U.S. aviation authorities proposed rules that could require airlines to spend billions of dollars replacing or updating critical flight safety equipment to accommodate the sale of new wireless spectrum mandated in President Donald Trump’s signature tax and spending package.

The Federal Aviation Administration said the regulations, released on Monday, would require airlines to upgrade radio altimeters — instruments that measure a plane’s altitude — so that they are able to withstand interference from wireless signals across neighboring spectrum bands.

The FAA estimates “the total undiscounted cost” to retrofit planes with the new equipment will be $4.49 billion. The rules will undergo a public comment period and won’t be effective until the FAA releases a final rule.

The Federal Communications Commission said in its own proposed rule in November that it was considering ways to help cover the cost of the retrofits, including through proceeds from its auction.

The FAA’s proposal comes just a few years after the airline industry and wireless carriers, such as AT&T Inc. and Verizon Communications Inc., clashed over the expansion of new 5G services on a part of the spectrum, known as the lower C-Band. At the time, the airlines and the FAA said the 2022 rollout could disrupt radio altimeters, which are critical for safe flight.

The crisis threatened to upend U.S. air travel but ultimately a compromise was reached under which wireless providers agreed to temporarily limit power levels near major airports and the FAA required airlines to install new protective equipment.

Trump’s Big Beautiful Bill required the FCC to sell at least 100 megahertz of spectrum in the upper C-Band. Senator Maria Cantwell, the top Democrat on the Senate Committee on Commerce, Science, and Transportation, warned the upper C-band spectrum is even closer to the band used by altimeters and poses similar risks.

Under the law, the FCC would be required to auction off the critical portion of spectrum by 2027.

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