The path and terminator concept is a means to permit coding of terminal area procedures, SIDs, STARs, and approach procedures. Simply put, a textual description of a route or a terminal procedure is translated into a format that is useable in RNAV systems. One of the most important concepts for pilots to learn regarding the limitations of RNAV equipment has to do with the way these systems deal with the path and terminator field included in complex route records.
The first RNAV systems were capable of only one type of navigation; they could fly directly to a fix. This was not a problem when operating in the en route environment in which airways are mostly made up of direct routes between fixes. The early approaches for RNAV did not present problems for these systems and the databases they used because they consisted mainly of DME/DME overlay approaches flown only direct point-to-point navigation. The desire for RNAV equipment to have the ability to follow more complicated flightpaths necessitated the development of the path and terminator field that is included in complex route records.
Path and Terminator Legs
There are currently 23 different leg types, or path and terminators that have been created in the ARINC 424 standard that enable RNAV systems to follow the complex paths that make up instrument departures, arrivals, and approaches. They describe to navigation avionics a path to be followed and the criteria that must be met before the path concludes and the next path begins. Although there are 23 leg types available, none of the manufactured database equipment is capable of using all of the leg types. Pilots must continue to monitor procedures for accuracy and not rely solely on the information that the database is showing. If the RNAV system does not have the leg type demanded by procedures, data packers have to select one or a combination of available lleg types to give the best approximation, which can result in an incorrect execution of the procedure. Below is a list of the 23 leg types and their uses that may or may not be used by all databases.
- Initial fix or IF leg—defines a database fix as a point in space and is only required to define the beginning of a route or procedure. [Figure 1]
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| Figure 1. Initial fix |
- Track to a fix or TF leg—defines a great circle track over the ground between two known database fixes and the preferred method for specification of straight legs (course or heading can be mentioned on charts but designer should ensure TF leg is used for coding). [Figure 2]
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| Figure 2. Track to a fix leg type |
- Constant radius arc or RF leg—defines a constant radius turn between two databases fixes, lines tangent to the arc, and a center fix. [Figure 3]
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| Figure 3. Constant radius arc or RF leg |
- Course to a fix or CF leg—defines a specified course to a specific database fix. Whenever possible, TF legs should be used instead of CF legs to avoid magnetic variation issues. [Figure 4]
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| Figure 4. Course to a fix or CF leg |
- Direct to a fix or DF leg – defines an unspecified track starting from an undefined position to a specified fix. [Figure 5]
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| Figure 5. Direct to a fix or DF leg |
- Fix to an altitude or FA leg-defines a specified track over the ground from a database fix to a specified altitude at an unspecified position. [Figure 6]
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| Figure 6. Fix to an altitude or FA leg |
- Track from a fix from a distance or FC leg-defines a specified track over the ground from a database fix for a specific distance. [Figure 7]
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| Figure 7. Track from a fix from a distance or FC leg |
- Track from a fix to a distance measuring equipment (DME) distance or FD leg-defines a specified track over the ground from a database fix to a specific DME distance that is from a specific database DME NAVID. [Figure 8]
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| Figure 8. Track from a fix to a DME distance or FD leg |
- From a fix to a manual termination or FM leg-defines a specified track over the ground from a database fix until manual termination of the leg. [Figure 9]
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| Figure 9. From a fix to a manual termination or FM leg |
- Course to an altitude or CA leg-defines a specified course to a specific altitude at an unspecified position. [Figure 10]
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| Figure 10. Course to an altitude or CA leg |
- Course to a DME distance or CD leg-defines a specified course to a specific DME distance that is from a specific database DME NAVAID. [Figure 11]
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| Figure 11. Course to a DME distance of CD leg |
- Course to an intercept or CI leg-defines a specified course to intercept a subsequent leg. [Figure 12]
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| Figure 12. Course to an intercept or CI leg |
- Course to a radial termination or CR leg—defines a course to a specified radial from a specific database VOR NAVAID. [Figure 13]
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| Figure 13. Course to a radial termination or CR leg |
- Arc to a fix or AF leg-defines a track over the ground at a specified constant distance from a database DME NAVAID. [Figure 14]
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| Figure 14. Arc to a fix or AF leg |
- Heading to an altitude termination or VA leg-defines a specified heading to a specific altitude termination at an unspecified position. [Figure 15]
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| Figure 15. Heading to an altitude termination or VA leg |
