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A few times, when flying into SFO, me and my fellow passengers were informed that due to foggy weather one of two parallel runways there is closed, causing delays. So, a few questions:

  • Why can only one runway be used during fog? During an instrument landing, if the instruments are precise enough to land the plane exactly in the middle of one runway, then surely they are precise enough to differentiate between two runways?
  • Is this standard practice in all airports or something specific to SFO? Is there some minimum distance between parallel runways above which it is safe to keep them both open in foggy weather?
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This might not answer your question, but you may find skybrary.aero/index.php/Parallel_Runway_Operation interesting. –  Greg Hewgill Apr 30 at 4:04

2 Answers 2

up vote 5 down vote accepted

While ILS is precise enough to guide the aircraft precisely onto the runway, it is only so precise in the immediate vicinity of the runway.

The instrument measures angular divergence from the runway axis. So while the one dot offset is just a few metres over threshold, it is much more at the point where the aircraft normally intercept the localizer, which is usually at least 10 miles out for large aircraft. At KSFO the runways are so close to each other that while the localizer could perhaps ensure separation in the final phase of landing, it can't ensure it in the earlier part.

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SFO makes use of simultaneous close parallel approach operations. This allows higher density operations than would otherwise be possible for runways spaced so close, but to make use of these ops, in SFOs case, requires 1600 ft ceilings and 4 sm visibility due to the requirement that aircraft maintain visual contact during close parallel operations.

It does look like SFO can run the ILS approaches simultaneously outside of close parallel operations, but is possible the required spacing makes its pointless to use both runways. If weather was bad enough that cat II ops were in effect, I don't see any verbiage on the cat II charts indicating simultaneous ops are approved.

Regardless of how precise your instruments are, there are strict requirements on how close you can be to another airplane in the approach phase and that separation be maintained if both parallel aircraft perform the missed approach.

The ability to run parallel approaches in low visibility depends on how far apart the runways are laterally. Airports like IAH and ATL can run triple parallel ops and if you take a look at their runway layout you will find they are spaced quite far apart and may be displaced relative to one another.

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