The Final Approach “Fix” on an ILS

Consider the ILS RWY 26 at Lewiston, ID (KLWS). This approach is a “pure” ILS; it doesn’t offer an “or LOC” option.

Here’s a question that came up recently during a presentation that I gave to a group of IFR pilots:

“Where’s the final approach fix?”

The profile view does not include the familiar “Maltese cross” that marks the FAF on charts for procedures that include minimums for both a full ILS (a precision approach with a glideslope) and a localizer-only, nonprecision approach to an MDA, as on the ILS RWY 20 at KPWT.

But because the chart for KLWS is only for a precision approach, it doesn’t have a charted “final approach fix.”

AIM 5−4−5. Instrument Approach Procedure (IAP) Charts includes a note that explains the situation:

The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude. This point marks the PFAF [precision final approach fix] and is depicted by the ”lightning bolt” symbol on U.S. Government charts. Intercepting the glide slope at this altitude marks the beginning of the final approach segment and ensures required obstacle clearance during descent from the glide slope intercept altitude to the lowest published decision altitude for the approach.

The Aeronautical Chart User’s Guide also explains:

Non-Precision Approaches
On non-precision approaches, the final segment begins at the Final Approach Fix (FAF) which is identified with the Maltese cross symbol. When no FAF is depicted, the final approach point is the point at which the aircraft is established inbound on the final approach course. Stepdown fixes may also be provided between the FAF and the airport for authorizing a lower minimum descent angle (MDA) and are depicted with the fix or facility name and a dashed line. On non-precision only approach procedures, the approach track descends to the MDA or VDP point, thence horizontally to the missed approach point.

The ACG offers the following additional distinction:

Precision Approaches
On precision approaches, the glideslope (GS) intercept altitude is illustrated by a zigzag line and an altitude. This is the minimum altitude for GS interception after completion of the procedure turn. Precision approach profiles also depict the GS angle of descent, threshold crossing height (TCH) and GS altitude at the outer marker (OM) or designated fix.

The plan and profile views for the KLWS approach may further confuse matters because they include a computer navigation fix (CNF); in this case (CFLSK). The Pilot/Controller Glossary explains CNF thus:

COMPUTER NAVIGATION FIX (CNF)- A Computer Navigation Fix is a point defined by a latitude/longitude coordinate and is required to support Performance-Based Navigation (PBN) operations. A five-letter identifier denoting a CNF can be found next to an “x” on en route charts and on some approach charts. Eventually, all CNFs will be labeled and begin with the letters “CF” followed by three consonants (e.g., ‘CFWBG’). CNFs are not recognized by ATC, are not contained in ATC fix or automation databases, and are not used for ATC purposes. Pilots should not use CNFs for point-to-point navigation (e.g., proceed direct), filing a flight plan, or in aircraft/ATC communications… (REFER to AIM 1-1-17b5(i)(2), Global Positioning System (GPS). [See below for a more detailed explanation.]

Back to AIM 5-4-5. If you’re flying an ILS, make sure you observe any altitude restrictions outside the published GS intercept altitude. The AIM cautions that:

Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent. If the pilot chooses to track the glide slope prior to the glide slope interception altitude, they remain responsible for complying with published altitudes for any preceding stepdown fixes encountered during the subsequent descent.

More about CNFs from AIM 1-1-17b5(i)(2):

A Computer Navigation Fix (CNF) is also a point defined by a latitude/longitude coordinateand is required to support Performance−Based Navigation (PBN) operations. The GPS receiver uses CNFs in conjunction with waypoints to navigate from point to point. However, CNFs are not recognized by ATC. ATC does not maintain CNFs in their database and they do not use CNFs for any air traffic control purpose. CNFs may or may not be charted on FAA aeronautical navigation products, are listed in the chart legends, and are for advisory purposes only. Pilots are not to use CNFs for point to point navigation (proceed direct), filing a flight plan, or in aircraft/ATC communications. CNFs that do appear on aeronautical charts allow pilots increased situational awareness by identifying points in the aircraft database route of flight with points on the aeronautical chart. CNFs are random five-letter identifiers, not pronounceable like waypoints and placed in parenthesis. Eventually, all CNFs will begin with the letters “CF” followed by three consonants (for example, CFWBG). This five-letter identifier will be found next to an “x” on enroute charts and possibly on an approach chart. On instrument approach procedures (charts) in the terminal procedures publication, CNFs may represent unnamed DME fixes, beginning and ending points of DME arcs, and sensor (ground-based signal i.e., VOR, NDB, ILS) final approach fixes on GPS overlay approaches. These CNFs provide the GPS with points on the procedure that allow the overlay approach to mirror the ground-based sensor approach. These points should only be used by the GPS system for navigation and should not be used by pilots for any other purpose on the approach. The CNF concept has not been adopted or recognized by the International Civil Aviation Organization (ICAO).

Note that a modern GPS-based navigator like the Garmin GTN 750 includes the CNF fix CFLSK in the flight plan when you load the ILS RWY 26. And it conveniently labels it as the FAF; that fix corresponds to the GS intercept altitude and marks the beginning of the final approach segment.

4 thoughts on “The Final Approach “Fix” on an ILS”

  1. First time that I have heard of a Computer Fix. I vaguely remember seeing an x or two but did not pay much attention. I was aware of the FAF ,strictly speaking, being an altitude on the GP for RNAV, but did not put that together for an ILS. I need to fly an ILS or two more often, very hooked on LPV RNAV. I assume that the last AIM reference in your post means that you can not blow through the floor of an outer step down at the inner corner of the step down. Trying to picture how it could happen.

  2. I appreciate your quick response to this good question that came up and thorough analysis of this use case. As a mid-winter rusty IFR pilot, these sessions are invaluable for filling in the many little details that are out there.

    Thanks for all your professional and high quality contribution to our continued learning Bruce!

  3. Interesting that the Jeppesen approach plate uses a large open star instead of the “x” found on the Gov plate. Also the CFLSK is bracketed instead of a parenthesis, maybe somewhat more “well defined.”

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