The FAA’s final radio altimeter rule changes the regulatory basis for managing C-band interference. Since 2021, operators have worked under aircraft-specific airworthiness directives, aircraft flight manual limitations, approved alternative methods of compliance, and temporary wireless mitigations. The new rule establishes a permanent interference-tolerance requirement for radio altimeter systems installed on aircraft operating in the contiguous United States.
For Part 121 aircraft and larger aircraft operated under Part 129, compliance is required by December 30, 2030. Other affected aircraft have until October 31, 2034. Upper C-band terrestrial service may begin in the first markets one day after the initial aircraft deadline.
The implementation work starts well before an aircraft enters modification. Operators and lessors need an accurate record of each installed configuration, a supported compliance determination, approved design data, an equipment and installation forecast, and control of the records that will support both continued operation and any FCC rebate claim.
Current compliance with a C-band airworthiness directive does not establish compliance with the final rule. Selection of a radio altimeter article advertised as interference-tolerant also does not settle the aircraft-level question. Compliance depends on the complete installed system and the approved data applicable to that installation.
The final rule was published on July 31, 2026, becomes effective on September 29, 2026, and adds 14 CFR 91.220, 121.326, and 129.16. The controlling document is Docket FAA-2025-5666, amendments 91-384, 121-396, and 129-56.
From temporary C-band controls to a permanent aircraft requirement
Radio altimeters operate between 4.2 and 4.4 GHz. They measure height above terrain and provide data to the flight crew and to aircraft systems that can include terrain awareness, traffic collision avoidance, predictive windshear, flight guidance, autoland, and enhanced flight vision functions. Rotorcraft applications can include hover and search-and-rescue autopilot modes.
The Lower C-band deployment between 3.70 and 3.98 GHz created the first broad interference concern. The FAA responded through airworthiness directives, operating limitations, and aircraft-specific interference-tolerance determinations. Wireless carriers accepted temporary restrictions near certain airports while the aviation industry modified susceptible aircraft.
The next phase involves spectrum closer to the radio altimeter band. The FCC has authorized terrestrial wireless use between 3.98 and 4.14 GHz. A 20 MHz guard band occupies 4.14 to 4.16 GHz, while fixed satellite operations will remain between 4.16 and 4.20 GHz.
The FCC deployment schedule is phased. Incumbent fixed-satellite operations must clear the relevant spectrum in the top 75 contiguous-U.S. Partial Economic Areas by December 30, 2030. Upper C-band terrestrial service may begin in those markets on December 31. They account for approximately 70 percent of the U.S. population.
The transition deadline for the remaining contiguous-U.S. markets is June 30, 2031. Terrestrial service may begin there on July 1, 2031, or earlier if the FCC’s completion-certification and incentive-payment conditions are satisfied. The temporary wireless commitments associated with the Lower C-band deployment are expected to end on December 31, 2030. The FAA therefore aligned its first aircraft deadline with the initial, highest-population phase of Upper C-band service rather than a single nationwide activation date.
The installed-system performance requirement
At the center of 14 CFR 91.220 is an interference tolerance mask, or ITM. The mask defines the maximum radio-frequency interference that the radio altimeter system must safely tolerate at the surface of the antenna between 3.0 and 5.6 GHz while operating from ground level through 500 feet above ground level. The specified values include a 6 dB safety margin.
The ITM should not be read as a test of external interference alone. Radio-altimeter-to-radio-altimeter interaction is not part of the mask itself, but the FAA requires the compliance showing to account for signals from every radio altimeter installed on the same aircraft. That distinction matters for dual- and triple-system installations because onboard radio altimeter signals can affect performance against the ITM.
Compliance is established for the installed system. The supporting data may need to account for the transceiver, software standard, antennas, cabling, installation losses, aircraft interfaces, and functions that use radio-altitude data. Each radio altimeter system installed on the aircraft must meet the requirement.
