Welcome Guest. Sign in or Signup

8 Answers

Computer Navigation Fix

Asked by: 11410 views Airspace, FAA Regulations

/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0in; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin;}

To aid in the approach chart/database correlation process, the FAA has begun a program to assign five-letter names to CNFs and to chart CNFs on various National Oceanic Service aeronautical products. (Instrument Procedures Handbook, 2012).

 

If a CNF is listed in the GPS database but not published on the approach chart, can you fly the approach if the CNF is not also on the approach chart?

It seems to me that both must be published (GPS database and approach chart)

Thank you for the feedback.

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.

    +7 Votes Thumb up 7 Votes Thumb down 0 Votes



  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    To make sure I understand regarding overlay approaches, a CNF may not be published/printed on the paper approach chart but the CNF may be in the GPS database.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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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?

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.