The protracted row between the Federal Communications Commission (FCC) and the FAA over the rollout of AT&T and Verizon 5G C-band service hit the pause button earlier this month, with the wireless carriers offering accommodations that at this writing appear to mollify the concerns of commercial air carriers at the U.S.'s largest airports. At issue, the impact of wireless signals in the 3.7 to 3.98 GHz range bumping up against radar/radio altimeters that live in the 4.2 to 4.4 GHz environment, causing those altimeters to give potentially false readings to pilots and the various aircraft systems they support, endangering aviation safety, particularly during takeoff and landing in low visibility conditions. The apparent solution on the horizon is to adopt a version of accommodations already in place at France’s largest airports and in Canada—powering down C-band base stations near runways and aiming wireless tower antennas downward at steeper angles.
Some variant of this solution was suggested by the wireless carriers, the airlines’ trade association, Airlines For America (A4A), and the Airline Pilots Association International (ALPA). But while this détente addresses the concerns of the major airlines, it does little to assuage the concerns of regional carriers, business and general aviation, and particularly the rotorcraft community. There are also national security implications as a good chunk of the U.S. military’s aircraft rely on older technology unmodulated pulse style radar altimeters more susceptible to C-band interference, as opposed to more modern frequency-modulated continuous-wave (FMCW) technology in newer models. But even those have some susceptibility. Existing radar altimeters cannot be economically modified to defeat this interference and new models that do are just now coming to market. If the recent ADS-B mandate is any indication, it could be a decade or more before a meaningful fleet retrofit is achieved and then, of course, there is the thorny issue of who should pay for it.
For all impacted aircraft, the inability to use radar altimeters makes them more reliant on less secure technology, such as GPS and ADS-B, points out Josh Lospinoso, CEO of transportation cybersecurity firm Shift5 and a founding member of U.S. Cyber Command. “The danger is the reliance it could create on other systems to provide altitude data. The problem is that those systems are vulnerable to a wide variety of cyber and spoofing attacks. It is really a bad idea to rely on other systems besides the radar altimeter for making sure you can land an aircraft safely.” In the event 5G C-band base stations were hacked, they could theoretically be manipulated to transmit at higher power “invalidating any mitigation,” Lospinoso, said. “No question about it, it would be a huge problem.
“Among the classes of aircraft to be concerned about with non-compliant radar altimeters," he added, "the military aircraft are certainly the ones that I would be most concerned about as an asset class.”
The aviation community voiced concerns about 5G C-band as early as 2018 and formally requested that the FCC delay the scheduled December 2020 spectrum auction to evaluate the impact on radar altimeters. In 2019, the Aerospace Vehicle Systems Institute (AVSI) published initial findings of C-band interference with radar altimeters. In October 2020, the Radio Technical Commission for Aeronautics (RTCA) published a comprehensive study that enumerated the dangers to radio altimeters from 5G C-band. The RTCA concluded, “This [interference] risk is widespread and has the potential for broad impacts to aviation operations in the United States, including the possibility of catastrophic failures leading to multiple fatalities.”
While elements of that study with regard to commercial aviation came under fire, the RTCA’s findings about business aviation and rotorcraft operations were on solid footing. Namely, that these categories of aircraft were far more likely to experience radar altimeter interference from 5G C-band.
Subsequent AVSI research of nine leading radar altimeters also found evidence of C-band interference, according to Clay Barber, Garmin’s principal engineer and a panelist at an NBAA webinar on the topic in early December. When standards for these altimeters were developed, largely in the 1970s, manufacturers and regulators “didn’t anticipate that the FCC would bring a lawnmower into the library in the spectrum neighborhood.“
Interference responses observed included the “no computing state,” meaning the radio altimeter does not have enough signal information to give an output. This would also disable linked avionics such as for Terrain Awareness and Warning Systems (TAWS). “In other cases, the rad alt simply gave misleading information and relayed that to the downstream systems. Then you can have very bad things happen” triggering erroneous TAWS warnings or actions from the automatic flight controls such as initiating a landing flare too high or too low. “That is what the FAA has the most concern with. It’s got a lot of variation,” Barber said. 
