The Federal Aviation Administration issued a notice of proposed rulemaking Wednesday that would require new minimum performance standards for aircraft radio altimeters to ensure continued safe operations amid expanding wireless services in the Upper C-band. The proposal follows recent congressional direction for the Federal Communications Commission to auction at least 100 megahertz of spectrum in the 3.98–4.2 GHz range, a band adjacent to frequencies used by aviation radio altimeters.
Radio altimeters, also known as radar altimeters, are commonly installed on commercial airliners, business jets, helicopters and some general aviation aircraft and provide critical data for applications such as terrain awareness and warning systems, traffic collision avoidance systems, wind shear detection and certain helicopter automation modes.
The FAA said interference affecting these systems could lead to inaccurate altitude readings during low-altitude phases of flight, including approach and landing.
Compliance timelines tied to aircraft type
Under the proposal, aircraft operated under Part 121 air carrier rules and certain large foreign-registered aircraft operating under Part 129 would be required to meet the new interference-tolerance standards by the date the FCC authorizes wireless service in the Upper C-band. All other aircraft equipped with radio altimeters would have an additional two years to comply.
The agency noted that existing airworthiness directives and voluntary agreements with wireless providers are interim measures and do not address long-term compatibility with expanded spectrum use. Wednesday’s proposal asserts that current radio altimeter designs were developed before the introduction of high-powered terrestrial wireless signals in nearby frequency bands, and thus could have issues reliably rejecting interference without upgrades or replacement.
Proposed performance requirements
The proposed rule would establish specific minimum interference tolerance levels that radio altimeter systems must meet while operating at altitudes from the surface to 500 feet above ground level. These requirements are intended to reflect the best achievable performance using current technology without degrading altimeter functionality.
Aircraft not currently equipped with radio altimeters would not be required to install them under the proposal.
Public comments on the proposal are due by March 9, after which the agency will consider feedback before issuing a final rule.
