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?

5 Answers 5


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.


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.

  • 2
    $\begingroup$ Yes, ATL can perform triple parallel approaches while also performing two additional parallel departures. It has five parallel runways. DFW also has 5 parallels. $\endgroup$
    – reirab
    Jan 22, 2015 at 19:46
  • $\begingroup$ @reirab: ORD is another airport with five parallel runways (with a sixth parallel on short final, expected arrival 2020). $\endgroup$
    – Vikki
    Nov 4, 2019 at 5:05
  • 1
    $\begingroup$ @Sean Yeah, it does now. The present 10R/28L was still under construction back in 2015 when I wrote that comment, though. I believe ATL now also has a 6th parallel in the plans, though it's not under construction yet, as far as I know. $\endgroup$
    – reirab
    Nov 4, 2019 at 16:50

I didn't see specific distances mentioned for the second part of the question.

If the runways (centerline to centerline) are separated by at least 4300 feet (and other conditions are met), the airport can conduct simultaneous instrument approaches.

If the runways are ate least 2500 feet apart, they can conduct staggered approaches (both runways in use, but aircraft do not approach alongside each other).

As a reference, the SFO runways have a centerline separation of 750 feet.

From the AIM http://tfmlearning.faa.gov/Publications/atpubs/AIM/Chap5/aim0504.html#?F8258ROBE

Runway separation diagram

It also shows the situation that waivers are possible for separations less than 4300 feet if additional conditions are met.

  • 1
    $\begingroup$ This should be marked as the correct answer $\endgroup$
    – Steve Kuo
    Nov 19, 2018 at 15:42
  • $\begingroup$ The finals controllers use FAA order 7110.308 which is simultaneous ILS approaches to closely spaced parallels. It’s waives all of that. Usually a slightly staggered operation, SFO has a waiver to go down to 1.0 mile offset between aircraft in the same “pair”. $\endgroup$
    – Murph
    Oct 16, 2021 at 11:40

It's not. Precise enough, that is.

There's a point in the ILS landing process called "Minimums". At that point the pilots must see the runway or throttle up and go-around. Assuming they have seen the runway, they must be prepared to stick-fly it onto the centerline, and then keep it on the centerline during braking, against crosswinds (still pushing on the tail), asymmetry in braking or reverse thrust, etc.

The precision of this varies by different classifications of wayside and on-airplane equipment, so both minimums and the "when you take over and stick fly" points also vary (separately). You can't decree that every landing airplane has the latest & greatest.

In fair weather, SFO ops (when my cubicle was directly under the flight path) involved bringing in airplanes near abreast, virtually wingtip to wingtip. That's not the computer either. That is the slightly trailing plane stick-flying it, using the Mark 1 eyeball for separation. And that depends on both fair weather and a high cloud ceiling, so they can visually line up (with each other) early in the approach.

The abreast (not staggered) approach was required for capacity reasons, not for landing but for takeoff: these two aircraft needed to land simultaneously and clear the runway intersections simultaneously. There were two departing aircraft lined up on the two cross runways to start their takeoff roll the moment this pair swished by.

I've even seen a triple: a Japan Air Lines 747 and two F-16s. Guess which day it was.


They don’t close a runway for fog or bad weather. That would never happen. They close runways for scheduled maintenance or if something happened like a bird strike and the port authority needs to get out there and clean it up. SFO can run dual ILS operations to 28L/R in very low visibility with a 1.0 mile stagger between aircraft. The arrival rate for that operation (FAA order 7110.308) and full blown sidebys in visual conditions is only a difference of 10-12 aircraft per hour. Which is enough to cause some delays. When the weather gets really bad pilots will start requesting category 3 ILS approaches due to company requirements(?) Our only cat 3 ILS is 28R which is also the longer runway. So more aircraft will request it. The left is still open, just not used as much.


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