How Do Planes Land in Fog? The Ground Systems Engineer’s Explanation

You’re looking out the airplane window and seeing absolutely nothing. Not the runway, not the ground, not even the wingtip. Just gray soup. And yet the flight attendant is cheerfully announcing that you’ll be landing in ten minutes.

Most passengers assume the pilot is either incredibly brave or the autopilot is doing some magic. Here’s what’s really happening: there’s a radio beam projecting up from the ground that’s creating an invisible pathway through the fog, and your plane is following it down with remarkable precision.

Understanding how planes land in fog is crucial for appreciating the technology that enables these safe landings.

I maintain those ground systems. Let me show you how they work.

When I tell people I’m an FAA electronics engineer who works on navigation aids, they usually assume I’m talking about something in the cockpit. They picture the autopilot doing all the work, or they imagine pilots using radar to “see through” the fog. Neither is quite right.

The most common misconception is that modern aircraft land themselves automatically in bad weather. And while that’s technically possible with the most advanced aircraft and airports, it’s not what’s happening on most foggy approaches. Even when the autopilot is flying the approach, it’s following instructions from my equipment on the ground, not making decisions on its own.

The second misconception is about what pilots can actually see. When airline passengers hear “low visibility conditions,” they picture pilots peering through light fog or rain, maybe squinting a bit but generally seeing where they’re going. In reality, during a true low-visibility approach, pilots might see absolutely nothing until they’re a few hundred feet above the runway, if that. Some approaches are certified down to zero visibility.

Here’s what passengers don’t realize: the decision to land in fog isn’t made in the cockpit. It was made years earlier when someone decided whether to install the right equipment on the ground.

This is what I call a black box—a system your life depends on that nobody has ever explained to you. Most travelers stay confused about this, not because they’re incapable of understanding, but because nobody’s ever told them. If you’re the kind of person who refuses to accept black boxes, who wants to actually understand what’s happening when you can’t see the ground, then keep reading. You’re about to cross over from confused to informed.

The Ground Systems You Never Hear About

Every landing, whether in perfect sunshine or zero visibility, follows the same basic principle: get the aircraft lined up with the runway centerline and descend at the correct angle until the wheels touch pavement: simple concept, tricky execution when you can’t see anything.

This is where the Instrument Landing System comes in. ILS is the workhorse system that’s been guiding aircraft through bad weather since the 1940s, and it’s still the standard at most airports worldwide. Think of ILS as creating an invisible three-dimensional pathway through the air. But instead of using light or radar, it uses two radio beams that intersect right at the point where pilots want to be.

My job is keeping those beams exactly where they’re supposed to be.

The ILS has two main components on the ground, and most passengers have walked right past both of them without knowing what they were looking at.

The localizer sits at the far end of the runway, past the departure end. It looks like a row of antenna poles, usually behind a fence in a restricted area. This system creates a radio beam that defines the runway centerline, extending outward for 18 miles or more. If the aircraft drifts left or right of this beam, the cockpit instruments tell the pilots (or autopilot) to correct back toward the centerline.

Here’s what makes this precise: the localizer isn’t just broadcasting a single signal. It’s actually transmitting two slightly different signals, one on the left side of the centerline and one on the right. When an aircraft is perfectly centered, it receives equal amounts of both signals. Drift to one side, and the balance shifts. The cockpit equipment turns this into a simple needle on a display that shows whether to turn left or right.

The glideslope sits beside the runway, usually 750 to 1,250 feet down from the landing threshold. It’s a smaller antenna array, typically tucked away near the runway edge lighting. This system creates a beam that defines the proper descent angle, usually three degrees.

The glideslope works on the same principle as the localizer, just rotated 90 degrees. It transmits two signals: one above the glide path and one below. Fly too high, and the cockpit instruments tell you to descend. Too low and they tell you to climb. The typical glideslope beam extends about ten miles from the runway.

When an aircraft is on both the localizer and the glideslope, it’s in what pilots call “the slot.” It’s perfectly positioned for landing, even if the pilots can’t see a thing outside the cockpit windows.

I work on the ground side now, but I spent years in military avionics working on autopilot systems, flight directors, and flight management systems. I’ve seen both sides of this equation, the aircraft systems and the ground infrastructure, and understanding how they work together is what makes low-visibility approaches possible.

Modern aircraft have a flight director, a display that shows pilots precisely which control inputs to make to stay on the localizer and glideslope. It’s like having an exact set of instructions: pitch up by 2 degrees, bank left by 1 degree, and add a 50-foot-per-minute descent rate. Many aircraft can couple the autopilot directly to the ILS signal, so the aircraft literally flies itself down the beam.

