The Federal Aviation Administration is proposing a solution to the long-running dust-up between the airline and cellular industries over 5G rollouts: stricter requirements for radio altimeters in airplanes, which would potentially go into effect early next year (via Bloomberg).

The piece of safety equipment, which ties into several systems for most commercial jets, operates using radio frequencies that are similar to the 5G C-band radio waves that Verizon and AT&T have been rolling out after acquiring licenses to use the bands for billions of dollars. C-band is almost critical for 5G to live up to all the hype; it’s what allows carriers to offer incredible speeds without the minuscule range of mmWave.

In theory, this shouldn’t be a problem. The altimeters and cell towers aren’t sharing spectrum; the radio waves they’re using are just in the same neighborhood — the altimeters should just ignore the 5G signals. In practice, though, the FAA has found that doesn’t always happen, potentially causing major issues now that there’s a lot more going on in those frequencies.

In its notice of proposed rulemaking set to be released on the 11th, the FAA cites around 100 reported “altimeter anomalies” where it couldn’t rule out 5G interference as the culprit. The incidents resulted in things like several types of erroneous warnings in the cockpit and incorrect altimeter data being displayed — the types of things that could be very bad if pilots were relying on those instruments during a low- or no-visibility landing.

The FAA says the warnings “increase flightcrew workload” as they try to figure out if they’re just caused by equipment acting up and that repeated false alarms could “lead to flightcrew desensitization to warnings from these safety systems.” This would likely lead to a “catastrophic incident” in the future, according to the regulator, as the 5G towers aren’t just a temporary thing that will be going away anytime soon.

These concerns aren’t new; in 2021, AT&T and Verizon delayed their C-Band rollouts, and 2022 saw the carriers going back and forth with regulators and airlines, trying to come up with a solution that pleased everyone. There were buffer zones around airports, more delays, and several mandates from the FAA that specified how planes had to act at airports with 5G. Much of the onus to act, however, has been on the cell carriers — they had to change plans about where and how they could roll out their 5G tech. And when deadlines loomed, the airlines warned that C-band being activated could cause “catastrophic disruption.”

Last summer, after months of negotiating and even congressional hearings, the FAA said that AT&T and Verizon would be able to fully roll out their C-Band networks by July 2023. While it seemed like a breakthrough at the time — it looked like everyone had started to work together — there were definitely still questions left around how it would work in practice and whether the steps already taken would be enough.

Now that the deadline is approaching, the FAA seems to want to push airlines and other aircraft operators to also change their equipment. It’s apparently determined that additional mitigations will be needed for some aircraft, which, in certain cases, could mean installing filters on existing altimeters that will block out C-Band signals before they reach the sensor itself. In others, it’ll mean replacing the altimeter entirely.

The FAA estimates around 820 planes would need filters, while 180 would need new altimeters altogether. According to its calculations, that would cost around $26 million, though the document doesn’t seem to specifically say who’ll be on the hook for that. Given that the FAA is soliciting comments on the rule, it’ll probably hear from airlines, carriers, and anyone else involved with opinions on who should have to pay.

If the rule passes as-is, the deadline for making sure airplanes have compliant equipment will be February 1st, 2024. Until then, the FAA has proposed putting restrictions on planes with altimeters that can’t properly filter 5G — starting July 1st, 2023, the rule says there are “certain operations” those planes won’t be able to carry out at airports with C-Band coverage.

If the rule is adopted, it won’t be the final step (as has been the case with many, many other actions throughout the saga). The regulator says it’s working on developing a Technical Standard Order for altimeters going forward and that the standard could change things down the line.