5G Is In The Air

In the last month or so, U.S. telecommunications companies have ramped up deployment of their 5G networks across the country, jolting commercial aviation operators into ensuring their radio altimeters are free from interference by emissions from hundreds of new cellular towers near airports.

The telecom giants agreed until July 1 to being powering up the 5G system and switching on new towers that provide high-speed data services. Commercial airlines, business jet operators, and helicopter fleet owners were given that date as a deadline to either retrofit, replace or otherwise modify radio altimeters, also called radar altimeters, or face landing restrictions at nearly 200 airports where the signals could interfere with altimeters tuned to a certain transmission range.

Radio altimeters, often shortened to rad-alts, feed information into just about every avionics system on a modern aircraft, explained Alex Haak, Associate Director of Programs: Avionics Flight Deck Aftermarket at Collins.

The yearlong voluntary extension by Verizon and AT&T—and about 20 other telecom companies—allowed commercial airlines and business jet operators ample time to mitigate the effects of 5G bleeding into the bandwidth used by rad-alts.

“The reality is that band has been sold to telecom and this is going to be a future state for us,” Haak said during a recent webinar hosted by Collins and Aviation International News. “It's really about how do we adapt to the future state. … Previously, that 3700 to 4200 band was really unoccupied. That was a guarded zone so that there wasn't an interference within the rad-alts, and now that 5G is starting to encroach a little bit closer on arrival. That's where you can get the potential for interference with your rad-alt which we'll say is highly undesirable on final approach and after takeoff which is a critical rad-alt phase of flight.”

Altimeters are mostly relied on by pilots and crew during final approach, below about 2,500 feet as they close in on an airport. They are also used on departure, but less so, Haak said. In both phases of flight, the rad-alt becomes a critical part of the navigational equation, he said.

“When we think though about rad-alts and 5G, it's not really the rad-alt alone in a silo we need to think about,” Haak said. “We need to think about what inputs into the greater avionics system the radio altimeter does.”

For starters, there is the flight management system, or FMS. It and the terrain alerting and warning system, or TAWS, both take height data input from the radio altimeter. So does the ground proximity warning system, or GPWS, which provides approach guidance to the pilot as the plane comes in to land, Haak said.

Also communicating with the rad-alt is the aircraft’s auto-throttle, autopilot, and other automatic landing systems. The rad-alt feeds height-above-ground readings to the traffic collision avoidance system or TCAS. If the aircraft is equipped with a head-up display or HUD, that system also provides altitude readings pulled from the onboard radio altimeter. As does a synthetic vision system (SVS), the stall protection system, and the onboard maintenance and diagnostics systems.

“While we have all these integrated avionics portions that were added, the rad-alts on your airplane really feed into these subsystems and it can affect them,” Haak said. “It can affect the safety of the flight as we come in to land. So we need to be very cautious about how 5G could affect us.”

Possible interference with radio altimeters by 5G service has been a concern for some time and the FAA requested that any anomalies be reported. So far, the agency has received more than 420 reports of radio altimeter anomalies occurring within a known location of a 5G C-band deployment, according to the FAA. About 315 of those reports were unrelated to 5G C-band interference and were resolved through normal continued operational safety procedures.

For the roughly 100 or so other anomalies occurring within areas where the FAA has issued a notification to air missions (NOTAM), the FAA has excluded other potential causes for the anomaly, but could not rule out 5G C-band interference as the potential source of the radio altimeter anomalies.

For the 100 incidents where 5G was found to likely be the cause of an anomaly, the transmissions produced possibly erroneous TAWS warnings, TCAS warnings, erroneous landing gear warnings, and the erroneous display of radio altimeter data, according to the FAA.

“Although these flight deck effects are less severe than the hazards associated with low-visibility landings, the FAA is concerned that to the extent 5G C-Band operations contributed to such events, the effects will occur more frequently as telecommunication companies continue to deploy 5G C-Band services throughout the country,” the FAA said in a statement.

