Definition
Instrument approach procedures originally designed around ground-based navigation aids that have been authorized for use by RNAV-equipped aircraft using two or more DME stations to compute position. The aircraft's flight management system measures slant-range distances from multiple DME ground stations simultaneously and triangulates an accurate position, allowing it to fly the published approach path without relying on the original primary navaid.
Plain English
An older approach procedure that an RNAV-equipped aircraft is allowed to fly by using distance measurements from two or more ground stations to figure out exactly where it is, instead of using the navigation aid the approach was originally built for.
Context Anchor
Seen when studying instrument approach procedures and how area navigation systems define paths between fixes.
Derivation
DME stands for Distance Measuring Equipment. 'Overlay' means one thing laid on top of another -- here, an RNAV-computed path laid over a procedure originally built for conventional navaids. The aircraft flies the same ground track, just using a different way of knowing where it is.
Why Pilots Care
Enables precise lateral navigation on approaches using legacy ground-based equipment without relying on satellite navigation.
Analogy
It is like tracing a new, easier-to-follow line over an existing route on a paper map. The route is based on the older path, but the airplane follows it using its onboard navigation system.
Intuition Check
Do not read “overlay” as meaning the approach is only a display layer on the screen. In this context, it means an area navigation procedure has been placed over an existing approach path and must still be flown using approved navigation equipment.
Example Sentence 1
With GPS unavailable, the crew confirmed the FMS was receiving two suitable DME stations and continued with the DME/DME overlay approach into the destination airport.
Example Sentence 2
DME/DME overlay approaches provide an alternative to RNAV procedures in areas with good DME coverage.