The navigation specification is the middle component of the last lesson's trio: the label that states what the airspace demands. The labels come in two families, and the difference between RNAV (area navigation) and RNP (required navigation performance) specifications is the requirement for on-board performance monitoring and alerting: an RNP system must watch its own performance and alert the crew when it can no longer be trusted; an RNAV system carries no such requirement. Same navigation, different self-awareness.
What the X means
Two families, then: RNAV without the self-monitoring requirement, RNP with it. Every label also carries a number, and the X in RNAV X or RNP X is the lateral navigation (LNAV) accuracy, the total system error in nautical miles, which is expected to be achieved at least 95 per cent of the flight time by the population of aircraft operating within the given airspace, route or procedure: RNP 1 means within 1 NM laterally, 95 per cent of the time, across the fleet flying it. One trap guards the family tree: aircraft approved to the more stringent accuracy requirements may not necessarily meet some of the functional requirements of a navigation specification that has a less stringent accuracy requirement. Accuracy and functionality are separate axes: the tighter number does not automatically include the looser specification's required functions.
The functional floor
Beneath the numbers sits a shared floor: the basic functional requirements of the RNAV and RNP specifications:
continuous indication of lateral deviation — on the deviation scales the Instrumentation course drew;
distance/bearing to the active waypoint;
GS (groundspeed) or time to the active waypoint;
navigation data storage;
failure indication.
Where each specification lives
The specifications map onto the phases of flight, and the exam asks the map:
RNAV 10 and RNP 4 — the oceanic/remote phase of flight;
RNAV 5 — the en-route and arrival phases;
RNP 2 — the en-route and oceanic/remote phases;
RNAV 2 — might be used in the en-route continental, arrival and departure phases;
RNAV 1 and RNP 1 — the arrival and departure phases;
RNP APCH (required navigation performance approach) and RNP AR APCH (authorisation required approach) — the approach phase;
RNP 0.3 — all phases of flight except oceanic/remote and final approach, primarily for helicopters.
Two footnotes complete the map: RNAV 2 and RNP 2 are also used as navigation specifications in their own right, listed by the syllabus, and RNAV 1, RNP 1 and RNP 0.3 may also be used in the en-route phases of low-level IFR (instrument flight rules) helicopter flights.