The Federal Aviation Administration is proposing a solution to the long-running spat between the aviation and mobile industries over 5G rollout: stricter requirements for aircraft radio altimeters, which may take effect early next year (through Bloomberg).

The piece of safety equipment, which connects to different systems for most commercial jets, operates at radio frequencies similar to the 5G C-band radio waves that Verizon and AT&T rolled out after acquiring billions of dollars in licenses to use the bands. C-band is almost crucial for 5G to live up to all the hype; it’s what allows carriers to offer incredible speeds without mmWave’s miniscule range.

In theory, this shouldn’t be a problem. The altimeters and transmission towers do not share a spectrum; the radio waves they use are just in the same neighborhood – the altimeters should just ignore the 5G signals. In practice, however, the FAA has found that this doesn’t always happen, which could potentially cause major problems now that there’s a lot more going on on those frequencies.

In his notification of proposed regulations set to release on the 11th, the FAA lists about 100 reported “altimeter anomalies” where 5G interference could not be ruled out as the culprit. The incidents resulted in things like various types of false cockpit warnings and incorrect altimeter data being displayed – the kind of thing that could be very bad if pilots relied on those instruments during a landing in low or no visibility.

The FAA says the alerts “increase the workload of the flight crew” as they try to determine whether they are caused solely by equipment that is working and that repeated false alarms “could lead to flight crew insensitivity to alerts from these safety systems.” This would likely lead to a “catastrophic incident” in the future, according to the regulator, as the 5G towers are not just something temporary that will soon disappear.

These concerns are not new; in 2021, AT&T and Verizon postponed their C-Band rollouts, and in 2022, the airlines went back and forth with regulators and airlines, trying to come up with a solution that satisfied everyone. There were buffer zones around airports, more delays, and several mandates from the FAA specifying how aircraft should behave at 5G-enabled airports. Much of the responsibility to act, however, lay with the mobile carriers – they had to change plans on where and how to roll out their 5G technology. And as deadlines approached, the airlines warned that activating the C-band could cause “catastrophic disruptions”.

Last summer, after months of negotiations and even congressional hearings, the FAA said AT&T and Verizon could fully roll out their C-Band networks by July 2023. Although it seemed like a breakthrough at the time, it seemed that everyone started working together — there were certainly still questions about how it would work in practice and whether the steps already taken would be enough.

As the deadline approaches, the FAA appears to be urging airlines and other aircraft operators to change their equipment as well. It has apparently been established that some aircraft require additional measures, which in certain cases could mean installing filters on existing altimeters that block C-Band signals before they reach the sensor itself. In other cases, this means replacing the altimeter completely.

The FAA estimates that about 820 aircraft would need filters, while 180 total would need new altimeters. According to the calculations, that would cost about $26 million, although the document doesn’t seem to say specifically who will be on the hook for that. As the FAA is seeking comment on the rule, it will likely hear from airlines, carriers and everyone else involved with opinions on who should pay.

If the rule passes unchanged, the deadline to ensure aircraft have compliant equipment is Feb. 1, 2024. Until then, the FAA has proposed placing restrictions on planes with altimeters that can’t properly filter 5G — starting July 1, 2023 , the rule says there are “certain operations” that those aircraft cannot perform at airports with C-Band coverage.

If the rule is adopted, it’s not the last step (as has been the case with many, many other actions throughout the saga). The regulator says it is developing a technical standard order for altimeters in the future and the standard may change things over time.