Some instrument approaches do not consist only of straight tracks joined by ordinary turns. Required Navigation Performance Authorisation Required — RNP AR — procedures can include precisely defined curved segments known as Radius-to-Fix, or RF, legs. An RF leg is a constant-radius path around a defined turn centre, beginning and ending at specified fixes. When an approved aircraft flies the procedure, its navigation system follows that coded curve rather than allowing the aircraft to create an arbitrary turn between two waypoints. [1]
RNP AR exists for operations that demand navigation capability, aircraft functionality, crew training and operator approval beyond ordinary RNAV use. The FAA’s controlling operational guidance is Advisory Circular 90-101A, which establishes criteria for obtaining authorisation to conduct RNP AR instrument approach procedures. The letters “AR” are therefore important: the presence of a procedure in a navigation database does not, by itself, authorise every RNAV-capable aircraft to fly it. [2]
What RNP actually adds to RNAV
Area navigation, or RNAV, allows an aircraft to navigate on a desired path without having to fly directly from one ground-based navigation beacon to another. Required Navigation Performance is a form of performance-based navigation that includes onboard performance monitoring and alerting. In practical terms, the aircraft must not only navigate accurately enough for the operation; the system must also monitor whether the required navigation performance is being achieved and alert the crew when it cannot be assured. [3]
The RNP value expresses the lateral navigation performance associated with the operation in nautical miles. FAA material for RNP AR procedures explains that minima can be associated with specified RNP values and that the operator’s authorisation identifies the minimum RNP capability it may use. An RNP 0.30 line, for example, is not simply a statement that the aircraft is expected to wander no more than 0.30 NM by chance; it belongs to a navigation specification with containment, monitoring and alerting requirements. [4]
Why a curved leg is useful
A fixed-radius turn lets procedure designers control where the aircraft will be during the turn far more precisely than a conventional fly-by waypoint whose exact ground track can vary with speed, bank angle and wind. FAA guidance describes the RF leg as a constant-radius circular path around a defined turn centre, with the turn beginning and ending at defined fixes. This can help a procedure remain inside protected airspace while routing around terrain, obstacles, restricted areas or other airspace constraints. [1]
The RF path is not created by a pilot manually selecting a bank angle that “looks about right”. The leg geometry is coded in the navigation database and interpreted by an approved flight-management/navigation system. The system knows the turn centre, radius and end points and generates steering guidance intended to remain on the defined curved path. ICAO’s Performance-Based Navigation Manual describes the RF path terminator as a fixed-radius arc defined by the required data elements. [3]
Radius-to-Fix is a path terminator
Modern navigation databases describe procedure legs using standard path-and-terminator concepts. “RF” identifies a leg whose path is a radius to a fix. The leg connects a defined start fix to a defined end fix along a circular arc centred on another defined point. This is different from simply telling the aircraft to turn left or right at a waypoint. The database contains enough geometry for the flight-management system to construct the intended ground track. [3]
Because the path is fixed relative to the ground, the aircraft has to vary the bank required to remain on it as groundspeed and wind change. Procedure design therefore considers aircraft speed, bank-angle capability and environmental assumptions. FAA guidance specifically notes that RF-leg design takes account of speed, wind and bank-angle criteria. The aircraft’s approved RNP AR functionality must be able to provide the path keeping and monitoring required for the procedure. [1] [2]
Why an ordinary RNAV turn is not always equivalent
On a conventional RNAV route, a fly-by turn can be anticipated before the waypoint so that the aircraft transitions smoothly onto the next leg. The actual turn path depends on navigation-system turn construction and operating conditions. That is acceptable where the protected airspace is designed for those variations. An RF leg is different because the procedure requires the aircraft to track a specifically defined arc. [1]
This fixed geometry can be particularly valuable where the available corridor is narrow. However, it also means the designer and operator have to know that the aircraft can fly the curve within the assumed performance envelope. A navigation system that can fly ordinary RNAV waypoints is not automatically approved for RF legs. FAA AC 90-101A addresses aircraft eligibility, system capability and operational approval for RNP AR procedures. [2]
Why RNP AR can use lower RNP values
RNP AR procedures can publish minima associated with different RNP values. FAA aeronautical information explains that each line of minima may carry an RNP value and that the pilot may use only a line compatible with the operator’s approved capability. A lower numerical RNP value represents a more demanding lateral navigation performance requirement. It does not mean a pilot may simply dial in a smaller number to gain lower minima. [4]
Authorisation is therefore a combination of aircraft and operator capability. The aircraft needs eligible navigation equipment and functions; the operator needs approved procedures, training and maintenance arrangements; and the crew needs to conduct the procedure according to the authorisation. FAA AC 90-101A exists specifically to define those operational-approval criteria. [2]
Onboard performance monitoring and alerting
The “RNP” part of RNP AR requires the aircraft to know whether its navigation performance remains good enough for the operation. The navigation system continuously assesses the performance of its position solution against the required value and provides an alert if it can no longer assure the specified performance. This is a defining feature of RNP navigation specifications in ICAO performance-based navigation. [3]
