The Federal Aviation Administration (FAA) this week began issuing specific, detailed warnings to pilots around the country about the threat that 5G operations in C-band spectrum might pose to their aircraft. According to Bloomberg, the effort represents one of the FAA's most sweeping sets of such notices in its history. The news outlet reported that the FAA has so far issued a total of 1,462 Notice to Air Missions (NOTAMs) at 50 airports around the country. Each NOTAM is directed at a particular location or type of aircraft or pilot. In general, they warn of how 5G operations in C-band spectrum might impact aircraft radio altimeters, which can be used during landings in low-visibility conditions.For example, one order at the Ronald Reagan Washington National Airport in Washington, DC, warns helicopter pilots that their altimeters can't be trusted under certain conditions. But, as with other US-government missives, the actual text of the notice is difficult for outsiders to understand: "POINT OF ORIGIN SFC-5000FT AGL. HEL OPS REQUIRING RDO ALTIMETER DATA TO INCLUDE OFFSHORE INSTRUMENT OPS, HOVER AUTOPILOT MODES, SAR AUTOPILOT MODES, AND CAT A/B/PERFORMANCE CLASS TKOF AND LDG NOT AUTHORIZED EXC FOR ACFT USING APPROVED ALTERNATIVE METHODS OF COMPLIANCE DUE TO 5G C-BAND INTERFERENCE PLUS SEE AIRWORTHINESS DIRECTIVE 2021-23-13."The notices cover all kinds of situations. For example, Bloomberg reported that pilots landing at Chicago's O'Hare International may be prohibited from using certain low-visibility instrument landing procedures unless aircraft manufacturers can show their equipment is safe. Similarly, pilots won't be able to use their instruments to land at the Mayo Clinic's St. Mary's Hospital in Rochester, Minnesota, when visibility is low.But, according to Reuters, other notices pave the way for GPS-guided lands at airports in Miami and Phoenix.The notices are based on information AT&T and Verizon have provided to the FAA about the location of their C-band transmission sites around US airports. Earlier this week, the FAA identified 50 airports around the country that could be affected by 5G in C-band spectrum.Verizon currently offers its Ultra Wideband 5G network in millimeter wave spectrum in select big cities. But the company plans to also apply the Ultra Wideband brand to its planned C-band network, which the company said will cover up to 100 million people in the US later this month. Click here for a larger version of this image. (Source: Verizon)The FAA said Wednesday it "expects to provide updates soon about the estimated percentage of commercial aircraft equipped with altimeters that can operate reliably and accurately in the 5G C-band environment."As noted by Bloomberg, aircraft and altimeter manufacturers can apply for an "alternative means of compliance" that could allow exemptions to some or all of the FAA's restrictions. The publication noted that some companies have begun making such applications, but so far the FAA hasn't approved any.The FAA "has made progress to safely reduce the risk of delays and cancellations as wireless companies share more data and manufacturer altimeter testing results arrive," the agency said in a release.High stakesThe agency's latest notices cap a remarkable, high-stakes public negotiation between several US government agencies and AT&T and Verizon. At issue are whether 5G operations in C-band spectrum will interfere with aircraft radio altimeters. On one side, the FCC, NTIA and a number of wireless companies argue that there won't be any interference. On the other side are airlines, the FAA and US Secretary of Transportation Pete Buttigieg, arguing that interference is possible and that 5G operations in the band should be delayed or curtailed.In response to those concerns, AT&T and Verizon agreed to a month delay in the launch of their 5G networks in C-band spectrum. However, they initially rejected requests for another two-week delay but quickly reversed themselves after last-minute urgings from top-level government officials in the White House. The issue even gained enough momentum to earn a statement from President Biden praising the latest 5G launch delay – to January 19 – and assuring those involved that there would be no more delays.Verizon, for its part, launched a major new promotional campaign around its C-band network launch, which is scheduled to occur sometime later this month. AT&T, meantime, has scheduled a media event for January 24.