Experts Say Many Radalts May Need To Be Retrofitted

Boeing 787-10


Credit: Boeing

Aircraft manufacturers and operators face the likelihood that many of the radio altimeters currently installed on aircraft will have to be retrofitted with filters or replaced to protect against potential interference from 5G wireless transmissions, aviation technical experts say.

Under airworthiness directives (AD) the FAA issued in December 2021 for passenger-category airplanes and helicopters, pilots are prohibited from conducting certain approach and landing procedures that require altitude data from radio altimeters (radalts), also called radar altimeters, when in the presence of 5G C-band transmissions. The agency has published nearly 1,500 Notices to Air Missions (NOTAM) that identify airports, heliports and airspace in the U.S. where flight restrictions will apply when wireless carriers begin operating new 5G networks.

“At some point, there is likely going to be a need for many altimeters to be retrofitted or replaced to improve the performance and improve the compatibility with 5G interference to be able to get complete relief from these restrictions,” said Seth Frick, a Honeywell Aerospace radar systems engineer, during a webinar Jan. 13. “That’s something that, within the industry and particularly as an altimeter manufacturer at Honeywell, we have very much in mind and are going to be dedicating significant effort toward in the near term.”

After agreeing with the FAA to postpone their network starts twice, telecommunications companies AT&T and Verizon plan to introduce 5G C-band service beginning on Jan. 19. At the FAA’s request, they will implement 5G exclusion zones around 50 airports for six months.

Already, the FAA has served notice that it will require Boeing 787-8/9/10 flight crews “to take additional precautions when landing on wet or snowy runways” at airports where 5G C-band service is deployed. The directive affects 137 U.S.-registered airliners and 1,010 worldwide.

In a Continued Airworthiness Notification to the International Community on Jan. 14, the FAA said it has determined that “anomalies” on 787s due to 5G C-band interference may affect multiple systems that use radalt data, “regardless of the approach type or weather.” 

While a 787 is landing, the notification states, “this interference could prevent proper transition from AIR to GROUND mode, which may have multiple effects. As a result, lack of thrust reverser and speedbrake deployment and increased idle thrust may occur; and brakes may be the only means to slow the airplane. Therefore, the presence of 5G C-band interference can result in degraded deceleration performance, increased landing distance and runway excursion.”

AMOC Application Process

FAA graphic
The FAA says 5G interference mitigations in the U.S. differ from those in France. Credit: FAA

During the Jan. 13 webinar hosted by trade group Helicopter Association International, technical experts provided a glimpse into intensive, behind-the-scenes meetings involving the FAA and industry to develop alternative methods of compliance (AMOCs) to the ADs and site-specific NOTAMs that will allow operators to continue flying into airports and heliports where 5G restrictions apply.

The initial emphasis is on transport-category airplanes, Frick said. To obtain an AMOC for a specific aircraft type, radalt manufacturers will provide test data to aircraft manufacturers indicating the susceptibility of their models to 5G C-band interference. The data will be used to define an aircraft “protection radius,” which will be matched against 5G base-station locations around airports, provided by the telecom companies.

“They will take that protection radius for a given aircraft and draw a circle around all those 5G base station locations,” Frick explained. “Then they will look at airports and draw additional circles with a 2 nm radius centered at each runway threshold for all the runway ends at that airport, and that gives you this big bubble or area around an airport.” 

“When you apply for an AMOC and provide that protection radius information, the criterion is that if there is no intersection between the base station bubbles and that airport bubble for any of the base stations around an airport, then that aircraft is going to be cleared to operate into that airport without those AD restrictions applying.”

When an aircraft’s protection radius intersects with a base-station bubble around an airport, for that particular airport and particular aircraft, the AD restrictions will apply. When the FAA processes and approves AMOC applications from aircraft manufacturers, it will provide in return a list of airports with NOTAMs at which an aircraft will be able to operate without AD restrictions, and airports where AD restrictions will apply. One issue that has been flagged with this initial approach is that when a 5G base station falls with the 2 nm radius around an airport, “no AMOC is going to be able to clear this airport, no matter how small the [aircraft] protection radius is,” Frick said.

“This is the first step that’s been identified as a way to try to clear some of these restrictions for big swaths of aircraft fleets as quickly as possible, but it’s going to be a continuously evolving [process],” Frick said. “In particular, the altimeter performance across different models varies very drastically. So, these protection radius values can be anywhere from a few hundred feet or less, to tens of miles depending on the actual altimeter and how it performs against potential 5G interference signals. There’s going to be a very wide range of different potential impacts and different levels of relief that can be obtained through this process.”

Retrofit Or Replace?

Honeywell ALA-52B radar altimeter
A Honeywell ALA-52B radar altimeter. Credit: Honeywell Aerospace

Frick also serves as co-chairman of a special committee formed by standards organization RTCA, which is working collaboratively with the European Organization for Civil Aviation Equipment (EUROCAE) to update the underlying minimum operational performance standards (MOPS) used in manufacturing radalts, to ensure they are protected from 5G and other external radio frequency interference. 

The current MOPS the FAA references in its technical standard order (TSO) design authorization for radalts—EUROCAE ED-30—dates to 1980 and is little changed from the original specification developed in 1966, according to Frick. Developing a new set of underlying performance standards and having it accepted by the FAA and baked into an updated TSO that guides manufacturers in building 5G-hardened radalts will take years.

In parallel with formulating an AMOC strategy, aviation technical experts are considering what can be done to improve radalt performance on the current operational fleet of aircraft. That could involve retrofitting radalts with band-pass filters—devices that allow certain frequencies to pass through within a given range while rejecting other frequencies—or swapping them out for better-performing models.

But the radalt retrofit-or-replace solution is complicated. 

“That’s something that we’re already working on in earnest and trying to fast-track as much as possible, understanding that installing a filter is not as easy as just going and picking something up off-the-shelf and slapping it on your certified aircraft,” Frick said. “One, the filters themselves are custom parts built to spec. They’re going to have to be qualified. We’re going to have to have adequate demonstration that installing that filter does not degrade the performance of the altimeter system to a point where it is no longer compliant with its requirements.” 

“And then, finally, to actually install that you’re going to have to have some sort of updated approval at the aircraft level, whether that’s an STC [supplemental type certification] or updated field approval,” Frick added. “We’ve been working on all of those things in parallel. We’ve had discussions with the FAA about them as well. It’s likely still not going to happen as fast as a lot of people hope that it could.”

Nick Kefalas, a technology fellow with Lockheed Martin-Sikorsky Aircraft, concurred with Frick that retrofitting existing radalts will be a challenge.

“It’s not as simple as slapping on a band-pass filter and going out and recertifying the aircraft,” Kefalas said. “It’s a very difficult and expensive process for a manufacturer to get this data with the band-pass filter and then have to go back and recertify an aircraft. That means all of the existing fleet will have to be grounded for the duration of that [process] or would have to function under VMC [visual meteorological conditions] in these specific areas of exclusion until a resolution or a supplemental type certification is completed.”

Bill Carey

Based in Washington, D.C., Bill covers business aviation and advanced air mobility for Aviation Week Network. A former newspaper reporter, he has also covered the airline industry, military aviation, commercial space and unmanned aircraft systems. He is the author of 'Enter The Drones, The FAA and UAVs in America,' published in 2016.