This monitoring matters because an RF leg may be positioned close to terrain or other constrained airspace. Procedure safety does not depend on the crew discovering a large track error visually. The aircraft’s certified navigation architecture has to support the required containment and alerting concept, while the crew has defined actions if the required performance is lost. Exact alert terminology and procedures are aircraft- and operator-specific and must come from approved manuals. [2]
The flight-management system needs the correct database
RNP AR procedure geometry is database driven. The fixes, RF-leg geometry, altitude constraints and other coded elements must correspond to the published procedure. Operators therefore need navigation-database control processes that meet their approval requirements. Pilots also cross-check the loaded procedure against the chart and applicable operating procedures rather than manually altering critical procedure geometry. [2]
For RF legs in particular, manual creation of an apparently similar turn would defeat the assurance provided by the validated database path. The system is intended to fly the coded fixed-radius leg as designed. ICAO PBN material treats the RF path terminator as a defined navigation-database element, not as a freehand pilot technique. [3]
Autopilot and flight director considerations
RNP AR approvals can specify use of the flight director or autopilot depending on the procedure, aircraft and authorised RNP value. The objective is to achieve the required path-tracking performance and reduce the chance that workload or manual-control variation takes the aircraft outside the expected performance. FAA AC 90-101A contains detailed operational criteria rather than a single universal rule that every RNP AR approach must always be flown identically. [2]
This is another reason RNP AR should not be reduced to “GPS approach with a curved line”. The approval covers the complete operational system: navigation sensors, flight-management functions, displays, guidance, alerting, crew procedures, training and maintenance. The curve is visually striking, but it is only one component of the performance-based-navigation concept. [2]
RF legs and terrain-rich airports
Curved RNP AR procedures are often associated with airports where terrain or airspace makes a conventional straight final-routing solution difficult. A fixed-radius path can allow a procedure designer to place the aircraft in a controlled lateral corridor before it reaches the final approach segment. The existence of an RF leg does not, however, mean the procedure is inherently “more dangerous”; the protected areas, obstacle assessment and required navigation performance are part of the approved procedure design. [1] [2]
Nor does RNP AR mean an aircraft is allowed to fly closer to terrain merely because satellite navigation is accurate. The procedure’s obstacle-clearance design, aircraft performance assumptions and authorisation requirements work together. The navigation specification is one part of a larger instrument-procedure safety case. [3]
Missed approaches can also carry demanding requirements
The RNP requirement does not necessarily end at decision altitude. FAA aeronautical information notes that RNP AR missed-approach segments may themselves have RNP requirements. The procedure chart and operator authorisation identify the applicable capability. A crew therefore has to consider the complete procedure, including the missed approach, when confirming that the aircraft is eligible. [4]
This is operationally logical: if the missed approach turns through constrained terrain or airspace, navigation performance remains important after the decision to discontinue the landing. The aircraft’s navigation system must continue to provide the guidance and monitoring needed for the published missed-approach path. [2]
Wind does not change the published radius
An RF leg is defined on the ground, so wind changes the aircraft’s required heading and bank to remain on that ground track rather than moving the published arc downwind. The flight-management and guidance system compensate within the capability and design assumptions of the operation. Procedure design accounts for wind and speed assumptions because the bank required to maintain a given radius rises as groundspeed changes. [1]
If conditions, equipment status or navigation performance mean the required path cannot be assured, the operation is governed by the applicable RNP AR procedures. The existence of sophisticated automation does not eliminate operating limits. It makes the path repeatable when the defined conditions and capabilities are satisfied. [2]
Why the curved line is technically significant
On a chart, an RF leg can look like nothing more than a smooth bend. Technically, it represents a major change from point-to-point navigation. The procedure designer has specified not only where the aircraft should be at the start and end of the turn but also the path it should follow between them. The navigation database encodes that geometry, the flight-management system constructs the arc, the guidance system commands the aircraft around it and the RNP monitoring function checks that the navigation performance remains acceptable. [3]
That is why RNP AR can produce approach paths that look unusually precise or curved when plotted on a map. The aircraft is not improvising around terrain. It is following an authorised, coded path whose navigation performance, aircraft capability, procedure design and crew qualification have been considered together. [2] [4]
Verified Sources / References
- Federal Aviation Administration — Aeronautical Information Manual, Instrument Approach Procedures. FAA description of RNP AR procedures and RF legs.
- FAA Advisory Circular 90-101A — Approval Guidance for RNP Procedures with AR. Primary U.S. operational-approval guidance for RNP AR.
- ICAO Doc 9613 — Performance-Based Navigation Manual. ICAO framework for RNAV/RNP navigation specifications and RF path terminators.
- FAA Aeronautical Information Publication — ENR 1.5 Holding, Approach and Departure Procedures. FAA information on RNP AR minima, authorisation and RF-leg requirements.
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