The 5G Airline Controversy: What Is It About?

It could be an issue. But it will probably be resolved soon. What to know the next time you hear it come up.

A Southwest flight passing directly over a cell phone tower, on its approach to landing at Pensacola airport last year. (Photo by John Herron.)

This post is a basic who-what-why primer on the controversy involving new 5G wireless networks, and airline operations at major U.S. airports. It’s not meant to be conclusive but instead an introduction, with links to more detailed discussions.

Update January 21 10am ET: Please see additional links, new information, and answers to some reader queries at bottom of this post.

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Short version: 5G versus the airlines is potentially a real issue, rather than a bogus threat. But it’s likely that the parties involved will work out adjustments soon. Which is a good thing.

Now, the longer version, Q-and-A style:

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Could cell phones really affect airline safety?

The new ones, yes.

(And to be clear, I’m referring not to individual phones but to the transmission systems and new broadcast towers that enable very high-speed 5G data speeds.)

We’ve all become numbed to routine, never-enforced warnings to turn off cell phones on takeoff and landing aboard airlines. They have seemed like security theater, and in practical terms the goal might have been mainly to pry people’s attention away from their phones during important phases of a flight.

The new 5G networks, which were activated yesterday, are different. That is because the part of the broadcast spectrum they use is closer-than-comfortable to a part used by a specific aviation device, called a radio altimeter. (Also radar altimeter or radalt. They all refer to the same thing; I’ll use radio.) The chart below, from a technical report that’s been the basis of recent controversy, introduces the overall idea.

The central question is: could transmissions and emissions on the newly authorized 5G part of the spectrum overlap and interfere with the signals that an airliner’s radio altimeter relies on, for safe guidance of a plane? Especially if the transmission towers are directly along the landing paths that aircraft follow?

The aviation world says: maybe they could, so let’s be careful before taking the risk.

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What is a ‘radio altimeter’?

If you’ve seen an airplane cockpit in photos or in real life, you have seen a regular (or “barometric”) altimeter. It’s the device that looks like a clock dial, with a hand indicating the plane’s altitude. In the shot below, of a pre-GPS-era cockpit, it’s the dial at top center (showing an altitude of 700 feet).

The kind of instruments you’d find in most Cessnas, Pipers, Beechcraft, etc before the GPS era. The altimeter (top center), attitude indicator (top left), and vertical-speed indicator (bottom center) on current planes still look more or less the same. (Getty Images)

A radio altimeter works on an entirely different principle. A barometric altimeter gauges the plane’s altitude by comparing air pressure outside the plane (which goes down, as the plane goes up) with sea-level pressure. It’s indispensable but is not a high-precision instrument.

A radio altimeter gives much more exact, moment-by-moment readings of the plane’s distance from the ground. It does so by transmitting signals downward and measuring how long it takes them to bounce back. Let’s say the Denver airport has an elevation of 5,500 feet. If a plane were 500 feet up, on final approach to land, the barometric altimeter would show 6,000 feet above sea level. The radio altimeter would say 500 feet above the ground, and it would keep ticking off the exact distance as the plane glided down.¹

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Why does a radio altimeter matter?

In clear weather, a flight crew could get a plane safely on the ground even if both kinds of altimeters had failed. You wouldn’t want that, but pilots are trained to use countless visual cues to judge their height above the runway and the path down to a landing. Nearly every airport big enough to handle airline flights has runways with visual “glideslope indicators” — a combination of red and white lights to show continuous guidance on whether you’re high, low, or on the right vertical path down.

It’s different in bad weather. Everything complicated in aviation involves guiding planes from takeoff to touchdown if the pilots can’t see where they are going.²

A big step in modern, high-volume, all-weather airline travel is equipping airplanes to land safely even if fog or clouds block the pilots’ view. That is where the radio altimeters crucially come in. With their very accurate second-by-second, foot-by-foot measurements of the plane’s distance above the runway, they can in principle allow a plane to “land itself.”

The aviation term for this kind of procedure is a “Cat III ILS landing.” The YouTube video below, by a Boeing 777 pilot named Juan Browne, gives a professional pilot’s view of the whole situation. The first two minutes distill what these landings are like, and the role a radio altimeter plays. In essence, the radalt is how the plane knows how far away the ground is, and how it can safely touch down.

