The move is the latest twist in a dramatic showdown that erupted in the last couple of months between the telecommunication and air travel sectors, and between the agencies that are supposed to regulate them, the Federal Aviation Administration (FAA) and the Federal Communications Commission (FCC). At the heart of the standoff lies some of the U.S.’s most prized technological real estate: radio spectrum that has traditionally been used for satellite communications but that is now being repurposed for 5G connectivity.

Here’s what you need to know about a clash that seemingly arose out of nowhere—with one side desperate to improve the quality of its cellular services, and the other claiming that planes might start crashing out of the sky.

What’s the frequency, Kenneth?

Kind of like how cars use lanes on a highway, wireless communications take place in defined sections of the electromagnetic spectrum, called frequency bands. In order to work properly, a system using one of these bands needs to be sure it won’t encounter interference from other users, either in that band or in adjacent bands. This is why radio spectrum is tightly regulated—and, given that it is a finite resource, why it is so extremely valuable.

As 5G connectivity becomes ubiquitous around the world, it needs more and more spectrum to ensure those high speeds. That means repurposing frequencies that were previously used for other things.

In the current debate, the focus is on so-called C-band spectrum that lies between 3.7 gigahertz (GHz) and 4.2 GHz, for which AT&T and Verizon paid a combined $69 billion in an auction nearly a year ago—a record-breaking sum that reflected their desperation to improve the speed and availability of their 5G networks.

Some of the money went to satellite companies that previously used the spectrum, to help them move to other frequencies. But that wasn’t the only sector that claimed to be affected: so too did the aviation industry, because some altimeters—the devices that planes and helicopters use to measure altitude—use the neighboring 4.2–4.4 GHz band. Turn on those new 5G networks, the airlines warned, and altimeters could start giving false readings, with catastrophic consequences.

Rumble becomes a roar

The potential clash entered the public consciousness at the end of October, when the Wall Street Journal reported that current and former aviation officials were worried that flight-control systems might stop working as intended, causing flight cancellations and diversions in the cities where the wireless carriers would be using the spectrum.

Several days later, the FAA issued a warning about Verizon and AT&T’s plan to start using the spectrum for their services on Dec. 5, saying airlines “should be prepared for the possibility that interference from 5G transmitters and other technology could cause certain safety equipment to malfunction, requiring them to take mitigating action that could affect flight operations.” It told altimeter and plane manufacturers to share more data about which altimeter models are being used in which planes. Within a couple of days, AT&T and Verizon agreed to delay their new networks’ activation for a month, so the issue could be further studied.

So, did the FCC—the agency that auctioned off the spectrum—somehow miss the potential for altimeters to stop working? Hardly. When the commission issued its report about repurposing the frequencies in March 2020, it specifically addressed the issue by pointing out that wireless carriers would only be able to use the spectrum up to 3.98 GHz, thus providing a very spacious buffer between their territory and the band that altimeters use. The FCC said these limits were “sufficient to protect aeronautical services.”

However, the FAA took the aviation sector’s side in the dispute, and tried to delay the FCC’s auction just before it began in December 2020. It failed. The auction went ahead, and the Dec. 5, 2021 go-live date was officially scheduled. That gave the two sides the better part of a year to figure out which equipment might need to be upgraded, and get it done. That didn’t happen, and the situation came to a head over the last week.

At the end of December, the FAA and Department of Transportation requested a further delay to the networks’ activation near airports, and trade group Airlines for America threatened to sue the FCC over the matter.

On Sunday, AT&T and Verizon promised to limit the power of the transmissions near airports for a period of six months, creating exclusion zones even wider than the very conservative exclusion zones required around French airports. However, they rebuffed the FAA and DOT’s request to further delay the activation for another two weeks.

And then, late Monday, they changed their minds, agreeing to the two-week delay.

“Safety is the core of our mission, and this guides all of our decisions. The FAA thanks AT&T and Verizon for agreeing to a voluntary delay and for their proposed mitigations,” the agency said in an emailed statement. “We look forward to using the additional time and space to reduce flight disruptions associated with this 5G deployment.”

So could 5G actually make planes crash?

The aviation industry has commissioned reports that purport to show potential radio interference, but the FCC and wireless industry experts have pushed back against the findings, saying they are based on incomplete data, opaque testing, and questionable modeling.

And as the wireless carriers have been yelling for months now, around 40 countries already run 5G services in the C-band without planes dropping out the sky. Even the FAA’s early-November warning noted that “there have not yet been proven reports of harmful interference [with aviation systems] due to wireless broadband operations internationally.”

“The laws of physics are the same in the United States and France,” wrote AT&T CEO John Stankey and Verizon CEO Hans Vestberg in their Sunday letter to Transportation Secretary Pete Buttegieg. “If U.S. airlines are permitted to operate flights every day in France, then the same operating conditions should allow them to do so in the United States.”

That sentiment was echoed by FCC Commissioner Brendan Carr in a letter he sent to Buttegieg on New Year’s Day. “Your request for delay is not backed up by the science, engineering, or law,” Carr wrote. “Indeed, your arguments are predicated on the claim that there are unresolved concerns about harmful interference from C-band operations into radio altimeters. That is not correct. The FCC—the expert agency charged by Congress with addressing precisely those types of concerns about harmful interference—resolved these issues all the way back in March 2020.”