Computer Navigation Fix

8 Answers

1. 
   

   
   

   John D. Collins on May 20, 2013

   A CNF is not a part of the approach definition. It was added to enable the GPS to be used on overlay approaches. An overlay approach has “or GPS” in the title. A GPS requires a FAF to sequence to the MAP in order to conduct an approach. Several things happen relative to the FAF using a GPS navigator. At a point 2 NM prior to the FAF, the CDI sensitivity changes from terminal mode to approach mode and the CDI changes fro, +/- 1 NM full scale to +/-.3 NM. At 1 minute prior to the FAF, the integrity is checked to determine if it meets criteria for the approach. So what do you do when an overlay approach doesn’t have a FAF defined but you want to fly it with the GPS? This would be the case if a VOR or NDB was located on the airport and the approach consists of the VOR/NDB as the IAF, then you fly outbound, then a procedure turn and descend back to the VOR/NDB as the MAP. You invent a computer fix and place it in a location far enough out from the VOR/NDB that it can serve the function of a FAF and the GPS can be kept happy.

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2. 
   

   
   

   Dan Chitty on May 20, 2013

   Thank you John.

   To make sure I understand regarding overlay approaches, a CNF may not be published/printed on the paper approach chart but the CNF may be in the GPS database. In this case, the CNF serves as the FAF or more specifically a sensor FAF.

   If I am comparing the paper approach chart against the GPS database waypoints and the approach chart does not list the five letter CNF but the GPS lists the CNF, I would question if the database is incorrect since the database does not match the approach chart.

   Let me know your thoughts.

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3. 
   

   
   

   John D. Collins on May 21, 2013

   A fair number of the CNF fixes have been added to the approach charts, but they are not required for the procedure if flown with a conventional Navaid. If a CNF is charted, it will have an x at its location and the name will be in parenthesis.

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4. 
   

   
   

   Mark Kolber on May 21, 2013

   Her’s an example of what John is talking about. For when the links get updated, it’s the ILS or LOC 26 at KFTG: http://155.178.201.160/d-tpp/1305/06851IL26.PDF

   Notice that it’s not an overlay approach. Just a garden-variety ILS. That’s not inconsistent with the limitations for use of GPS on an ILS approach – the only restriction is that you must use the LOC (or VOR or NDB) for lateral guidance on the final approach course.

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5. 
   

   
   

   Dan Chitty on May 21, 2013

   Thank you Mark.

   What is the purpose of the CNF (CFNRM) for this approach at KFTG?

   Also, refer to KGHM VOR/DME RWY 2 overlay approach:

   Since ATC is not aware of CNFs and CNFs are not to be used for ATC communication, how do I communicate with ATC if I want to fly to a CNF for this approach?

   Example: I want to fly direct to CNF (OSYER) to begin the arc.

   Thank you for the feedback.

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6. 
   

   
   

   Mark Kolber on May 21, 2013

   As John (and the AIM at 1-1-19.j.) explained, they have no ATC function whatsoever. CNFs are really only so that the computer navigation system can orient itself. Think of it as situational awareness for the GPS rather than for you. When charted, it’s so you understand what the GPS is doing, so you can monitor it for errors.

   The IAF on the east side is “8 DME on the GHM 105° radial”, just as it was before the days of GPS. And, if you wanted to specifically request it, you’t tell ATC you’d “like to intercept the arc on the GHM 105° radial,” just like you would do if you did not have a GPS.

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7. 
   

   
   

   Dan Chitty on May 21, 2013

   Mark and John,

   Thank you for the excellent explanations. I now have a great understanding, especially of CNFs on DME arcs.

   One additional question:

   What is the purpose of the CNF (CFNRM) for the ILS / LOC 26 approach at KFTG?

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8. 
   

   
   

   John D. Collins on May 21, 2013

   I really don’t know, it is not used in the Garmin database as part of the approach. It may have been requested by one of the FMS systems manufacturers to enable them to code the location where the PT commences.

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