An aircraft to land needs three things, Once the weather gets below basic VFR (visual flight rules) of 3000′ ceilings and 5 miles visibility,
- Qualified airfield
- Equipped aircraft
- Trained pilots
Each instrument-rated airfield has specific equipment that defines the approach and landing environment. This usually includes radio direction indicators, lighting, and visibility measurement gear (which rates how far one can see, translated as Runway Visual Range, or RVR, and measured in meters).
Some of the approaches can lead a pilot down from above 15,000′, while others are meant to be picked up at just a few thousand feet. Air traffic controllers are in constant conversation to the pilots, have them on radar, and are directing them to the airfield, either giving them specific instructions just for them, or telling them to follow an existing procedure that is published for the field. Some fields have approaches that are not rated below 200′ ceilings, while others don’t require any defined ceiling and can allow the right combination of aircraft and pilot to land with only 75 meters of visibility. (Ref-)
All aircraft today utilize the Instrument Landing System (ILS).
This is when pilots tune into a frequency for the runway they are landing at which will appear on various instruments in the cockpit. For lateral navigation to the runway, part of the ILS system includes the localizer, which basically tells the pilot if he/she is to the left or right of the runway. The second part to the ILS system is the glide slope, which is the vertical navigation to the runway. If the pilot follows the instrument, it should guide them down to the correct runway.
Most modern commercial aircraft, whether it’s a Boeing or an Airbus have auto land capabilities. The same principle is applied when using the ILS, except the autopilot can land the plane all by itself. Aircraft such as the Boeing 737 NG have the Heads Up Display (HUD), which can be used to land the airplane in bad conditions. Airlines such as Southwest will only have pilots use the HUD to manually fly the plane in fog conditions, not auto land. This all depends on how the airline wants pilots to operate their aircraft. ILS approaches are categorized into a few areas. The worst of it being a CATIIIc approach, then IIIb, IIIa, CATII, and CATI being the approach with more ideal conditions.
This is how all pilots make safe landings when visibility is less than ideal.
The landing also depends on the type of airplane. There are some airplanes that are certified to land when the pilot cannot see the runway until a couple of seconds before touchdown. These highly automated airplanes use redundant computers to fly the approach, flare the airplane, land and track the center line.
All airports have visibility requirements for takeoff. The required visibility depends on the type of runway lighting and obstacles. The normal minimum visibility for takeoff at a major airport is 600 feet visibility on the runway. With very specialized equipment and training it can be reduced a bit below that, but it is rare. If the fog impairs visibility below the necessary requirement, operations are affected. Otherwise pilots train to fly in low-visibility conditions and mitigate the problems caused by fog.