Area Navigation and RNAV/PBN
The framework
Area navigation allows flight on any desired path rather than toward or away from ground aids. Performance-Based Navigation (PBN) is the umbrella: the navigation specification defines the required accuracy, integrity, continuity and functionality. RNAV specifications (RNAV 10, RNAV 5, RNAV 2, RNAV 1) require the aircraft to stay within the containment of the specification; RNP specifications (RNP 4, RNP 1, RNP 0.3, RNP APCH) additionally require on-board performance monitoring and alerting, so the crew is warned if accuracy degrades.
Core concepts
- RNP value is the lateral accuracy in nautical miles to be maintained for 95 per cent of the flight time; the containment limit used for obstacle clearance is generally twice that value.
- ANP, the actual navigation performance computed by the system, must stay below the RNP value. If ANP exceeds RNP, an alert is raised and the procedure must not be continued.
- Sensors: GNSS is primary; DME/DME with VOR/DME, INS/IRS, and hybrids are used to update or back up.
- Waypoints are fly-by (turn anticipation, the aircraft rolls out on the new track) or fly-over (used where overflight is required, such as a beacon or a hold).
- RNP APCH minima include LNAV, LNAV/VNAV, LP and LPV. Baro-VNAV has a temperature limit, typically not below ISA minus 15 at the aerodrome, because cold temperatures make the computed path steeper than the published one.
Worked example
On an RNP 0.3 approach the system shows ANP 0.22 nm: accuracy is inside the specification, so the approach may continue. If ANP rose to 0.35 nm the containment is no longer assured, the alert appears, and a missed approach or diversion to a conventional procedure is required.
Traps
Treating RNAV as equivalent to RNP (RNP requires monitoring and alerting); assuming GPS alone satisfies RNP AR; ignoring temperature limits on baro-VNAV; and mishandling fly-by turn anticipation at closely spaced waypoints.