United States mobile networks have already been rolling out 5G connectivity for years, but more recently, AT&T and Verizon have been excited about deploying 5G on the new C-Band spectrum. The C-Band rollouts have been continuously delayed due to concerns from the Federal Aviation Administration (FAA) and various airlines, and now global air travel has been thrown through a loop.

The C-band spectrum operates between around 3.7 and 4.2 GHz, offering a middle ground between the slow far-reaching bands (like those often used for LTE and sub-6GHz 5G) and fast short-range bands (like the ones utilized by mmWave 5G). AT&T and Verizon both won sections of the C-Band spectrum in an auction, and both carriers originally planned to start deploying 5G on C-Band by the end of 2021, potentially allowing them to have their own version of T-Mobile's mid-band 5G.

C-Band 5G could interfere with the radio-powered altimeter used in some planes.

The Federal Aviation Administration, which regulates civil aviation and manages all air traffic in the United States, is concerned that C-Band 5G could interfere with the radio-powered altimeter used in some planes. Altimeters are used to determine how far a plane is from the ground, so it's critical that they work correctly, especially near airports during take-off and landing.

The Department of Transportation (DoT) and the FAA sent a joint letter to AT&T and Verizon on December 31, asking them to delay the rollout of C-Band service at certain airports, until the FAA has verified each airport won't be affected by possible radio interference. The carriers agreed, but by January 16, the FAA had only cleared "an estimated 45 percent of the U.S. commercial fleet" to land at airports where C-Band 5G has been deployed. The FAA said, "the airplane models approved include some Boeing 737, 747, 757, 767, MD-10/-11 and Airbus A310, A319, A320, A321, A330 and A350 models."

Airlines are either switching passengers to different plans or cancelling flights.

AT&T and Verizon finally activated their C-Band service on Wednesday, and even though both networks are still working with the FAA to keep it off near select airports and runways, some airlines have cancelled flights anyway. Reuters reports that Emirates has stopped flights to Boston, Chicago, Dallas-Fort Worth, Houston, Miami, Newark, New Jersey, Orlando, San Francisco and Seattle, while service to Los Angeles, New York, and Washington are unaffected. Emirates primarily uses the Airbus A380 and Boeing 777 — neither of which were mentioned in the FAA's current list of approved aircraft. Korean Air Lines, Taiwan's China Airlines, Japan Airlines, and other airlines are either switching passengers to different plans or cancelling flights.

AT&T told XDA Developers in a statement, "At our sole discretion we have voluntarily agreed to temporarily defer turning on a limited number of towers around certain airport runways as we continue to work with the aviation industry and the FAA to provide further information about our 5G deployment, since they have not utilized the two years they’ve had to responsibly plan for this deployment. We are frustrated by the FAA’s inability to do what nearly 40 countries have done, which is to safely deploy 5G technology without disrupting aviation services, and we urge it do so in a timely manner. We are launching our advanced 5G services everywhere else as planned with the temporary exception of this limited number of towers."