Plans to deploy 5G wireless broadband services in a portion of radio spectrum known as the C-band are on hold over concerns it could interfere with the instruments aircraft use to measure altitude.

Amid months of regulatory confusion, the Federal Aviation Administration is requesting public comments on new airworthiness directives it has issued prohibiting airplane and helicopter flight operations that use radio altimeter data in the presence of 5G C-band interference. The directives warn that flights to low-visibility areas with a 5G presence are unsafe and could be canceled or diverted because a report by a technical association suggested interference from 5G telecommunications systems could cause altimeters to become inoperable or produce misleading data.

The directives are among a series of recent statements by the FAA that have disrupted long-standing plans by the Federal Communications Commission and wireless carriers to transition the 3.7–3.98 GHz portion of C-band radio spectrum from satellite television to 5G services, starting on December 5. AT&T and Verizon have now voluntarily delayed deploying 5G in the band until January and have pledged to limit some 5G services in the new band for six months in an attempt to appease the FAA and allow for further study of potential altimeter complications.

The FAA’s late interruption of the FCC’s plans has caused consternation among many stakeholders and observers. In mid-December, six former FCC chiefs wrote to the FCC’s current leadership detailing their concerns over the FAA’s eleventh-hour efforts to challenge the deployment. “The FAA position threatens to derail the reasoned conclusions reached by the FCC after years of technical analysis and study,” said the group, which included Ajit Pai and Tom Wheeler, FCC chairmen under the Trump and Obama administrations, respectively.

Taking the FAA’s side in the dispute were Reps. Peter DeFazio (D-OR) and Rick Larsen (D-WA), who sent a letter of their own to the FCC, urging the agency to work with the FAA to provide any requested data and ensure that deployment is halted until after the FAA concludes that potential risks are safely mitigated. The lawmakers blasted what they called a “deploy now, fix later” approach by the FCC, implicitly accused the agency of not prioritizing safety.

The expansion of available 5G bands has been in the works since 2017, and the official decision to open up the 3.7–3.98 GHz band dates to February 2020, leading to an auction earlier this year to license portions of the band to wireless carriers. Then, despite the years of advance planning, the FAA released a bulletin in early November voicing concerns over the 5G deployment and requesting additional materials to review the risk of interference for radio altimeters.

Wireless carriers hope that the 3.7–3.98 GHz band will boost speeds and increase range compared to the existing GHz and millimeter-wave spectrum. Although the C-band extends from 3.7 to 4.2 GHz, radio altimeters operate from 4.2 to 4.4 GHz, so a 220 MHz buffer was put in place to guard against interference.

The FAA is expected to release more details and specific restrictions prior to the January 5 deployment of the C-band, according to The Wall Street Journal.