There are three categories of ILS approaches, namely I, II, and III :-). The difference between them is what we call the "minimums", which are conditions defining at what point of the approach the crew must have runway in sight, or go arround and take another chance or divert.
I won't explain the working or ILS here, but just give some ideas of the minimums and what is required to go there, and how it looks like.
Most airlines operate with ILS up to category II, which is a minimal ceiling of 100 ft above ground level (30 meters), and an horizontal visibility of 350 meters. I don't know if you realise what that means for a pilot to gain sight of where he will have to land only from 30 meters, and to land at a speed of 150 kts (about 300 km / h) in a visibility of 350 meters !
Category III is even more impressive, as it can be up to ... zero visibility. In this case, the landing is fully automatic, with two autopilots and a third computer managing the power automatically, plus a radio altimeter measuring accurately the vertical distance.
There are a lot of conditions associated to Cat II / III operations, including:
- Crew certification
- Plane certification
- ILS being powered with backup local generators to ensure no interruptions
- No one within the ILS shelter
- No visitor in the cockpit
- No plane / vehicle in close vicinity of the runway to ensure top quality ILS signals
God bless the ILS and autopilots.
But then you will ask... how can fog create so huge delays then ?
This is mostly because all airliners are not operating Cat III, and sometimes fog creates conditions that does not allow for Cat II landings... but the main reason is ground operations then.
To give you an idea of what fog is, what is seen on this picture is not problematic at all. A plane at the category II minimas would gain sight of runway approximately 30 meters above ground, where the approach lights seen on this picture are red !
It is really hard for pilots and ground controllers to manage proper taxi operations within fog. Final approach and landing along the ILS is easy, but there is no guidance other than visual for taxiing, so how would you taxi if you can't see more that 200 meters or even less what's come in front of your 400 tons plane ?
I remind being the tour guide for visitors once in a control tower on a foggy day... they were quite disapointed as the runway was not visible from tower because the fog was so dense. But they could testify that the ground radar was working properly.
