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SFO's ILS isn't always functioning. It was turned off for months when Asiana crashed (June 1st - Aug 2nd)[1].

[1] http://www.popularmechanics.com/flight/a9124/what-went-wrong...

I'm pretty sure SFO always has functioning ILS, otherwise the airport would have to close every time there was even mildly bad weather. Each runway has its own system, though, so an individual runway's ILS can be turned off.

If there was fog, they wouldn't have been using 28L.

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