U.S. aviation regulators welcomed steps proposed by AT&T Inc. and Verizon Communications Inc. to mitigate potential risks to aircraft from new 5G services.

“This is an important and encouraging step,” the Federal Aviation Administration said in a statement. “We look forward to reviewing the AT&T and Verizon proposal. The FAA believes that aviation and 5G C-band wireless service can safely co-exist.”

The companies said they will roll out new 5G wireless service at temporarily reduced power in coming months to alleviate fears the signals may interfere with the electronics of jetliners and other aircraft. The measures, described Wednesday in a letter to the U.S. Federal Communications Commission, follow an alert to the aviation industry from the FAA regarding the new service.

It is a significant sign of compromise after months of growing tension between the aviation industry and wireless carriers. They have been at loggerheads over the use of a set of airwaves known as the C-band. 

The aviation industry has said the new 5G signals could disturb safety equipment on aircraft, while the FCC and the mobile industry have said there is no evidence of a problem. 

Mobile providers previously said they’ll delay using the airwaves for a month, until early January, in response to the concerns. On Wednesday they went further, spelling out in detail how they could mitigate potential risks.

“We have voluntarily agreed to certain precautionary protection measures,” AT&T said in a statement. “Though there is no credible evidence that a legitimate interference problem exists, we agreed to take these additional steps to alleviate any safety concerns.”

The action was viewed by the aviation industry as a positive step that minimizes the chance of draconian flight limits, said three people familiar with the at-times tense discussions. They asked not to be identified because of the sensitive nature of the talks.

The FCC also responded warmly to the communication.

“These technical mitigations represent one of the most comprehensive efforts in the world to safeguard aviation technologies,” the FCC said in a statement after the letter from the carriers was released. “With these measures in place, the FCC will continue to work productively with the FAA so that 5G networks deploy both safely and swiftly.”

AT&T and Verizon in their letter pledged to operate the new 5G service at reduced power everywhere for six months, with even lower power levels and limited antenna heights near airports and along landing paths. Transmissions would also be limited for antennas pointed skyward and at locations near “public helipads,” the companies said.

AT&T and Verizon also opened the door to extending the restrictions beyond six months if “credible evidence emerges that real-world interference would occur if the measures were relaxed,” they said in the letter.

At the same time, the companies’ letter took pains to defend their initial plans and repeatedly criticized an earlier analysis raising concerns about safety risks. 

That report, by RTCA Inc., a Washington-based nonprofit that studies technical aviation issues, last year concluded that the potential for interference created a safety hazard. But without detailed information on power levels and how cell tower antennas would be positioned, industry analyses had been forced to make worst-case assumptions, said four people familiar with discussions on the issue. 

Certification requirements for radar altimeters, which bounce radio beams off the ground to determine an aircraft’s height, never anticipated risks of interference from nearby frequencies, so some models may not be protected from nearby C-band waves. 

Radar altimeters are critical for jetliners making landings in low visibility, and provide data to assist pilots during touchdown in clear weather as well. They also feed systems that warn pilots as they approach mountains shrouded in darkness or clouds and other technology that warns of potential midair collisions. Some aircraft, such as the Boeing Co. 737, use them to automatically set jet-engine power levels. 

The issue of interference may be even more thorny for helicopters, which operate at low altitudes near cell towers the majority of the time. The FAA mandates the use of radar altimeters on commercial helicopters and those carrying patients on emergency medical flights. Copter pilots using night-vision goggles must also have a working radar altimeter. 

The letter from AT&T and Verizon is aimed at satisfying requests from aviation groups for data to allow for more precise risk analysis. Radar altimeter manufacturers should be able to determine relatively quickly whether most devices are susceptible, said one person familiar with the process. Some older models might be more vulnerable and difficult to assess, according to the person.

The dispute represents a hitch in mobile providers’ pursuit of 5G airwaves to serve billions of connected homes, factories and gadgets with the latest generation of technology. AT&T and Verizon are in a race to catch up to T-Mobile US Inc., which is about a year ahead on 5G network deployment using other airwaves not suspected of causing interference.

Mobile carriers have permission to use the C-band beginning Dec. 5. The FCC awarded wireless network providers access to the radio bands in a February auction. Verizon spent US$45 billion on the airwaves in question, and AT&T devoted US$23 billion in an FCC auction.