Substitute Airway En Route Flight Procedures  Substitute Airway or Route Structures  2-9  Figure 2-17 Non-Part 95 sub-routes.  Figure 2-16 14 CFR Part 95 sub-routes.  all pilots in filing preferred routes results in fewer air traffic  delays and better efficiency for departure, en route, and  arrival air traffic service. [Figure 2-15]  ARTCCs are responsible for specifying essential substitute  airway or route segments (sub-routes) and fixes for use  during scheduled or unscheduled VOR/VORTAC shutdowns.  Scheduled shutdowns of navigational facilities require  planning and coordination to ensure an uninterrupted  flow of air traffic. Aeronautical Information Services, in  coordination with the ARTCCs, determine when the length  of outages or other factors require publication of sub- routes and Flight Program Operations (AJW-3) provides  flight inspection services, obstacle clearance verification,  certification, and final approval of substitute routes.  A schedule of proposed facility shutdowns within the region  is maintained and forwarded as far in advance as possible  to enable the substitute routes to be published. Substitute  routes are normally based on VOR/VORTAC facilities  established and published for use in the appropriate  altitude strata. In the case of substitute routes in the upper  airspace stratum, it may be necessary to establish routes by  reference to VOR/VORTAC facilities used in the low altitude  system. Non-directional (radio) beacon (NDB) facilities may  only be used where VOR/VORTAC coverage is inadequate  and ATC requirements necessitate use of such NAVAIDs.  Where operational necessity dictates, NAVAIDs may be  used beyond their standard service volume (SSV) limits that  define the reception limits of unrestricted NAVAIDs, which  are usable for random/unpublished route navigation,  provided that the routes can be given adequate frequency  protection.  Uncontrolled airspace  BCD vortac (shutdown)  ABC vortac  Off-airway sub-route  CDE vortac  V-98  V-98  Obstacle study and flight  inspection of sub-route required  Uncontrolled airspace  BCD vortac (shutdown)  ABC vortac  Sub-route V-98  CDE vortac  V-98  V-98  Obstacle study and flight  inspection of sub-route required  (Centerline must be in  controlled airspace) 