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At KPHX, there's a Runway 25L/R and a runway 26. What would the reasoning be for using this instead of what seems to be the more common L/C/R designation?

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There is flexibility on the numbering of parallel runways based on a number of factors as described in FAA advisory_circular/150-5340-1J -page 4 (see excerpt below). In the case of KPHX for example, there is the main terminal between runways 8/26 and 7R/L-25R/L. Also, when approaching from the east the threshold for runway 25L is a significant distance from the threshold for runway 25R (about 2500 feet inset).

I would guess that minimizing possible pilot confusion factored into the decision not to have a "center" runway designation.

Excerpt from FAA advisory_circular/150-5340-1J -page 4

KPHX Airport Diagram

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  • $\begingroup$ Funnily enough, with a 10-degree declination, 8/26 is the closest designation, and the7/25 L/R pair is "the one off". Interesting. Was the declination larger when they built the runways? $\endgroup$
    – yo'
    Mar 8 at 14:51
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While it is going to be very hard to pinpoint the exact reason for KPHX, there are several logical reasoning for why it is done in general (I must disagree with your observation that XL/C/R is more common than XL/R, X+/-1 configuration, especially when there are more than 3 parallel runways).

  • Avoiding confusion. During rush hour it might be too confusing to have 3 runways with the same number

  • Separate complex. There is a good 1KM (3,500 ft) between 8-26 and 7L-25R centerlines. Using a different number helps to artificially (or not so much) separate the airport to different areas (some airports literally use N/S or W/E "complex"/"area" on the official charts)

  • Preparation for the future. If KPHX is ever going to get a fourth parallel runway to the north of 08-26, it will be much easier to add L/R to the existing runway than renaming the supposedly 07C-25C

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    $\begingroup$ I think "separate complex" is most likely. Consider KATL, 26L/R north of the terminals, 27L/R south, and 28 much farther south. In both cases, "the 25's" or "the 27's" are close together, while "the 26's" or "26" is "over there." Three moderately close parallels in the same complex, like SEA or DFW, make the case where a Center runway fits rather than disrupts a clear mental picture of things. $\endgroup$
    – Ralph J
    Mar 7 at 15:36
  • $\begingroup$ Airports with three runways in the same direction close to each other are probably pretty rare, because you'd need non-conflicting arrival, departure and missed approach patterns for all of them, and "center" runways would likely end up with a long corridor of airspace that all traffic needs to go through, the area used for sequencing would be far back, a missed approach would be a long detour, and emergency procedures would involve clearing the airspace of another runway. It's an operational nightmare you need a good reason for. $\endgroup$ Mar 8 at 1:04

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