Figure: Example "Descent Angle NA" approach, from SAIB HQ-14-25, p. 2.
It makes sense to use a Continuous Descent Final Approach (CDFA) on most non-precision approaches, for more about that see: Instrument Procedures / Continuous Descent Final Approach (CDFA). But what about for straight in approaches where the FAA says "Descent Angle NA" on the chart?
There are two things to keep in mind here:
- Once you descend below any published altitude, including the MDA, obstacle clearance is your responsibility. If the approach has a certified visual descent segment, such as on an ILS, an RNAV(GPS) with LNAV/VNAV minimums, or one with PAPI or VASI vertical guidance, you have the additional reassurance that the vertical path is okay within the specified margins.
- You may want to consider flying an approach other than one with "Descent Angle NA" noted if your FMS has issues with these types of approaches. I can't answer this for you, but as of October 2015, we in the Honeywell world have to be wary of these approaches. (More about that below.)
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
[SAIB HQ-14-25, p. 1.]
- The Federal Aviation Administration (FAA) believes advisory vertical guidance can aid the pilot when flying the final approach segment of IAPs without a glideslope or an approved glide path. As a result, there were multiple public requests for the FAA to publish vertical descent angles on these IAPs. Many manufacturers then chose to use the published vertical descent angle when providing advisory vertical guidance for these IAPs.
Translation: the FAA supports using a continuous descent final approach.
- The types of IAPs the FAA publishes with a vertical descent angle to help provide advisory vertical guidance include: 1) conventional IAPs (i.e., very-high frequency omni-directional receiver (VOR), localizer-type directional aid (LDA), simplified directional facility (SDF), etc.); and 2) required navigation performance IAPs titled “RNAV (GPS) RWY XX” with stand-alone lateral navigation (LNAV) and/or localizer performance without vertical guidance (LP) lines of minima.
Translation: The FAA lumps togehter VOR, LDA, SDF, and RNAV(GPS) approaches without VNAV minimums into this discussion. The guidance gets murkey because this paragraph covers "IAPs the FAA pbulishes with a vertical descent angle."
- The published vertical descent angle and navigation equipment-generated advisory vertical guidance offers no guarantee of meeting altitude constraints. Advisory vertical guidance does not guarantee obstacle protection or compliance with procedural altitudes. Advisory vertical guidance solely offers an aid to help pilots establish a continuous, stabilized descent during the final approach of the IAP and avoiding the traditional “dive and drive” method. Pilots must use the primary barometric altimeter to comply with all air traffic clearances and altitude constraints.
Translation: A published vertical descent angle on one of these approach types requires that you crosscheck performance against altitude and obstacle constraints; the FAA is not here to help in the case of these approaches. Note that we are again talking about "published vertical descent angle" approaches.
- When the FAA charts these IAPs, they do not show a vertical descent angle in the profile view. The charts currently include the following statement: “Descent Angle NA”. Like flying any other IAP, the pilot must see and avoid any obstacles in the visual segment during transition to landing.
Translation: Once you depart the MDA, it is up to you to spot any obstacles. This paragraph addresses IAPS that "do not show a vertical descent angle," switching gears on us.
- The affected IAPs may create a hazard if the pilot continued to reference the advisory vertical guidance while transitioning to the visual segment of the approach. To avoid creating a possible hazard, FAA procedure designers began excluding a vertical descent angle from these IAPs and coding a “0” (zero) in the appropriate ARINC 424 database format specifications to communicate the absence of the vertical descent angle.
So now we are talking about the approaches with "Descent Angle NA" noted. This "0" coding can be problematic with some FMS because some systems will translate this zero into a default 3° angle, which may not be true, or do something with that zero (such as divide by zero) that the FMS cannot understand. So what to do?
- The FAA recommends operators and pilots carefully review departure, destination and alternate airport IAPs during pre-flight operations. During this review, operators and pilots should look for IAPs published with no vertical descent angle and charted with the statement: “Descent Angle NA” or “Descent Angle NA – Obstacles” in the profile view of the procedure. For these IAPs, the FAA recommends finding and planning to use another IAP (if available) not affected by this airworthiness concern.
Honeywell Update (October 2015)
We in the Honeywell world are adversely impacted . . .
[Honeywell Direct-To, October 2015]
- As stated in the SAIB HQ-14-25, there may be unintended consequences with the use of a zero value coded in the FPA data field. In summary, the SIL describes how Honeywell Business and General Aviation flight management systems will treat a 0 degree FPA as an invalid angle and will insert a valid angle as calculated by the FMS.
- The procedures that have a zero coded as the FPA will still contain altitude constraints. Because the altitude constraints are included in the approach data, the FMS will make use of these and will insert the default vertical angle, typically 3.00 degrees, and continue to provide advisory VNAV to the MAP. The altitude encoded on the MAP may not reflect an appropriate altitude from which an aircraft can make a normal landing (see figure).
Figure: Advisory VNAV Path, from Honeywell Direct-To, figure 2.
Honeywell FMS users will get a 3° angle, but the angle may not necesarily end at 50' height above touchdown at the end of the runway.
- Furthermore, many databases do not contain intermediate step down fixes (SDF) and the default vertical angle may not provide adequate guidance above the SDF altitude. When flying procedures that have the statement “Vertical Angle NA,” the use of advisory VNAV should not be used (see figure).
Figure: Advisory VNAV Path Violating SDF, from Honeywell Direct-To, figure 3.
The drawn angle may fail to honor stepdown altitudes.
- Pilots should remain vigilant when utilizing VNAV on non-precision procedures and should understand the following:
- Flight crews are responsible for ensuring obstacle clearance in the visual segment for procedures identified by the FAA as ‘Descent Angle NA’ or ‘Descent Angle NA – Obstacles’.
- As with all step down fixes, flight crews must comply with published step down altitudes.
- Honeywell is working to either include the descent angle to ensure the FMS will clear those step down fix altitudes, or to remove the affected procedures from the navigation database.
- Until these procedures are either removed or mitigation is approved, operators are encouraged to follow the FAA recommendation found in the SAIB.
Honeywell Direct-To October 2015, Advisory Vertical Guidance with No Published Vertical Descent Angle.
Special Air Worthiness Information Bulletin (SAIB) HQ-14-25, FMS and GPS Equipment Advisory Vertical Guidance with no Published Vertical Descent Angle, August 26, 2014.