FAA Final Rule on Interference-Tolerant Radio Altimeters
Requirements for Interference-Tolerant Radio Altimeter Systems, Docket FAA-2025-5666, signed July 24, 2026 and issued with the FCC’s Upper C-band Report and Order. Federal Register publication will set the effective date.
FreeFlight Systems participated in the rulemaking through the Joint Aviation Community and in its own comments, advocating for a compliance path that works for operators. Sections below cover why the rule exists, what it requires, what it means if your aircraft was already upgraded, and how it affects the current Terrain Series and future models.
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What the Final Rule Means for Your Fleet
The FAA has found that Lower C-band wireless signals cause unsafe conditions for certain aircraft. That finding is why more than a dozen airworthiness directives are in force today. Upper C-band service will increase the risk, and this rule is the response.
A radio altimeter does not fail loudly. It reports a wrong number, and everything downstream believes it. TAWS, TCAS, autoland, autothrottle, EFVS and HUD to touchdown, windshear detection. On rotorcraft, the hover and search and rescue autopilot modes.
In the docket, ALPA pointed to Turkish Airlines 1951, where a faulty radio altimeter drove the autothrottle to idle on approach, as evidence that corrupted RA data can seed an automation failure that only becomes apparent when there is no altitude left to recover it.
The FAA built a six decibel safety margin into the mask, above the maximum interference environment its own coordination with the FCC permits, to cover uncertainty in how base stations are designed and deployed.
The Lower C-band sits at 3.7 to 3.98 GHz. The Upper C-band sits at 3.98 to 4.2 GHz, directly against the RA band at 4.2 to 4.4 GHz. The FCC order makes 160 MHz available with service beginning December 31, 2030, permits base station EIRP up to 65 dBm/MHz at heights to 450 feet AGL, and ends the voluntary wireless commitments that have shaped deployment near airports since 2021.
The FAA stated in the rule that regulating the interference environment is outside its authority; it can regulate only the aircraft. NOTAMs, AFM limitations, and area-by-area mitigation worked while deployment was partial. A performance standard for the equipment is the durable answer once the band is in service nationally.
The rule adds three new sections to the federal aviation regulations. Section 91.220 sets the standard and reaches every aircraft. Sections 121.326 and 129.16 apply that same standard to the airlines and to covered foreign carriers on an earlier deadline.
The requirement itself is short. No aircraft may operate in the 48 contiguous United States and the District of Columbia with a radio altimeter installed unless that altimeter keeps working from the ground to 500 feet AGL in the interference environment the rule defines. The rule does not require an aircraft to carry a radio altimeter. It sets the performance standard for the ones that do.
The ITM matches the NPRM with one addition, a requirement in the 4170 to 4180 MHz range, and carries requirements to 5.6 GHz so a single retrofit holds against future spectrum changes. No performance requirement was adopted above 500 ft AGL, given the FCC’s 450 ft emitter height limit and the additional path loss at altitude.
Where an operator cannot meet the schedule, the Administrator may authorize continued operation with limitations. The FAA does not expect this to be routine.
December 30, 2030 for Part 121, and Part 129 with 30 or more passenger seats or more than 7,500 lb payload capacity.
October 31, 2034 for all other aircraft subject to Part 91, including Parts 125, 133, 135, 136, 137, and 194, and the remaining Part 129 fleet.
Upper C-band service begins December 31, 2030 and the voluntary wireless commitments end the same date. From that point the FAA expects to update the current directives for airplanes that are not compliant yet, restricting low-visibility approach and landing. The FAA notes it does not expect much operational impact, since few operators outside Part 121 are authorized for those approaches. For operators working to 2034, 2030 is still the practical planning anchor.
The rule shifts the burden from the operation to the equipment. Rather than restricting where and how an aircraft may fly, it requires the altimeter itself to keep working in the interference environment.
That is a change from how this has been handled since 2021. RA-dependent operations have carried an administrative and geographic overlay: AFM limitations tied to NOTAMs, restrictions in 5G deployment areas, and a spectrum dimension to dispatch and approach planning.
An altimeter meeting the mask is recognized by the FAA as satisfying the directives in force today, and the FAA plans to remove those directives after the final compliance date. Low-visibility approaches, autoland, and head-up display or enhanced vision to touchdown return to being a function of the aircraft’s capability and the weather. For rotorcraft, so do the procedures that depend on radio altimeter data, including hover and search and rescue autopilot modes and night vision goggle operations.
For transport operators that includes CAT II and CAT III, and SA CAT I and SA CAT II.
The directives in force address the Lower C-band. This rule addresses the Upper C-band, so an Upper C-band compliant system will be required by your compliance date. Commenters noted that filters fitted to existing systems may not be sufficient at these frequencies.
The FAA will recognize a system meeting § 91.220 as an approved alternative method of compliance with the current directives for transport and commuter category airplanes, so the next upgrade closes both.