Several equipment and approval routes can lead to compliance. An existing design may be changed under a TSO-C87 or TSO-C87a authorization. A new or substantially changed transceiver design may use TSO-C87a or the planned TSO-C221. The FAA also plans a separate TSO for radio altimeter antennas. Equipment can instead be developed under the authority of a type certificate or supplemental type certificate holder.
The joint RTCA and EUROCAE minimum operational performance standard is scheduled for publication in March 2027, after which the FAA plans to publish TSO-C221 for transceivers. The final rule does not require operators to wait for that standard, nor does it make TSO-C221 the exclusive route. Equipment that meets the applicable aircraft requirements and the ITM can comply through another approved path.
A technical standard order authorization approves an article. Aircraft installation approval remains a separate matter. A new radio altimeter can be incorporated through an amendment to a type certificate or supplemental type certificate, an amended STC, or a new STC, as applicable.
Where an installation remains within the parameters substantiated by the radio altimeter supplier, the FAA says an additional aircraft-level ITM demonstration may not be necessary. The type certificate or STC holder still has to address the installation and its intended functions. Installations outside the substantiated parameters require additional assessment and supporting data.
For an operator or lessor, a model or part-number statement is therefore an incomplete compliance record. The useful evidence connects the approved article, its modification and software status, the installation configuration, applicable design approval, and aircraft effectivity.
Applicability and compliance dates
The rule applies to operations in the airspace of the 48 contiguous states and the District of Columbia.
| Compliance date | Aircraft and operations |
|---|---|
| December 30, 2030 | Aircraft with a radio altimeter operated under Part 121. The same date applies to Part 129 aircraft with 30 or more passenger seats or a payload capacity greater than 7,500 pounds. |
| October 31, 2034 | Other radio-altimeter-equipped aircraft subject to Part 91 in the covered airspace, including operations under Part 91 subpart K and Parts 125, 133, 135, 136, 137, and 194. It also applies to the remaining Part 129 fleet. |
The rule does not create a general radio altimeter carriage requirement. An aircraft without a radio altimeter does not need one solely because of 14 CFR 91.220. Removal may be possible where the type design, operating requirements, and dependent aircraft functions permit it, but the removal still requires applicable approved data and a properly recorded configuration change.
Operations conducted only in Alaska, Hawaii, Puerto Rico, other U.S. territories, and the excluded offshore airspace are outside the rule’s geographic scope. A foreign operator may choose to modify only the subfleet assigned to the contiguous United States. That approach requires dependable fleet assignment controls after the applicable deadline.
For lessors, geographic applicability affects the future operating market for the aircraft. A noncompliant aircraft may retain utility outside the covered airspace, while its ability to serve a U.S. operator or conduct U.S. operations will be constrained. Lease placements, extensions, redeliveries, and planned retirements near the compliance dates should account for that distinction.
How the final rule relates to the current C-band ADs
The current C-band airworthiness directives address the Lower C-band environment. The principal directives are AD 2023-10-02 for transport and commuter category airplanes and AD 2023-11-07 for various helicopters.
Those directives use aircraft-specific interference-tolerance data, operating limitations, and approved alternative methods of compliance. They remain relevant until the FAA withdraws or supersedes them. The final rule addresses the Upper C-band environment through a different performance basis.
The relationship between the two compliance determinations runs in one direction. The FAA intends to recognize radio altimeter systems that meet 14 CFR 91.220 as an acceptable basis for an AMOC to the current transport and commuter airplane ADs. An AMOC approved under the existing AD does not establish that the installation meets the new ITM.
Operators should maintain separate status fields for current AD compliance and final-rule compliance. A notation such as “5G tolerant” is too broad for either determination. The final-rule record should identify the transceiver part number, modification and software status, antenna and cabling configuration, approved design data, ITM substantiation, and aircraft effectivity.
Lessors should obtain the same evidence for the technical records. Relying on an operator status report without the associated configuration and approval data can create a gap at lease return or during the next placement.
