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Based on FAA advisory circular AC 150/5300-13A, Airport Design, when operating triple simultaneous precision instrument approaches the parallel runway separation could be reduced from 5000 feet to 4300 feet if the airport elevation is above than 1000 feet:

Triple simultaneous precision instrument approaches for airports below 1,000 feet (305 m) elevation normally require parallel runway centerline separation of 5,000 feet (1524 m) between adjacent runways. Triple simultaneous precision instrument approaches for airport elevations at and above 1,000 feet (305 m) and reduction in separation are currently under study by the FAA. In the interim, the FAA will, on a case-by-case basis, consider proposals utilizing separations down to a minimum of 4,300 feet (1311 m) where a 5,000-foot (1524 m) separation is impractical or the airport elevation is at or above 1,000 feet (305 m). Reduction of separation may require special radar, monitoring equipment, etc.

What is the theory behind the advice?

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  • $\begingroup$ My guess is that since true airspeed increases as airport elevation increases, they need more space (time) in order to maintain separation if someone does something wrong. The 1000' mark was probably arbitrary and they are now reviewing each one on a case-by-case basis to see what will actually work. $\endgroup$ – Lnafziger Aug 8 '16 at 15:13
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    $\begingroup$ @Lnafziger But the advice is that separation reduce when sea level rise........ $\endgroup$ – Him Aug 8 '16 at 15:15
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    $\begingroup$ The way that I read it, currently 5000 feet separation is required for airports below 1000 foot elevation, and above 1000 foot elevation is not currently permitted at all. That restriction is probably in place because of what I mentioned. The section that you quoted says that they are now considering other proposals, and being more lenient than in the past (which is a good thing)! $\endgroup$ – Lnafziger Aug 8 '16 at 15:26
  • $\begingroup$ 1000 feet seems really low to have a material effect on TAS $\endgroup$ – rbp Aug 8 '16 at 19:33
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What the text means is for triple simultaneous approaches for airports above 1,000 feet MSL, the FAA needs to conduct a case-by-case study. I.e. there is no automatic approval, yet.

Also, but not related, for airports below 1,000 feet MSL, reduction in separation from 5,000 feet also requires a case-by-case study.

... impractical or the...

I do think the paragraph you quoted can benefit from better punctuation.


Runway-to-runway separation does increase with elevation due to increased ground-speed and reduced reaction time.

The case-by-case study is usually done via numerous trials.

As for why 5000 feet, I was stuck in a loop between Annex 14 Volume I, PANS-ATM (Doc 4444), and the AC 150/5300-13A. Each quoting the other. Literally.

The Manual On Simultaneous Operations On Parallel Or Near-Parallel Instrument Runways does indeed discuss reaction times.

So, it's fair to deduce the increased separation is to give more time for ATC to notice any deviation and have time to react.

The FAA AIM does confirm the OFZ (Obstacle Free Zone) increases by 20 feet per 1000 feet elevation. The OFZ is in turn quoted as the reason for the runway separation increase the higher the airport is in the AC you quoted.


In short, increased elevation/ground speed requires case-by-case study to approve or bring down the separation. It's down to the reduced reaction time and airport equipment—e.g. radar accuracy, refresh rate, and so on.

The values themselves seem to be derived from trials plus added factor of safety plus rounding up. I can't find information on the trials.

Separation is not reduced with increased elevation, it's the opposite—if the airports (low and high) have the same exact equipment.

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The 4300 ft separation is to allow wingtip vortices to dissipate from one runway to the next. This was posted about fairly recently.

Why do simultaneous-ops parallel runways need to be so far apart?

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  • $\begingroup$ "This was posted about fairly recently." Could you, er, be a little more specific? Posted where would be a good start. On this site? In a year's time, even that won't be enough to help people find what you're talking about. $\endgroup$ – David Richerby Jun 5 '18 at 18:36
  • $\begingroup$ Maybe, but not for "parallel triple simultaneous precision approaches". Then 4300 ft would be needed. $\endgroup$ – CrossRoads Jun 6 '18 at 0:59

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