Definition
An instrument approach procedure originally designed for ground-based navigation aids (such as VOR, NDB, or TACAN) that has been authorized for flight using GPS or RNAV equipment in place of the underlying navaid. The approach retains its original name and ground-based design, but the pilot navigates the same course, fixes, and altitudes using approved area navigation equipment.
Plain English
It's an old-style approach built around a ground radio beacon, but you're allowed to fly it using your GPS instead of tuning that beacon. The path and altitudes are the same — only the equipment guiding you down is different.
Context Anchor
Seen in instrument procedure and RNAV/GPS discussions, especially when older ground-based approaches can be flown using approved onboard navigation equipment.
Derivation
Overlay' means one thing placed directly on top of another. Here, GPS guidance is laid over the existing ground-based approach — the procedure underneath is unchanged, but a new way of flying it sits on top.
Why Pilots Care
Allows pilots to use precise GPS navigation on legacy approach procedures, increasing accuracy and situational awareness without requiring a completely new chart.
Intuition Check
Do not read overlay approach as simply an approach drawn on top of a moving map. In this context, it means approved onboard navigation guidance used to follow an existing published instrument approach.
Example Sentence 1
With the VOR out of service, the pilot flew the overlay approach using GPS guidance to the same final approach fix and minimums.
Example Sentence 2
Many older airports still publish overlay approaches so RNAV-equipped aircraft can use the same procedure as legacy navigation users.