Mark Huber is an aviation industry veteran who has worked in the flight training, aircraft sales, aircraft component, and airport development businesses. He has flight time in more than 50 different and diverse aircraft, including helicopters, aerobatic aircraft, vintage warbirds, jet fighters, business jets and turboprops, and transport-category aircraft. He has jumped out of several of these—on purpose—with a parachute. His defense and aviation journalism has been published in a variety of national and international periodicals.

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Mark Huber

This summer the spectrum wars have continued between the wireless and aviation industries in the U.S. Initially, stakeholders tried to tackle the issue of 5G C-band interference on radio altimeters (radalts). Now the field of play shifts to the potential of the Ligado Wi-Fi system to disrupt GPS/GNSS satellite navigation on the L-band.
After delays triggered by aviation industry concerns over 5G C-band interference on aircraft radalts and cellular companies’ temporary agreement to restrict a wider roll-out of the service and/or the full power of certain transmitters, the FAA-imposed July 1 deadline came for aircraft to either be equipped with interference-tolerant radalts or refrain from operations susceptible to 5G interference (under Airworthiness Directive 2023-10-02). That includes low-visibility approaches. Air carriers not equipped with interference-tolerant radalts will not be allowed to operate in U.S. airspace beginning in 2024.
Restrictions had been previously in place via an ever-expanding list of notams only in areas of proven 5G C-band interference via an earlier AD, 2021-23-13. Issued in late May, AD 2023-10-02 supersedes that earlier action. The date of the restrictions reflects that on or after July 1 up to 21 wireless companies were expected to “power up” their 5G C-band cellphone network transmitters.
The impact of all of this on business aviation was immediate. OEMs issued advisories for aircraft flight manuals (AFMs) to be modified in accordance with the new restrictions. Typical was Gulfstream, which advised customers via a maintenance and operations letter issued on May 31: “As no Gulfstream aircraft are considered radio-­altimeter compliant at this time, operators will be required to comply [with the restrictions] on or before June 30, 2023.”
A separate AD governing helicopters (AD 23-11-07) could have a profound impact on that industry. Those limitations prohibit using radalt minimums for offshore approaches and instead using barometric limitations; using autopilot hover mode that requires radalt data; engaging search and rescue autopilot modes that use radalt data; and performing any takeoffs or landings in accordance with any procedure in the RFM or OpsSpecs with radalt data. Helicopter air ambulance operators using night vision goggles are exempt (under Exemption&nbsp; Number 18973B).
Representing the airlines, the International Air Transport Association (IATA) previously noted that the July 1 deadline was unrealistic, given ongoing supply-chain issues, and said the FAA had wildly underestimated the cost of compliance, which the trade association pegged at $638 million. The FAA had estimated $26 million. For rotorcraft, the Helicopter Association International (HAI) estimated that installing new filters to modify existing radalts can cost up to $10,040 per helicopter and a new radalt carries a price of up to $40,000.
While some cell carriers have voluntarily agreed to signal-strength mitigation efforts around some airports through 2028, even that may not be enough to reduce travel disruptions, IATA warned, noting that the technological fixes to comply with the July 1 deadline were merely a stop-gap solution, while a new standard is being developed for radar altimeters and is due next year.&nbsp;
IATA senior v-p Nick Careen classified the July 1 requirement as a “temporary holding action,” noting that, “under current scenarios, airlines will have to retrofit most of their aircraft twice in just five years. And with the standards for the second retrofit yet to be developed, we could easily be facing the same supply-chain issues in 2028 that we are struggling with today.”
While the FAA is continuing to study the problem, it appears to be shifting much of the burden related to information generation and gathering aimed at resolving the continuing issue over 5G C-band radio altimeter interference to avionics and aircraft OEMs, aircraft operators, and a civilian contractor to the agency. In a special airworthiness information bulletin (SAIB) issued May 29, the FAA requested that manufacturers of radio altimeters and aircraft, as well as operators, continue to voluntarily provide “specific information” related to those altimeters’ design, functionality, and usage; continue to test the equipment in cooperation with federal authorities; and report results to both civil aviation authorities and spectrum regulators.
The FAA said it is continuing to collaborate with the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA) on the issue as it assesses options beyond its related ADs. The agency called the requirements interim actions pending the development of a “Technical Standard Order for radio altimeters…that meets international minimum operational performance standards.”

