The FAA is continuing its education campaign to safeguard operators from the effects of potential 5G interference in the C-Band, this time with a Safety Alert for Operators (SAFO 21007) warning of systems that could have faulty information and advising of forthcoming notams that will identify affected geographic regions.
Released last week, SAFO 21007 stated the notams will specify where radio altimeter operations will be prohibited per airworthiness directives issued this month (AD 2021-23-12 and AD 2021-23-13).
Covering all transport and commuter category airplanes, and all helicopters equipped with a radio altimeter, the ADs “were prompted by a determination that radio altimeters cannot be relied upon to perform their intended function if they experience interference,” the FAA said. It also noted, “The radio altimeter is an important aircraft instrument, and its intended function is to provide direct height-above-terrain/water information to a variety of aircraft systems.”
The FAA explained that because the radio altimeter is more precise than a barometric altimeter, it is used where aircraft height over the ground needs to be precisely measured, such as during autoland or other low-altitude operations. It must detect faint signals reflected off the ground, making it vulnerable to interference with out-of-band signals.
“Anomalous (missing or erroneous) radio altimeter inputs could cause...other systems to operate in an unexpected way during any phase of flight—most critically during takeoff, approach, and landing phases,” the agency said and listed a number of such systems that could be affected.
These included: Class A terrain awareness and warning system (TAWS-A), enhanced ground proximity warning systems, traffic alert and collision avoidance systems (TCAS II), takeoff guidance systems, flight control, tail strike prevention systems, windshear detection systems, envelope protection systems, altitude safety call out/alerts, autothrottle, thrust reversers, flight director, primary flight display of height above ground, stick pusher, engine and wing anti-ice systems, and automatic flight guidance and control systems.
Rollout of the 5G C-band wireless broadband is anticipated on January 5 initially in 46 predetermined areas. The notams will identify the areas, airports and heliports, and procedures where the radio altimeter may be unreliable and also specify where certain operations would be prohibited, along with exceptions for operators that have FAA-approved alternative methods of compliance.
An airspace notam will delineate a three-dimensional area where the radio altimeter is unreliable from 5G C-Band wireless broadband interference. An aerodrome notam will outline public airports and heliports where radio altimeters would be unreliable in the presence of the 5G C-band. The FAA further is developing notams identifying the public and special instrument approach procedures affected by 5G C-Band interference.