ICAO Doc 9613
5. SPECIFIC RNAV AND RNP SYSTEM FUNCTIONS
5.1 Fixed radius paths
5.1.1 The fixed radius paths take two forms: one is the RF leg type (see Figure I-Att A-4). The RF leg is one of the leg types described that should be used when there is a requirement for a specific curved path radius in a terminal or approach procedure. The RF leg is defined by radius, arc length, and fix. RNP systems supporting this leg type provide the same ability to conform to the track-keeping accuracy during the turn as in the straight line segments.
Note.— Bank angle limits for different aircraft types and winds aloft are taken into account in procedure design.
5.1.2 The other form of the fixed radius path is intended to be used with en-route procedures. Due to the technicalities of how the procedure data are defined, it falls upon the RNP system to create the fixed radius turn (also called a fixed radius transition or FRT) between two route segments (see Figure I-Att A-5).
5.1.3 These turns have two possible radii, 22.5 NM for high altitude routes (above FL 195) and 15 NM for low altitude routes. Using such path elements in an RNAV ATS route enables improvement in airspace usage through closely spaced parallel routes.
Two types of fixed radius paths:
- Radius-to Fix legs (RF) - used in terminal or approach procedures (defined by radius, arc length and fix);
- Fixed Radius Transition (FRT) - used with en-route procedures.
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