RNP Approaches (RNP APCH and RNP AR)

An RNP approach is an instrument approach flown by area navigation, almost always using GNSS, to a navigation specification that requires the aircraft to monitor its own position accuracy and to alert the crew when it can no longer guarantee it. It gives runways without an ILS a straight-in approach, often with vertical guidance, and gives runways with one a back-up that needs no ground transmitter at the airport.

ICAO's performance-based navigation (PBN) concept has two approach specifications: the standard RNP approach (RNP APCH) and the RNP authorization required approach (RNP AR APCH), which needs a specific operational authorisation. Chart titles differ. FAA charts use RNAV (GPS) RWY 27 for RNP APCH and RNAV (RNP) RWY 27 for RNP AR. ICAO's PANS-OPS has moved to RNP RWY 27, adding (AR) for authorisation-required procedures, and older European charts say RNAV (GNSS). One chart may carry several lines of minima, LPV, LNAV/VNAV, LP and LNAV, each tied to different equipment and flown differently.

RNP APCH and the PBN framework

PBN defines navigation by the performance required rather than by the sensor used. An RNAV specification sets an accuracy; an RNP specification adds on-board performance monitoring and alerting, so the system warns the crew when it cannot assure that accuracy. The RNP value is the distance either side of the intended path within which the aircraft must stay for at least 95% of the flight time.

RNP APCH requires 1 NM in the initial, intermediate and missed approach segments and 0.3 NM in the final approach segment. GNSS is the sensor (see GNSS). Basic GNSS, with integrity from receiver autonomous integrity monitoring (RAIM), supports LNAV. A satellite-based augmentation system (SBAS), such as WAAS in North America or EGNOS in Europe, broadcasts corrections and integrity data through geostationary satellites and is required for LP and LPV. LNAV/VNAV takes its vertical guidance from barometric VNAV or from SBAS.

Europe has written the transition into law. Commission Implementing Regulation (EU) 2018/1048, the PBN implementing rule, requires air navigation service providers to implement RNP APCH procedures with LNAV, LNAV/VNAV and LPV minima at all instrument runway ends: by 3 December 2020 where the runway had no precision approach, and by 25 January 2024 where it had one. Where terrain, obstacles or traffic make vertically guided minima impractical, LNAV minima alone are acceptable. From 6 June 2030, conventional procedures may be kept only for contingencies such as a GNSS outage.

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Approach design and the basic T

Many RNP APCH procedures follow the basic T approach design. A straight-in initial segment and two base-leg initial segments, roughly perpendicular to the final approach course, meet at an intermediate fix (IF), which may itself be an initial approach fix (IAF). The intermediate segment leads to the final approach fix, in GPS terms the final approach waypoint (FAWP), and the final segment to a missed approach waypoint, usually at or near the threshold. Where a course reversal is needed, a holding pattern at the IF replaces the procedure turn.

The T lets aircraft join from any direction. FAA charts surround it with a terminal arrival area (TAA), divided into straight-in, left-base and right-base areas bounded by magnetic courses to the IF or IAF, each with its own minimum altitude; ICAO procedures use terminal arrival altitudes on the same principle. Once cleared for the approach within an area, the pilot may descend to that area's altitude.

The procedure is loaded by name from the current navigation database, never built waypoint by waypoint, because the coding carries the fly-by and fly-over waypoints, the leg types and the FAWP that triggers approach sensitivity.

Minima lines: LNAV, LNAV/VNAV, LP and LPV

LNAV minima are lateral only and end at a minimum descent altitude (MDA). Localizer performance (LP), localiser performance in British spelling, uses SBAS for sharper, angular lateral guidance but still ends at an MDA; the FAA publishes it where terrain or obstacles prevent vertically guided LPV minima. LNAV/VNAV adds approved vertical guidance and a decision altitude (DA). LPV (localizer performance with vertical guidance) uses SBAS for angular lateral and vertical guidance that behaves like an ILS.

LPV is the line closest to an ILS. The FAA allows LPV decision altitudes as low as 200 ft above touchdown, with visibility as low as 1/2 statute mile, where terrain and airport infrastructure support them, matching a CAT I ILS. In Europe the EGNOS LPV-200 service began on 29 September 2015, allowing LPV approaches to a 200 ft decision height, known as SBAS CAT I and meeting ICAO Annex 10 CAT I performance requirements. Many LPV minima are nevertheless higher, because obstacles, lighting or runway infrastructure limit them: the chart, not the receiver, sets the minimum. In ICAO terminology LNAV/VNAV and LPV have been classed as approaches with vertical guidance (APV), between non-precision and precision approaches.

DA versus MDA decides the technique. On LNAV/VNAV and LPV the go-around starts at the DA, and the procedure design allows for the slight descent below it during the go-around. On LNAV and LP the MDA is a floor; commercial operators fly the final segment as a continuous descent to a derived decision altitude so that the aircraft never breaks it (see controlled flight into terrain). The receiver annunciates the level of service it can provide, such as LPV or LNAV; if that degrades, only the minima matching the remaining level may be used, and the operator's procedures say whether to continue or go around.

