The Federal Aviation Administration (FAA) suspects new 5G cellular networks may have caused roughly 80 instances of aircraft system interference this year, with pilots reporting a range of malfunctions since the latest generation of mobile connectivity went live in January.

“The FAA has received several hundred reports of possible 5G interference and, as of mid-September, we have been unable to rule out 5G in approximately 80 cases,” the FAA tells FlightGlobal.

“None of these resulted in safety-related effects, and none affected a direct aircraft control input such as autothrottle or speed brakes/spoilers.”

But the revelation of the reports comes as the aerospace industry has in recent weeks asked the US Federal Communications Commission (FCC) to require cellular provides to take steps to prevent 5G signals from interfering with radio altimeters.

Southwest Airlines Boeing 737

Source: Southwest Airlines

The FAA says  it has not conclusively determined that 5G interference was responsible for the 80 issues reported, but it is assuming so for the purpose of risk analysis.

It adds that aviation is safe thanks to steps taken to mitigate interference, noting cellular companies have deployed 5G in areas nationwide using tens of thousands of antennae without serious flight problems. The cellular industry also insists 5G is safe for aircraft.

The FAA was responding to an inquiry from FlightGlobal relating to nearly 90 flight incident reports filed in the USA this year by pilots who cited “5G” as a possible cause.

Collected by the government’s Aviation Safety Reporting System (ASRS), the reports list about 50 cases of radio altimeter problems. Others detail failures of altimeter-dependent avionics and cockpit systems, and many involved multiple system malfunctions. Numerous incidents occurred at low altitudes during critical phases of flight. By comparison, pilots reported just three radio altimeter failures in all of 2021.

The ASRS reports do not draw conclusions about actual causes, meaning other factors could be to blame. But they suggest 5G has caused some troubling aircraft system failures.

“Captain reported navigation systems malfunctions due to suspected 5G interference [that] led to an altitude overshoot during departure,” a pilot noted of a May incident.

“Distraction resulted in climbing to 10,300ft, before correcting to 10,000ft during intermediate level off,” the pilot wrote. “The issue may have been corrupt radio altimeter data in the aircraft sensors which caused aircraft to think it was still on the ground.”

IEEE Spectrum, a publication of the Institute of Electrical and Electronics Engineers, reported such incidents on 13 October.

Another pilot wrote of receiving a “40ft call-out followed by additional alerts” while at 300ft during approach to Salt Lake City in June. “Maintenance indicated that other crews had experienced the same problem and that it may be caused by 5G interference.”

The cellular industry notes that 5G services have been active for years in Europe and elsewhere without problems. In late 2021, trade group CTIA, which represents the sector, accused the aerospace industry of “fearmongering”.

“FCC rules pertaining to operating 5G in the C-band have been shown to provide the necessary protection for aviation operations, and there have been no known safety-related impacts,” CTIA tells FlightGlobal.

The cellular industry is collaborating with the FAA, the FCC and others “to continue to ensure that C-band 5G and air traffic safely coexist”, the group adds. The FCC did not respond to a request for comment.

Installation of a

Source: AT&T

A worker installs a 5G antenna

Cell providers have much at stake, having invested billions of dollars in their 5G networks, which use advanced components and transmit at higher frequencies than previous generations, providing faster data transfer and more capacity.

US firms only secured the required bandwidth in 2021 after bidding $81 billion for access to the 3700-3980MHz range through an FCC auction. They can initially transmit at up to 3800MHz, gaining access up to 3980MHz in late 2023. Radio altimeters transmit in a very similar range, at around 4200-4400MHz.

The FCC said a 220MHz “guard band” between the cellular and altimeter spectrums would “protect” altimeters but still encouraged the aviation industry to study risks.

AT&T and Verizon were first out of the gate, firing up 5G on 19 January.

“The 5G antennas that are starting to come out have higher gain and more power… in a band that is next door to the radio altimeter band,” says Shawn Carpenter, electromagnetic programme director at engineering simulations company Ansys, which studies 5G-altimeter interference.

He also says cellular companies increasingly employ “beam-spotlighting” and other technologies to focus transmissions at users. If a passenger flips on their phone during landing, “you would have the potential where the base station would try to put a beam right on that aircraft”, Carpenter adds.

The issue is complex because interference can vary as aircraft pitch and roll, ascend and descend.

Concern within the aviation industry initially appeared muted. But a “5G Task Force” formed by radio-standards group RTCA analysed risks and raised alarm in an October 2020 report.

