taxiway by operators of large aircraft.

"It really puts all the challenges in one place that pilots have," Ketcham said. "There’s just a multitude of things that all take up a little piece of our cognitive bandwidth. And when you add that up and make it a very challenging airport, where’s the spillover going to be?”

That spillover might be behind Logan’s runway incursions — while Boston ranks seventh, other similarly busy airports, like Detroit and Minneapolis St. Paul, didn’t make the top 20.

Richard Marchi, the director of aviation planning and development at Massport from 1977 until 1993, says he's not surprised by Logan's ranking. He says Massport, the state agency that runs Logan Airport, has been dealing with the same confusing layout for decades.

“When Logan was built in the '20s, airplanes couldn’t handle crosswinds very well, so they had to have runways that pointed in each direction," Marchi said. "That’s no longer necessary, but it’s the legacy layout that they’re stuck with. And that means that there’s a lot of places where a taxiing aircraft has got multiple taxiways or multiple runways in front of it. It makes it more difficult to put it in the marking and the signage to make all of that stuff clear."

These days, most modern airplanes can operate into crosswinds of up to 45 knots, or 50 miles per hour, so Marchi says runways pointed in different directions are obsolete.

“Those multiple runway headings are no longer needed, but of course that’s what you've got," he said. "You’re not going to change the layout of the runways at an old airport like Boston because it’s no longer required by crosswind considerations.”

Massport says none of the incursions at Logan over the last 10 years posed a real risk of collision. All of them were category C and D events, which puts them in the two lowest-risk categories. 

Even though runway incursions are usually harmless, pilot and host of Airline Pilot Guy podcast Jeff Nielsen says they’re still an important focus for the aviation world. 

"To say that incursions are not near misses or near collisions are actual collisions is, I think, simplifying it a little too much, because they can result in that," he said. "But most — the vast majority of incursion events — are relatively benign, but could have the potential to become a serious incident or accident."

The FAA has identified three hot spots at Logan that are more vulnerable to incursions or collisions. It awarded the airport a $1.3 million grant in 2016 to study its runway incursions and come up with solutions.

Massport documents reviewed by WGBH News illustrate it isn’t planning on overhauling the existing layout — it’s using the FAA grant to look into design alternatives that would make small changes to the runway instead.

Massport says its on track to finish the study by the end of this summer.

Air traffic control audio from this story was made available by LiveATC.net.