- Heading to a DME distance termination or VD leg-defines a specified heading terminating at a specified DME distance from a specific database DME NAVAID. [Figure 16]
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| Figure 16. Heading to a DME distance termination or VD leg |
- Heading to an intercept or VI leg-defines a specified heading to intercept the subsequent leg at an unspecified position. [Figure 17]
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| Figure 17. Heading to an intercept or VI leg |
- Heading to a manual termination or VM leg-defines a specified heading until a manual termination. [Figure 18]
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| Figure 18. Heading to a manual termination or VM leg |
- Heading to a radial termination or VR leg-defines a specified heading to a specified radial from a specific database VOR NAVAID. [Figure 19]
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| Figure 19. Heading to a radial termination or VR leg |
- Procedure turn or PI leg—defines a course reversal starting at a specific database fix and includes outbound leg followed by a left or right turn and 180° course reversal to intercept the next leg. [Figure 20]
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| Figure 20. Procedure turn or PI leg |
- Racetrack course reversal or altitude termination (HA), single circuit terminating at the fix (base turn) (HF), or manual termination (HM) leg types—define racetrack pattern or course reversals at a specified database fix. [Figure 21]
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| Figure 21. Racetrack course reversal or HA, HF, and HM leg |
The GRAND JUNCTION FIVE DEPARTURE for Grand Junction Regional in Grand Junction, Colorado, provides a good example of different types of path and terminator legs used. [Figure 22] When this procedure is coded into the navigation database, the person entering the data into the records must identify the individual legs of the flightpath and then determine which type of terminator should be used.
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| Figure 22. Grand Junction Five Departure |
The first leg of the departure for Runway 11 is a climb via runway heading to 6,000 feet mean sea level (MSL) and then a climbing right turn direct to a fix. When this is entered into the database, a heading to an altitude (VA) value must be entered into the record’s path and terminator field for the first leg of the departure route. This path and terminator tells the avionics to provide course guidance based on heading, until the aircraft reaches 6,000 feet, and then the system begins providing course guidance for the next leg. After reaching 6,000 feet, the procedure calls for a right turn direct to the Grand Junction (JNC) VORTAC. This leg is coded into the database using the path and terminator direct to a fix (DF) value, which defines an unspecified track starting from an undefined position to a specific database fix.
Another commonly used path and terminator value is heading to a radial (VR) which is shown in Figure 23 using the CHANNEL ONE DEPARTURE procedure for Santa Ana, California. The first leg of the runway 19L/R procedure requires a climb on runway heading until crossing the I-SNA 1 DME fix or the SLI R-118, this leg must be coded into the database using the VR value in the Path and Terminator field. After crossing the I-SNA 1 DME fix or the SLI R-118, the avionics should cycle to the next leg of the procedure that in this case, is a climb on a heading of 175° until crossing SLI R-132. This leg is also coded with a VR Path and Terminator. The next leg of the procedure consists of a heading of 200° until intercepting the SXC R-084. In order for the avionics to correctly process this leg, the database record must include the heading to an intercept (VI) value in the Path and Terminator field. This value directs the avionics to follow a specified heading to intercept the subsequent leg at an unspecified position.
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| Figure 23. Channel One Departure |
The path and terminator concept is a very important part of airborne navigation database coding. In general, it is not necessary for pilots to have an in-depth knowledge of the ARINC coding standards; however, pilots should be familiar with the concepts related to coding in order to understand the limitations of specific RNAV systems that use databases.
Path and Terminator Limitations
How a specific RNAV system deals with Path and Terminators is of great importance to pilots operating with airborne navigation databases. Some early RNAV systems may ignore this field completely. The ILS or LOC/ DME RWY 3 approach at Durango, Colorado, provides an example of problems that may arise from the lack of path and terminator capability in RNAV systems. Although approaches of this type are authorized only for sufficiently equipped RNAV systems, it is possible that a pilot may elect to fly the approach with conventional navigation, and then re-engage RNAV during a missed approach. If this missed approach is flown using an RNAV system that does not use Path and terminator values or the wrong leg types, then the system will most likely ignore the first two legs of the procedure. This will cause the RNAV equipment to direct the pilot to make an immediate turn toward the Durango VOR instead of flying the series of headings that terminate at specific altitudes as dictated by the approach procedure. [Figure 24]
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| Figure 24. ILS or LOC/DME RWY 3 in Durango, Colorado |
Pilots must be aware of their individual systems Path and Terminator handling characteristics and always review the manufacturer’s documentation to familiarize themselves with the capabilities of the RNAV equipment they are operating. Pilots should be aware that some RNAV equipment was designed without the fly-over capability which can cause problems for pilots attempting to use this equipment to fly complex flightpaths in the departure, arrival, or approach environments.