Another NBAA panelist, Andrew Roy, director of engineering for Aviation Spectrum Resources, said not all aspects of the French and Canadian interference mitigation models are applicable in the U.S. as carriers here use much higher power levels on their transmitters. “No two 5G rollouts are the same,” he said. He also expressed concern that mitigation strategies were being rolled out without proper testing. “One of the arguments they [the wireless carriers] make is that they [other countries] have deployed [C-band] and we are not seeing any issues. What’s the problem? And that misses the part about aviation safety. You don’t say, ‘Well I’ve been flying for six months and everything’s fine.’ You need to do that analysis and assessment beforehand to make sure you are meeting the certification.
“Aviation is an industry that needs that safety assurance and if you don’t have that assurance you aren’t going to want to fly,” he said. “It’s really key that we have the data from the 5G [wireless providers] so that we know what is going on and we know what is happening as opposed to saying, ‘Well it hasn’t happened on a few flights so everything should be okay.’ Aviation doesn’t plan its flights that way.”
Various aviation industry sources interviewed by AIN repeatedly and bitterly complained about the short shrift their concerns received from the FCC, especially in the months leading to the spectrum auction, and the subsequent lack of technical cooperation received from the wireless carriers. For example, through early December, the wireless carriers had yet to present a list of C-band tower locations to aviation interests. What little cooperation the carriers offered was couched on the condition of a virtual gag order, a strategy made public by their CEOs in a letter to the DOT and FAA in early January, offering cooperation “on the condition that the FAA and the aviation industry are committed to doing the same without escalating their grievances, unfounded as they are, in other venues.” 
For the two domestic wireless carriers, which spent $67 billion acquiring the C-band rights last year and are poised to spend billions more rolling it out, the stakes couldn’t be higher: 5G is the wireless holy grail and C-band is the frequency “sweet spot” of range and coverage that is most efficient. Both carriers trail their partly German-owned competitor T-Mobile—which does not use the C-band—when it comes to 5G subscribers in the U.S., as the latter’s public relations minions pointed out in a snarky e-mail sent to journalists in January, that noted “Verizon and AT&T are TWO YEARS Behind. And the gap keeps getting wider.” It would not be an exaggeration to say that the futures of Verizon and AT&T are riding on C-band.
Going into the December holidays, the two wireless carriers already had agreed to a one-month delay to their planned C-band rollout—to Jan. 5, 2022—and had agreed to operate C-band base stations at reduced power at and around major airports for six months to give the FAA time to evaluate the impact on safety and operations at those locations. But for the FAA, that was not enough. It issued a series of airworthiness directives and advisories warning of safety issues and potential air traffic delays due to potential radar altimeter interference related to the 5G C-band rollout. The cost of those delays to the airlines alone was estimated annually at $4 billion. The FAA documents, issued from December 7 through 23, set in motion a series of holiday week letters and filings between the opposing sides.
Following a pair of airworthiness directives issued December 7 that warned of forthcoming notams where 5G C-band interference could be potentially an issue, the FAA doubled down, issuing a safety alert for operators (SAFO) warning of all the attendant avionics that could be compromised by erroneous radio altimeter data.
“Anomalous (missing or erroneous) radio altimeter inputs could cause these other systems to operate in an unexpected way during any phase of flight—most critically during takeoff, approach, and landing phases. These anomalous inputs may not be detected by the pilot in time to maintain continued safe flight and landing. Operators and pilots should be aware of aircraft systems that integrate the radio altimeter, and should follow all Standard Operating Procedures related to aircraft safety system aural warnings/alerts.
“These systems include, but are not limited to: Class A Terrain Awareness Warning Systems (TAWS-A), Enhanced Ground Proximity Warning Systems (EGPWS), Traffic Alert and Collision Avoidance Systems (TCAS II), takeoff guidance systems, flight control (control surface), tail strike prevention systems, wind shear detection systems, envelope protection systems, altitude safety call outs/alerts, autothrottle, thrust reversers, flight director, primary flight display of height above ground, alert/warning or alert/warning inhibit, stick pusher/stick shaker, engine and wing anti-ice systems, and automatic flight guidance and control systems (AFGCS).”
On December 31, the FAA pleaded with the wireless carriers for more delay and study. Transportation Secretary Pete Buttigieg and FAA Administrator Steve Dickson wrote, "Our goal would then be to identify mitigations for all priority airports that will enable the majority of our large commercial aircraft to operate safely in all conditions. This will allow for 5G C-band to deploy around these airports on a rolling basis, such that C-band planned locations will be activated by the end of March 2022, barring unforeseen technical challenges or new safety concerns.”
