Definition
Existing non-precision instrument approach procedures — originally designed around ground-based navigation aids such as VOR, NDB, or TACAN — that have been authorized for flight using GPS as the primary navigation source. The approach chart retains its original title (for example, 'VOR or GPS RWY 27') and uses the same fixes, courses, and altitudes, but the pilot may track the procedure using a suitable IFR-approved GPS receiver instead of the underlying ground-based aid.
Plain English
These are older instrument approaches, built for ground-based navigation aids like VORs, that the FAA has cleared pilots to fly using GPS instead. The approach itself doesn't change — only the navigation tool used to fly it.
Context Anchor
Seen when studying GPS instrument approaches and when reading an approach chart that allows GPS use on a procedure based on older ground navigation.
Derivation
Overlay' here means one thing laid on top of another. The GPS procedure is laid over the existing ground-based approach — same path, same fixes, just flown with a different navigation source.
Why Pilots Care
It expands the number of usable approaches at airports that lack standalone GPS procedures, improving access and safety in instrument conditions.
Intuition Check
Overlay does not mean “a better GPS version of the approach.” It means GPS is authorized to guide you along the same published approach path and instructions.
Example Sentence 1
With the VOR out of service, the pilot briefed the VOR or GPS RWY 27 as a GPS overlay approach using the panel-mounted IFR GPS.
Example Sentence 2
Because the airport had no RNAV approach, the crew used the GPS overlay on the existing NDB procedure.