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Turbine aircraft operators that upgraded radio altimeters just a few years ago to withstand interference from an expansion of 5G telecommunications frequency spectrum now face another round of costly modifications or replacements under recently issued mandates from the Federal Aviation Administration (FAA) and Federal Communications Commission (FCC).Per the FAA's final ruling published July 31, Part 121 commercial airlines and certain large foreign aircraft must be equipped with radio altimeters hardened against Upper C-band frequency interference by Dec. 30, 2030. All other affected aircraft operating under Part 91, including Part 135 operations, must comply by Oct. 31, 2034.These latest modifications are required to eliminate interference from signals in the newly reallocated Upper C-band spectrum (3.98-4.14 GHz), which is scheduled to be auctioned to wireless telecommunications providers next year and authorized for new wireless services beginning Dec. 31, 2030. This follows similar mitigations required to block interference across Lower C-band frequencies (3.7-3.98 GHz) that became active in January 2022.The later implementation date for general aviation operators should not invite complacency. "The 2034 deadline provides significant lead time, but operators may still face delays involving aircraft-specific certification, parts availability, limited avionics shop capacity and competition for qualified technicians," said a spokesperson for Vertical Aviation International (VAI). "We encourage operators to begin planning well before the deadline."Jens Hennig, vice president of operations, safety and security for the General Aviation Manufacturers Association (GAMA), also noted affected operators should be "motivated" to install modified or replacement radio altimeters well before the deadline. "That said, your pathway forward obviously depends on your airframe, installed equipment and available approved data," he added. "Some units may be modified internally or paired with filters; others will need replacement."FCC Rebates Offer Some Cost ReliefBoth upgrade paths carry substantial costs. In the agency's initial notice of proposed rulemaking (NPRM), the FAA forecast more than 33,000 radio altimeters in general aviation and business aircraft would need modification or replacement at an average price tag of $80,000, with total costs to operators ranging from $2.46 billion to $3.58 billion.A coalition of industry stakeholders responded those numbers were "likely underestimated," however, with estimated equipment and labor costs potentially reaching as high as $120,000-$160,000 per radio altimeter.Equipment prices also remain in flux, as avionics manufacturers continue to finalize their offerings after the release of the FAA's final rule. "Manufacturers have been executing based on the FAA's earlier proposal," Hennig said, "but they really needed the ink dried on the final regulation before committing to their respective solutions."The FCC's companion mandate holds the promise of potential cost relief, outlining a post-installation rebate program funded by the winning bidders in next year's Upper C-band auction. An NPRM expected this fall will codify how equipage funds will be allocated to operators, with a public comment period inviting further input from stakeholders.Although final costs remain unknown, the earlier Lower C-band implementation offers some direction. Behlul Poonawalla, vice president of products, programs and contracts at FreeFlight Systems, said modification and replacement costs will depend "on configuration, primarily whether the existing antennas and cabling can be reused.""As far as downtime, a direct replacement in an aircraft with serviceable antennas is generally a matter of a 24-to-48-hour turnaround," he added. "An installation that requires new antennas or rewiring runs longer, typically five business days."Aircraft and avionics OEMs are also determining the best regulatory paths for equipage."Some aircraft manufacturers have already issued service bulletins that install 5G bandpass filters, while others are moving toward complete radio altimeter replacements through supplemental type certificates (STCs)," said Bruno Mazzani, Duncan Aviation's business development manager-satellites. "The industry is expected to continue utilizing a combination of OEM service bulletins and STC-approved upgrade solutions, depending on the aircraft model and avionics configuration."How We Got HereIn addition to providing real-time measurements of actual height above terrain, modern radio altimeters also feed information to aircraft systems including terrain awareness and warning, traffic collision avoidance and predictive windshear alerts. These units operate in the 4.2-4.4 GHz band previously reserved for aviation users, which for decades offered sufficient protection from lower frequencies.However, ever-increasing demand for 5G wireless bandwidth has led telecommunications providers to encroach into that buffer zone, where those signals can overwhelm inadequately filtered radio altimeter receivers or cause erroneous readings. That interference carries substantial impacts on commercial and business aviation operations.Following the January 2022 expansion into the Lower C-band, operators utilizing non-hardened radio altimeters were prohibited from conducting low-visibility landings, use of autoland systems and other automated flight controls and terrain awareness systems wherever 5G interference was reported by NOTAMs. While overall disruptions were minimal, the situation initially caught many operators unaware.This time around, industry stakeholders report a much more collaborative process with the FAA and FCC in establishing workable solutions and timelines. "Across the aviation industry, we have worked closely with Congress, the FAA, U.S. Department of Transportation and the FCC to support the new rule, order and operator reimbursement framework," said a representative with Honeywell Aerospace. "The FAA's rule reflects meaningful industry input and establishes a practical path forward."Heidi Williams, vice president of air traffic services and infrastructure for the National Business Aviation Association (NBAA), welcomed the rebate program as "a clear win for our members," and thanked the FAA and FCC for "working closely with NBAA and others to develop a practical timeline for radio altimeter equipage that protects safety while recognizing the significant impact on aircraft operators.""Radio altimeter modifications or replacements can represent a substantial, unplanned expense," she added, "and ensuring that all affected operators have access to this funding will help accelerate equipage, support compliance and preserve access to the nation's aviation system."What Operators Should Do NowWith the clock ticking down, operators should identify every affected aircraft and altimeter now, engage with manufacturers and maintenance providers on which approved solution is planned, and preserve invoices and installation records for the rebate process. When possible, they should coordinate the work with scheduled inspections to further reduce operational impacts.Mazzani stressed that waiting until 2034 risks competing with thousands of aircraft for equipment, certification resources and shop capacity. "Early evaluation of aircraft requirements and coordination with maintenance providers can help mitigate potential scheduling challenges, supply chain constraints and aircraft downtime," he concluded.