The FAA, AT&T, and Verizon have collaborated extensively to ensure 5G C-Band radio frequency transmissions and aircraft operations can safely co-exist. The agreement with Verizon, AT&T, T-Mobile US (TMUS.O), and UScellular (USM.N) followed extensive discussions with the FAA, allowing carriers to increase power levels to get to full C-Band use by July 1.

In early January 2022, the FAA began to enforce “tailored runway protection zones” around airports where aircraft were most heavily reliant on radiator altimeters in the below-2,500-feet phase of flight prior to landing.

AT&T and Verizon coordinated their deployment around 5G C-Band mitigated airports “including in some cases reducing emission power around airports and committing to antenna pointing angles in the vertical plane to limit the potential for interference within the tailored runway safety zones.”

The companies eventually extended the rollout of 5G at full power until July 1, 2022, a date which Transportation Secretary Pete Buttigieg said would not be extended further.

The International Air Transport Association, representing more than 100 air carriers that fly to the United States, said in May that "Supply chain issues make it unlikely that all aircraft can be upgraded by the 1 July deadline, threatening operational disruptions during the peak northern summer travel season."

Heidi Williams, senior director of air traffic at the National Business Aviation Association (NBAA), said that given the voluntary deadline extension, aircraft operators simply have to adapt to the new 5G encroachment at certain airports, while expecting the interference to spread.

“As of July 1, those mitigations will be eliminated and the telco companies are going to go to greater power and full-scale deployment,” she said. “So at that point, all of those systems … are at greater risk potentially if you have not installed a filter or upgraded your rad-alt.”

Then-Acting FAA Administrator Billy Nolen said in June the FAA has "given airlines until July of this year to retrofit. Now upon July 1, if they haven't retrofitted, they will not be able to take advantage of lower visibility approaches that may result in a divert." If airlines have not retrofitted by next year, "they will not be able to operate" in U.S. airspace, Nolen said.

The proposed directives impact 4,800 U.S. registered airplanes and 14,600 worldwide. They require revising aircraft flight manuals by June 30 to prohibit some landings and include specific operating procedures for calculating landing distances and certain approaches when in the presence of 5G C-band interference.

At first, the FAA defined a rectangular airspace area around runways to protect aircraft from 5G interference as they landed. The rectangular area was later reshaped as a trapezoidal area, which “allowed for geographically expanded 5G C-Band transmissions that would not affect radio altimeter functions within the area,” the FAA said.

“The FAA is now able to assess the 5G C-Band transmissions' impact to aviation operations in a specific area, taking into account the particularities of the signal and the airport environment,” the administration said in a statement.

While the concern over 5G interference with flight systems is focused on the U.S., the technology is already widely deployed in other countries, like France and Japan. But U.S. airspace is the most complex in the world and deployment of 5G elsewhere is done at lower power levels. The telecom antennas in France are also specifically adjusted to reduce potential interference with avionics systems and are generally placed farther from airfields.

Despite the focus on mitigating interference with commercial airliners and business jets, which fly in affected airspace at lower altitudes only at the beginning and end of flights, helicopters are particularly vulnerable to 5G interference.

The FAA allows air ambulance operators to use night vision goggles in areas where the aircraft’s radio altimeter could be unreliable due to 5G C-band interference as identified by NOTAMs. Operators must comply with specific conditions and limitations. Similar to commercial aircraft, helicopters can perform day and night operations that do not require the use of a radio altimeter.

Most effects of potential 5G interference are felt in the cockpit. The good news is, the technology will enhance the passenger experience by allowing faster, more seamless streaming of data to the cabin, Haak said.

“Owners and operators are spending a lot of money to upgrade to 5G connectivity in the back of their aircraft to help the principals or to help the owners on their missions,” he said. “There is really a non-conflict here. They use a different bandwidth of signal than ground-based telecoms. So this actually is not an issue when we're talking about in-flight connectivity for 5G. Even though it's the same band, meaning high-speed internet service, it's really not the same thing and application from a technical perspective. So that is the good news. If you spend or your operator has spent a lot of money on the cabin side, this doesn't cause a problem for your avionics.”