RNAV and RNP Requirements in Detail

The course's last lesson is the fine print: what each specification actually demands of the aircraft, the crew and the database, from the ocean down to the runway.

Oceanic: RNAV 10

Far from every aid, redundancy is the requirement: RNAV (area navigation) 10 requires that aircraft operating in oceanic and remote areas be equipped with at least two independent and serviceable LRNSs (long-range navigation systems), each comprising one of:

an INS (inertial navigation system);
an IRS (inertial reference system)/FMS (flight management system);
a GNSS (global navigation satellite system).

The inertial members of that list drift with time, which sets a clock on the capability, and operators may extend their RNAV 10 navigation capability time by updating: a position update over a known point resets the drift and restarts the clock.

En route: RNAV 5's little freedom

One concession marks the en-route workhorse: manual data entry is acceptable for RNAV 5. At a five-mile tolerance, a crew may type the route in; in the terminal and approach specifications of the next sections, the database becomes the only source.

Terminal: the database rules

In the terminal specifications the database becomes law: pilots must not fly an RNAV 1, RNAV 2, RNP (required navigation performance) 1 or RNP 2 SID (standard instrument departure) or STAR (standard instrument arrival) unless it is retrievable by route name from the on-board navigation database and conforms to the charted route. The rule names RNP 2 alongside the terminal pair: whichever of the four flies a SID or STAR, the discipline is the same. The procedure is loaded whole, by name, and checked against the chart. After that, the route may be modified through the insertion (from the database) or deletion of specific waypoints in response to ATC (air traffic control) clearances, but the door stays shut on invention: the manual entry, or creation of new waypoints by manual entry, of either latitude and longitude or place/bearing/distance values is not permitted. Every fix flown must have come from the database, because only the database carries the coding the last lesson's reasonableness checks can vouch for.

Approach: the RNP APCH family

The same by-name rule guards the approach: pilots must not fly an RNP APCH (required navigation performance approach) unless it is retrievable by procedure name from the on-board navigation database and conforms to the charted procedure. One loaded procedure then offers up to three lines of minima:

LNAV minima (LNAV: lateral navigation) — a non-precision IAP (instrument approach procedure) designed for two-dimensional approach operations;
LNAV/VNAV minima (VNAV: vertical navigation) — a three-dimensional operation, with lateral guidance based on GNSS and vertical guidance based on either SBAS (satellite-based augmentation system) or Baro-VNAV (barometric vertical navigation);
LPV minima (localiser performance with vertical guidance) — a three-dimensional operation with the tightest lateral performance of the family.

Baro-VNAV's cold-weather clause

The vertical channel of an LNAV/VNAV approach carries conditions. First, certification: an RNP APCH to LNAV/VNAV minima may only be conducted with vertical guidance certified for the purpose. Second, temperature: an RNP APCH to LNAV/VNAV minima based on Baro-VNAV may only be conducted when the aerodrome temperature is within a promulgated range, if the barometric input is not automatically temperature-compensated. The why is the altimeter's oldest lie: in air colder than standard, the true altitude is lower than the indicated one, so a vertical path computed from barometric heights sits shallower over the ground than the designer drew it, and the obstacle clearance under the path quietly erodes. Cold enough, and the published path no longer guarantees the surveyed protection: hence the promulgated temperature floor. The same vulnerability makes the setting itself critical: the correct altimeter setting is critical for the safe conduct of an RNP APCH using Baro-VNAV: a wrong QNH displaces the entire computed path, not just a needle.

LPV and the FAS data block

The family's most capable line has the most specific needs: RNP approaches to LPV minima require SBAS, and RNP APCH to LPV minima requires a FAS (final approach segment) data block. The FAS data block is a standard data format used to describe the final approach path: the same object the GBAS of the augmentation lesson transmitted by radio, here carried in the database and flown against SBAS corrections.

Authorisation and A-RNP

Two closing facts. RNP AR APCH (required navigation performance authorisation required approach) requires authorisation: the name is the requirement, and the operator must hold the specific approval before the procedure may be flown. And the family has a bundle: A-RNP (advanced required navigation performance) incorporates the navigation specifications RNAV 5, RNAV 2, RNAV 1, RNP 2, RNP 1 and RNP APCH: one approval covering the working set from en route to the approach.