How 5G could make a mess of your next flight

The regulatory agency announced Dec. 7 that possible interference with aircraft radio altimeters from upcoming C-Band cell-site transmitters would require it to prohibit pilots from relying on those instruments to track their altitude above the ground near particular airports.

Workers are seen on a 5G tower at Shougang Park, one of the sites for the Beijing 2022 Winter Olympics, in Beijing on December 1, 2021. 

Or in plainer language than that of this FAA Airworthiness Directive: If bad weather means pilots can’t see a runway near those cell sites after AT&T and Verizon light up C-Band frequencies Jan. 5, expect them to land elsewhere. Or not take off at all.

“You’ll be forced to divert the flight to an airport that is not 5G-covered,” said Robert Mann, president of the aviation-industry consulting firm R.W. Mann. & Co. “Or you’ll have to not dispatch the plane.”

But while we won’t know which flights might get an FAA veto until the agency issues NOTAMs (“Notice to Air Missions”) naming airports, the weather allows forecasts. John Cox, a retired US Airways pilot who writes USA TODAY’s Ask the Captain column, said the D.C.-to-Boston megalopolis and the Pacific Northwest were especially at risk of cascading interruptions.

“The whole Northeast Corridor can be down when you have the right fog conditions,” he said. “The other one that concerns me is the Northwest: Seattle, Portland, those areas where the fog is a matter of routine.”

The actual risk of interference remains unclear. As the FAA directive notes, the 3.7-3.98 GHz frequencies for which AT&T and Verizon paid $23 and $45 billion early this year don’t overlap radio altimeters’ 4.2-4.4 GHz frequencies. Some models of altimeter might get confused anyway by adjacent signals–but we still don’t know which ones.

The FAA says bandwidth recently assigned to 5G can interfere with a plane’s radio altimeter, which uses radio frequencies to help determine how close a plane is to the ground when it’s coming in for a landing.“There’s no excuse for the FAA’s delay in its survey of existing altimeter models,” complained Harold Feld, senior vice president at the tech-policy nonprofit Public Knowledge, who wrote last month that this research should have started a year ago.

AT&T and Verizon already pushed back their C-Band launches – which will fill a gap in their 5G networks compared to T-Mobile, which already offers fast “midband” 5G on 2.5 GHz frequencies – and in November agreed to reduce C-Band signals’ power for six months.

(None of this changes the FAA’s ban on using cell service in flight.)

The wireless-industry trade group CTIA also points to successful C-Band 5G deployments in dozens of other countries, but the FAA directive notes that regulators elsewhere require “temporary technical, regulatory, and operational mitigations.”

The Air Line Pilots Association supports the agency; union spokesperson Corey Kuhn emailed a list of those mitigations, most featuring much tighter restrictions on C-Band power. For example, in November Canada announced limits on C-Band power around airports and a ban on C-Band cell sites closer to runways; implementing those rules here would leave many AT&T and Verizon subscribers with lesser 5G service.

Cox defended the FAA’s caution: “Just to categorically say, ‘Oh, it’ll be okay, we think,’ that is not consistent with the way we’ve conducted aviation safety.”

The FAA directive does invite airlines to certify that they’ve tested their equipment against C-Band interference, although Mann noted that replacing susceptible altimeters would demand extra testing and certification: “It’s more than just plugging in a new radio altimeter.”

Feld, however, said airline self-certification represents the most likely solution because airlines will take the heat for flight interruptions.

“If people’s flights are delayed, they’re not going to blame Verizon and AT&T,” he said. “They’re going to blame Southwest Airlines and Delta.”