How Planes Land in Fog: The ILS, Approach Categories and Autoland

What an instrument landing system is

An instrument landing system (ILS) uses radio transmitters at the runway. The localizer gives left-right guidance to the centreline. The glideslope gives up-down guidance along a descending path, normally 3°. In the cockpit, one needle shows left or right of the centreline, another above or below the path. The pilot keeps both centred by hand, or the autopilot follows the beams.

Why it works in cloud and fog

The beams let the crew descend in cloud without seeing the ground. An ILS approach normally has a decision height: the height above the runway by which the crew must see the required runway lights or markings. If they can, they continue; if not, they go around. Visibility along the runway is measured as runway visual range (RVR).

CAT I, CAT II and CAT III approaches

Approaches are grouped into categories. The lowest standard US values are: CAT I, decision height not lower than 200 ft (about 60 m) and RVR 2,400 ft (about 730 m), or 1,800 ft (about 550 m) with extra lighting or guidance equipment; CAT II, decision height 100 ft (about 30 m) and RVR 1,200 ft (about 370 m).

CAT III uses a decision height below 100 ft, or none. As of 2018, the lowest RVR the FAA had authorised for US operators was 300 ft (about 90 m). CAT II and III need special equipment and approval. Limits depend on the runway, aircraft and operator, and other countries set their own.

Autoland: can a plane land by itself?

Autoland is an automatic landing. The autopilot flies the approach, the flare (raising the nose just before touchdown) and the touchdown, and on some aircraft the rollout. The pilots monitor, ready to take over or go around. Crews also use it in good weather, for example to stay in practice.

It needs a certified aircraft that tolerates a single failure without a dangerous deviation, a suitable runway with a protected ILS signal, and a trained, current crew. So yes, a plane can land itself, but only because the crew chose it, set it up and watches it. If the beam is disturbed or systems fail, the crew takes over or goes around.