Area Navigation
Core concepts
Area navigation (RNAV) allows an aircraft to fly any desired path within the coverage of navigation aids, rather than tracking directly to or from a station. The system computes position from multiple inputs (VOR/DME, DME/DME, GNSS, inertial) and then guides along a defined route of waypoints. Key concepts are the flight path definition (track, distance, turn anticipation), the navigation specification (RNAV 5, RNAV 2, RNAV 1, RNP with on-board performance monitoring and alerting), and the distinction between RNAV and RNP: RNP includes a requirement for on-board performance monitoring and alerting. Also important are waypoint types, fly-by versus fly-over, and the effect of database currency.
Traps in questions
Common traps include: confusing RNAV with RNP; forgetting that RNAV 1 and RNP 1 require GNSS or equivalent; assuming all RNAV routes are GPS-based; and forgetting that the aircraft must remain within the specified lateral and longitudinal containment. Questions often ask about the effect of a stale navigation database or about fly-by waypoint turn anticipation. Also remember that an RNAV system does not remove the need for terrain awareness.
Worked example
An aircraft cleared on an RNAV 1 SID must achieve a total system error of 1 NM for 95% of the flight time. If the navigation solution is degraded and the system cannot guarantee that, the crew must notify ATC and revert to conventional navigation or request an amended clearance.
Rule of thumb
RNAV defines the path; RNP adds on-board performance monitoring and alerting. Always check that the navigation specification matches the aircraft capability and the database cycle.