Information requested in the SAIB is detailed, potentially voluminous, and in many cases likely proprietary. The FAA further asks that all retrofit plans and details be forwarded to the Mitre Corporation—the FAA’s chief civilian technical contractor for collection, aggregation, and de-identification. It places a heavy burden on avionics and aircraft manufacturers in particular for equipment and procedure testing and analysis, updating aircraft flight manuals, reporting anomalies, and developing retrofit plans. Part 121 operators are requested to incorporate the potential for 5G C-band interference into their safety management systems and flight planning and coordinate information on interference with avionics and aircraft manufacturers.
The SAIB also notes that “operators should consider the potential loss of pilot trust in dependent aircraft safety systems in the assessment of existing, and the development of new, crew procedures.” Pilots are encouraged to report all radio altimeter anomalies to air traffic control and submit detailed incident reports to the FAA.
Pilots began doing so last year and AIN obtained more than 500 pages of those reports earlier this year via a Freedom of Information Act request. The concern over “loss of pilot trust” in key systems looks real in those reports as suspect 5G interference seems to degrade or disrupt function of key safety and aircraft control systems in all categories of aircraft, particularly regional jets, business jets, and helicopters.
A sampling of the reports reveals that interference triggered alarms and otherwise had the potential to distract pilots during critical phases of flight, especially low-­visibility operations. Malfunctioning aircraft systems included traffic and terrain alerting systems, windshear detection systems, and head-up displays. In addition, reports pointed to delayed deployment of autospoilers, erroneous master warning and caution alarms, muting of aural automated altitude call-outs on approach, and false altitude readings that at times fluctuated wildly from reality. For rotorcraft, the problem was even more critical, with some operators reporting that the impact of interference forced them to terminate missions.
The following typify the reports contained in the AIN FOIA request:
On Jan. 21, 2022, while at 200 agl on short final to Runway 16R at Midland, Texas (KMAF), a Bombardier Challenger 300 got an erroneous aural call for “40” (feet agl), and then the digital and moving tape readout for the radalt vanished on both primary flight displays. At 100 agl, the digital RA display reappears with an erroneous readout of “30,” then “15,” then “40” before vanishing again, reappearing after touchdown rolling at 60 knots. The crew noted, “We are supposed to get aural calls of ‘50, 40, 30, 20, 10,’ but did not. No voice calls and no RA [radalt] displays. After touchdown, the autospoilers are supposed to deploy immediately but did not due to no RA input/failure.”
On June 1, 2022, an Embraer E175 regional jet taking off from Sacramento, California (KSMF) Runway 35R reported erratic radalt fluctuations while climbing at 2,000 fpm through 1,000 feet with readings of between 0 and 2,500 agl with accompanying “bank angle” and “terrain” audible warnings.
On Sept. 25, 2022, an Airbus Helicopters EC135T2+ departing Indian Head, Maryland (2W5) on a medevac flight experienced an RA that displayed “zero altitude” shortly after takeoff while the aircraft was at 1,200 feet agl. As a result, the flight was canceled and the helicopter returned to base
The FAA’s approach to the 5G C-band radalt problem is not being emulated in at least two key aviation markets—Australia and Canada. Australian authorities are focusing on continued 5G C-band restrictions around airports and other critical areas until at least 2026 to give operators time to either modify or replace interference susceptible radalts. But even after that deadline, technical measures will be taken by local wireless carriers to mitigate potential interference including the establishment of a permanent 200 MHz “buffer zone” between wireless broadband and radio altimeter frequencies and limits on C-band power and emissions.
However, Australian aircraft operators continue to report a broad spectrum of 5G C-band interference problems including uncommanded autopilot disengagement, spurious altitude indications, erroneous warnings, muting altitude aural call-outs on approach, and problems with the autoland system.

GPS issues too
Now comes the potential problem with GPS. Canada’s decision in June to deny Ligado (formerly LightSquared) use of frequencies in the L-Band spectrum for its 5G network is fueling hopes in the U.S. that the FCC may reverse its controversial 2020 approval to allow it in the U.S. Aviation groups including NBAA, HAI, and the Aircraft Owners and Pilots Association have opposed the use of portions of the L-Band for wireless communication because of its potential for interference with global positioning system (GPS) and other global navigation satellite system (GNSS) technology. As recently as April 2023, these aviation groups, along with 90 other organizations, sent a letter to President Biden urging revocation of the FCC’s “Ligado Order,” citing interference evidence from a 2022 study by the National Academies of Sciences, Engineering, and Medicine (NAS).
Ed Bolen, NBAA’s president and CEO, said the collective evidence mandates that the FCC revoke the Ligado decision. “With numerous studies now reaffirming our industries’ concerns about dangerous interference to these vital systems from the Ligado network, it’s long past time for the FCC to do the right thing and rethink this ill-considered decision,” he said.
The Biden Administration opposed the order via a petition for reconsideration issued by the NTIA. The opposition groups maintain that the order “is fundamentally flawed, incompatible with the FCC’s rules, and inadequate in protecting incumbent services from the harmful interference from Ligado’s proposed operations.” The groups note that the order also is opposed by the Department of Defense (DOD) and a bipartisan group of federal legislators.
Due to national security considerations alone, the groups maintain that the order should be stayed, noting the NAS study showed that “the terrestrial network authorized by [the Ligado Order] will create unacceptable harmful interference for DOD missions.”
NAS’s 2022 study found that Ligado would likely trigger “harmful interference to mobile satellite service” provided by Iridium. The U.S. DOD is Iridium’s largest customer and employs high-precision GPS receivers, which would be the most vulnerable to L-band interference from the Ligado system. The committee that prepared the NAS report concluded that interference mitigation could be problematic due to test, certification, and time constraint issues, and overall costs. However, it found that Ligado would not impact most commercially available GPS systems.
In denying Ligado’s application in Canada, Canada’s Innovation, Science, and Economic Development agency noted concerns from aviation OEMs and groups, in particular the ancillary terrestrial component (ATC) impact on aircraft safety systems, such as helicopter terrain awareness warning systems below 300 feet agl.thamultiple Ligado ATC base stations could have related aviation impacts in ways yet to be fully understood. z

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