The FAA’s voluntary fleet-planning request in 2026
The first operator action identified in the final rule arrives at the end of 2026. The FAA plans to ask all Part 121 and Part 129 operators to submit voluntary fleet equipage plans to their respective FAA oversight offices and to maintain those plans during the transition.
The request is voluntary because it does not appear as a requirement in 14 CFR 121.326 or 129.16. The FAA also determined that the final rule creates no new information-collection requirement. It has not prescribed a submission form, mandatory data fields, reporting interval, approval process, or enforcement consequence.
The request is intended to expose schedule risk. Radio altimeter suppliers need credible demand before setting production plans. Type certificate and STC holders depend on equipment data to complete aircraft approvals. Operators depend on those approvals, equipment deliveries, maintenance capacity, and access to suitable installation slots.
The FAA plans to convene transition meetings similar to the industry coordination used before the ADS-B Out mandate. Fleet plans will allow the agency and operators to identify certification, production, and installation constraints while there is still time to address them.
A useful plan should begin with a controlled, tail-level fleet list. For each aircraft or common configuration group, it should record:
- the installed radio altimeter, software, antenna, cabling, and approved installation configuration;
- current AD and AMOC status;
- expected 14 CFR 91.220 compliance status and supporting evidence;
- the planned equipment or modification path;
- applicable TC, amended TC, STC, or amended STC activity;
- required quantities, procurement assumptions, and expected delivery dates;
- planned maintenance inputs, installation capacity, and forecast completion; and
- open technical or commercial assumptions, with an owner and next decision date.
The installation forecast should compare the rate required to complete the fleet with the operator’s demonstrated modification rate. Aircraft awaiting an equipment or approval decision should remain visible in the schedule instead of being included in an assumed common solution.
An initial plan can contain unresolved items. Its value depends on showing which information is confirmed, which remains provisional, and what event will close each gap. Since the FAA has not prescribed an update interval, the operator can agree on a practical review cadence with its FAA oversight office.
Submitting the plan does not establish compliance, reserve equipment, create an approval, or determine FCC rebate eligibility. It gives the operator a defensible planning baseline and gives the FAA an earlier view of industry-wide constraints.
Lessors are not included in the FAA’s planned request unless they also operate under Part 121 or Part 129. They should still maintain a corresponding view for leased aircraft, particularly where the operator’s modification schedule overlaps a lease return, lease extension, or transfer to another jurisdiction.
Converting the plan into completed aircraft
The industry’s technical and commercial framework will continue to develop during 2027. The new consensus standard and TSO route are expected to mature, the FCC must conduct the Upper C-band auction, and approval holders will define aircraft-specific modification paths.
Operators do not need to wait for every path to be final before starting the fleet work. The configuration baseline, aircraft effectivity review, current AD reconciliation, maintenance-capacity analysis, and lease review can proceed while equipment and design approvals develop.
Aircraft should be grouped under a common modification path only after the underlying configuration has been verified. Aircraft of the same model can carry different transceiver standards, prior modifications, software, antennas, wiring, or approved installation data. Leased and acquired aircraft are common sources of variation.
For each confirmed configuration group, the program schedule should connect the expected design approval with equipment lead time, kit availability, engineering instructions, scheduled maintenance, installation labor, testing, records completion, and return to service. Planned aircraft retirements and lease returns also belong in the forecast.
Using April 2025 MITRE data, the FAA estimates 58,514 radio altimeters across 40,730 aircraft in the current operating civil fleet, including State-owned aircraft and excluding military aircraft. At 2025 fleet levels, the December 30, 2030 group consists of 12,457 Part 121 and qualifying Part 129 aircraft carrying 27,222 radio altimeters. Applying the FAA’s fleet-growth forecast increases that group to 13,526 aircraft and 29,558 radio altimeters by 2030. The 2030 forecast comprises 8,662 Part 121 aircraft with 18,423 radio altimeters and 4,864 qualifying Part 129 aircraft with 11,135 radio altimeters. That volume puts pressure on certification resources, production capacity, modification kits, avionics facilities, and airline maintenance schedules.
