U.S. Secretary of Transportation Pete Buttigieg warned that air travel this summer could face delays tied to airlines failing to update altimeter equipment in some aircraft so they do not interfere with rapidly expanding 5G networks.

“There’s a real risk of delays or cancellations,” Buttigieg said in an interview, according to a new report from The Wall Street Journal. “This represents one of the biggest — probably the biggest — foreseeable problem affecting performance this summer.”

The issue is tied to wireless operators using C-band spectrum to boost their 5G network capacity and coverage, which is close to spectrum bands used in some airplane altimeters. The C-band spectrum falls in the 3.7 GHz-3.98 GHz bands while the radar altimeters use spectrum in the 4.2 GHz-4.4 GHz bands.

While there are channels in between those bands, older radar equipment sometimes operates outside of its assigned bands, which means operators beaming 5G signals near the top-end of the C-band could potentially interfere with the older radar equipment. A similar issue impacted Ligado Networks’ ability to use spectrum it controlled near the 1.5 GHz spectrum band due to potential interference with older GPS equipment.

U.S. operators spent more than $80 billion to gain access to the C-band spectrum licenses to power their 5G network deployments, and tens-of-billions more to infuse that spectrum into their networks. But the full use of that spectrum has so far been limited near airports due to interference concerns. The operators had been working with the Federal Communications Commission (FCC) and Federal Aviation Authority (FAA) to gain full access to those assets, which initially included competing proposals that mostly resulted in contentious language and accusations exchanged between the differing parties.

A resolution was achieved in late March when the four largest U.S. operators — Verizon, AT&T, T-Mobile US and U.S. Cellular — filed a letter with the FCC stating they would limit the propagation of their cell towers transmitting C-band signals near many airports. This included 188 larger airports and military bases around the country.

“As C-band licensees, we will abide by the attached voluntary commitments,” representatives from the operators wrote in the filing. “These voluntary commitments will support full-power deployments across C-band and are crafted to minimize the operational impact on our C-band operations.”

The government filing noted the operators would implement power-reduction capabilities at their radio access network (RAN) towers beaming C-band spectrum near airports. They will also coordinate with the FAA and airport officials on those power limitations for initial deployments, with ongoing monitoring to allow for increased power output where possible.

The filing also stated that those power limits would start to be relaxed beginning July 1.

Buttigieg, according to The Wall Street Journal report, noted that more than 80% of domestic commercial airplanes and 65% of international aircraft that fly to the United States have radar altimeters that won’t interfere with the 5G signals.

Delta Airlines said up to 190 of its more than 900 aircraft will not have the updated altimeters by July 1, citing supply chain issues. It plans to re-route those impacted aircraft to routes that will eliminate possible interference issues.

JetBlue provided similar commentary to The Wall Street Journal.

United Airlines, Southwest and Air India all said their planes were in compliance, while American Airlines said its aircraft would be updated by the July 1 deadline.