US Transportation Secretary Pete Buttigieg said Friday there may be some airline delays or cancellations starting July 1st if the last remaining passenger jets haven’t upgraded their altimeters to deal with 5G interference, per a report from The Wall Street Journal report. The Federal Aviation Administration (FAA) maintains that 5G C-band signals could interfere with radio wave emitters passenger jets use to measure how far they are from the ground, which pilots rely on when landing with low visibility.

Though airlines aren’t actually required to get the new equipment in place until February 2024, those passenger jets that haven’t been certified for operation around C-band 5G signals by the first of July will not be allowed to land in certain low-visibility situations.

Most of the US domestic airline fleet is prepared, with over 80 percent of planes having been upgraded, but about 65 percent of international jets flying to the US still need fixing. The global airline group International Air Transport Association told WSJ that carriers would do their best to avoid disruptions, and that they were favoring aircraft with the required altimeters for flights to the US. Air India says all of its planes are so equipped.

Broadly, the article says airlines believe there will be little-to-no impact. In the US, most airlines say they expect to have their fleets fully upgraded by July 1st, though Delta Air Lines and JetBlue will both miss that date, with 190 planes and 17 planes outstanding respectively, says the WSJ report. The Airlines for America trade association blamed global supply chain issues for the difficulties hitting the deadline.

The Federal Communications Commission and wireless carriers AT&T and Verizon hit a wall with 5G expansion in 2021 when the FAA initially expressed its concerns about 5G C-band interference with aviation gear, kicking off a lengthy feud over where carriers could turn on their towers and with how much signal power.

The full expansion of the crucial band, which strikes a balance between the slow-yet-ubiquitous low-band 5G and the ultrafast-but-easily-stifled millimeter wave 5G, was initially paused until January 2022, but it saw further delays — first to July 5th, 2022 and then to July 1st of this year.

At the moment, the only flights that could face setbacks are those aboard planes that haven’t had the 5G-interference-busting equipment installed and would be landing in low-visibility circumstances. For instance, a JetBlue spokesman told WSJ there could be “limited impact” in Boston on low-visibility days beginning on July 1st.