REQUIRED NAVIGATION PERFORMANCE

The operational advantages of RNP include accuracy and integrity monitoring, which provide more precision and lower minimums than conventional RNAV. RNP DAs can be as low as 250 feet with visibilities as low as 3/4 SM. Besides lower minimums, the benefits of RNP include improved obstacle clearance limits, as well as reduced pilot workload. When RNP-capable aircraft fly an accurate, repeatable path, ATC can be confident that these aircraft will be at a specific position, thus maximizing safety and increasing capacity.

To attain the benefits of RNP approach procedures, a key component is curved flight tracks. Constant radius turns around a fix are called “radius-to-fix legs,” or RF legs. These turns, which are encoded into the navigation database, allow the aircraft to avoid critical areas of terrain or conflicting airspace while preserving positional accuracy by maintaining precise, positive course guidance along the curved track. The introduction of RF legs into the design of terminal RNAV procedures results in improved use of airspace and allows procedures to be developed to and from runways that are otherwise limited to traditional linear flight paths or, in some cases, not served by an IFR procedure at all. Navigation systems with RF capability are a prerequisite to flying a procedure that includes an RF leg. Refer to the notes box of the pilot briefing portion of the approach chart in figure 5-17.

In the United States, all RNP procedures are in the category of Special Aircraft and Aircrew Authorization Required (SAAAR). Operators who seek to take advantage of RNP approach procedures must meet the special RNP requirements outlined in FAA AC 90-101, Approval Guidance for RNP Procedures with SAAAR. Currently, most new transport category airplanes receive an airworthiness approval for RNP operations. However, differences can exist in the level of precision that each system is qualified to meet. Each individual operator is responsible for obtaining the necessary approval and authorization to use these instrument flight procedures with navigation databases.

RNAV APPROACH AUTHORIZATION

Like any other authorization given to air carriers and Part 91 operators, the authorization to use VNAV on a conventional nonprecision approach, RNAV approaches, or LNAV/VNAV approaches is found in that operator’s OpsSpecs, AFM, or other FAA-approved documents. There are many different levels of authorizations when it comes to the use of RNAV approach systems. The type of equipment installed in the aircraft, the redundancy of that equipment, its operational status, the level of flight crew training, and the level of the operator’s FAA authorization are all factors that can affect a pilot’s ability to use VNAV information on an approach.

Because most Part 121, 125, 135, and 91 flight departments include RNAV approach information in their pilot training programs, a flight crew considering an approach to North Platte, Nebraska, using the RNAV (GPS) RWY 30 approach shown in figure 5-18, would already know which minimums they were authorized to use. The company’s OpsSpecs, Flight Operations Manual, and the AFM for the pilot’s aircraft would dictate the specific operational conditions and procedures by which this type of approach could be flown.

There are several items of note that are specific to this type of approach that should be considered and briefed. One is the terminal arrival area (TAA) that is displayed in the approach planview. TAAs, discussed later in this chapter, depict the boundaries of specific arrival areas, and the MIA for those areas. The TAAs should be included in an IAP briefing in the same manner as any other IFR transition altitude. It is also important to note that the altitudes listed in the TAAs should be referenced in place of the MSAs on the approach chart for use in emergency situations.

In addition to the obvious differences contained in the planview of the previous RNAV (GPS) approach procedure example, pilots should be aware of the issues related to Baro-VNAV and RNP. The notes section of the procedure in the example contains restrictions relating to these topics.

Figure 5-17. RNAV (RNP) Approach Procedure with Curved Flight Tracks.

Figure 5-18. North Platte Regional (KLBF), North Platte, Nebraska, RNAV (GPS) RWY 30.

Baro-VNAV avionics provide advisory VNAV path indications to the pilot referencing a procedure’s vertical path angle (VPA). The computer calculated vertical guidance is based on barometric altitude, and is either computed as a geometric path between two waypoints or an angle from a single waypoint. If a flight crew is authorized to conduct VNAV approaches using an RNAV system that falls into this category, the Baro-VNAV temperature limitations listed in the notes section of the approach procedure apply. Also, since Baro-VNAV is advisory guidance, the pilot must continuously crosscheck the primary barometric altimeter to ensure compliance with all altitude restrictions on an instrument procedure.

Considering the pronounced effect of cold temperatures on Baro-VNAV operations, a minimum temperature limitation is published for each procedure for which Baro-VNAV minimums are published. This temperature represents the airport temperature below which the use of Baro-VNAV is not authorized to the LNAV/VNAV DA. The note “Baro-VNAV NA below -20°C (-4°F)” implies that the approach may not be flown at all using Baro-VNAV when the temperature is below -20° Celsius. However, Baro-VNAV may be used for approach guidance down to the published LNAV MDA. This information can be seen in the notes section of the previous example.

In the example for the RNAV (GPS) RWY 30 approach, the note “DME/DME RNP-0.3 NA” prohibits aircraft that use only DME/DME sensors for RNAV from conducting the approach.

Because these procedures can be flown with an approach approved RNP system and “RNP” is not sensor specific, it was necessary to add this note to make it clear that those aircraft deriving RNP 0.3 using DME/DME only are not authorized to conduct the procedure.

The lowest performing sensor authorized for RNP navigation is DME/DME. The necessary DME NAVAID ground infrastructure may or may not be available at the airport of intended landing. The procedure designer has a computer program for determining the usability of DME based on geometry and coverage. Where FAA Flight Inspection successfully determines that the coverage and accuracy of DME facilities support RNP, and that the DME signal meets inspection tolerances, although there are none currently published, the note “DME/DME RNP 0.3 Authorized” would be charted. Where DME facility availability is a factor, the note would read, “DME/DME RNP 0.3 Authorized; ABC and XYZ required,” meaning that ABC and XYZ DME facilities are required to assure RNP 0.3.