A four-year calendar does not provide four years of installation capacity. Time spent waiting for an approved path, production allocation, or the next suitable maintenance input reduces the period available for physical modification. The operator’s forecast should be driven by executable slots and completion rates rather than the number of months remaining.
Operating consequences beginning in December 2030
After December 30, 2030, Part 121 operators and the affected larger Part 129 operators may not operate a radio-altimeter-equipped aircraft in the covered airspace unless the system meets 14 CFR 91.220. Upper C-band service may begin the following day in the first 75 Partial Economic Areas, with the remaining markets scheduled to follow no later than July 1, 2031.
The regulations permit the FAA Administrator to authorize continued operation in a limited case. The FAA does not expect such authorizations to become routine, and any authorization can contain operating restrictions.
The agency also plans to supersede existing airplane and helicopter ADs for susceptible aircraft that have not reached the new standard. Expected airplane restrictions include CAT II and CAT III approaches, special authorization approaches, autoland, enhanced flight vision operations to touchdown, and head-up-display operations to touchdown.
For affected rotorcraft, restrictions can include Category A or Category B takeoffs and landings, search-and-rescue autopilot modes, hover autopilot modes, and procedures that use radio altimeter minima. The FAA expects these restrictions to apply throughout the covered airspace rather than around a maintained list of transmitter locations.
The remaining affected fleet has until October 31, 2034. After that date, the general prohibition in 14 CFR 91.220 applies. The FAA has said that it does not intend to authorize noncompliant systems for operations that require radio altimeter input to TAWS, TCAS or ACAS, predictive windshear, or night-vision-goggle functions.
For an operator, late compliance can remove low-visibility capability or prevent operation in the covered airspace. For a lessor, the same condition can affect placement options, residual value, and the work required before delivery to the next operator.
The FCC radio altimeter rebate framework
The FCC rebate program is separate from the FAA compliance determination. It is intended to support eligible owners and operators after they complete the required modification, but it does not provide advance funding and does not approve an aircraft installation.
The FCC’s Upper C-band order rejected actual-cost reimbursement for aircraft. Instead, the FCC adopted set rebate amounts for categories of aircraft, based on factors such as the number of radio altimeters and the general effort involved in the modification.
The FCC order reserves its defined “lump sum” election for incumbent earth-station operators and treats the aircraft program as a rebate structure with set-category amounts. The FAA preamble nevertheless uses “lump-sum payment” and “lump-sum payment rebate” when discussing the aircraft benefit. To avoid conflating the two mechanisms, this article uses “set-category rebate” as the more precise description of the aircraft program.
The Wireless Telecommunications Bureau has not yet defined the aircraft categories, rebate values, documentation requirements, or claim procedures. It must publish draft proposals by October 6, 2026, take public comment, and finalize the program afterward.
The FCC estimates total aircraft rebate costs between $3.83 billion and $5.71 billion. Approximately $2.21 billion is associated with the first FAA compliance group, while the second group is subject to a $3.5 billion aggregate cap. These figures are estimates and a cap, rather than cash available to operators today.
Upper C-band auction winners will fund the program in proportion to their winning bids. No entity becomes legally obligated to provide the rebates until the auction has occurred and the FCC has granted the licenses. The auction is required to conclude by July 4, 2027.
