January 5 is the day on which AT&T T +0.4% and Verizon plan to commence operation of 5G cellular services, using frequencies approved by the Federal Communications Commission in 2020.
However, the federal regulatory agency that oversees airlines has determined that cellular base stations generating the 5G signals near airports could impair radio altimeters used by pilots to determine how close they are to the ground.
The Federal Aviation Administration issued an "airworthiness directive" earlier this month requiring that commercial and commuter aircraft cease relying on radio altimeters when operating in the presence of 5G interference.