FAA Proposes New Radio Altimeter 5G ‘Tolerance Standards,’ Upgrade Plans

The FAA is set to propose new 5G C-band tolerance requirements for radio altimeters (radalts) and will mandate that Part 121 aircraft be equipped with systems that meet the new standards by Feb. 1, 2024 so they can keep operating into airports near 5G C-band radio towers.

A draft rule set for publication Jan. 11 lays out the new standards—expressed as an “effective isotropic power spectral density (PSD) curve” relative to an aircraft’s height above ground—for all U.S-registered “transport” and “commuter” category aircraft. Those operating under Part 121 must either add filters to existing radalts or upgrade them completely to comply. 

Aircraft in other categories can upgrade, but it’s not required. Aircraft that do not meet the tolerance requirements will be subjected to restrictions, including no Instrument Landing System approaches, “automatic landing options” and limitations on head-up display and enhanced flight vision system usage.

“Airplanes meeting these proposed minimum performance levels would be allowed to use the prohibited operations at the airports identified by an FAA Domestic Notice after July 1, 2023,” the draft rule explained. “Airplanes operating under [part 121] would also be required to have a radio altimeter that meets the proposed minimum performance standards (i.e., tolerance requirements) on or before Feb. 1, 2024.”

The rule affects about 8,000 aircraft. All of them will need flight manual revisions that detail the new flight restrictions that begin July 1. But only about 1,000 would need modifications—820 with filters and 180 with new radalts, the agency estimated. The rest are either upgraded, in line for changes or do not operate under Part 121. FAA’s cost estimates are $5,000 for adding new filters, and as much as $80,000 for an upgraded radalt.

The FAA and wireless industry have grappled with mitigating risks from the rollout of new so-called “mid-band” 3.7-3.98 GHz 5G wireless services near airports since late 2020. The agency last year pieced together a phased upgrade plan for affected aircraft, based on which models were seen as most vulnerable to 5G signals.

The latest standards take into account both lessons learned and the expected rollout of more 5G wireless services that are closer to the radalt’s 4.2-4.4 GHz spectrum. Wireless providers agreed to reduced transmission power near airports through June 2023 and delayed deployments in the upper range of their spectrum as part of a series of voluntary moves made to help give aviation stakeholders more time to assess risks and come up with mitigation plans.

The FAA’s primary concerns are both interference from 5G signals disrupting radalts, which bounce signals off the ground to help an aircraft determine its exact height, and nuisance alerts that pilots might receive from onboard systems not performing as expected.

“The erroneous warnings increase flight crew workload as they try to ascertain the validity of the warning,” the FAA said. “Repeated determinations that the warning occurred in error will lead to flight crew desensitization to warnings from these safety systems. In other words, as the flight crew becomes more desensitized to erroneous warnings, they are less likely to react to an accurate warning, negating the safety benefits of the warning altogether and likely leading to a catastrophic incident.”

The new standards replace current airport-by-airport limits contained in Notices to Air Missions (NOTAMS). 

“Given 5G C-Band signals are not expected to be temporary and that 5G signals will cover the contiguous U.S., NOTAMs are no longer the best means of communicating the location of the 5G C-Band environment,” the FAA said. “In addition, given the information gleaned over the past year, the FAA is now able to identify the conditions under which radio altimeters can be relied on to perform their intended function in the presence of a 5G C-Band environment.”

The FAA plans to take public comments on its proposal for 30 days.