FAA Proposes to Require C-Band-Tolerant Radio Altimeters Starting Next Year

The FAA is proposing that passenger and cargo aircraft in the U.S. have 5G C-band-tolerant radio altimeters or install approved filters by early 2024. The requirement is proposed in a notice for Wednesday's Federal Register, with comments due Feb. 10. Industry experts saw the latest as a net positive for telecom carriers, but some warned of a dangerous precedent being created by the FAA.

“The FAA proposes to supersede Airworthiness Directive 2021-23-12, which applies to all transport and commuter category airplanes equipped with a radio … altimeter,” the notice says: “Since the FAA issued AD 2021-23-12, the FAA determined that additional limitations are needed due to the continued deployment of new 5G C-Band base stations whose signals are expected to cover most of the contiguous United States at transmission frequencies between 3.7-3.98 GHz.” The proposed directive would “require modifying certain airplanes to allow safe operations in the United States 5G C-Band radio frequency environment.”

“The wireless industry continues to work closely and collaboratively with the FAA and, as this notice makes clear, the FAA’s schedule for altimeter updates is reasonable and practical,” a CTIA spokesperson emailed: “5G in the C-band coexists safely with air traffic, and we look forward to continuing to work together with all stakeholders to meet the FAA’s deadlines.”

“All’s well that that ends well and this is ending well for carriers,” New Street’s Blair Levin told us. “But to really end well the government stakeholders should do a deep dive into what went wrong, why the FAA did not issue a similar directive when the C-band auction was gearing up, and how to avoid such problems in the future,” he said: “There are hopeful signs, but we still need a national spectrum plan to institutionalize better planning processes.”

The development is “positive in the sense that it shows FAA's willingness to acknowledge that the solution to potential interference is to ensure that altimeters are operating in their assigned band,” emailed Joe Kane, Information Technology and Innovation Foundation director-broadband and spectrum policy. But the proposed directive “still only requires altimeters that comply with unwanted-emissions levels under the voluntary mitigation measures the wireless carriers have been providing,” he said: “In that sense, this is really just FAA streamlining and making permanent the standards they have been enforcing with AMOCs [alternative method of compliance requests]. But the mitigation measures are set to expire this summer, and what happens then remains an open question.”

Kane noted the FAA now estimates the cost of altimeter upgrades at just $26 million nationwide. “This highlights the importance of improving the coordination and information-sharing processes between the FCC, NTIA, and federal agencies,” he said. “Including that price with other relocation costs in the C-band order would have saved a lot of time, money and headaches.”

Dangerous Precedent

Recon Analytics’ Roger Entner said the latest development is “just ridiculous.” The airline industry is “approaching this as if their tardiness was someone else’s problem and the FAA provides cover for the slow walking of the solution,” he said: “This is a dangerous precedent because the FAA, de facto, becomes the regulator of wireless airwaves” and is “superseding the statutory power of the FCC,” he said: “The wireless carriers paid more than $80 billion for the spectrum in the expectation of free and clear use as the FCC allocated the spectrum, and now they are not allowed to use it in areas with the densest wireless traffic. The way this is going the FAA will provide another extension to the airlines when the 2024 deadline approaches.”

The FAA said since it issued AD-2021-23-12 in late 2021, it has developed a more sophisticated understanding of radio altimeters and the C band. Since 5G deployment started, the agency said it has received more than 420 reports of radio altimeter anomalies occurring within a known location of a 5G C-band deployment: “Approximately 315 of these reports were determined to not be related to 5G C-Band interference and were resolved through normal continued operational safety procedures.” But for about 100 reports the FAA was unable to rule out C-band interference.

"The airworthiness directive seems like a step in the right direction for the carriers, as it acknowledges that outdated altimeters will need to be upgraded to ensure that they don't experience harmful interference,” emailed Jeffrey Westling, American Action Forum director-technology and innovation policy. “This doesn't completely solve the issues as there is still disagreement regarding the power level carriers should be able to operate, and this directive is tied to the voluntary agreements of the carriers,” he said: “Until we can sort out the specific power levels, I don't think we will see resolution of this issue entirely."

“Three years after the C-Band rollout?” tweeted a spokesperson for 5G Americas: “Geez … aviation needs to move more quickly than this.”