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Similar to this question but more so!

The Enroute H-5 chart shows two RNAV waypoints over Spinner VORTAC (SPI): SOHOW and USIRE. Neither waypoint appears on the Low IFR charts, nor on the VFR charts.

SPI, USIRE, and SOHOW on H-5
Enroute H-5 as of Feb. 25, 2021, via Skyvector

Searching the online Form 8260-2 database I see that Spinner is used for many different instrument procedures:

Some of these procedures only use Spinner to define a waypoint on the route, but in other procedures Spinner is actually flown over.

SOHOW does not have a Form 8260-2 listed in the database, but from a general search I notice it is used in:

USIRE is used in:

Comparing the location of the three waypoints:

Waypoint Latitude Longitude
Spinner 39 50 23.04N 089 40 39.85W
SOHOW 39 50 25.37N 089 40 37.43W
USIRE 39 50 23.04N 089 40 39.06W
  • SOHOW is ≈300 feet northeast of Spinner
  • USIRE is ≈062 feet due east of Spinner
  • USIRE is ≈270 feet southwest of SOHOW

So the question is: Why do SOHOW and USIRE exist? And in particular why do they both exist? I can think of three reasons:

  1. There is a rule saying that RNAV procedures must only use RNAV waypoints.
    Counterargument: Spinner is a waypoint on the NATCA (RNAV) SID.
  2. Spinner will be decommissioned soon, and the RNAV waypoints will be used instead. Neither waypoint is exactly on top of Spinner, but perhaps it doesn't need to be; the LBNON waypoint will replace Lebanon VOR/DME soon and those do not have the same exact coordinates.
    Counterargument: Spinner is used in several VOR approaches and to define fixes on other instrument procedures; also I haven't heard of any plans to decommission it.
  3. The specific configuration of the LUCIT and KOLTS arrival procedures necessitate the distinction between two points 270 feet away from each other (less than 0.05 nautical miles).
    Counterargument: Really?
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Your Questions: Why do SOHOW and USIRE exist? And in particular why do they both exist?

Why do they exist: SOHOW and USIRE are published Waypoints on RNAV STARs into Gary, Ind., and Indianapolis, Ind., respectively. They exist and are published because they are part of the procedure being flown (as assigned by ATC).

Why do they both exist: Even though they are in extremely close proximity to each other in space, they are not physically located anywhere. The fact that both of these RNAV Waypoints are only "points in space" (geographic coordinates in a computer) reflects the value and flexibility of Area Navigation. Perhaps both STARs (to Gary and Indianapolis) could have used either SOHOW or USIRE, but there is no physical reason to do this. Also, since each of the Waypoints (SOHOW and USIRE) are part of different STARs going to different airports, there is less opportunity for confusion as to which procedure is being flown.

As a note, you mention in reason number 1 of your post (your Counterargument) that Spinner Vortac is a waypoint on the NATCA (RNAV) SID. Spinner is actually a "transition" on the NATCA SID, which technically ends at NATCA. See image below.

Good questions.

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  • $\begingroup$ This all makes sense. I suppose I was asking if anyone knows of a policy or procedure spelling out what you said, for example "a non-RNAV waypoint shall not be used in an RNAV procedure except in a transition route" and/or "No two procedures shall be begin at the same waypoint/navaid." Especially as new RNAV approaches come into being with some regularity, it seems strange to use up two pronounceable five-letter combinations when each waypoint sits (almost) exactly at the location of an existing navaid. $\endgroup$ – randomhead Mar 16 at 1:20
  • $\begingroup$ @randomhead - There are many examples of two (or more) different RNAV procedures using the same "point in space" Waypoint. But, as far as I can tell, when the same Waypoint is used in two or more RNAV procedures (e.g., STAR) they are for the same airport, not different airports, as in the case you have identified. . $\endgroup$ – 757toga Mar 16 at 15:24
  • $\begingroup$ @randomhead - Also, it is not unusual to have a Vortac as a fix/Waypoint integrated into an RNAV procedure (not just as a transition fix). However, keep in mind that when being flown by RNAV equipped aircraft (GPS, DME/DME/IRU, etc.) the Waypoints, including a Vortac (if part of the procedure) is being portrayed/displayed to the pilot/crew as a Waypoint along the RNAV route being flown. In other words, the crew would not have to change their navigation system to abandon the RNAV navigation and tune to a Vortac frequency to fly over the Vortac Waypoint. $\endgroup$ – 757toga Mar 16 at 15:54
  • $\begingroup$ "the crew would not have to change their navigation system to abandon the RNAV navigation and tune to a Vortac frequency"—I wasn't imaging they did; the VOR is just another lat/long coordinate in the database as far as the RNAV equipment is concerned. I think your point about having approaches to separate airports start at distinctly-named waypoints makes a lot of sense... except SPI is the beginning of feeder routes on the MOTIF into MDW and the Braymer into MCI. Those are both non-RNAV STARs though, I guess they could use the flexibility of RNAV to have new names for those STARs. $\endgroup$ – randomhead Mar 16 at 16:43

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