Required Navigation Performance (RNP) Approaches: Guidance for Flight Crews
Required Navigation Performance (RNP) Approaches: Guidance for Flight Crews
Definition
This article provides general guidance on flying Required Navigation Performance (RNP) approaches in transport-category aircraft equipped with modern electronic flight instrument systems (EFIS), and a flight management system (FMS). The following information serves as broad methodology and does not supersede aircraft-specific guidance in the relevant standard operating procedures (SOP).
Description
The naming of approaches that use RNP technology is not yet standard across the world. In the United States, an RNP approach chart might be labeled, for example: RNAV (RNP) Z 13R. In Europe, the same approach might be labeled: RNP Z 13R. Regardless of naming convention, RNP implies a GPS-based approach that requires a certain level of accuracy from an aircraft's navigational systems. On-board monitoring alerts pilots if required performance is not met. This can happen due to system malfunction or a lapse in reception from an adequate number of GPS satellites. An alert that warns of an unreliable GPS signal is known as a Receiver Autonomous Integrity Monitoring (RAIM) alert.
A typical standard RNP level for approach segments is 0.3 to 1.0, corresponding to a primary route width (centerline to boundary) of 0.3 nm to 1.0 nm. If the displayed RNP level falls below the limit, the FMS will generate a message that reads "DEGRADE," "UNABLE RNP," or something similar. Crews must immediately advise air traffic control (ATC) if this happens. Typically, a missed approach must be flown unless the landing can be completed in visual conditions.
With this higher level of GPS accuracy, approaches can be built to ease arrivals into airports with terrain or other obstacles that would otherwise make for challenging flying, especially at night or in instrument meteorological conditions (IMC). These approaches can include curvilinear segments, known as radius-to-fix (RF) legs. Special requirements come with flying such approaches, including adherence to speed limits that keep the aircraft within protected airspace and on the intended ground track.
Another important restriction is that crews cannot fly directly to a fix that starts, stops, or intercepts an RF leg. Doing so can put the aircraft well away from the intended ground track.
Put simply, an RNP procedure can make a difficult approach easy, but it must be set up and flown properly. Both the aircraft and the crew must be certified for RNP approaches. Some of the more demanding RNP procedures, such as those with an RNP level more restrictive than 0.3, can require specific authorization. They are labeled RNP (AR), indicating special authorization is required.
RNP Approach Minima
Pilots should check their operations specifications for the approach minima to use during an RNP approach. Typical approach charts list multiple lines of minima, which apply to various levels of aircraft equipment and certification. Those lines may include LNAV/Vertical Navigation (VNAV), Localizer Performance with Vertical Guidance (LPV), and Localizer Performance (LP).
Technique
Depending on aircraft type and configuration of the FMS and guidance panel, the acronym LAVAS can be used as a memory aid for important steps in setting up an RNP approach:
L - Select LNAV (or whatever guidance panel setting is used for FMS/GPS navigation).
A - Set the altitude of the final approach fix (FAF).
V - Select VNAV, or vertical navigation.
A - Arm the approach.
S - Check speed restrictions for radius-to-fix legs or any other approach segments.
Some operators' SOPs require an altimeter cross-check prior to the FAF, to ensure altimeter accuracy before beginning an approach with vertical guidance. Even if not specifically required, good airmanship would tend toward such a check.
Further Reading
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