As the Juan Browne video develops at length, the concern is that 5G transmitters near airports could block, scramble, or distort the radio altimeter signals. This could, in turn, interfere with operations where radio altimeters are crucial—namely, approaches in low visibility and bad weather.

And it could matter more broadly. John Herron, a former naval aviator who is now an airline pilot, sent me an email today on the larger importance of radio altimeter signals:

It’s difficult for lay people to grasp the importance of radio altimeters to modern commercial aircraft. Many think, what’s the big deal? After all, the pilots have a barometric altimeter too?

Well, the big deal is this gem of an instrument gives pilots the real precise distance from the ground at the precise moment in time, and has been incorporated into so many systems that have been developed and evolved to improve safety for everyone - the pilots, travelers and those who reside below. This is not just an issue of low visibility approaches. 

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What, exactly, is the airlines’ concern about 5G?

It is that the new ground-station transmitters for 5G are so much more powerful than the radio-altimeter transmitters aboard airplanes, and have been located so close to major runways and on approach paths, that interference is possible. And that it’s better to recognize this problem before a close call (or worse), rather than after.

In the meantime, “better safe than sorry” flight cancellations have ripple effects through an air-travel industry that has relentlessly boiled away excess capacity and slack. A disruption anywhere becomes congestion everywhere.

Below you see an illustration from a 2020 report by the technical group RTCA on possibilities for 5G interference with aviation. The red and yellow lines are an aircraft’s approach path to Runway 27L at O’Hare airport. The blue markers are cell phone transmission stations. The point of the illustration is how closely they match up.

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Isn’t this all about bureaucracy, or business?

There’s a huge range of institutional, economic, and cultural background themes entangled here. If this led to a mishap—which, again, I think is unlikely—someone would do a gripping documentary or investigation about the personalities, incentives, and practices that led to tragedy. (As Peter Robison did with his excellent recent book on the 737 Max tragedies, Flying Blind.)

The FCC is promoting rapid spread of 5G and has collected billions in its spectrum auctions. The telecom companies obviously have bet their future on this technology. The aviation industry has an understandable bias toward caution in changing equipment, standards, or practices. Airlines and aircraft companies would naturally prefer to postpone spending billions on updating their own equipment. The FAA has been chastened by its experience with the 737 MAX. As John Herron also wrote today:

First, we must recognize the agencies battling out, their respective histories and purpose, and current culture…

The FAA serves to promote aviation safety and at the same time promote airline travel and commerce. Human beings travel a lot now. But the FAA is coming off the B737 MAX debacle, and a needed culture shift has occurred and there’s no going back. Too many people needlessly died. 

Until recently, Verizon and AT&T treated aviation risks from C-band interference with radalts the same way they treated home value impacts from 5G cell poles interfering with neighborhood aesthetics

These are all different organizations, with different incentives and imperatives, and different interests to protect and advance. That’s life in businesses and bureaucracies. But, better late than never, a standoff among them appears to have been averted this week.

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What comes next? [And see update below]

I know better than to make predictions. But here are a set of informed sources to follow:

• The FAA’s own site, which is informative and detailed;

• Several updated YouTube videos by Juan Browne;

• An op-ed by John Herron in Aviation Week;

• An update from the ever-informed journal The Air Current;

• Today’s story by Andrew Tangel and Ryan Tracy from the Wall Street Journal;

• A letter to the FCC this month from Rep. Peter DeFazio, about the issue.

Technology; safety; bureaucracy; commerce — always an engrossing mix. I’m enormously relieved that we can discuss their interaction before something has gone grievously wrong.

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Update on January 21, 10:15am ET.

Here are a few more links readers have pointed out, and replies to some questions that have come in.

• New link: Today Peter Coy has a column in the NYT going into the conflict of bureaucratic and organizational interests that led to the standoff.

• New link: From Michael Koziol in IEEE Spectrum, a detailed technical look at the spectrum issues involved, and an explanation of why Verizon and AT&T are having these airport problems, but not T-Mobile.

• New link: Another airline pilot’s perspective, from Patrick Smith at the always informative “Ask the Pilot” site.

