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Way-points
Another significant difference between RNAV and conventional routes is that RNAV routes are defined by way-points as opposed to conventional fixes. (Note, however, that a conventional fix may also be defined as an RNAV way-point). Unlike conventional routes which are usually defined by tracks between fixes, an RNAV route is defined by tracks between way-points.
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Route Information
A third noteworthy difference between RNAV and Conventional terminal routes is the way in which route information is provided to the operator. Whilst route information for both conventional and RNAV routes is provided to operators in ‘original’ AIP format consisting of charts and explanatory text, RNAV route information needs to ‘translated’ into a format which can be stored in a navigation database before it can be used by the aircraft navigation system.
This transformation of aeronautical data from ‘State’ published format into usable data for the operator occurs in a series of steps. Using State-originated aeronautical information, data base suppliers collect and code this information in a standard data format known as ARINC424 (Navigation System Database Specification). This data format, which is usable by navigation system databases, is then ‘packed’ by the original equipment manufacturer (OEM2) for use in the database of a particular operator (the ‘end’ user).
This transformation of route information into ARINC 424 format is made possible by the use of ‘Path and Terminators’ developed by ARINC. Simplistically, ‘Path Terminators’ can be described as industry standard for describing a route information. These Path Terminators are two-letter codes: the first describes the type of flight path (e.g. a track between two way-points) and the second the route termination point (e.g. a fix). Thus, for example, track to a fix (TF) path terminator would be used to “code” a route between two way-points.
2 Original Equipment Manufacturer of the RNAV system.
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. Turns
From an airspace designer’s perspective, it is useful to understand that the design of turns on RNAV routes by PANS-OPS designers is different to conventional routes. As with straight segments of routes, turns also have to be coded into the route information using the Path and Terminator system. Turns can be coded in one of four ways:
Fly-By Transitions Fly-Over Transitions Fixed-Radius Transitions Conditional Transitions
The navigation system anticipates the turn onto the next leg. In en route mode (see below) turn anticipation can start as much as 20NM before the (turning) way-point. The aircraft over-flies the way-point before starting the turn onto the next leg. This type of turn is exclusive to Terminal Airspace, and then only when it is not possible to use a fly-by or fixed-radius transition e.g. to define an extended centre line. In this instance, the aircraft flies a specific turn with a defined radius. This type of turn provides the most accurate, predictable and repeatable turn performance by all aircraft and is, generally, the preferred method for transitions with large track angle changes. Most current RNAV systems cannot accommodate this coding at present. where the RNAV system initiates a transition once a specific altitude has been reached. Conditional transitions that involve a turn are defined by the preceding leg, the subsequent leg and an altitude restriction.
Note: From the designer’s perspective - particularly that of the PANS-OPS specialist – it is useful to be aware that the way in which the RNAV system executes the turn is determined by whether the RNAV system (or FMS) is operating in ‘en route’ or ‘Terminal’ mode. Generally, it may be said that when in ‘en route’ mode, the turn anticipation for fly-by transitions will be considerably greater in Terminal mode. Significantly, the designer should be aware that the all RNAV systems (and FMS) do not necessarily define ‘en route’ and Terminal’ mode the same way. Being aware of these aspects, the PANS-OPS procedure designer strive to design routes so that its coding ensures the greatest track predictability for air traffic control.
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