# Which Navigation Specifications support an RF leg?

I am studying FAA's rules concerning navigation specifications and the utilization of RF legs in an Instrument Flight Procedure. However, I am having trouble understanding if an RF leg is supported by RNAV procedures (RNAV 1 specifically) or only by RNP procedures. After some search in FAA's IFP coded database (CIFP file), I realised that only SIDs labeled as RNP 1 support RF legs, at least as far as FAA's procedures are concerned. However, an introductory page on FAA's site, concerning RNAV and PBN procedures, includes the RF as a general RNAV leg type.

Am I missing something? Could someone point me in the right direction?

EDIT: I think the question that pretty much sums up my confusion is if it's possible for a SID or STAR with only an "RNAV 1", instead of "RNP 1", requirement to contain an RF leg. Because so far I have only encountered RNP 1 SIDs with RF legs, such as STAYY ONE

First is understanding the difference between RNAV and RNP. This related question: What is the difference between RNAV and RNP? provides a basic answer to that part of your question.

To your specific question, RNAV procedures predate the acceptance of RNP standards. Since RNP standards have been established as the basis of navigation, you should expect that all procedures published in the future would be RNP procedures.

As a reference, I can point to RTCA Document DO-236C, Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation (MASPS). This document describes how RNP standards will be applied in the airspace. That covers airspace design, procedures and equipment used to fly within RNP airspace. Performance Based Navigation (PBN) adds aircraft specific performance requirements to the RNP concept.

Definitions from DO-236C which references ICAO Doc 9613:

RNAV system (as defined by ICAO Doc 9613): A navigation system which permits aircraft operation on any desired flight path within the coverage of station-referenced NAVAIDs or within the limits of the capability of self-contained aids, or a combination of these. An RNAV system may be included as part of a flight management system (FMS).

RNP system (as defined by ICAO Doc 9613): An area navigation system which supports onboard performance monitoring and alerting. An area navigation system capable of achieving the performance requirement of an RNP specification is referred to as an RNP system.

PBN (as defined by ICAO Doc 9613): Area navigation based on performance requirements for aircraft operating along an ATS route, on an instrument approach procedure or in a designated airspace.

Also from DO-236C, section 3.2 Path Definition Requirements, we find:

3.2.1.1 Navigation Database Flight Path Legs for Terminal Area Procedures

Permissible leg types for the definitions of the RNP flight paths for all navigation database procedures (e.g. SIDs, STARS, Approaches including missed approaches), with the exception of airways, are limited to the following:

Permissible Leg Types ARINC 424 Leg Type
- Track to Fix leg TF
- Radius to Fix leg RF
- Initial Fix IF
- Hold legs HM, HA and HF
- Fix to Altitude leg FA
- Direct to Fix leg DF
- Course to Fix leg CF

Note 1: RNP procedures that require the repeatability and predictability of
a specified ground track should be constructed using IF, TF, and RF legs only.

Note 2: If the FA, DF and CF types are used, they may introduce less repeatability and predictability.

As you can see from the above, RNP procedures are built using RNAV leg types. RF legs are an RNAV leg type unlike the similar AF leg which is a DME arc that can be flown with a DME.

Realistically, you will only find RF legs within SIDs, STARs, and IAPs as that's where they are needed. They are useful when you need a large change of direction which is common in the terminal environment but not so much while enroute.

• @StephenS you are correct. I should have included approaches in my statement. RF legs are a common part of PBN approach procedures. – Gerry Apr 2 '19 at 17:22
• @Gerry I really appreciate the answer, helped me to clear up some things. But since I would like to make sure that I have understood this, could you clarify one more thing: Is it possible for a SID or STAR with only a "RNAV 1 required", instead of "RNP 1 required", to contain an RF leg? Because so far I have only encountered RNP 1 SIDs with RF legs, such as STAYY ONE – Vector Zita Apr 3 '19 at 6:51
• I don't think it is. All the procedures I see that say "RNAV 1" include the note DME/DME/IRS or GPS required. That implies older FMSes that aren't up to the RNP standard. Most FMS of that era do not support RF legs. Even in the current RNP universe, RF legs are still not all that common. – Gerry Apr 3 '19 at 12:04