Definition
An FAA Advisory Circular that provides airworthiness approval guidance for positioning and navigation systems, including GPS and other Global Navigation Satellite System (GNSS) equipment installed in aircraft. It sets out the standards an installation must meet for the equipment to be approved for the navigation operations it will be used to perform, such as en route, terminal, and approach phases of flight.
Plain English
An FAA document that lists the rules a GPS or similar navigation system has to meet before it can be installed and used legally in an aircraft for different stages of flight.
Context Anchor
Seen in instrument procedure and area navigation discussions when the handbook points to the approval basis for installed navigation equipment.
Derivation
Advisory Circular comes from 'advisory' (offering guidance) and 'circular' (a document circulated to interested parties). The number 20-138 identifies the topic series (20 covers general airworthiness), and the letter C marks the third revision of that document.
Why Pilots Care
Confirms that installed navigation equipment meets regulatory standards before it can be used for IFR procedures.
Grounding Statement
When the handbook cites AC 20-138C, it is pointing to FAA guidance behind approved navigation equipment, not giving a cockpit procedure.
Intuition Check
Here, AC does not mean alternating current or aircraft. It means Advisory Circular, an FAA guidance document.
Example Sentence 1
The avionics shop confirmed the new GPS navigator was installed in accordance with AC 20-138C, so it could be used for RNAV approaches.
Example Sentence 2
Chapter 2 points to AC 20-138C when describing equipment requirements for random RNAV routes.