Why a problem on the ground can affect flights thousands of miles away among other issues

Regional U.S. air traffic control centers communicate using a complex network that's “one of the largest telecommunications systems in the country,” and disruptions like those seen on Friday with an hourslong, nationwide grounding of flights can reverberate widely, according to an aviation expert.

Michael McCormick, a former airspace manager with the Federal Aviation Administration, said it could take days for airlines — many of which had to divert flights — to resume normal operations after officials lifted the grounding order. The order was lifted late Friday.

Here's a look at what happened, how regional air traffic control centers work, how controllers communicate with pilots and why a problem on the ground can affect flights thousands of miles away among other issues:

New technology being added to the nation’s air traffic control system overloaded antiquated equipment and caused “intermittent” communications and radar gaps, leading to the ground stop that lasted more than four hours Friday, Trump administration officials said Saturday in a press briefing.

Secretary of Transportation Sean Duffy said a contractor had been working on replacing a section of the system’s old copper wire with new fiber wire, and that somehow resulted in the system overload. Duffy said officials were probing whether contractor “incompetence” or a faulty router was the cause.

“Specifically, we had over 1,000 connection attempts every 4 to 5 seconds,” Duffy said. “And what that did was gave us issues with our communication with airplanes, and it also caused some disruption with our radar feeds. And when we see that, your safety is paramount, we shut down the airspace.”

Planes flying at the time of the ground stop were able to land safely using “well-known procedures” in place for such circumstances, Duffy said Friday.

The government has been working to overhaul the outdated technology air traffic controllers rely on since Congress approved $12.5 billion for the effort last year. Miles of old copper wires have been replaced with fiber optic lines, while old radio systems are gradually being replaced at airports. But the work is far from complete, and Duffy used the news conference on Friday and Saturday to urge Congress to provide more funding.

The FAA grounded flights nationwide Friday due to issues at regional air traffic control centers, an FAA notice said.

The U.S. operates 22 regional air traffic control centers, managed by the FAA, oversees long-distance aircraft operating between departure and arrival airports. according to McCormick, associate professor and program coordinator in the Department of Applied Aviation Sciences at Embry-Riddle Aeronautical University. McCormick estimates these control centers take over flights at altitudes over 10,000 feet.

These centers, which operate in such places as Anchorage, Alaska, Houston and Indianapolis, are one of the three layers that guide pilots. The airport towers direct pilots at the takeoffs at the 500-plus airports that have towers. Then, the so-called Terminal Radar Approach Control centers take over and guide pilots as they get further away from the airport. And then the regional control centers take over. When descending, the reverse happens, experts say.

Experts say that the controllers at the regional centers are sitting at radar displays and are responsible for only that airspace.

“As you fly, your flight is handed off from one controller to another controller,” said Stephen Creamer, president of the Air Traffic Control Association, a trade group for equipment manufacturers involved with air traffic control. “That handoff process is like taking the screen and the aircraft on one screen and making sure it’s on someone else’s screen before you send the communications over to that next controller and have the pilot start talking to them.”

McCormick said the country's regional, high-altitude air traffic control centers need to be connected to remote radio transmitter or receiver sites. That's to allow for radio frequency coverage to communicate with the aircraft.

Those transmitter sites are connected to the centers via telecommunications systems, the likes of which are in the process of moving from copper wires to fiber optic lines. The two-way communication is important to ensure — among other things — that planes maintain safe spacing, he said.

“What is especially problematic in this case is that it’s what’s known as a rolling outage,” McCormick said. “In other words, it's unpredictable and it may impact one air traffic control center now and then it gets better and then impacts another air traffic control center.”

But he and other aviation experts said that the FAA is now transitioning to what's known as controller pilot data link — a digital, text-based messaging system that allows controllers and pilots to exchange routine clearances, instructions and requests through the data link instead of voice radio.

In September, Newark Liberty International Airport temporarily lost radar communications with aircraft, after a backup fiber optic line was inadvertently cut by railroad construction workers. That forced the FAA to immediately cancel hundreds of flights going in and out of the airport.

But airline experts say that scenario was different from what occurred on Friday. Federal Aviation Administration Administrator Bryan Bedford told reporters Saturday that intermittent frequency instability affected as many as 17 of the 22 U.S. regional air control centers.

“The thing that was unique about this is that it was a type of failure that wasn’t local because it was part of the communication system,” he said. “It propagated around the country.”

Andrew Charlton, an aviation consultant, said the “knock-on” effect is incredible.

“The aircraft doesn’t just fly from A to B and back to B and then stop,” he said. “It then moves onto C and then over to E and around and around and around. It is, of course, exacerbated if you’re talking about long-haul flights, international flights over the Atlantic or over the Pacific.”

Aviation experts say that pilots can still communicate with aircraft even during intermittent communications disruptions, noting as an example that many planes have satellite phone systems.

John Hansman, aeronautics professor at MIT whose research includes information technology in aviation, said that pilots can still talk directly to other pilots in airplanes around them via radio. And there's also equipment on the airplane that tells pilots if there are airplanes around them, he said.

If a control tower loses radio capabilities, it can also communicate with pilots with high intensity light beams, which send signals to pilots to alert whether it's safe to descend, he said.

“We have a number of backup systems that allow the airplanes to fly safely to the extent that air traffic control can still communicate directly with the radio,” said Hansman.