5G poses a “major risk” of “harmful interference to radar altimeters on all types of civil aircraft”, it concluded. “This risk is widespread and has the potential for broad impacts to aviation operations… including the possibility of catastrophic failures leading to multiple fatalities, in the absence of appropriate mitigation.”

Many pilots think they have experienced such scenarios.

“While at [a Tampa] gate, the captain’s radar altimeter fluctuated from approximately -90ft to 400ft. Multiple call outs were observed such as ‘retard’ and various altitudes,” a pilot said of a January incident. “I’ve been flying an aircraft with a radar altimeter for years, and never once have I seen a malfunction such as this until the 5G turn-on. Coincidence? Probably not.”

“Possible 5G event,” another pilot reported of a February incident. “At 2500ft… and again at 900ft, the [Captain’s] radar altimeter become inoperative with a red flag… In the flare I noticed a resistance to pitching up. It almost felt like the autopilot was still engaged.”

Late last year, in the weeks leading up to 5G’s launch, the FAA and aerospace industry seemed to be scrambling to catch up. In November 2021, the FAA warned of interference, asking aerospace manufacturers to study the issue. In December it essentially prohibited aircraft – including large jets – from using some aircraft systems near 5G antennae pending further action.

Meanwhile, the Department of Transportation (DoT) and FAA began negotiating with AT&T and Verizon, which then agreed to delay 5G’s start by 30 days until 5 January and to reduce the power of upward 5G transmission and transmissions near airports.

“We have a case where the bureaucracy couldn’t keep up.. to fully understand what was going on,” says Ansys’ Carpenter.

Still concerned, on 31 December – five days before 5G’s planned activation – the DoT warned air travel could still be wildly disrupted. AT&T and Verizon caved again, delaying 5G’s start until 19 January and agreeing to create “C-Band radio exclusion zones” near 50 airports.

The companies called interference “utterly unfounded”, accusing the Aerospace Industries Association (AIA) of seeking to force the telecoms industry to fund altimeter upgrades, and blaming the FAA for dragging its feet. The AIA and Air Line Pilots Association International (ALPA), however, insist they did raise concerns, which the FCC failed to address.

The FAA says the 5G mitigations lessened, but did not eliminate, risks at the 50 airports. So it began identifying which aircraft had sufficiently robust altimeters to safely perform low-visibility landings at those fields, eventually clearing most US airliners, with regional jets and Boeing DC-9-family aircraft being the exceptions.

But even for cleared aircraft, the FAA required airlines to adopt 5G-specific dispatch and landing procedures, saying faulty altimeter data can corrupt autothrottles, autopilots, flight controls, traffic alert and collision avoidance systems, aircraft configuration warnings, and ground proximity warning systems.

The agency insists such measures have kept air travel safe. But the ARSA reports catalogue troubling incidents.

“Second time this happened today in two separate aircraft at two separate airports. While on final approach, just about at the final approach fix, the autothrottles disengaged. Concerned about possible 5G issues,” a pilot said of a January 2022 event.

“After rotation from Runway 7L at [Phoenix], the captain’s radio altimeter appeared to be frozen at the normal ‘on the ground’ indication,” another pilot said of a June flight. “The preselected pitch and roll modes did not engage automatically on climb out.”

“I feel it’s important to make these events known… for the safe implementation of the 5G network,” the pilot wrote.

The reports include instances of faulty landing gear alerts and erroneous “too low – terrain” and “pull up” warnings. Speed brakes and thrust reversers failed to deploy. Engines unexpectedly spooled up during approach. Pilots disconnected auto-systems to fly manually, with some reporting that the distraction caused them to deviate from air traffic control instructions.

“I went heads-down to report the occurrences to the company… and failed to recognise we climbed through [18,000ft] and failed to complete the after take-off checklist,” a pilot said of a January incident. “Most importantly, failed to reset the altimeters to 29.92. We levelled at [37,000ft].”

The issue remains far from settled. More cellular firms are poised to launch 5G, and in late 2023 they gain access to the 3800-3980MHz range – closer still to the band used by altimeters.

In recent months the FAA has continued negotiating, saying in June that cellular companies agreed to maintain “some level of voluntary mitigations” for another year, through 5 July 2023. By that time, it said, most aircraft should be fitted with altimeter “filters” or new altimeters.

And in early October, AIA, ALPA, and other aerospace bodies asked the FCC to pass rules limiting above-horizon 5G transmissions and caps on “spurious emissions”. Last week, Reuters reported the FAA had made a similar request.