Definition
An FAA Advisory Circular titled 'Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures.' It provides guidance and operational limitations for pilots using IFR-approved RNAV systems (such as GPS) as a substitute for ground-based navigation aids like VORs, NDBs, DMEs, and TACANs when flying conventional routes, departures, arrivals, and approaches.
Plain English
It is the FAA document that tells pilots when and how they are allowed to use GPS in place of older ground-based navigation equipment on standard IFR routes and procedures.
Context Anchor
You may see AC 90-108 in instrument equipment discussions when deciding whether the aircraft’s installed navigation system is acceptable for a route, approach, or fix requirement.
Derivation
Advisory Circular' is the FAA's term for non-regulatory guidance documents. The number '90-108' identifies the subject series (90 = general flight operations) and the specific publication. Knowing 'AC' means 'Advisory Circular' helps the reader recognise that this is FAA guidance, not a regulation in itself.
Why Pilots Care
It gives pilots more flexibility to use modern cockpit equipment on legacy procedures without requiring every ground-based navaid to be operational.
Intuition Check
Do not read “advisory” as “unimportant.” An Advisory Circular is usually guidance rather than a regulation by itself, but it explains what the FAA considers an acceptable way to operate.
Example Sentence 1
Under AC 90-108, the pilot used the aircraft's GPS to fly the VOR airway without tuning the underlying VOR station.
Example Sentence 2
During preflight planning the instructor confirmed the aircraft met all AC 90-108 criteria for flying the ILS using the GPS overlay.