Definition
An area navigation (RNAV) method in which the aircraft determines its position primarily by measuring distances from two or more ground-based DME stations, supplemented by an onboard Inertial Reference Unit (IRU) that provides position information when adequate DME coverage is not available. The IRU bridges gaps in DME signal coverage so that RNAV accuracy is maintained throughout the route.
Plain English
A way for the aircraft to know where it is by listening to two ground stations that tell it how far away they are, and using an internal motion-sensing unit as a backup when those ground stations can't be heard.
Context Anchor
Seen on IFR en route charts and in RNAV equipment discussions, especially when a route or operation depends on what navigation sources the aircraft can use.
Derivation
DME comes from 'Distance Measuring Equipment'; IRU from 'Inertial Reference Unit' (inertial = based on motion sensed inside the aircraft itself); RNAV from 'Area Navigation,' meaning navigation along any chosen path rather than only station-to-station. The combined name simply lists the position sources the system uses, in order of priority.
Why Pilots Care
Allows precise direct routing and RNAV operations in areas with limited VOR coverage.
Analogy
It is like finding your location by knowing your distance from two known landmarks, while also carrying a device that tracks your movement when the landmarks are briefly harder to use.
Intuition Check
Don't assume this is GPS-based. DME/DME/IRU RNAV is a non-GPS method — it uses ground DME stations as the main position source, with an inertial unit filling the gaps.
Example Sentence 1
The aircraft was certified for DME/DME/IRU RNAV, so the crew could accept the en route clearance without needing GPS.
Example Sentence 2
High altitude charts show DME/DME/IRU RNAV routes that aircraft can fly without traditional VOR navigation.