Definition
An authorized procedure under which a properly certified IFR GPS receiver may be used in place of an Automatic Direction Finder (ADF) or Distance Measuring Equipment (DME) to determine the aircraft's position relative to navigation fixes, intersections, and station-referenced distances during instrument flight operations. The substitution is permitted for specific functions such as identifying named fixes, holding at NDB or DME fixes, flying DME arcs, and determining distance from a navaid, provided the underlying fix or facility is loaded from the current onboard navigation database.
Plain English
Pilots are allowed to use their IFR-approved GPS to do the jobs that an ADF or DME radio would normally do, like finding a fix, holding over a point, or measuring how far they are from a station, as long as the GPS is set up correctly and the fix is in its current database.
Context Anchor
Seen when flying instrument procedures that name an NDB, compass locator, DME distance, or DME fix, especially when the aircraft does not have a working ADF or DME receiver installed.
Derivation
Substitution comes from a Latin idea meaning “to put in place of.” That fits the aviation use: the GPS is being used in place of ADF or DME information for a specific allowed purpose, not as a loose backup for anything the pilot wants.
Why Pilots Care
It permits completion of certain instrument procedures without ADF or DME equipment, lowering the cost and complexity of the aircraft while preserving safe navigation standards.
Intuition Check
Do not read “substitution” as “any GPS can replace any required equipment.” Here it means an approved IFR GPS may replace ADF or DME information only in the ways FAA guidance and the procedure allow.
Example Sentence 1
Because the aircraft had no DME installed, the pilot used GPS substitution to fly the 15 DME arc onto the final approach course.
Example Sentence 2
Because the aircraft lacked an ADF receiver, GPS substitution for ADF was applied to locate the NDB holding fix.