The quick-thinking maneuver by an airline pilot to steer clear of a B-52 bomber in North Dakota’s airspace has raised awareness about how small airport control towers, often operated by private firms without onsite radar systems, manage operations.

Neither the B-52 bomber nor the Delta Air Lines jet pilots seemed to be aware of the other’s presence until the airline pilot visually spotted the B-52 approaching. The episode, which happened last Friday, is still under scrutiny. The Air Force has stated that the Minot airport controller failed to inform the bomber’s team about the airliner, and the SkyWest pilot handling the Delta flight noted to passengers his surprise.

Passengers were startled by the abrupt turn and descent carried out by the pilot of Delta Flight 3788 to steer away from the bomber, which had just completed a state fair flyover in Minot. A video recorded on the plane later included the pilot’s explanation of the event.

“Sorry about the aggressive maneuver. It caught me by surprise,” the pilot can be heard saying on the video posted on social media. “This is not normal at all. I don’t know why they didn’t give us a heads-up.”

This close call is just the latest incident to raise questions about aviation safety in the wake of January’s midair collision over Washington, D.C., that killed 67 people. Here’s more about the way small airports like Minot operate:

Many small airports lack radar

It is common for small airports across the country to operate without their own radar systems because it would be too costly to install them at every airport. But there generally aren’t many problems with that.

The controllers at small airports are able to guide planes in to land visually with binoculars and radios as long as the weather is clear. Aviation safety consultant Jeff Guzzetti, who used to investigate crashes for both the National Transportation Safety Board and Federal Aviation Administration, said if the weather is bad, a regional FAA radar facility may be able to help, but ultimately planes simply won’t land if the weather is too bad.

Sometimes small airport towers do have a video feed that gives controllers a view of a radar screen at an FAA facility miles away. Because the radar is so far away the display may not be as detailed about planes flying close to the ground, but the system does give controllers more information. It’s not clear if the Minot tower has one of those systems because the company that runs it, Midwest Air Traffic Control Inc., hasn’t responded to questions since the incident.

The overlapping network of FAA radar facilities across the country also keeps track of planes flying between airports, and an approach control radar center in Minneapolis helps direct planes in and out of Minot before controllers at the airport take over once they see the planes. The Minot airport typically handles between 18 and 24 flights a day. That’s how it works at many small airports.

“Most times it works just fine,” Guzzetti said.

Some of these small airports could gain radar as part of a massive overhaul of the air traffic control system, but that will depend on how busy the airports are and how much funding Congress ultimately approves for the multibillion-dollar project. So far, $12.5 billion was included in President Trump’s overall budget bill.

Private companies operate the towers

The FAA says that 265 airport towers nationwide are operated by companies as part of the contract system. The Transportation Department’s Inspector General has said the contract towers that handle more than one quarter of the nation’s flights are more cost effective than comparable FAA towers and have similar safety records. In some cases, local governments help pay the costs of contract towers.

The program began in 1982 at five less busy towers that had closed because of the air traffic controller strike the previous year, and it has expanded significantly over the years because it has been so successful. Most of the airports with contract towers would have no controllers without the program.

“Common sense would tell you that having an extra set of eyes controlling the local traffic — especially in good weather — would be safer than having no controller and just having the pilots talking to each other,” Guzzetti said.

That’s exactly how it works at the vast majority of the 5,100 public airports nationwide that are smaller than Minot. Pilots at those uncontrolled airports use their radios to coordinate takeoffs and landings with other planes in the area. Only about 10% of all airports have towers.

The FAA says it works closely with the companies that run contract towers to ensure their controllers are properly trained. It is easier to get certified at a contract tower because they handle fewer flights than FAA towers even though controllers are held to the same standard.

More controllers are needed nationwide

The ongoing shortage of air traffic controllers has persisted for years partly because it takes so long to train and certify new controllers. The FAA has said that it is roughly 3,000 short of the number of controllers it should have at its facilities.

The staffing situation at private towers is similar because they hire from the same pool of candidates. All the roughly 1,400 controllers at these smaller airports have to meet the same qualification and training requirements. But contract towers also have the ability to hire controllers who retired from an FAA tower before the mandatory retirement age of 56. The contract towers don’t have a retirement age.

The FAA has been working for a long time to hire more air traffic controllers to replace retiring workers and handle growing air traffic. But it can be hard to find good candidates for the stressful positions who can complete the rigorous training.

Transportation Secretary Sean Duffy has announced several efforts to hire and retain more controllers. The FAA is trying to shorten the time it takes between when someone applies to the air traffic controller academy in Oklahoma City and when they start training, and the agency is also trying to improve the graduation rate there by offering more support to the students. The candidates with the highest scores on the entrance exam are also getting top priority.

The FAA is also offering bonuses to experienced controllers if they opt not to retire early and continue working to help ease the shortage.