• Question: Why isn’t this happening in Europe?
  A combination of differences — power settings, antenna placement, airport buffer zones, spectrum allocations — appear to make the larger difference.
  For instance, the FAA’s “5G and Aviation Safety” site has this chart, comparing U.S. and French standards:
  

  

  And the IEEE Spectrum article says this:

  “In Europe, for example, 5G midband rollouts have proceeded without much concern for radio altimeters, because the spectrum allocated is at just slightly lower frequencies (3.4–3.8 GHz in Europe, as opposed to the mentioned 3.7–3.98 GHz in the United States). Meanwhile, countries like Canada have installed buffer zones like the ones AT&T and Verizon have agreed to. The Australian Communications and Media Authority has said it believes that a 200-MHz guard band (like the one in the United States) between 5G networks and radio altimeters is sufficient in itself.”
  Also see this story by Charles Riley and Joseph Ataman in CNN Business on the U.S./European contrast.
  Might this week’s U.S. showdown eventually be seen as a case of the FAA giving in to panicky over-reaction, or other bureaucratic impulses? I don’t know. But, as John Herron pointed out, through its history the FAA has emphasized “what could possibly go wrong??” thinking about any technological changes. A bias toward worst-case-scenario thinking is part of the reason air travel has become so exceptionally safe. And in the wake of the 737 MAX disasters, no one wants to be the subject of the next “were regulators asleep at the wheel?” investigation.

• Question: What’s the short-term answer?
  AT&T and Verizon have agreed to temporary delays in activating their 5G systems near the airports the FAA was most concerned about. The FAA in turn has lifted or amended its emergency orders banning certain instrument approaches at those airports. (You can see a list of the affected airports in a PDF downloadable from this FAA page. A day-by-day update on the aircraft types and airports that have been cleared for operation is at the very bottom of this page.)
  My guess is that for the foreseeable future, all affected parties—FAA, airlines, FCC, wireless companies—will agree on some limitations on 5G towers right around airports that all involved can live with. And eventually more evidence will come in about whether, how, and to what degree the 5G systems really are a problem for today’s planes. Wireless companies and airlines can adapt to what that evidence shows.

• Question: What’s the long-term answer? Ultimately there will be a fleetwide upgrade of avionics, notably including radio altimeters, to modern versions that are much better shielded against off-frequency interference. As many articles have pointed out: 5G technology is new, and a lot of aviation technology is very old, because of the extreme regulatory caution of the industry as a whole. (The airplanes I trained in during the 1990s, were built in the 1970s, and were designed in the 1950s. I still see those very same airplanes being used at the same airport on training flights these days.) But a complete turnover in equipment will take time—for regulatory approval, among other reasons—and a lot of money.
  The FAA has already cleared the radio altimeters used in most of the US commercial fleet for operations even in 5G areas. As it announced just yesterday:
  “The FAA issued new approvals Thursday that allow an estimated 78 percent of the U.S. commercial fleet to perform low-visibility landings at airports where wireless companies deployed 5G C-band. This now includes some regional jets….
  “The FAA is working diligently to determine which altimeters are reliable and accurate where 5G is deployed in the United States. We anticipate some altimeters will be too susceptible to 5G interference. To preserve safety, aircraft with those altimeters will be prohibited from performing low-visibility landings where 5G is deployed because the altimeter could provide inaccurate information.”

If and as new information arrives, I’ll do a separate follow-up post. Thanks for many readers for sending questions and leads.

1. A barometric altimeter works even when the plane is at high altitudes. The radio altimeter starts working when it gets within about 2,500 feet of the ground.

2. As an aside: getting comfortable with landing a plane is the part of learning to fly that resembles learning to ride a bike. But as with bicycling, the assumption is that once you’ve grasped the “sight picture” for the descent path, and how to manage the “flare” just before touchdown, it should be a routine skill. The premise of “instrument approaches” in aviation is to provide vertical-and-horizontal guidance until you get far enough below the clouds that you can see the runway. From that point on, the hardest part of the flight is supposed to be done. What passengers experience as the difference between a “rough” and “smooth” landing generally comes down to the final inches of descent.