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.
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:
- ROCKY SID off of Indianapolis
- LAKES SID off of Kansas City
- NATCA SID off of St Louis (an RNAV procedure!)
- Several Victor airways and J-routes
- Several instrument approaches to SPI and other nearby airports (including use as an IAF or feeder fix for RNAV approaches!)
- MOTIF STAR into Chicago/Gary
- Bradford STAR into Chicago
- Braymer STAR into Kansas City
- RIVRS STAR into St Louis
- Vandalia STAR into St Louis
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:
- LUCIT STAR into Gary
USIRE is used in:
- KOLTS STAR into Indianapolis
Comparing the location of the three waypoints:
|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:
- There is a rule saying that RNAV procedures must only use RNAV waypoints.
Counterargument: Spinner is a waypoint on the NATCA (RNAV) SID.
- 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.
- 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).