How Do Planes Land in Fog and Bad Weather?

One of the most common questions people ask about aviation is deceptively simple: how does a plane land when the pilot can't see the runway? Dense fog, low cloud, driving rain, and blizzards all create conditions where visual approaches are impossible. Yet aircraft land safely in these conditions every single day. Here's how.

The Short Answer

Modern aircraft don't rely on the pilot seeing the runway to land. They use a combination of:

Ground-based radio navigation systems that beam precise guidance signals toward the aircraft
On-board computers that interpret those signals and can fly the approach automatically
Strict operational procedures that define exactly how and when different equipment can be used

In the most extreme cases — think zero-visibility fog at major airports — aircraft can land with the autopilot still engaged and the runway only becoming visible at the very last moment, or not at all until after touchdown.

The ILS: The Workhorse of Instrument Approaches

The Instrument Landing System (ILS) has been the global standard for precision approaches since the 1950s. Almost every major runway in the world has one.

How It Works

An ILS system consists of two radio transmitters near the runway:

The Localiser is aligned precisely with the runway centreline and broadcasts a signal that tells the aircraft (and autopilot) whether it's too far left or right.

The Glide Path (or Glide Slope) transmitter, typically located about 300 metres from the threshold, broadcasts a signal that defines the correct descent angle — usually 3 degrees for most airports, though London City uses 5.5 degrees.

The two signals together create an invisible "funnel" in the sky leading directly to the touchdown zone. The aircraft's instruments (and autopilot) follow this funnel continuously, making small corrections left-right and up-down to stay perfectly centred.

ILS Categories

Not all ILS systems are equal. They're classified into categories that define how low and how far the pilot needs to see before committing to land:

CAT I — The most common category. The aircraft must have the runway in sight before reaching 200 feet above the runway (the "Decision Height"). Visibility must be at least 550 metres (or runway visual range of 800m). Used at virtually every commercial airport in the world.

CAT II — For worse conditions. Decision Height is between 100 and 200 feet. Visibility requirement drops to 300 metres RVR (Runway Visual Range). Requires more sophisticated aircraft equipment and additional crew training.

CAT III — For very low visibility. This is where it gets impressive.

CAT IIIa — Decision Height of 50–100 feet, RVR as low as 200 metres. The pilot may see the runway approach lights just before landing.
CAT IIIb — Decision Height as low as 15 feet (or no decision height at all), RVR as low as 50 metres. In IIIb conditions, the entire approach and touchdown can be completed by the autopilot with the runway barely visible from the cockpit.
CAT IIIc — No decision height, no visibility minimum. Fully automatic landing even in zero visibility. This is theoretically certified but rarely approved in practice because taxiing after landing becomes the limiting problem.

Autoland: When the Autopilot Lands the Plane

In CAT IIIb conditions, the autopilot doesn't just assist — it flies the entire approach and lands the aircraft. This is called autoland or automatic landing.

For an autoland to work:

The aircraft must be certified for Cat III operations — this requires multiple redundant autopilot channels that cross-check each other. If any channel fails, the system disengages safely.
The ILS must be Cat III certified — with higher-power transmitters, more precise alignment, and protection zones around the equipment.
The runway must have special lighting — Approach Lighting Systems (ALS), centreline lights, and touchdown zone lights that remain visible even at very low RVR.
The crew must be trained for Cat III operations — additional simulator training is required.

During an autoland, both autopilots engage (on aircraft like the Boeing 777 and Airbus A380, three autopilot channels engage simultaneously for redundancy). The aircraft flies the ILS, flares automatically, cuts thrust, and touches down on the runway — all without pilot input on the controls.

The pilot's job during autoland is to monitor the systems, be ready to go-around if anything goes wrong, and take control after touchdown to steer and brake.

Other Navigation Systems Used in Bad Weather

RNAV / RNP Approaches

GPS-based approaches (using RNAV — Area Navigation — or the more precise RNP — Required Navigation Performance) are increasingly common. These use GPS and the aircraft's flight management system to fly very precise curved paths to the runway without needing ground-based transmitters.

RNP approaches can achieve accuracies similar to CAT I ILS — useful at airports where it's impractical to install traditional ILS equipment.

GLS (GBAS Landing System)

A newer ground-based augmentation system that uses a single VHF transmitter to broadcast GPS corrections to multiple runways simultaneously. More flexible than ILS but still being rolled out at major airports.

VOR and NDB Approaches

Older, less precise approaches using VOR (VHF Omnidirectional Range) beacons or NDB (Non-Directional Beacons). These are non-precision approaches — they provide lateral guidance but not a precise glide path. Decision heights are much higher (typically 400–600 feet), requiring better visibility. Being phased out at major airports.

What About Crosswinds?

Fog and low visibility are one challenge. Crosswinds are another entirely.

Every aircraft has a demonstrated crosswind component — the maximum crosswind at which it has been test-flown. For large commercial jets, this is typically around 30–38 knots.

Landing in a crosswind requires a crab approach (the aircraft flies at an angle to the runway to counter the wind drift, then straightens up just before touchdown) or a sideslip (banked into wind with opposite rudder applied). Both techniques are taught thoroughly and practised in simulators.

In very strong crosswinds, ATC may switch to a different runway, delay aircraft, or divert them to alternates where conditions are better.

Windshear and Microbursts

Windshear — a sudden change in wind speed or direction — is one of the most dangerous weather phenomena for aircraft on approach. It can cause sudden loss of lift or unexpected increases in descent rate.

Modern airports use LLWAS (Low Level Wind Shear Alert Systems) — networks of anemometers around the airport that detect wind variations. Pilots receive wind shear alerts from ATC and can also receive warnings from their own GPWS (Ground Proximity Warning System) or EGPWS (Enhanced GPWS).

If a windshear warning is received on approach, the correct procedure is always a go-around — full power, pitch up, climb away and reassess.

Winter Operations: Ice and Snow

De-icing Before Departure

Aircraft cannot take off with ice on wings. Ice disrupts airflow, reduces lift, and adds weight. Before departure in icy conditions, aircraft are sprayed with:

Type I de-icing fluid — glycol-based, orange coloured, removes existing ice
Type IV anti-icing fluid — thicker, green coloured, prevents ice from forming again (has a hold-over time after which it must be reapplied)

The process is carefully timed — there's a "holdover time" after which the fluid loses effectiveness and the aircraft must be re-treated or return for more.

Contaminated Runways

Snow and ice on runways dramatically reduce braking effectiveness. Airports measure runway friction coefficients and report them to crews. Pilots adjust approach speeds and landing distances accordingly. Most large airports have significant snow clearing equipment — Heathrow's snow plan, for example, involves dozens of vehicles and years of detailed contingency planning.

Seeing the Results on Your Tracking App

When conditions are bad at a major airport, you'll often see the effects clearly on What Plane? and similar tracking apps:

Stack holding patterns — aircraft circling for extended periods as the flow rate drops
Long final approach queues — aircraft spaced further apart in low visibility (the ILS CAT III separation minima are larger)
Diversions — aircraft destined for one airport landing at an alternate

If you see an aircraft that looks like it's heading for one airport but landing at another, or flying an unusual circling path, weather operations are often the reason.

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