Definition
An alternate navigation source approved for use when the primary navigation system required for a procedure is unavailable or unreliable. In current FAA practice, this most commonly refers to using a suitable RNAV (GPS) system in place of ground-based navigation aids such as VOR, DME, ADF, or NDB, when permitted by regulation and aircraft equipment authorization.
Plain English
It's an approved backup way to navigate when the navigation aid the procedure was originally designed around isn't working or isn't installed in the aircraft.
Context Anchor
Seen in instrument procedure and equipment requirement discussions, especially when deciding whether GPS may legally replace another navigation source for a route or procedure.
Derivation
From Latin substituere, meaning 'to put in place of another.' The term simply means a stand-in navigation source — one that takes the place of the primary one when needed.
Why Pilots Care
It allows a flight or approach to continue safely and legally instead of diverting or canceling when primary navigation equipment fails.
Intuition Check
Do not assume “substitute” means “use anything instead of anything.” Here it means an approved navigation source may replace a specific unavailable source only when the rules and procedure allow it.
Example Sentence 1
With the VOR out of service, the pilot used an approved GPS as a substitute means of navigation to identify the intersection.
Example Sentence 2
ATC cleared the flight to use a substitute means of navigation for the arrival when the ground station was unavailable.