Potential Risk of Interference of 5G Signals on Radio Altimeter - Clarification on RadAlt Retrofit Mandate - Civil Aviation Safety Alert (CASA) No. 2025-05

Attention:

All Canadian Operators Equipped with a Radio Altimeter and Air Traffic Service

File Classification No.:	Z 5000-35 U
RDIMS No.:	20910798
Document No.:	CASA 2025-05
Issue No.:	01
Effective Date:	2025-04-08

Purpose:

The purpose of this Civil Aviation Safety Alert (CASA) is to clarify Transport Canada’s assessment of the current 5G environment in Canada, as well as Radio Altimeter Tolerance and retrofit expectations described in CASA No. 2024-05 regarding potential risk of interference of 5G signals on radio altimeters (RadAlts).

Transport Canada (TC) has studied the equivalency of the Canadian spectrum environment to the U.S. and has accepted the scientific conclusions from Innovation, Science, and Economic Development (ISED) Canada validated by the Communications Research Centre Canada (CRC) related to the safe operation of radio altimeters in the planned Canadian spectrum environment.

TC has determined that 5G tolerance requirements for RadAlts set by the Federal Aviation Administration (FAA), known as “Radio Altimeter Tolerant” can be leveraged for the current Canadian environment until 5G mitigations measures sunset in 2026.

TC has issued Airworthiness Directive (AD) CF-2024-14 (for fixed-wing aircraft) and CF-2024-15 (for rotorcraft) that describe current RadAlt tolerance requirements in Canada. These ADs also confirm that aircraft meeting FAA AD 2023-10-02 RadAlt tolerance requirements (for fixed-wing aircraft) and FAA AD 2023-11-07 (for rotorcraft) are considered RadAlt tolerant in Canada.

TC has encouraged and continues to strongly encourage all aircraft operators to retrofit their aircraft to be RadAlt tolerant.

AD CF-2024-14 stipulates that fixed-wing aircraft not considered RadAlt tolerant are prohibited from performing the following operations requiring RadAlt data:

• Instrument Landing System (ILS) Instrument Approach Procedures (IAP) Special Authorization (SA) CAT I, SA CAT II, CAT II, and CAT III.
• Automatic Landing operations.
• Manual Flight Control Guidance System operations to landing/head-up display (HUD) to touchdown operation.

AD CF-2024-15 stipulates that rotorcraft aircraft not considered RadAlt tolerant are prohibited from performing the following operations requiring RadAlt data:

• Performing approaches that require radio altimeter minimums for rotorcraft offshore operations. Barometric minimums must be used for these operations instead.
• Engaging hover autopilot modes that require radio altimeter data.
• Engaging Search and Rescue (SAR) autopilot modes that require radio altimeter data.
• Performing takeoffs and landings in accordance with any procedure (Category A, Category B, or by Performance Class in the RFM or Operations Specification) that requires the use of radio altimeter data.

Despite strongly encouraging all aircraft operators to retrofit their aircraft to be RadAlt tolerant in order to be alleviated from the aviation limitations set-out in ADs CF-2024-14 and CF-2024-15, TC clarifies that these ADs do not impose aircraft operators to comply with RadAlt tolerance requirements in order to operate in Canadian airspace, provided that they comply with existing aviation limitations listed in effective ADs.

Background:

ISED is the spectrum regulator in Canada. Since December 2021, ISED has allowed flexible use networks and technology (including 5G) in the frequency bands 3450 to 3650 MHz (3500 MHz). ISED allowed (5G) in the frequency band 3650-3900 MHz (3800 MHz) following its auction on October 24, 2023. Deployment in the 3800 MHz band has begun since May 29, 2024. Additionally, ISED has allowed non-competitive licensing (NCL) framework to operate in the frequency band 3900-3980 MHz. The frequency bandwidth allocated to these services are close to one used by aircraft radio altimeters (4200-4400 MHz).

5G is also already deployed in the United States as well as in several countries in Europe and Asia, with power levels and spectrum mitigations varying from country to country.

Given the worldwide expansion of this technology, data gathered by the FAA, ISED, the Radio Technical Commission for Aeronautics (RTCA) and others, Transport Canada concluded there exists a possibility of disturbance for certain radio altimeter (RadAlt) models by 5G radio waves in numerous operational scenarios in the frequency band 3450-3980 MHz at or above a specific power spectral density (PSD) curve threshold, which may affect aviation safety.