Eligibility differs between the two compliance groups:
| FAA group | Expected claimant and aircraft conditions |
|---|---|
| December 30, 2030 group | The claimant is generally the operator holding a U.S. air carrier or operating certificate under Part 119. The aircraft must operate under Part 121 in the contiguous United States, have at least one installed radio altimeter, have an original certificate of airworthiness or original export certificate of airworthiness issued before April 1, 2030, and be subject to the first FAA deadline. |
| October 31, 2034 group | The claimant is generally the owner identified in the FAA Aircraft Registry. The aircraft must be U.S.-registered, operate under Part 91 in the contiguous United States, have at least one installed radio altimeter, have an original certificate of airworthiness or original export certificate of airworthiness issued before July 1, 2031, and be subject to the second FAA deadline. |
There is a drafting conflict in the FCC material that is material to imported aircraft. The order’s preamble includes an original export certificate of airworthiness in both eligibility groups, while the codified definition of “Eligible Aircraft” in 47 CFR 27.1411(b)(5), as published, refers only to an original certificate of airworthiness. Until the FCC or the Wireless Telecommunications Bureau resolves that discrepancy, an imported aircraft should not be treated as rebate-eligible on the preamble language alone.
This distinction matters for leased aircraft. The Part 119 operator is generally the claimant for an eligible Part 121 aircraft even when the aircraft is leased. For the later group, the registered owner is generally the claimant. A Part 135 operator should not assume that it will receive the rebate when the FAA Registry identifies another entity as owner.
Foreign-registered aircraft are excluded from the FCC program even when the FAA rule requires them to comply for operations in the contiguous United States. Spare radio altimeter inventory is also excluded. The rebate applies after completion of necessary work on an eligible aircraft.
Claims must contain documentation showing that the modification was completed by the applicable FAA deadline. The current outer submission dates are June 30, 2031, for the first group and April 30, 2035, for the second. The Wireless Telecommunications Bureau can establish earlier deadlines.
The FCC’s 30-day payment provision begins after the clearinghouse approves a claim and incorporates it into invoices submitted to the licensees. It does not promise payment within 30 days of an operator or owner filing a claim. Portions of the claims and payment framework also require Office of Management and Budget approval before becoming operative.
Until the FCC completes the remaining rulemaking, a fleet budget should not assume a particular rebate value or payment date. Actual configuration and installation costs should remain separate from the projected rebate.
Operator and lessor records should support four separate determinations
A controlled fleet record should allow the operator or lessor to determine four things without reconstructing the aircraft history each time:
- whether the aircraft complies with the current Lower C-band AD and any applicable AMOC;
- whether the installed system meets 14 CFR 91.220;
- whether the aircraft and claimant meet the FCC eligibility conditions; and
- how the lease allocates modification authority, cost, records delivery, and any rebate proceeds.
These determinations can draw from the same source records, but they should not be collapsed into a single “5G status.” Each has a different legal or commercial basis.
The supporting record should include equipment and software identification, antenna and cabling configuration, approved design data, aircraft effectivity, modification work orders, component release documents, test results where applicable, aircraft-record entries, return-to-service evidence, invoices, proof of payment, original certificate-of-airworthiness date, and registry information.
For leased aircraft, the parties should resolve modification consent, access to technical data, planned maintenance input, cost allocation, rebate rights, and delivery-condition language before the installation window becomes constrained. A configuration accepted at lease return may still be unsuitable for the next operator’s intended geography or operating approvals.
How Endeavor Elements can support the program
Endeavor Elements can support an operator or lessor at the aircraft-type, subfleet, or complete-fleet level.
The work can begin with reconciliation of the installed configuration and current AD status by tail. From that baseline, we can determine rule applicability, identify unsupported configurations, map the available approval paths, and connect equipment demand to the operator’s maintenance forecast.
For Part 121 and Part 129 operators, that analysis can be converted into the voluntary fleet equipage plan expected by the FAA at the end of 2026. As technical paths mature, the same program record can track certification gates, procurement assumptions, modification inputs, completion rates, and schedule exposure.
For operators and lessors preparing for the FCC process, we can maintain a separate eligibility and evidence record tied to each completed aircraft. That record preserves the documents likely to be required by the clearinghouse without treating a prospective rebate as confirmed revenue.
This work supports planning and evidence control. Final compliance remains dependent on the applicable FAA regulations, airworthiness directives, approved design data, and aircraft installation.