Airline industry leaders have intensified warnings that the introduction of a specific 5G service could interfere with aircraft radio altimeters.
Radio altimeters determine an aircraft’s height above the ground by measuring the time it takes for a radio wave to travel from the aircraft to the surface, reflect, and return. They are critical safety instruments, particularly during approaches, landings, and low-altitude operations.
In February, the Federal Communications Commission (FCC) completed an auction for C-band spectrum (3.7–3.98 GHz) and granted licenses to wireless carriers to deploy 5G services in that band. Radio altimeters operate in the 4.2–4.4 GHz range. The FCC concluded that a 220 MHz separation between C-band 5G and altimeter frequencies provides sufficient protection.
AT&T and Verizon announced plans to begin C-band 5G deployments on January 5, 2022.
“Come January 5th—unless something changes—we will not be able to use radio altimeters at 40-something of the largest airports in the country,” United Airlines CEO Scott Kirby told reporters after a Senate Commerce Committee hearing. “It is a certainty. This is not a debate.”
Southwest Airlines CEO Gary Kelly told lawmakers the impact “would be a significant setback” for airline operations. Executives from aircraft manufacturers have expressed similar concerns. In a joint letter to U.S. Transportation Secretary Pete Buttigieg, Airbus Americas and Boeing leaders Jeffrey Knittel and Dave Calhoun warned that “5G interference could adversely affect the ability of aircraft to safely operate.”
Questions about the interference studies
Some airline concerns are based on studies from the RTCA (Radio Technical Commission for Aeronautics) and the Aerospace Vehicle Systems Institute (AVSI) that indicate mid-band 5G signals could disrupt altimeter performance. Regulators and industry groups, however, have disputed the studies’ conclusions and methodologies.
The FCC reviewed the AVSI analysis and stated that the study “does not demonstrate that harmful interference would likely result under reasonable scenarios (or even reasonably ‘foreseeable’ scenarios …).” Critics of the AVSI and RTCA reports point to several methodological issues, including:
- Insufficient information to allow replication of tests, such as not identifying the specific altimeter models used.
- Application of large cumulative margins—for example, adding a six dB safety margin, six dB for cable loss, and four dB for 5G, resulting in a combined 16 dB penalty that may overstate risk.
- Inconsistency in applying loss assumptions—for instance, applying losses in altimeter assessments while not accounting for them in evaluations of other onboard wireless systems like WAIC (Wireless Avionics Intra Communications).
- Use of unrealistic test scenarios, such as a 20-degree aircraft roll at extremely low altitude or placing aircraft and cell towers at unlikely relative positions and orientations.
- Failure to account for altimeter filtering and other design features that are intended to reject energy from adjacent frequency bands.
If the report’s assumptions were accurate, other existing mid-band systems—such as AN/SPN-43 radar and other legacy services—would already be causing measurable interference with radio altimeters, but such widespread problems have not been observed.
The Federal Aviation Administration (FAA) issued a Special Airworthiness Information Bulletin (SAIB) urging manufacturers and operators to share information about altimeter design, usage, and test results with federal authorities. The FAA asked industry participants to work with regulators to test and assess equipment and report findings to civil aviation authorities and spectrum regulators.
“This SAIB recommends that radio altimeter manufacturers, aircraft manufacturers, and operators voluntarily provide to federal authorities specific information related to altimeter design and functionality, specifics on deployment and usage of radio altimeters in aircraft, and that they test and assess their equipment in conjunction with federal authorities. Results from that testing and assessment should be reported to the appropriate civil aviation authorities (CAAs) and spectrum regulators. The FAA is currently collaborating with the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA) to assess the need for mitigation beyond the recommended action in this SAIB.”
At the same time, the FAA noted that mid-band frequencies have been used for cellular networks elsewhere in the world without reported incidents of harmful interference to aviation equipment. The agency pointed out that wireless broadband services have operated in the 3.65–3.7 GHz range in the United States since 2007 without documented interference to altimeters.
Wireless industry perspective and mitigation steps
Roughly 40 countries already deploy C-band for 5G, and several have conducted interference tests without finding harmful effects on aviation systems. Wireless carriers argue there is no evidence that properly deployed C-band 5G threatens flight safety.
A Verizon spokesperson said, “Air safety is of paramount importance, but there is no evidence that 5G operations using C-band spectrum pose any risk to aviation safety, as the real-world experience in dozens of countries already using this spectrum for 5G confirms.”
To address concerns while testing continues, Verizon and AT&T voluntarily agreed to temporarily reduce power levels on national C-band transmitters and to impose stricter power limits near major airports and public helipads. These measures aim to allow further FAA evaluation without an immediate nationwide service disruption.
Some former FCC chairs warned that the FAA’s cautious stance could undercut the FCC’s technical conclusions after years of analysis. Industry groups emphasize that delaying C-band deployment would have economic consequences and would slow the benefits 5G promises across multiple sectors. CTIA CEO Meredith Attwell Baker estimated that postponing rollout by one year could reduce economic growth by roughly $50 billion during a critical recovery period following the pandemic.
The FCC’s original conclusion—that properly regulated C-band 5G is unlikely to cause real-world interference with aircraft radio altimeters—remains the prevailing technical position among many regulators and carriers. Nevertheless, manufacturers, airlines, and aviation authorities continue to press for additional tests, information sharing, and targeted mitigations to ensure flight safety while enabling 5G expansion.
(Photo credit: Ross Parmly on Unsplash)
Interested in learning more about 5G and industry perspectives? Events such as the 5G Expo bring together sector leaders to discuss developments and opportunities. Upcoming dates previously announced include events in 2022 in Santa Clara, Amsterdam, and London.