Aviation experts warn 5G rollout could result in wildly wrong altimeter readings

By Dan Parsons | January 17, 2022

Signals emanating from a nationwide forest of 5G cellular towers set for activation in the U.S. this week could interfere with radar altimeters, potentially causing them to report wildly inaccurate measurements of an aircraft’s height above the ground, aviation experts are warning. Such erroneous data could also be passed onto other flight control systems.

AT&T and Verizon Communications are set to activate the first set of 5G communications towers on Jan. 19, and while much concern has been aired over how the new signals could affect commercial air travel, rotorcraft that fly and land at public airports are particularly vulnerable to interference. 

As well as measuring a helicopter’s distance from the ground, radar altimeters feed altitude data to other critical safety systems like terrain avoidance warning systems (TAWS), enhanced ground proximity warning systems (EGPWS) and automatic flight controls that perform hover holds and other maneuvers. 

The Jan. 19 rollout marks the debut of C-band 5G service, which promises speedier cell service for smartphones. Of particular concern in the U.S. is 5G c-band in the range of 3.7 to 3.98 megahertz, which the telecom companies spent millions to secure for their own use. 

When flying near 5G towers, spoofed radar altimeters, often shortened to “rad alt,” could feed other flight control systems erroneous data, leading to cascading failures and potential catastrophe, according to Nick Kefalas, a technology fellow at Sikorsky, which is owned by Lockheed Martin.

“Operators have learned to depend on the rad alt, or more importantly, they’re depending on other systems, and interpreting them,” Kefalas said during a Jan. 13 webinar on the impacts of 5G on helicopter operations hosted by Helicopter Association International (HAI). “When that doesn’t happen, you can have a cascade effect of failures. So, while losing rad alt itself, or having an erroneous undetected value, may not seem a critical failure, when you start adding additional systems that depend on the validity and accuracy of that number, then you have a cascade effect of failure buildup that can lead to a catastrophic event.”

Kefalas said helicopter operators can expect two possible types of altimeter interference — in engineering speak, “failure condition manifestations” — when operating in the vicinity of 5G towers. The first is an altitude reading that is “erroneous but detected” in which the instrument recognizes it is measuring an inaccurate altitude. In that case, the altimeter would alert the pilot of the problem. 

A second, and potentially more hazardous form of interference, is “erroneous, undetected” in which the altimeter is providing inaccurate altitude but isn’t aware of it, “in which case, you would continue to get values that the system would declare as valid, but they would not correspond to the correct altitude. And that’s the most dangerous of the two, obviously,” Kefalas said. “It’s the erroneous undetected, particularly whether you’re within a few feet or a few tens of feet, that is probably the most dangerous one of all.”

HAI went so far as to suggest the 5G rollout could ground the nation’s entire helicopter fleet, including airborne law enforcement and air ambulance services. Late on Jan. 12, the FAA issued more than 1,450 notices to air missions (NOTAMs) that prevent pilots from operating aircraft near known hazards, including new 5G towers. 

Commercial air carriers have successfully raised concerns about 5G interreference near airports, where a miscalculation of altitude could be catastrophic. As a result, the Federal Communications Commission (FCC) and FAA have designated buffer zones around major airports where 5G service will be blocked. But helicopters operate at relatively low altitude, often far from major airports, and for much longer duration than commercial airliners. Many of the towers are located near smaller airports and hospitals from which medevac, law enforcement, news gathering and other helicopters operate. 

HAI has partnered with the FAA to maximize the efficiency of an existing regulatory relief process, where operators can demonstrate alternative methods of compliance (AMOC) with the agency’s 5G-related restrictions.

The association also successfully lobbied the FAA for an exemption for air ambulance operations to the 5G C-band restriction that requires a normally functioning radio altimeter for certain operations. This exemption will allow Part 119 certificate holders authorized to conduct helicopter air ambulance (HAA) operations under Part 135 to continue operations while employing radar altimeters that may not function normally due to 5G C band interference. The relief will also allow the use of night vision goggles (NVGs) in HAA operations. 

“This exemption will allow HAA operators to continue to do what they do best — save lives,” HAI president and CEO James Viola said in a statement. “There is no question that it is in the public interest for these lifesaving operations to continue. HAI’s top priority will always be safety, and we will continue to work with FAA to determine the best solutions, whether through exemptions or through alternative methods of compliance, to provide all our members with the means for continued safe operation.” 

Rolling out the 5G network will happen in stages. Some towers are already operational, and Jan. 19 is the launch date for the initial U.S. nationwide activation. Every three months, the telecom companies will update the FAA, through the FCC, with the location of new tower activations, steadily increasing the density of 5G infrastructure, which requires more towers to provide the same coverage as the previous 4G signal, according to Kefalas. 

“Everything’s changing hour-by-hour, day-by-day here with every meeting we have,” Kefalas said. “It’s a very fluid situation. Right now, if the rad altimeters meet the criteria that we as the aviation industry manufacturers have put together that show that an aircraft model with this particular altimeter operates with success, then the OEM should proceed with an AMOC application and companies like Boeing, Airbus and Sikorsky and others are doing [that] right now.”

Seth Frick, a radar systems engineer with Honeywell, said altimeters altered by 5G signals could produce a disparity of several feet to thousands of feet from a helicopter’s actual altitude, depending on the distance from a tower. 

“You could have things bumping around plus or minus 50 feet, 100 feet, to complete loss of function … to sustained erroneous output that is not indicated as erroneous, which could be hundreds of feet, or thousands of feet away from the true altitude,” Frick said during the HAI webinar. Certain altimeters could see relatively little or no interference from 5G, while others could be rendered completely useless, he added. 

“I don’t know if there’s any cases where we can say there is absolutely no interference,” Frick said. “There certainly are altimeter models that we think we have data that shows they perform very well and are very robust. Any errors, either undetected, erroneous altitude output or loss of function for those particular models is going to be extremely unlikely. But there’s a whole lot of other cases where there is a likelihood of interference, but it kind of depends on how the aircraft is being operated, where you are relative to the 5G towers, and so on. So, it’s not really a binary thing of this altimeter model is susceptible, this other model is not.”