Substitute Route

A route assigned to pilots when any part of an airway or route is unusable because of NAVAID status. These routes consist of:
  1. Substitute routes which are shown on U.S. Government charts.
  2. Routes defined by ATC as specific NAVAID radials or courses.
  3. Routes defined by ATC as direct to or between NAVAIDs.

Source: Pilot Contoller Glossary (PCG)