Flight Standards Service of FAA has issued SAFO - Safety Alert For Operators - 21007, 23 December 2021. Subject and Purpose sections quoted verbatim:
"Subject: Risk of Potential Adverse Effects on Radio Altimeters when Operating in the Presence of 5G C-Band Interference.
Purpose: This SAFO provides information and guidance to operators regarding the risk of potential adverse effects on radio altimeters when operating in the presence of 5G C-Band wireless broadband signals, and the role of Notice to Air Missions (NOTAMs) in identifying the geographic areas where certain operations requiring a radio altimeter are prohibited in the presence of 5G signals (in the C-Band) by Airworthiness Directives (ADs) 2021-23-12 and 2021-23-13."
Link to FAA website and the SAFO document:
SAFO21007, Risk of Potential Adverse Effects on Radio Altimeters when Operating in the Presence of 5G C-Band Interference (faa.gov)
FAA
also issued an SAIB - Special Airworthiness Information Bulletin, AIR-21-18R1, "Risk of Potential Adverse Effects on Radio Altimeters"
Here is, verbatim, its opening paragraph:
"This Special Airworthiness Information Bulletin (SAIB) informs aircraft manufacturers, radio altimeter manufacturers, operators, and pilots of the planned deployment of wireless broadband networks in the 3700-3980 MHz bands (C-Band).1 C-Band wireless broadband deployment is permitted to occur in phases with the opportunity for operations in the lower 100 megahertz of the band (3.7-3.8 GHz) in 46 markets beginning as soon as December 5, 2021; however, the FAA does not expect actual deployment to commence until January 5, 2022.
This SAIB recommends that radio altimeter manufacturers, aircraft manufacturers, and operators voluntarily provide to federal authorities specific information related to altimeter design and functionality, specifics on deployment and usage of radio altimeters in aircraft, and that they test and assess their equipment in conjunction with federal authorities. Results from that testing and assessment should be reported to the appropriate civil aviation authorities (CAAs) and spectrum regulators. The FAA is currently collaborating with the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA) to assess the need for mitigation beyond the recommended action in this SAIB." (emphasis added)
Link to FAA website and SAIB:
AIR-21-18R1.pdf (faa.gov)
(This SLF/att'y credits FlightGlobal Daily Briefing (24 Dec. 2021) for noting this development.)