But even with the autopilot flying, pilots are actively monitoring the approach. They’re watching their airspeed, their altitude, their position relative to the ILS beams, and scanning their instruments for any sign that something isn’t right. The automation doesn’t replace the pilots; it allows them to focus on management and decision-making rather than hand-flying in conditions without external visual references.

The critical moment comes at what’s called decision height. This is the predetermined point at which pilots must decide: continue the landing or go around. The decision isn’t based on how they feel or whether they want to give it a try. It’s binary. Either they have the required visual references to continue, or they don’t.

For a basic approach, at 200 feet above the runway, pilots need to see one of several things: the approach lights, the threshold, the threshold markings, the runway end identifier lights, or the visual approach slope indicator. They don’t need to see the entire runway. They need enough visual reference to complete the landing safely.

If they don’t have that visual reference at decision height, they must immediately execute a missed approach procedure. There’s no “let’s descend another 50 feet to see if it gets better.” The rules are absolute, and they exist because the alternative is controlled flight into terrain.

Why Your Flight Diverts (And the Other One Doesn’t)

Not all ILS approaches are created equal. There are three categories, and the difference between them determines whether your flight diverts to another airport or lands in the fog.

Category I (CAT I) is the basic level. It allows pilots to descend to 200 feet above the runway before they need to see anything. If they can’t see the runway environment at 200 feet, they must abort the approach and either try again or go somewhere else. CAT I requires visibility of at least 1,800 feet down the runway, which is roughly a third of a mile.

Category II (CAT II) allows descents down to 100 feet above the runway with visibility as low as 1,200 feet. This requires more precise ground equipment, more capable aircraft systems, and specially trained and qualified pilots.

Category III (CAT III) is where things get serious. There are three subcategories here. CAT IIIa allows descents to 50 feet with 700-foot visibility. CAT IIIb drops to just 50 feet, with only 150 feet of visibility. CAT IIIc is zero visibility, zero decision height, and an actual automatic landing with no visual reference required at all.

Here’s the insider detail that matters: most airports in the United States only have CAT I ILS, if they have ILS at all. Only the busiest airports invest in CAT II or CAT III systems because the equipment is expensive and the maintenance requirements are substantial.

When you hear the pilot announce “we’re holding for fog to lift” or “we’re diverting to another airport,” it’s often because your destination airport doesn’t have the ground equipment to support lower visibility approaches. The aircraft might be competent, the pilots might be qualified, but if my equipment on the ground isn’t certified to CAT II or CAT III standards, the approach can’t be flown.

Here’s where it gets even more interesting. There’s another category most passengers have never heard of: Special Authorization, or SA CAT operations. This is how two planes can land at the same airport in the same weather while a third one diverts.

SA CAT I and SA CAT II allow lower minimums than the airport’s ground equipment would usually support—but only if the aircraft has enhanced equipment to compensate. An SA CAT I approach might get you down to 150 feet decision height at an airport that only has basic CAT I ground systems. SA CAT II can give you 100-foot minimums at airports without full CAT II lighting infrastructure, as long as the aircraft has autoland capability or a certified heads-up display.

The ground navigation signals I maintain still have to meet the same precision standards. What’s different is that the aircraft’s sophisticated systems compensate for reduced lighting and visual aids on the ground. So when you see one airline’s plane land while yours diverts to another airport, it might not be pilot skill or courage. It might be that their aircraft has autoland and SA CAT II authorization, and yours doesn’t.

Same fog. Same airport. Same ground equipment. Different capabilities in the cockpit.

Here’s the part nobody tells you. Those radio beams I mentioned? They can’t just drift around. They need to stay within incredibly tight tolerances, and it’s my job to make sure they do.

Every ILS installation is flight-checked regularly. An FAA aircraft equipped with precision test equipment flies the approach while measuring exactly where the localizer and glideslope beams are. The localizer centerline needs to be accurate to within 0.0175 degrees at the runway threshold. For context, that’s about one-fifth of the width of a runway at that point.

The glideslope is even more critical. The beam that defines the three-degree descent path must be accurate to within 0.075 degrees. If the glideslope is transmitting a beam that’s too high, aircraft following it will land long and potentially run off the far end of the runway. Too low and they’ll hit short of the runway, which is catastrophically worse.

When pilots report “ILS unreliable” or when we take a system out of service, it’s usually because something has shifted out of tolerance. Maybe a support pole on the localizer shifted slightly in soft ground. Maybe moisture got into a glideslope antenna, altering the signal characteristics. Perhaps a new building was constructed too close to the signal path, causing reflections.

