Definition
An FAA Advisory Circular titled 'National Route Program (NRP),' which provides guidance and procedures for the National Route Program. The NRP allows operators flying at or above FL390 (and in some cases FL290) to file user-preferred routes between city pairs rather than being restricted to published airways, helping them choose more efficient routes based on winds, weather, and traffic.
Plain English
An FAA guidance document that explains how high-altitude flights can file their own preferred routes between two cities instead of being forced to follow the standard published airways.
Context Anchor
Seen in instrument flight planning discussions about routes that do not simply follow standard published airways from start to finish.
Derivation
An 'Advisory Circular' is exactly what it sounds like — a circular (a document issued to a wide audience) that advises. ACs are non-regulatory FAA publications that explain, recommend, or clarify. The number 90-91 is just the FAA's filing system: the '90' series covers air traffic control and general operations.
Why Pilots Care
It supplies approved methods pilots can legally use to select more efficient or practical IFR routes while remaining compliant with air traffic control expectations.
Intuition Check
Do not read advisory as casual advice you can ignore without thought. In FAA use, an Advisory Circular is official guidance that explains an accepted way to understand or comply with aviation procedures, even though it is not usually a regulation by itself.
Example Sentence 1
The dispatcher referenced AC 90-91 when filing a user-preferred route from Los Angeles to New York at FL410.
Example Sentence 2
During preflight planning the instructor pointed out how Advisory Circular (AC) 90-91 supports certain helicopter routing options in busy airspace.