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In What is the number in a closed box right below a victor airway? I noticed:
What is an "Alternate Airway"?
Is it a leftover from something that used to exist? What is it?
(My gut feeling tells me it's something so obvious but I'm really not seeing it.)
I found an old definition in the All-Weather Flight Manual published by the US Navy in 1957:
Alternate Airways
Alternate airways are occasionally used for lateral separation when traffic conditions require it. The alternate airways are located not far from the associated main airway but are at least 15° on either side, or both sides, of the main airway. The alternate airway number includes the main airway number, plus an indication of the geographic position in which it lies with respect to the main airway. V-20-S is the alternate airway south of Victor 20. Your clearance will include Omnirange stations which form any particular alternate airway so that you will have no doubt when using an alternate airway.
The current AIP Appendix: ATS Routes does not list any alternate airways as far as I could find, so I guess they are not used anymore. They are however still mentioned in FAA Order JO 7110.65:
b. VOR/VORTAC/TACAN alternate airways. State the word “Victor” followed by the number of the airway in group form and the alternate direction.
EXAMPLE- "Victor Twelve South."
Thanks to @757toga's comment, I was able to find more:
Circa mid-80s the FAA agreed with ICAO to "revoke all Alternate Airways" (Federal Register Vol. 51, 1986), and for example in 1986 was in the process of renaming such an airway in Hawaii.
And before that, from a related study of the NAS (National Airspace System):
ALTERNATE AIRWAY REDUCTION AND REIDENTIFICATION - (TG 1-3.2) completed.
Study Date: November 8, 1982. (2 weeks)
The present alternate airway structure is still based largely on nonradar separation standards. With the increased use of radar, a study is needed to evaluate the possibility of eliminating unnecessary alternate airways and reidentify remaining routes taking into consideration ICAO standards. This would also contribute to a reduction in chart clutter.
— ADA150743 National Airspace Review Enhancement Plan. Revision 3. 1984. (PDF misfiled under ADA159743)
To add to the information from the other answers to this question:
Years ago, prior to the extensive availability of ATC radar, it was not uncommon to use Alternate Airways for IFR enroute lateral separation between aircraft .
In order to achieve and maintain separation between aircraft in a non-radar environment ATC procedures could be complex and inefficient (compared to the use of radar separation). One procedure that reduced the complexity was the use of Alternate Airways. These airways were located in areas where, based on the traffic flow, were beneficial for both ATC and pilots.
For example, consider if one aircraft is at a higher altitude than a second aircraft and they are in close proximity and assigned the same (Victor Airway) route. If the aircraft at the higher altitude needs to descend to an altitude below the second aircraft some form of non-radar separation (longitudinal or lateral) needs to be achieved prior to allowing the aircraft at the higher altitude to descend. One method to achieve this as efficiently as possible was the development and use of alternate airways. (see the sample image below for an example)
If the angle between the primary Airway and its alternate was 15 degrees the point at which ATC lateral separation would exist would be 16 NM from the Navaid. If the angle between the primary Airway and its alternate was greater than 15 degrees the point at which lateral separation would exist would be less than 16 NM. (see FAA Order JO 7110.65Z, Section 5, Lateral Separation Non-Radar)
The image below is just one example of the use of Alternate Airways, which likely no longer exist anywhere in the U.S. due to the availability of radar throughout most of the enroute structure.
Lastly, the foundational design of the IFR ATC system was (and is) based on Non-Radar routings and loss of communication between ATC and pilots (with some exceptions). The elimination of Alternate Airways is a product of the now expansive availability of ATC radar coverage resulting in the reduced need for using Non-Radar separation (longitudinal and Lateral) procedures. (Vertical separation is Non-Radar separation even when used in a radar environment)
