How Flying Works
How Autoland Lands a Plane in Zero Visibility
In the thickest fog an airliner is certified to land in, a pilot might not see the runway at all until the wheels are almost on it. At that point the autopilot isn’t helping with the landing. It is flying the whole approach, the flare and the touchdown, tracking radio beams more precisely than any human hand could while the pilots see nothing outside. This is autoland, and its lowest-visibility version, CAT IIIb. It’s a different way of flying from anything a pilot does by hand, and it’s built around one system: the ILS.
The two beams that make it possible
The Instrument Landing System (ILS) sends out two radio beams from equipment at the runway. The localizer marks the runway’s extended centreline: left-right guidance. The glideslope marks a precise 3-degree path down to the runway: up-down guidance. Together the two beams pin down one exact path through the air. An aircraft with both needles centred is exactly on the planned approach, in any visibility, because radio waves don’t care about fog. On a standard instrument approach, a human pilot hand-flies along those beams. Autoland goes further: the autopilot flies the same beams, continuously and automatically, all the way to the runway.
Try it
Follow the beams: an ILS approach in fog
Two radio beams guide the aircraft: the glideslope sets a steady 3° descent and the localiser lines it up with the centreline. Each category lowers the decision height, the point where the crew must see the runway or go around, and the minimum visibility (RVR). CAT IIIB: decision height below 50 ft or none, visibility down to about 50 m. In CAT III the autopilot flies all the way to touchdown, flaring at roughly 30 to 50 ft. Vertical scale exaggerated; ICAO category values.
The real numbers
CAT III visibility categories
CAT I
Visibility down to ~550m
Decision height ~200ft — still hand-flown by most crews
CAT IIIa
Visibility down to ~175-200m
Decision height under 100ft, or none
CAT IIIb
Visibility down to 50m (some operators 75m)
The lowest category in routine airline use
~30-50ft
Radar altimeter flare trigger
Roughly where autoland begins its flare (varies by aircraft type), measured directly
What actually happens in the final seconds
A radar altimeter measures height above the ground directly, by bouncing a signal off it rather than using air pressure. It tells the autoland system when the aircraft reaches the flare height, roughly 30 to 50 feet depending on the type. That’s when the system starts the flare, gently raising the nose to slow the descent for a smooth touchdown. A pilot does the same by feel. Here it’s worked out from precise, constant sensor data instead of a judgment made through a windscreen showing almost nothing. After touchdown, the system can keep using the localizer to hold the runway centreline until the crew disconnects the autopilot. One deliberate choice runs through the whole design. Once autoland has engaged and locked onto valid ILS signals, it carries on to landing with no further pilot input. Input in that final phase would bring back exactly the imprecision the system exists to remove.
Why the crew still matters completely
None of this replaces the pilots. Every CAT III autoland has a full crew actively monitoring the approach. They cross-check that the aircraft, the approach and the systems are all behaving as expected, ready to disconnect and go around at the first sign of trouble. The certification is three separate things that must all line up. The runway needs CAT III-certified ILS equipment. The aircraft needs autoland certification for that category. The airline and crew need their own approval to fly it. A capable aircraft at an uncertified airport can’t do it, and neither can an uncertified crew in a capable aircraft. Autoland is an engineering achievement built to remove the limits of human eyesight, the one real obstacle left in fog. It isn’t a step towards removing the humans.
Why fog delays flights even when autoland works
If aircraft can land themselves in fog, why do foggy mornings still cause delays? The answer is the radio beam.
The ILS beams that guide an autoland can be bent by large metal objects near the runway, including other aircraft. On a clear day that doesn’t matter, because pilots can see. In fog, the autopilot is relying on a clean beam all the way to touchdown.
So when visibility drops, air traffic control switches to low-visibility procedures. Aircraft waiting to take off hold further back from the runway. An arriving aircraft can only get landing clearance once the one ahead has landed and taxied far enough clear that it no longer disturbs the signal. In the UK, NATS says the spacing between arrivals can grow by up to 50%.
Fewer landings per hour means flights from elsewhere get held at their departure airports. Autoland makes fog landings possible. It doesn’t make them as frequent.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Does autoland mean no pilot is needed?
No. Every autoland has a full flight crew actively monitoring the approach, ready to take over by hand or go around at any sign of a fault. Once autoland is engaged and locked onto the ILS signals, it flies to touchdown without further pilot input. But “without input” doesn’t mean “without a crew”.
What stops a pilot landing by hand in thick fog?
Reaction time and what the eye can see, not skill. Below a certain visibility, a pilot can’t see the runway early enough or clearly enough to line up and flare safely. The autoland system tracks the ILS beams with a constant precision no human eye and hand could match in those conditions.
Can every airport and every aircraft do a CAT III autoland?
No, on both counts. The airport needs CAT III-certified ILS equipment on that particular runway. The aircraft, the crew and the airline’s operating licence must each be separately approved for CAT III. It’s a three-way certification, not a feature you just switch on.
What's the actual difference between CAT IIIa and CAT IIIb?
How poor the visibility can be. CAT IIIa allows landing in visibility down to about 175-200 metres. CAT IIIb goes lower, to as low as 50 metres under ICAO rules (airlines often use higher minima, around 75 metres, depending on approval and equipment): fog so thick that a pilot looking down the runway at touchdown may see little beyond the nose.
Last reviewed: October 2026. Facts that change, such as fleets, airspace and airline ownership, are re-checked on a schedule. Spotted an error? Email contact@skyplus.news and we’ll fix it. See how we check facts.