The US Federal Aviation Administration (FAA) reportedly prepared to issue a warning stating 5G transmissions in the C-Band may interfere with flight safety, projecting flight delays and cancellations could result from operators’ upcoming launches.

A report by The Wall Street Journal revealed the FAA is drafting guidance to explain how 5G in the C-Band spectrum may impact the performance of radar altimeters on aircraft. The agency projected airline schedules will be impacted if pilots do not use cockpit systems relying on the altitude equipment.

Radar altimeters use spectrum in the 4.2GHz to 4.4GHz band. In the US, mobile operators are expected to use C-Band n77 spanning 3.3GHz to 4.2GHz, with the Federal Communications Commission (FCC) allocating blocks in the 3.7GHz to 3.98GHz following an auction earlier this year.

Satellite companies are in the process of clearing the spectrum for mobile operators, with 5G services set to begin in 46 US markets on 6 December.

The US aviation industry expressed concern before the C-Band auction. Early last month, FAA deputy administrator Bradley Mims wrote about “deep concern” regarding potential interference, Reuters reported adding the FCC argued its track record demonstrated a continuing commitment to aviation safety.

Outside the US, several operators operate C-Band 5G networks.