Baro-VNAV vertical guidance

Barometric vertical navigation (baro-VNAV) builds the final approach path from the aircraft's barometric altitude: the flight management system computes a path at the published vertical path angle down to the threshold and flies it by reference to the altimeter. Two things move that path in space. A wrong altimeter setting shifts it by the setting error, and non-standard temperature tilts it. In air colder than ISA the aircraft is lower than the altimeter shows, so the real path is shallower and closer to obstacles; in hot air it is higher and steeper.

Charts therefore publish temperature limits. An FAA chart may read, for example, "Uncompensated Baro-VNAV NA below −8 °C (+18 °F) or above 47 °C (117 °F)". Outside that range an aircraft without an approved temperature compensation system may not use the LNAV/VNAV minima. The limits do not apply when the vertical path comes from SBAS, which, like an ILS glide path, is fixed in space and unaffected by temperature.

EASA's AMC 20-27A applies the same principle: APV baro-VNAV is not permitted below the promulgated minimum aerodrome temperature unless the system has approved cold temperature compensation for the final approach. It leaves the pilot responsible for cold temperature corrections to the published minimum altitudes, including those of the initial and intermediate segments, the DA/H and the missed approach (see minimum safe altitudes and the guide to cold weather altimetry).

Advisory vertical guidance (LNAV+V)

Many SBAS receivers generate an advisory glidepath on procedures that have only LNAV or LP minima, annunciated as LNAV+V (advisory vertical guidance) or LP+V. It helps the pilot fly a stable, continuous descent from the final approach fix (see stabilised approach), but it is not approved vertical guidance. The approach remains a non-precision approach to an MDA. The barometric altimeter stays the primary reference, every step-down fix altitude must still be respected, and the advisory path gives no obstacle protection below the MDA. Following it, or a published vertical descent angle, below the MDA without the required visual reference is a recognised path to controlled flight into terrain.

CDI scaling and approach mode

A GNSS receiver changes the sensitivity of its course deviation indicator automatically as the flight progresses. This CDI scaling (approach sensitivity), known as GPS CDI scaling in FAA material and GNSS CDI scaling elsewhere, follows the phase of flight:

En route the scale is wider. The receiver confirms the change with an approach mode annunciation, such as LPV, LNAV/VNAV or LNAV. If approach mode is not annunciated by the FAWP, the crew must not descend on the final segment. An integrity alert before the FAWP means the approach is not started; one after it means a missed approach. EASA material frames the same transition through the RNP value, which changes from 1 NM to 0.3 NM for the final approach segment.

Overlay approaches

An overlay approach is a conventional procedure, typically VOR, VOR/DME or NDB, coded into the navigation database so that an approved GNSS receiver can fly it. The GPS overlay approach began as an FAA initiative in the 1990s that added "or GPS" to the titles of existing procedures, as in VOR or GPS RWY 12. With "or GPS" in the title, GNSS may be the navigation source for the whole approach, including the final segment, but the protection areas and minima remain those of the conventional design. Stand-alone RNAV (GPS) procedures, by contrast, are designed for GNSS from the start.

Without "or GPS", paragraph 1-2-3 of the FAA's AIM still lets a suitable RNAV system substitute for ground aids in many roles, and allows it to navigate the final segment of a VOR, TACAN or NDB approach provided the underlying aid is operating and is monitored for course alignment. A procedure whose ground aid is out of service therefore cannot be flown to its final segment on GNSS alone.

RNP AR approaches

RNP authorization required (RNP AR) procedures, spelt authorisation in EASA texts, use tighter navigation performance than RNP APCH to reach runways hemmed in by terrain or constrained airspace. Each line of minima carries its own RNP value, 0.3 NM or less in the final approach segment. The obstacle evaluation area extends only twice the RNP value either side of the path, with no secondary area or added buffer, so the aircraft must hold the path precisely. Procedures may include radius-to-fix (RF) legs, curved paths of constant radius, inside the final approach segment, and some require an RNP below 1 NM in the missed approach.

Equipment alone does not qualify an aircraft or crew. FAA charts state "Authorization Required", and the AIM compares the authorisation with that needed for CAT II or III ILS operations; each operator's authorisation, issued using AC 90-101A, names the lowest RNP value it may use. In Europe, RNP AR APCH is the only PBN specification for aeroplanes that still needs a specific approval, under SPA.PBN.100 of Part-SPA. RNP APCH needs none, although the aircraft must be certified for it and the crew trained. EASA certifies the aircraft under CS-ACNS, which in 2019 took over the earlier AMC 20-26 (RNP AR), AMC 20-27A (RNP APCH and APV baro-VNAV) and AMC 20-28 (LPV).

An instrument landing system relies on a ground monitor that shuts the signal down if it drifts out of tolerance; an RNP approach relies on RAIM or SBAS integrity and on the aircraft's own performance monitoring. Before the approach begins, the crew confirms which line of minima the equipment, the approval and the temperature allow.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.