Understanding Advanced RNP
Understanding Advanced RNP
The RNP Corner
Advanced Required Navigation Performance (A-RNP) is the latest navigation specification in the evolution of Performance Based Navigation (PBN). The following information is intended to provide a basic understanding of A-RNP and required aircraft capabilities.
PBN is comprised of two kinds of navigation specifications: RNAV and RNP (figure 1). The important difference between the two is that the RNP specification requires on-board performance monitoring and alerting as part of the avionics functionality. The use of RNP systems offer significant safety, operational, and efficiency benefits. While RNAV and RNP applications will co-exist for several years, a transition to RNP is being made as the proportion of aircraft equipped with RNP systems increases and the cost to transition is reduced.
Figure 1. Navigation Specification Designations
A-RNP is simply a combination of several Navigation Specifications, along with additional functions described in detail further below. A-RNP encompasses all phases of flight from departure and en route to arrival and approach. The A-RNP specification is intended to provide for an internationally harmonized standard. For the operator, it provides specific criteria used to qualify for operations on RNP routes, SIDs, STARs or approaches.
A-RNP encompasses the following RNP specifications:
> RNP-2 for domestic, offshore, oceanic and remote continental en route
> RNP-1 for departures and arrivals (ODPs, SIDs and STARs)
> RNP Approaches (including RNAV (GPS) approaches)
Note: RNP AR (Authorization Required) approaches are not covered under A-RNP and in the U.S., are defined under AC 90-101A. Charter and commercial operators seeking A-RNP approval under OpSpec C063, B035, or B036, that have a current RNP AR approval under OpSpec/LOA C384, meet the criteria for A-RNP defined in AC 90-105A, without the need for re-examination of aircraft eligibility.
A-RNP recognition is based on navigation systems meeting the performance and functional criteria for RNP-2, RNP-1 and RNP APCH to LNAV minima. Additional to the RNP requirements, AC 90-105A and ICAO PBN Doc 9613, list the following six functional capabilities that also define A-RNP eligibility.
> RF legs (Curved Paths)
Radius-to-fix (RF) leg capability allows for a constant radius turn starting and ending on a fix or waypoint. The FMS computes the actual flight path, providing for repeatable and predictable turn performance. RF legs are currently used in terminal and approach procedures.
> Parallel offsets
Parallel offsets provide a capability to fly offset from the parent track route segments and are intended to replicate the track at the desired offset to the left or right of the centerline route.
> Scalable RNP
RNP scalability refers to the avionics systems ability to automatically retrieve and display the required RNP value for each leg segment of a route or procedure from the navigation database. RNP values are depicted on the primary flight display and the CDI is scaled to the RNP value for the current leg. Two-dot (full scale) deflection of the CDI equals the RNP value displayed on the PFD for the current leg. RNP scalability is applicable for all SIDS, STARS, approaches and en route airways defined by an RNP requirement.
> RNAV Holding
RNAV holding is intended to give aircraft the ability to fly either ATC-defined or published holding patterns with the performance-monitoring, and alerting abilities associated with RNP.
> Fixed Radius Transitions (FRT)
FRTs are waypoint turn transitions between en route segments using a defined radius. FRTs are like fly-by turns, but use a fixed radius track with performance boundaries, creating a predictable, repeatable path associated with RNP. The purpose is to apply closer route spacing along turns on airways, or to transition from one airway to another.
> Time of Arrival Control (TOAC)
TOAC is an advanced function of the FMS designed to calculate and adjust the speed of the aircraft in an attempt to arrive at a point within a defined time limit. This function is not yet well defined for either equipment requirements or airspace implementation. Currently, no Honeywell FMS can claim TOAC capability. This is referenced in Service Information Letter (SIL) D201707000018, which states Honeywell Flight Management System (FMS) PBN capabilities in accordance with AC 90-105A.
NOTE: At present, the U.S. only intends to implement RF legs, Parallel offsets, and Scalable RNP. RNAV holding, FRT, and TOAC are presently considered optional A-RNP functions in the U.S.
Other countries may choose to implement a different subset of functions, or all six functions for their A- RNP definition. Therefore, before conducting A-RNP operations, pilots should refer to the country’s Aeronautical Information Publication (AIP) to confirm the aircraft has the required capability.
In the United States, an LOA is not required for A-RNP for Part 91 (General Aviation) operators, but the aircraft must meet the functional requirements for the specification defined in AC 90-105A. Charter and commercial operators need Ops Specs for RNP-2, RNP-1, and RNP APCH procedures for A-RNP eligibility. Non-US aviation authorities may require approval from the state of registry to use A-RNP. Operators must check with their country’s regulatory agencies for A-RNP eligibility requirements.
A-RNP provides many safety and efficiency benefits across all phases of flight, including:
> Closer route spacing for en route structure allows for higher airspace capacity and more efficient transitions to the terminal environment (Optimum Profile Descents)
> Reduction in the size of holding areas permits holds to be placed closer together or in more optimum locations
> Aircraft ability to comply with tactical parallel offset instructions as an alternative to radar vectoring (fuel and time savings)
> RF leg capability providing repeatable and predictable turn performance, enabling the use of RNP SID’s, STARS and approaches with curved RF legs
These procedures are in use in non-U.S. countries and being developed in the U.S. (figures 2, 3).
Figure 2. Copyright Jeppesen, Inc. – Used by Permission
Figure 3. Copyright Jeppesen, Inc. – Used by Permission
Honeywell FMS A-RNP Status
For a detailed listing of Honeywell avionics platforms by aircraft type that currently meet or do not meet the requirements for A-RNP under AC 90-105A and ICAO PBN Manual 9613, please reference SIL D201707000018 on the Pilot Gateway.
In summary, A-RNP applications allow qualified aircraft to take full advantage of their navigation and performance capabilities. This will allow the development of more efficient and safer terminal procedures and a higher capacity en route structure.
Please contact Honeywell Flight Technical Services with any questions or operational issues.
Derek Fiedler supports RNP, Datalink, and Forge Flight Services for Honeywell Flight Technical Services. He can be reached via email at Derek.Fiedler@Honeywell.com.