Bulgaria Ski discussion board

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Heavy fog has caused the cancellation of scores of flights at Sofia airport during the Christmas holidays, disrupting the travel plans of thousands of people.

Sofia airport's runway is outfitted with a category II instrument landing system, which allows landing when vertical visibility is 30 metres and horizontal visibility of at least 350 metres.

But during the day, the fog blanket cut horizontal visibility at the runway to 200 metres, which would require at least a category III A certification."

Avalanche:

Flight cancellations at Heathrow are different to the Sofia situation - at Heathrow the visibility was low but still good enough to permit landing.

Imagine driving your car in the fog - you slow down and leave a larger gap between yourself and the car in front of you. The same is true for aircraft on approach - they are spaced farther apart than usual as after landing it takes longer to find the taxiway exit and vacate the runway before the next aircraft can land as well as the tower controller working without being able to see the aircraft.

As Heathrow is at capacity a small change in procedures has a massive knock on effect on ops - if (for example) aircraft are usually spaced 3 miles apart and this is increased to 4 miles in fog, for every 4 aircraft landing in normal conditions you can now only handle 3 aircraft.

That leads to flight cancellations and therefore some departures cannot leave there is not enough capacity for every flight scheduled to land therefore cancellations occur.