In July 2023, ISED established technical rules in the 3450-3900 MHz bands to mitigate potential interference to radio altimeters, which extends the technical rules for the 3450-3650 MHz band (3500 MHz) that were initially published by ISED in November 2021. The current protection measures, which will be maintained until January 1, 2026, include:

• Exclusion and protection zones to mitigate interference to aircraft having a certain level of 5G tolerance at certain runways (see Appendix B) at 35 airports covering 93% of air traffic in Canada;
• Reduced fundamental emissions power levels at the runways at the 35 airports having protection and exclusion zones, implemented by E.I.R.P. elevation mask^{Footnote 1}; and
• Reduced fundamental emissions power levels based on degree of antenna uptilt above the horizon, nationwide, to minimize skyward emissions from 5G base stations.

Protected runways (in Appendix B) only offer protection to aircraft that can withstand a certain level of 5G fundamental and spurious emissions.

Upcoming 5G environment:

January 1st, 2026 represents the first sunset date of current spectrum mitigations. Reduced fundamental emissions power levels based on degree of antenna uptilt above the horizon, nationwide, will no longer exist.

Exclusion and protection zones will no longer exist in any of the 35 initially protected airports. Reduced fundamental emissions power levels at the runways of the 35 initially protected airports will be kept only for the 3800 MHz band, implemented by specific E.I.R.P. elevation masks.

In 2026, all aircraft may be subject to stricter aviation limitations and/or retrofit expectations due to the sunset of current 5G mitigations. On January 1st, 2028, remaining spectrum mitigations will also sunset, which may further alter aviation limitations and/or retrofit expectations. TC does not intend to impose a broad-based retrofit mandate but will instead enable aircraft operators to make this determination for their own aircraft.

Further details on the 5G mitigations being sunset are included in Appendix D.

Ongoing recommended action:

1. All operators are strongly encouraged to retrofit their aircraft to meet radio altimeter tolerance requirements described in ADs CF-2024-14 (for fixed-wing aircraft) and CF-2024-15 (for rotorcraft) by securing maintenance slots, parts, and supplemental type certificates (STCs) if necessary.

2. Operators should also continue to follow the Reporting Recommendations in No. 2024-05. In the event of an actual disturbance of radio altimeter, it is imperative that flight crew report the event to the Air Traffic Service as soon as possible, regardless of the location of the occurrence be it domestic or foreign airspace. Pilot in command and operator are to complete the ‘Radio Altimeter Disturbance/Interference Report’ in Appendix C: Radio Altimeter Disturbance/Interference Report.

3. Transport Canada is committed to ensuring the safety of the aviation system during the deployment of 5G networks while minimizing impacts to the Canadian aviation industry and ensuring aviation safety.

4. Transport Canada continues to engage its United States counterparts as well as its aviation stakeholders to assess existing aviation safety risks in all weather conditions, particularly for aircraft that are equipped with Radio Altimeters that are not 5G-tolerant and develop aviation-side mitigations to safeguard Canada’s aviation system while protecting the continuous flow of air traffic.

5. Aviation safety is paramount to Transport Canada and the department is committed to strengthening Canada’s aviation system and promote aviation safety throughout Canada.

Contact office:

For more information concerning this issue, contact the AART Documentation Services AARTDocServices-ServicesdocAART@tc.gc.ca

Original signed by
Charles Lanning
For

Linda Kovacic
Director, Standards
Civil Aviation

Appendix A: Criteria for Radio Altimeter Tolerance in Canada

In Canada, Radio Altimeter tolerance requirements are established in TCCA ADs CF-2024-14 (for fixed-wing aircraft) and CF-2024-15 (for rotorcraft).

These ADs also confirmed that aircraft meeting FAA AD 2023-10-02 RadAlt tolerance requirements (for fixed-wing aircraft) and FAA AD 2023-11-07 (for rotorcraft) are considered RadAlt tolerant in Canada.

FAA ADs may tailor future 5G-related Canadian ADs.

Appendix B: List of protected runways

Table 1 identifies the runways which are protected by ISED 5G Protection/Exclusion Zones for the 3500 MHz band and 3800 MHz band until 31 December 2025.

Note: Airports indicated in Bold typeface are subject to medium-power deployment in the 3900 MHz band.

Appendix C: Radio Altimeter Disturbance/Interference Report

Available online: https://wwwapps.tc.gc.ca/Corp-Serv-Gen/5/forms-formulaires/searchrs.aspx?formnumber=26-0870

Appendix D: List of effective 5G mitigations in Canada

In Canada, today

In Canada, on 1 January 2026: (changes from today marked in bold)

In Canada, on 1 January 2028: (changes from 2026 marked in bold)