Modern airliners do not need to see the runway to land on it. The key is the Instrument Landing System, or ILS: ground-based radio transmitters at the runway that broadcast two invisible beams — the localizer for horizontal alignment with the centerline, and the glide slope for the correct descent angle. The aircraft's instruments follow these beams down through cloud and fog with far more precision than human eyes ever could.

For a manual landing, the rules are strict. Pilots must be able to see the runway at the decision altitude — typically around 200 feet above the ground — and the reported visibility must meet a minimum of roughly 550 meters. If the runway is not in sight at that point, the crew must execute a go-around: climbing away and trying again or diverting. It is this rule that causes so many weather cancellations when fog settles in.

Below those minimums, the crew switches to autoland. In a Category III approach, the autopilot flies the entire final approach, flare and touchdown without pilot input, guided by the ILS beams and refined by onboard sensors. Fleets are increasingly certified for this — even the next-generation aircraft flowing from Avolon's massive 250-jet order will arrive autoland-ready — which is why major hubs can keep operating in fog that would have grounded everything a generation ago.

The final seconds are the system's masterpiece. Around 200 feet the aircraft transitions from radio-beam guidance to its radar altimeter, which measures height above the ground to within a few feet. At about 50 feet the autopilot raises the nose into the flare to soften touchdown, and near 30 feet the autothrottle pulls the engines to idle. Triple-redundant flight computers cross-check every calculation; if any one disagrees, the pilots are alerted instantly to take over.

Pilots are anything but passengers during an autoland. They monitor the instruments, call out every altitude, keep their hands near the controls, and are ready to execute a manual go-around the moment anything looks wrong. After touchdown, the autopilot can even steer the rollout along the centerline using the rudder and nose-wheel steering — critical when the crew may not see the runway until the aircraft is already rolling on it.

The catch is infrastructure: autoland only works at runways and airports certified for Category III operations, and only on suitably equipped aircraft. Smaller regional airports and older runways cannot support it, which is why thick fog still diverts flights to alternates. During events like the October storm chaos over Istanbul, where low visibility and violent rain combined, some flights landed smoothly on autoland while others diverted — the difference was the airport's equipment and the crew's certification, not the weather alone.