The FAA today announced two new airworthiness directives (ADs)—one covering transport and commuter-category airplanes and the other directed at helicopters—that address the threat of potential radar altimeter interference from 5G cellular in the 3.7 to 3.98-GHz frequency (C-band). The ADs require insertions into the limitations sections of aircraft and rotorcraft flight manuals (A/RFMs) that will limit flight operations when such interference is encountered.
Telecommunications companies in the U.S. are expected to activate additional 5G frequencies as early as January 5. Last month, these companies announced that they would voluntarily restrict power output at cell towers using these frequencies near 46 airports and other strategic locations in the U.S. for six months.
In a prepared statement issued with the ADs, the FAA said it “believes the expansion of 5G and aviation will safely coexist. Today, we took an important step toward that goal by issuing two airworthiness directives to provide a framework and to gather more information to avoid potential effects on aviation safety equipment. The FAA is working closely with the Federal Communications Commission and wireless companies and has made progress toward safely implementing the 5G expansion. We are confident with ongoing collaboration we will reach this shared goal.”
But the FAA also acknowledged the exigency and risk posed by some 5G by waiving the comment period for implementation and warning of the potential for air travel and emergency air services disruption triggered by the compliance of the ADs. Where radar altimeter inference is noted or has the potential to exist, the FAA plans to issue location-specific notams prohibiting certain aircraft operations.
Specifically, the FAA noted the potential for some 5G frequencies to interfere with radar altimeter-linked systems such as automatic flight control systems and autoland required for safe aircraft operations in low visibility situations. The agency warned, “Anomalous (missing or erroneous) radio altimeter inputs to these systems may cause the aircraft to be maneuvered in an unexpected or hazardous manner during the final stages of approach and landing, and may not be detectable by the pilot in time to maintain continued safe flight and landing. Inaccurate radio altimeter data can result in pilots not trusting their instruments, eroding the foundation on which all instrument flight training is built.”
For helicopters, the risk from certain 5G frequencies has the potential to be even more acute. The FAA noted, “automatic and/or manual flight guidance systems on helicopters facilitate low visibility operations and rely on accurate radio altimeter inputs.”
In areas of 5G interference identified by notams, the related AD would prohibit the following operations: “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 Rotorcraft Flight Manual or Operations Specification) that requires the use of radio altimeter data.”
The new ADs follow the November 2 FAA issuance of a special airworthiness information bulletin that requested information from aircraft operators and OEMs on the potential for 5G interference.