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An American Airlines commercial aircraft flies past a cellphone tower as it approaches to land at John Wayne Airport in Santa Ana, California, on Jan. 18.MIKE BLAKE/Reuters

Two of the biggest cellphone carriers in the United States said Tuesday they would temporarily delay deployment of some 5G towers near airports, as international airlines began to cancel flights to U.S. destinations over concerns new wireless signals could make it unsafe to land planes.

The U.S. telecom sector will on Wednesday turn on new 5G service in what is known as C-band spectrum, which promises faster and more extensive mobile internet.

Testing by the aviation industry, however, has shown the possibility for interference between C-band signals and instruments that show an aircraft’s distance above the Earth, information critical to pilots and automated landing systems. U.S. airlines warned this week that activation of C-band service near airports would cause air travel “chaos.” Canadian aviation experts say the potential risks have been better addressed north of the border.

But in the U.S. on Tuesday, hours before the C-band activation, Verizon VZ-N said it has “voluntarily decided to limit our 5G network around airports.” AT&T said it would “temporarily defer turning on a limited number of towers around certain airport runways.” It’s the third time the two carriers have announced C-band delays. Both said they would activate the service as scheduled on Wednesday farther away from airports.

T-Mobile TMUS-Q “does not use the C-band spectrum the FAA is concerned about, and we don’t anticipate any limitations when we are ready to deploy it in late 2023,” the company said in a statement.

The AT&T T-N and Verizon announcements seemed likely to avert a broader crisis in U.S. aviation.

But some foreign airlines said they would keep some planes from U.S. airspace.

Dubai-based Emirates said it would suspend flights to nine U.S. airports beginning Jan. 19, citing “operational concerns” over the deployment of 5G networks. On Tuesday, Air India, Japan Airlines and All Nippon Airways also warned passengers about rescheduling or cancellations of U.S. flights assigned to Boeing 777 aircraft, citing a warning from the manufacturer. Jim Proulx, a Boeing spokesman, declined to comment.

The Federal Aviation Administration said even with the delayed C-band rollout near airports, it anticipates “there will be some impacts” because of limitations in some aircraft altimeters.

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Air travel advocates, meanwhile, said more care is needed. “Airline passengers and shippers deserve a commitment from the telecom companies not to launch the new 5G service at any of the airport locations identified by the FAA as being susceptible to 5G interference until a permanent fix is found,” Joe DePete, president of the Air Line Pilots Association, said in a statement.

The aviation industry has pointed to particular risks for helicopters, such as an air ambulance seeking to land near a 5G tower. Earlier this month, the FAA issued more than 1,400 notices to prohibit certain flight operations near new 5G towers.

Executives at U.S. wireless carriers have criticized airline concerns, asking why the country is different from others that have safely deployed C-band networks. U.S. telecoms spent US$81-billion to secure C-band spectrum in a government auction last year.

“We are frustrated by the FAA’s inability to do what nearly 40 countries have done, which is to safely deploy 5G technology without disrupting aviation services, and we urge it to do so in a timely manner,” AT&T said Tuesday.

The aviation industry has said the U.S. has authorized use of higher 5G power levels and a broader bandwidth than other countries.

“It’s a mess,” said Robert Kokonis, president of independent Canadian aviation consultancy AirTrav Inc. For the “airlines especially, it’s been bad enough during COVID, but now to have this potentially at play is crazy,” he said.

In Canada, authorities have restricted use of some 5G spectrum near 26 airports. Wireless carriers will also initially use a lower frequency that is more distant from the bands used by radar altimeters.

Canadian military, medical and rescue aircraft, which often fly at low altitudes, are protected by rules governing the angles at which the 5G antennas are erected.

“It’s not a big issue in Canada but it’s an issue, nonetheless,” said John Gradek, who teaches aviation leadership at McGill University.

He said airlines that land in Canada are not as affected as their U.S. counterparts, in part because the U.S. has far more new pilots who are unaccustomed to landing planes without the use of automation.

Transport Canada highlighted the potential for interference with aircraft systems, particularly altitude gauges, in a safety alert to operators of private and commercial aircraft on Dec. 29.

Air Canada AC-T said it is not cancelling flights to any U.S. destinations. WestJet did not immediately respond to a request for comment, but its website showed it has not followed Emirates’ move to reduce U.S. service as a result of the 5G change.

Peter Fitzpatrick, an Air Canada spokesman, said, “While we have procedures in place and use aircraft types that ensure safe operations, we are engaged on this matter as we continually work with Canadian, U.S. and other regulators, and aircraft manufacturers, to further enhance safety.”

The aviation sector operates with a far more conservative risk tolerance than other industries, said David Redman, director of the Aerospace Vehicle Systems Institute at Texas A&M University, which began testing radar altimeter interference with C-band 5G in 2018.

“A dropped call is a lot less concern than a dropped airplane,” he said.

Experiments with nine different radar altimeter systems discovered “interference we didn’t really expect to see,” Mr. Redman said. Such interference could produce incorrect altitude readings, a situation regulators describe as “hazardously misleading information,” or cause an altimeter to report “no computed data.”

Failures could produce “catastrophic results resulting in multiple fatalities,” RTCA Inc., a non-profit U.S. aviation safety organization, wrote in a 2020 report.

It is unclear how regularly interference might be expected from new 5G towers, but the aviation industry has pointed to its safety record as evidence that caution is required.

“Are you going to put your kids on that flight if it’s probably not going to crash?” asked Mr. Redman.

If an error “happens for a safety-critical Class A system more than one flight hour in a billion, then it’s too many.”

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