How Flying Works
How Autoland Lands a Plane in Zero Visibility
In the thickest category of 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 assisting the landing — it is flying the entire approach, the flare, and the touchdown itself, tracking radio beams with a precision no human hand could match while looking at nothing. This is autoland, and specifically the lowest-visibility version of it, CAT IIIb, and it is a genuinely different mode of flying than anything a pilot does by hand, built entirely around one instrument system: the ILS.
The two beams that make it possible
The Instrument Landing System, ILS, transmits two separate radio beams from equipment at the runway. The localizer marks the extended centerline of the runway — lateral guidance, left-right. The glideslope marks a precise 3-degree descent path down to the runway — vertical guidance, up-down. Where those two beams intersect at any given moment defines one exact point in three-dimensional space, and an aircraft tracking both needles centered is, by definition, exactly on that intended approach path, in any visibility at all, because radio waves don’t care about fog. A standard instrument approach uses those beams as guidance for a human pilot to hand-fly toward. Autoland goes further: it lets the autopilot fly those exact same beams, continuously and automatically, all the way to the runway.
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 ~75m
The lowest category in routine airline use
~50ft
Radar altimeter flare trigger
Precisely where autoland begins its flare, measured directly
What actually happens in the final seconds
A radar altimeter, which measures height above the ground directly by bouncing a signal off it rather than relying on barometric pressure, tells the autoland system precisely when the aircraft reaches roughly 50 feet — the point where it initiates the flare, gently raising the nose to slow the rate of descent for a smooth touchdown, exactly as a pilot would by feel, except calculated from continuous, precise sensor data rather than a hand-flown judgment call made through a windscreen showing almost nothing. After touchdown, the system can continue using the localizer signal for lateral control down the runway centerline until the crew disengages the autopilot manually. One deliberate design choice runs through the whole system: once autoland has engaged and acquired valid ILS signals, it proceeds to landing without further pilot input required — input during that final phase would introduce exactly the kind of imprecision the entire system exists to remove.
Why the crew still matters completely
None of this replaces the pilots. Every CAT III autoland is flown with a full crew actively monitoring the approach, cross-checking that the aircraft, the approach, and the systems are all behaving exactly as expected, and ready to disconnect and go around immediately at the first sign anything is wrong. And the certification is three separate, independent things that all have to line up: the specific runway needs CAT III-certified ILS ground equipment, the specific aircraft needs autoland certification for that category, and the specific airline and crew need separate operational approval to fly it — a capable aircraft at an uncertified airport, or an uncertified crew in a capable aircraft, still can't do it. Autoland is a genuine engineering achievement built entirely around removing human visual limitation from an environment where visual limitation is the only real obstacle left — not a step toward removing the humans themselves.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Does autoland mean no pilot is needed?
No — a full flight crew is required for every autoland, actively monitoring the approach and ready to take manual control or go around at any sign of a fault. Once autoland is engaged and has acquired the ILS signals, it flies to touchdown without further pilot input, but "without input" is not the same as "without a crew."
What actually stops a pilot from landing manually in thick fog?
Human reaction time and visual reference, not skill. Below a certain visibility, a pilot simply cannot see the runway environment early enough or precisely enough to align and flare safely — the autoland system tracks the ILS beams with a level of continuous precision no human eye and hand could match under 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 specific runway, and the aircraft, the specific crew, and the airline's operating certificate all need to be separately qualified for CAT III operations — it's a three-way certification, not a feature that's simply switched on.
What's the actual difference between CAT IIIa and CAT IIIb?
Visibility minimums. CAT IIIa allows landing in visibility as low as roughly 175-200 meters; CAT IIIb pushes that down to as little as about 75 meters — thick enough fog that a pilot looking straight down the runway at touchdown still might not see much beyond the nose of the aircraft.