Definition
An FAA Advisory Circular that establishes the National Route Program (NRP), a procedure allowing eligible IFR flights operating at or above FL390 within the contiguous United States to file user-preferred routes between specified fix-to-fix points, rather than being restricted to the published airway and jet route structure.
Plain English
It is the FAA document that lets high-altitude flights pick their own preferred path across the country instead of being forced to follow the standard published routes.
Context Anchor
Seen in Instrument Flying Handbook discussions of IFR routing options, especially under “Other Routing.”
Derivation
AC' stands for Advisory Circular, the FAA's standard format for non-regulatory guidance. The number '90-91' is a filing identifier: the '90' series covers air traffic control and general operations, and '91' is the sequential number within that series.
Why Pilots Care
Using these routes reduces ATC workload, speeds up flight planning, and often provides more direct paths than conventional airways.
Intuition Check
Do not read “AC” here as aircraft or alternating current. In this context, AC means Advisory Circular, an FAA guidance document.
Example Sentence 1
The dispatcher filed a route under AC 90-91 to take advantage of a strong tailwind at FL410.
Example Sentence 2
Before departure the dispatcher checked AC 90-91 to confirm the selected route was still valid for the planned altitude.