The directives in force today cover:
- Transport and commuter category airplanes generally (AD 2023-10-02, 88 FR 34065)
- Boeing 747-8 and 777 (AD 2023-12-05)
- Boeing 787 (2023-12-10)
- Boeing 737, newer models (2023-12-11)
- Boeing 757 and 767 (2023-12-12)
- Boeing 737, older models (2023-12-13)
- Boeing 747, older models (2023-12-14)
- Legacy McDonnell Douglas (2023-12-15)
- MHI RJ, formerly CRJ (2025-04-08)
- Bombardier BD-700 series, Global family (2023-14-01, 2023-13-15, 2023-06-13)
- Airbus BD-500, A220 (2023-14-02)
The FCC Report and Order creates a radio altimeter retrofit rebate program funded by Upper C-band auction winners. Domestic aircraft owners installing compliant equipment can obtain a lump-sum payment defraying system and alteration cost. Funding is expected within six to twelve months of the auction, which must be complete by July 4, 2027. The FAA’s analysis assumes $80,000 to $120,000 installed per unit. Foreign operators are not covered.
All Terrain Series models are certified to TSO-C87a and recognized as tolerant under the FAA directives in force and the corresponding Transport Canada directives. Aircraft flying them today are equipped for today’s environment.
The rulemaking record reflects industry agreement that no radio altimeter in service meets the new interference tolerance mask. FreeFlight Systems is evaluating the Terrain Series against the finalized criteria and has already started the upgrade path to meet the new requirements. At this time, we anticipate a service bulletin hardware and software upgrade for existing customers. Product updates will be posted on freeflightsystems.com as they become available.
FreeFlight recommended that the FAA keep the ITM consistent with the NPRM and its safety analysis, provide clear means of compliance in AC 20-199, recognize § 91.220 compliance as a means of compliance for the existing directives, and allow a minor or major design change approval under TSO-C87a alongside the planned TSO-C221 path. The FAA adopted the last two.
Those two provisions are what make this manageable and practical. One upgrade closes both the new rule and the directives already in force, and a modified TSO-C87a design can move as a minor change under the existing authorization rather than requiring a new one. FreeFlight also told the FAA it will apply to amend its STCs for retrofit installations to meet the ITM and the deadlines.
For operators, FreeFlight is building two routes to that upgrade. Aircraft already flying the Terrain Series are anticipated to upgrade by service bulletin, hardware and software, sized to fit a scheduled maintenance visit. For new installations, we are developing a next-generation radar altimeter for certification to TSO-C221 once that standard is published.
Tell us your aircraft types and your transition timeline, and we will send you the relevant program updates.
Planning Your Compliance Path
Several paths exist. An RA supplier may modify equipment under an existing TSO authorization, obtain a new authorization under TSO-C87a, or obtain authorization to the new industry standard once it is available. For a design already authorized under TSO-C87 or TSO-C87a, the FAA confirmed the modification qualifies as a minor change under the existing authorization where the scope does not require a substantially complete investigation, and that data establishing ITM compliance does not affect that eligibility.
The FAA plans to publish TSO-C221 immediately after the new RTCA and EUROCAE consensus standard is published. New designs can be incorporated as amendments to an existing type certificate or STC, or approved through a new STC.
Draft Advisory Circular AC 20-199, Installation of an Airborne Low-Range Radio Altimeter System, was issued for comment alongside the NPRM. The FAA states the final version will be published to support implementation of the rule. The FAA also confirmed that previously installed antennas will not require requalification where the transceiver demonstrates the frequency selectivity of the mask, and that approval of RA systems is eligible for delegation to designees and ODA holders.
FAA will approve next-generation RA systems that meet the performance requirements in § 91.220 as an AMOC with current ADs that were issued to assure safety in the presence of Lower C-band wireless signals for transport and commuter category airplanes.
The current directives remain in full effect until the initial compliance date. The FAA plans to update them after December 30, 2030 for aircraft not yet compliant, and to remove them after the final compliance date. For Part 129 operators whose aircraft are not US registered, equivalent limitations would be applied through Operations Specifications or other deviation authority.
Where the Administrator authorizes continued operation past a compliance date, the rule states the restrictions will include, at minimum, a prohibition on CAT II and III approaches, SA CAT I and II approaches, EFVS to touchdown, autoland, use of a Head Up Display to touchdown, rotorcraft Category A and Category B takeoff operations, search and rescue and hover autopilot modes, and certain rotorcraft procedures requiring RA minima.
The FAA plans to convene RA transition meetings with industry to track equipage progress, and plans to ask Part 121 and Part 129 operators to voluntarily submit fleet equipage plans to their certification management offices by the end of 2026 and to maintain those plans through the transition. That is the nearest-term action item in the rule.
Several operators recommended in the docket that retrofits proceed by service bulletin rather than supplemental type certification to reduce delay. FreeFlight told the FAA it will apply to amend its existing STCs for retrofit installations rather than build new ones, so for aircraft already flying the Terrain Series the intended path is a service bulletin against the existing installation, sized to fit a scheduled maintenance visit.
Operators working to the October 31, 2034 date should plan against December 31, 2030, when Upper C-band service begins and the current voluntary wireless commitments end.
Federal Register publication, which sets the effective date at 60 days after publication and locks the official text. The Office of the Federal Register may make edits during that process.
The RTCA SC-239 and EUROCAE WG-119 minimum operating performance standards, which commenters expected in early 2027, and TSO-C221, which follows them.
Publication of the final AC 20-199, which the FAA states will support implementation.
FCC rebate eligibility terms and funding timing, which follow completion of the Upper C-band auction.
FreeFlight’s compliance findings for the Terrain Series, which will be available to customers after completion of our evaluation.
Program updates for the Terrain Series will be available as our evaluation completes. Let us know what you operate and when you plan to transition, and we will send what applies.
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