Initially, the wireless companies would have none of it. Two days later, on January 2, the CEOs of AT&T and Verizon—John Stankey and Hans Vestberg—wrote to Buttigieg and Dickson: “AT&T and Verizon agreed to wait until January 5 to begin using the C-band and to implement additional restrictions on our use of the spectrum through July 5 over and above the operational restrictions the FCC already had found sufficient to protect radio altimeters. Although there was no requirement for us to adopt these measures, we did so voluntarily in the spirit of cooperation and good faith.
“Now, on the evening of New Year’s Eve, just five days before the C-band spectrum will be deployed, we received your letter asking us to take still more voluntary steps—to the detriment of our millions of consumer, business, and government customers—to once again assist the aviation industry and the FAA after failing to resolve issues in that costly 30-day delay period, which we never considered to be an initial one.”
The FCC ignored emergency petitions—filed over the holidays—from both A4A and ALPA, that requested further delay. Less than 48 hours before C-band was scheduled to be switched on, airline interests prepared an emergency filing to stay C-band deployment with the U.S. Court of Appeals. Mere minutes before the suit was going to be filed, both sides agreed to the two-week delay requested by Buttigieg and Dickson.
While all sides initially reacted positively to the development, behind-the-scenes business and general aviation, in particularly the rotorcraft industry, harbored serious misgivings, believing the airlines had cut a side deal and left them in the breeze. NBAA president and CEO Ed Bolen publicly spun diplomacy, stating, “The NBAA welcomes this short-term reprieve from the Verizon and AT&T 5G rollout so that we can better understand and communicate its potential impact on aircraft, airports, and airspace across the system. We need answers to key questions in order to ensure we remain the world’s largest, safest, and most efficient aviation system, and we will utilize this time to gather and share much-needed information about this development for all aviation segments, including business aviation.” Many NBAA members have equipped their aircraft with expensive and sophisticated landing and vision systems that would be worthless should 5G C-band deploy without mitigation near thousands of U.S. airports, not just the few dozen large commercial ones currently under discussion.
However, the helicopter industry clearly has the most to lose, and rotorcraft interests pulled no punches following the announcement of the two-week armistice. Writing for the Helicopter Association International (HAI), John Shea, its government affairs director, blasted the wireless carriers’ plan to reduce power at only a handful of public heliports.
“The effects of 5G deployment are not limited to the nation’s busiest airports, and mitigations by wireless carriers should not be limited to those locations either.” HAI pointed out that “the voluntary measures proposed by the wireless carriers would provide modest 5G limitations at the surface of public-use heliports, of which there are only 55 in the country. That number is dwarfed by the estimated 6,533 to 8,533 helicopter landing sites in the United States, with more than 4,000 being private-use heliports co-located at hospitals.” HAI emphasized, “All over the country, from densely populated cities to oil rigs 200 miles offshore, helicopters are used to save lives, serve and protect American citizens, and support critical industries in demanding environments—and many of those missions are conducted from start to finish without the use of airports.” The rotorcraft lobby noted that the impact of 5G C-band radio altimeter interference could be particularly harmful to helicopter air ambulance (HAA) operations and bring with it loss of life. Helicopter air ambulances are required to be equipped with radio altimeters by law. HAI has petitioned the FAA for an exemption to this requirement due to the ongoing 5G controversy.
“The loss of a single life because of misguided 5G-related policies would be reprehensible,” HAI said, noting that HAA operators transport roughly 1,000 injured or critically ill patients every day. “Up to 50,000 of the more than 300,000 people transported by HAA operators during 2021 were transported from off-airport/unimproved areas at night—meaning the mitigations proposed to maintain an equivalent level of safety at airports will have no effect on those operations.”
Andrew Roy echoed Shea’s concerns. “Smaller operators, particularly helicopters, generally have lower-performing avionics and operate considerably closer to [cell] towers. Take an emergency medevac helicopter. If that is going in [on scene] to extract from a road accident or something else they are not going to have time to go into a proper approach angle. They are going to get in or get out. If they are hovering within 150 feet of a cell tower pointed right at them with—and these 5G main beams can have a signal amplification of 10,000 times—then you will start to see some issues with that going forward.”
And those “issues” could very well end in an NTSB accident report.