I can’t see the radio beams. Nobody can. But I can measure them, analyze them, and adjust the equipment until they’re exactly where they need to be. That’s the job. I’ve climbed antenna towers in ice storms to fix a faulty component. I’ve troubleshot faults at 2 AM while aircraft held overhead waiting for the system to come back online. I’ve sat in equipment shelters running tests while planes land 100 feet above my head, following beams I’ve just calibrated.

Not every airport has ILS, and not every ILS is created equal. This frustrates passengers when their flight diverts for weather, but there are real reasons behind it.

Cost is the obvious factor. A basic CAT I ILS installation runs into the millions of dollars. CAT III systems cost significantly more because they require backup power systems, redundant equipment, specialized monitoring, and more complex maintenance programs. Small airports with limited airline traffic can’t justify the expense.

Terrain plays a role, too. Hills, buildings, or other obstacles near the approach path can disrupt ILS signals. Some airports are in locations where it’s simply not possible to site the equipment properly. The localizer needs a clear area behind the runway, and the glideslope needs to be positioned where its signal won’t be blocked or reflected by terrain.

Runway length matters as well. CAT II and CAT III approaches require longer runways because the calculations assume pilots might need to land from a no-flare, autopilot-flown touchdown. Shorter runways can’t accommodate the necessary safety margins.

When your flight diverts because of fog, it’s not that the airline doesn’t want to land at your destination. It’s that the infrastructure isn’t there to do it safely. The pilots and the aircraft might be fully capable of a CAT III approach, but if the destination airport only has CAT I equipment, that’s as low as they can legally go.

The next time you’re sitting on a foggy runway waiting for departure clearance, or you’re approaching an airport where you can’t see the ground, you’ll know what’s actually happening.

Those airline announcements about “low visibility procedures” aren’t just weather talk. They’re referring to specific protocols that change how air traffic control sequences arrivals, how much spacing is required between aircraft, and what minimum equipment must be operational. The announcement about a delay isn’t the airline being overly cautious; it’s the system adapting to reduced capacity when weather requires more precise procedures.

When the pilot announces “we’ve been cleared for the ILS approach,” they’re not just using aviation jargon. They’re telling you that air traffic control has confirmed the ground-based navigation system is operational and accurate, that the aircraft is appropriately equipped. The crew is qualified, and they’re about to fly a precisely defined path through the clouds to the runway.

And when you hear “we’re holding for weather” or “we’ll be diverting,” it’s not a failure of nerve or skill. It’s a recognition that the ground infrastructure at your destination isn’t capable of supporting approaches in the current conditions. The pilots would happily land if the equipment on the ground could safely guide them in.

Most passengers never consider the invisible infrastructure that makes all-weather operations possible. They assume it’s all in the aircraft or all about pilot skill. Both matter, but neither works without the invisible radio beams that create a precise pathway through the worst weather.

I’ve spent sixteen years as an FAA electronics engineer and seven years before that as a technician, maintaining and calibrating these systems. There’s a moment I think about sometimes. The fog was so thick you couldn’t see the runway from the control tower. Flights are diverting everywhere. And then a CAT III-equipped aircraft followed our signals down—trusting equipment I’d calibrated the week before—and touched down perfectly.

The passengers deplaned, annoyed because they’d circled for an hour. They had no idea what just happened. No idea that the system worked exactly as designed, that the invisible infrastructure did its job, that the reason they landed at all was precision equipment and the people who maintain it.

The next time you land in fog, you might not see the runway until the last few seconds before touchdown. That’s by design. The pilots aren’t hoping for the best or feeling their way down. They’re following a radio beam that I’ve verified precisely where it needs to be, with tolerances measured in hundredths of a degree.

You were never in danger. You were in the slot.

Ready to Cross Over?

You just learned what 99% of travelers will never understand—the invisible system that gets you safely through fog. The ground equipment. The tolerances. The categories. The real reason flights divert.

You’re not confused anymore.

This is what it means to be a Black Box Survivor. You refuse to accept systems you don’t understand. You want to know how things actually work, not just accept vague explanations.

Every Tuesday, I open another black box—the systems your life depends on, but nobody explains. Navigation aids. Airport infrastructure. The invisible technology that keeps aviation working.

Join the other Black Box Survivors who refuse to stay confused:

Or get the complete guide: “Why Your Plane Can’t Land” – Learn the real reasons behind weather delays, runway closures, and diverted flights from someone who maintains the ground equipment.

FAA Employee – All views are personal. Not official guidance. For official information, visit FAA.gov.