Domestic air travel is hurtling towards a crisis beginning January 5, and neither policymakers nor the public seem to grasp the gravity of the situation.
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.
The directive takes effect January 4—the day before AT&T and Verizon activate their 5G networks—and if nothing changes, domestic air travel could be crippled.
Radio altimeters play a critical role in enabling the safe operation of aircraft, especially when they are close to the ground but the ground cannot actually be seen by pilots.
Like, for example, in bad weather or after nightfall.
The airworthiness directive thus has the potential to disrupt up to half a million flights annually, forcing airlines to cancel or divert flights traveling to airports with nearby cellular towers when conditions dictate use of the altimeters.
An aviation expert explained to me that “if a radio altimeter is interfered with, it could result in bad data being propagated throughout the plane”—with potentially catastrophic consequences.
The problem can be minimized by replacing existing altimeters, changing 5G frequencies, or reducing the power output of cellular base stations.
Replacing altimeters on thousands of aircraft currently operating within U.S. airspace would take the better part of a decade.
Changing the frequencies of AT&T and Verizon’s 5G networks would be similarly difficult—although T-Mobile’s 5G offering operates on a different, non-threatening part of the radio-frequency spectrum.
So, the only near-term solution to avoid the disruptions entailed by the airworthiness directive would be to turn down the power on certain base stations.
The problem is less pronounced the farther aircraft are from the ground, because signal strength dissipates with distance.
Unfortunately, the biggest airports and most heavily used approach paths are located near the biggest concentrations of base stations, because that’s where the people are.
For instance, if you are traveling I-495 across the American Legion Bridge between Virginia and Maryland, a large cellular tower can be seen within a few hundred yards of the river.
When the river is visible, it is the preferred flight path to Reagan National Airport, and is also favored by rotorcraft including the president’s helicopter.
When the river is not visible, like at night or in a rainstorm, pilots still use the same path to the airport but must rely on their radio altimeters to maintain proper altitude.
Hundreds of commercial flights fly over that cellular tower during a typical week, but if it starts generating the power output planned for 5G signals, the airworthiness directive would kick in, severely restricting pilot options.
Planes might need to divert to other airports, however all of the nearby options have their own potential 5G problems, so there could be some lengthy detours for passengers.
One thing the FAA is determined will not happen is that pilots rely on spurious altimeter data when on final approach, which means steering clear of potentially disruptive 5G signals.
The directive allows for workarounds if pilots can avoid relying on potentially compromised altimeters, but the alternatives would increase cockpit workload during the most dangerous phase of flight (final approach).
The telecom industry insists that operating in the problematic portion of the radio-frequency spectrum is essential to fielding 5G services in a timely fashion, and argues that the Federal Communications Commission exhaustively reviewed spectrum options before settling on what is known as C-Band.
Dozens of other countries have done the same.
However, the FAA states in the official rationale for its directive that “no information has been presented that shows radio altimeters are not susceptible to interference caused by C-Band emissions permitted in the United States.”
AT&T and Verizon have offered to reduce the power levels generated by their 5G networks somewhat for a limited period of time, but the aviation community says that proposal is inadequate to meet the concerns driving the FAA directive.
A briefing generated by the aviation industry finds that “at least 70% of air transport aircraft would still be unable to safely operate under the wireless carriers’ proposal.”
So, a coalition of aviation stakeholders led by the Aerospace Industries Association has proposed bigger reductions in base-station power output for a longer period of time, until the FAA’s concerns can be fully addressed.
The telecom companies are resisting, saying that the power reductions would impair the functioning of 5G networks to a point where U.S. technology might not be competitive with that used in other countries (most notably China).
There doesn’t seem to be any easy way out of this impasse, and people on the telecom side of the debate are often dismissive of the FAA concerns; they say there is adequate separation between 5G and radio altimeter locations on the spectrum to prevent interference.
Maybe—but there’s a big difference between a dropped phone call and a dropped airplane, so don’t be surprised if your next flight to a major city ends up being diverting to avoid 5G disruption of vital flight systems.
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