Still, a handful of carriers canceled a few flights on the first day. Air-safety regulators have been working to clear aircraft to operate in low-visibility conditions and said some flights could still face delays as that work continues.

What’s happening in the 5G flight-safety fight?

Telecom companies were supposed to roll out a new, faster wireless internet service in early December. The plan hit some hurdles after air-safety regulators prepared to impose disruptive flight restrictions because they said the 5G service could potentially pose risks to cockpit equipment in aircraft. The telecom industry disputes that the service would create any hazards.

So now what?

After two delays tied to the battle,VerizonVZ -0.20%and AT&T started to deploy the new service Jan. 19—but it won’t cover all the areas the companies had anticipated, at least for the short term.

The day before activation, both companies said they would temporarily pause delivering the service near certain airports. The deferral came after chief executives at several large airlines said in a Jan. 17 letter to Biden administration officials that they could face significant flight disruptions if the 5G service was turned on within 2 miles of major airport runways.

Have any airlines said they plan to alter flights?

Yes. On Jan. 18, several international carriers said they planned to suspend some flights to the U.S. beginning the following day, partly as a result of operational concerns tied to restrictions the FAA issued in response to the 5G rollout. Two Japanese carriers, however, reversed their decision to pause flights, citing an update from the FAA thatBoeing Co.777 jets could be used. Emirates Airline separately said it restored some service to three of the nine U.S. cities where it planned suspensions.

A handful of other flights involving Boeing 777 planes were canceled, such as British Airways’ service to Chicago O’Hare International Airport from London Heathrow.Delta Air Lines Inc.has said it is planning for possible flight cancellations in certain weather conditions.

Why has the aviation industry been worried about flights?

At issue is whether the new 5G signals affect aircraft equipment that helps planes land in difficult weather conditions and avoid crashes. The equipment, called radar or radio altimeters, uses radio frequencies to measure the distance between aircraft and the ground.

Why 5G Rollout Is Concerning Airlines and Flight-Safety Regulators: What to Know

Aviation officials worry that the cellular frequencies that telecom companies plan to activate could interfere with altimeters, potentially throwing off readings the devices take and endangering planes. The 5G service will operate in a frequency known as the C-band, which is close to the airwaves the altimeters use.

Telecom-industry representatives say connections over the C-band won’t interfere with cockpit devices, citing Federal Communications Commission technical experts and analyses by other regulators around the world.

The FAA recently issued rules that prevent planes from landing in certain low-visibility conditions at sites where the 5G service has been activated. Flight operators and manufacturers are able to follow steps to demonstrate to the FAA that their altimeters can operate reliably and accurately in the new 5G environment, the agency has said.

So will the new 5G service make flights unsafe or not?

The FAA has said it is taking steps to ensure aircraft don’t operate in what it considers to be an unsafe situation because of the 5G service. The point of its restrictions on aircraft using certain radar altimeters, the agency has said, is to make sure the devices aren’t potentially impaired.

Telecom industry representatives say signals over the C-band won’t cause harmful interference to cockpit devices. They have pointed to the fact that the U.S. included a buffer band to separate the new 5G signals from those used by radar altimeters.

Does the new 5G service only affect commercial flights?

No. Helicopters and private planes also rely on altimeters. Some law-enforcement and fire-service helicopters could be grounded if the new 5G services are switched on, according to Helicopter Association International, a trade group. The FAA has approved an exemption allowing air ambulances to keep flying, removing the requirement for them to rely on altimeters.

Helicopters typically have more-diverse flight paths than planes and fly more slowly, exposing them to more cell towers and longer bursts of potential interference, radar engineers and manufacturers said. Helicopters and private planes use a far larger number of airports and takeoff and landing sites than commercial planes.

What is 5G and why is it a big deal for the telecom industry?

Fifth-generation cellular technology is substantially faster than previous services, easing activities such as downloading software and streaming video. Such links are already available in parts of the U.S. from Verizon, AT&T andT-Mobile US Inc.TMUS -0.22%in several spectrum bands. Coverage isn’t uniform, and AT&T and Verizon in particular need more frequencies to keep their networks from becoming congested.

Verizon, for example, bid $45.5 billion for its C-band licenses, and plans to use them to offer more 5G service for mobile customers and provide home internet links in some areas where cable broadband isn’t available or costs too much.

Why did this battle emerge now?

The tussle is occurring in large part because federal agencies didn’t act earlier to work through their disagreements.

Concerns about potential interference with cockpit equipment had been on the FCC’s radar, according to a commission order published in March 2020. That order found, in part, that one study didn’t demonstrate that harmful interference was likely under reasonable scenarios and suggested that an industry task force continue to analyze the issue.

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The FAA brought up its worries in a letter sent shortly before an FCC auction began in December that year, and asked for the sale to be postponed. Trump administration officials didn’t agree with the FAA’s last-minute concerns and the C-band auction went forward. But the aviation industry’s focus on the issue didn’t go away. By last autumn, the two regulators began sharing information both had been seeking for months.

What does all of this mean for airline passengers?

For some passengers, the new 5G service’s activation has already meant changes to flight plans, including the pre-emptive cancellation of some flights. The question remains whether other airlines make similar decisions. Weather conditions could play a role.

(This article may be updated.)

—Drew FitzGerald and Doug Cameron contributed to this article.

Write to Micah Maidenberg at