Potential Risk of Interference of 5G Signals on Radio Altimeter - Civil Aviation Safety Alert (CASA) No. 2021-08


All Canadian Private, Commercial Air Operators with a Radio Altimeter and Air Traffic Service

File Classification No.: Z 5000-35
RDIMS No.: 17921072
Document No.: CASA 2021-08
Issue No.: 02
Effective Date: 2021-12-23


The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness of the potential risk of 5G interference and to recommend precautionary operational measures before confirmation of impact of 5G radio waves on radio altimeters.


Innovation, Science and Economic Development Canada (ISED) is the spectrum regulator in Canada. ISED will allow flexible use networks and technology (including 5G) in the frequency band 3450-3650 MHz following its auction in June 2021. Additionally, ISED recently decided to allow flexible use wireless systems, including 5G systems, to operate in the frequency band 3650-3980 MHz for 2023. The frequency bandwidth allocated to these services are close to one used by aircraft radio altimeters (4200-4400 MHz). In some countries in Europe and Asia, 5G is already deployed. Given the worldwide expansion of this technology, Transport Canada was drawn to a recent Radio Technical Commission for Aeronautics (RCTA) report which concludes to a likelihood of disturbance for certain radio altimeter models by 5G radio waves in numerous operational scenarios and most notably at low altitude (less than 1000 ft) in the frequency band 3700-3980 MHz.

The most undesirable outcome of interference is the indication of undetected erroneous height information given by the radio altimeter. Depending on the type operations, the equipment model and aircraft type, this kind of erroneous information could have significantly adverse impacts on flight safety. Erroneous radio altimeter information may impact Terrain Awareness Warning Systems (TAWS), Traffic Alert and Collision Avoidance Systems (TCAS) and Airborne Collision Avoidance Systems (ACAS), Wind Shear detection systems, flight control systems, and autoland systems (including auto-throttle and automated landing flare and rollout) and cause the loss of situational awareness due to erroneous or unexpected behaviour.

Section 602.08 of the Canadian Aviation Regulations (SOR/96-433) and Advisory Circular (AC) 700-005 Use of Transmitting and Non Transmitting Portable Electronic Devices are still applicable, where no air operator shall permit use of a Portable Electronic Devices (PEDs) on board an aircraft except with the permission of the operator of the aircraft and permit the use of non-transmitting PEDs on board aircraft during critical phases of flight.

On November 18, 2021, ISED established interim technical rules in the 3450-3650 MHz band to mitigate potential interference to radio altimeters but will continue to monitor and study this issue as it develops. The main protection measures include:

  • exclusion and protection zones to mitigate interference to aircraft around certain airport runways where CAT I/II/III automated landing is authorized; and
  • a national antenna down-tilt requirement to protect aircraft used in low altitude military operations, search and rescue operations and medical evacuations all over the country.

Recommended action

Other than possible interference to radio altimeter, Transport Canada has not identified unsafe effects on aviation from 3450-3650 MHz deployment. Operators should identify all possible indications that might be evidence of possible radio altimeter disturbance in their aircraft and ensure this information is provided to the flight crews. In addition, operators should perform risk assessments and if required implement procedure and/or operation restrictions that could lead to the following precautions being taken, for example:

  • RNP AR procedures not protected by the exclusion and protection zones should not be conducted in instrument flying conditions (IMC) unless there is an alternate means to identify obstacles and terrain (weather radar for example) because the TAWS may not be reliable.
  • Steep Approach at Salluit, QC, should not be conducted in IMC unless there is an alternate means to identify obstacles and terrain (weather radar for example) because the TAWS may not be reliable.
  • Night Vision Goggles (NVG) or Night Vision Imaging Systems (NVIS) operations should not be conducted without external lighting to identify ground proximity because the radio altimeter may not be reliable.
  • Helicopter Offshore RNAV (GNSS)/ARA procedures to a location where 5G in the C-band is deployed should not be conducted in IMC unless there is an alternate means to identify obstacles and terrain (weather radar for example) because the radio altimeter may not be reliable.
  • Category A helicopter procedures should not be conducted without alternative altitude reference (known barometric height for example) because the radio altimeter may not reliable.
  • Helicopter auto hover should not be conducted without clear external drift reference because the radio altimeter may not be reliable.

Operators should remind passengers and flight crew that all electronic devices should be carried in the cabin, on their person or in the luggage. If these were placed in checked baggage, they should be turned off and protected from accidental activation.

All 5G PEDs carried in the aircraft should be set to non-transmitting mode so they do not transmit on the cellular networks (e.g. airplane mode) or switched off.

For essential communications, e.g. during emergency medical service operations (EMS), crew should only use 3G or 4G communication devices.

In the event of an actual disturbance of radio altimeter, it is imperative that flight crew report the event to the Air Traffic Service (ATS) 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 Annex A: Radio Altimeter Disturbance/Interference Report.

International operators need to be aware of the potential risk of interference to radio altimeters when operating in countries with different 5G networks and their respective mitigations measures, if any are in place. In jurisdictions where no mitigations measures are established, operators need to confirm that their radio altimeter is sufficiently robust against 5G potential interference before relying on radio altimeter information below 200 ft AGL for: CAT I/II/III auto land operation, RNP AR approach/departure, Steep Approach (above 4.5 deg), NVG operation, offshore RNAV (GNSS)/ARA procedure, helicopter Category A take-off/approach procedure, auto hover, automatic landing operations and head-up display (HUD) to touchdown operation. Granted that operator are to comply with States NOTAMs that may prohibit instrument approach procedure or operation unless alternative methods of compliance (AMOC) are approved.

Transport Canada continues to closely coordinate with the manufacturers, regulator and international stakeholder to determine if additional actions are warranted.

Contact office:

For more information concerning this issue, contact the Civil Aviation Communications Centre:

Original signed by

Félix Meunier
Director, Standards
Civil Aviation


The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.