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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.