Advisory Glidepaths

Reprinted from IFR Magazine June 2013 –

A Safety Compromise

Visual Descent Angles (VDAs) are the angle between the runway threshold at the crossing height (TCH) and the minimum altitude at the final approach fix.  Stepdown fixes are normally located so the VDA crosses the fix at or above the fix minimum altitude.  AeroNav charts show these with the VDA and TCH anywhere in the profile view.

But sometimes a straight path from a FAF to the runway threshold crosses a stepdown fix below its minimum altitude.  For these, the VDA is computer from the threshold to the stepdown fix minimum altitude rather than the FAF.  AeroNav charts show this with the VDA and TCH squeezed between the stepdown fix and the runway in the profile view.

VDAs enable Continuous Descent Final Approaches (CDFA, also known as Constant-Angle Non-Precision Approach, CANPA) on non-precision approaches.  These are safer than the ‘dive and drive’ method and most airlines have adopted CDFA for non-precision approaches.  However, on typical general aviation GPS units with WAAS, CDFA techniques are limited to flying GPS approaches to LNAV minimums with an advisory glidepath indication (LNAV+V).

But the comfort of flying a stabilized glidepath on the CDFA conflicts with the reality that obstacle clearance may not exist below MDA.  This is enough of a concern that the FAA has started to remove VDA information from approaches without obstacle clearance on the VDA below the MAP.

For these approaches, the VDA is replaced on the approach plate with a note stating “Descent Angle NA”.  Removing the VDA, and thus the derived glidepath, solves he problem of pilots potentially descending into obstacles mindlessly following that derived glidepath.  But, it also negates any benefits from the CDFA techniques prior to the MDA.

VDA

What’s in Your Database?

Properly flying a continuous descent on approach requires a glidepath indication, which is computed based on the VDA and TCH values stored in a GPS database.  We spoke to Jeppesen to learn about how they process instrument procedures into their aeronautical databases.

Jeppesen receives its US source data from the FAA and encodes the data in ARINC 424 format, the standard aeronautical data format used in aircraft.  This data is then provided to vendors, such as Garmin.  Although the format includes a lot of information, some of the details found on the actual approach chart are omitted – minimums, in particular – which is why pilots must still have a chart for the procedure being flown.

Jeppesen will include VDAs that are supplied in the source data, and compute then if not in the source data, unless the source data specifically deletes them by showing a zero angle as the FAA has begun to do.  Jeppesen also ensures that supplied VDAs will meet all stepdown fixes.  When the FAA’s VDA is computed based on a stepdown fix, Jeppesen data applies that VDA back to the altitude of the FAF.  A pilot level at the published FAF minimum altitude will intercept this vertical angle after the FAF resulting in a delayed, but continuous, descent.

Inside Garmin Navigators

We also talked to Garmin about how they use the navigation data.  Garmin receives navigation data from Jeppesen and converts it into a format for the actual devices in the cockpit.  This is what you install.

Garmin requires specific conditions before the depict an advisory glidepath.  Advisory glidepaths will only be displayed for GPS approaches (including overlay approaches) that have LNAV minimums and no charted LNAV/VNAV or LPV minimums.  There is currently no support for advisory glidepath to LP minimums.  The unit must also be receiving WAAS and integrity at the same level as is required for LNAV/VNAV minimums.  Finally, the procedure must have a non-zero VDA.  The last requirement means that LNAV+V will be unavailable for procedures that have had VDAs removed by the FAA.

Procedures that meet these criteria will depict a derived glidepath.  Because the VDA is relative to the runway threshold and independent of the MAP or approach minimums, the glidepath is shown all the way down to the threshold.  This is where pilots might get into trouble if they continue to follow the glidepath without regard to potential obstacles.

Why not discontinue display of a VDA below the MDA?  Unfortunately, minimums are not included in the ARINC 424 source data.  Additionally, the avionics could not know the applicable minimums for a given approach.  The published minimums might be raised, for example by NOTAM or due to a remote altimeter source in use or even just circling vs straight in.

Glidepath Interpretation

Pilots must realize that once past the MAP, they are responsible for their own obstacle clearance and should not assume they will be safe on the glidepath, as they are with an ILS.  Any glidepath provided on a non-precision approach is for advisory use only.  The AIM reflects this, avionics manuals and aircraft flight manual supplements reflect this, yet it remains a source of confusion.

This is a classic human factors problem with pilots being pulled in conflicting directions.  Many pilots have been conditioned to follow the glidepath by tears of flying ILSes and human nature is to remember best what we learned first.  Since there are now glidepaths for advisory use only, it’s not surprising that a pilot under stress toward the end of an instrument approach might fall back onto their familiar ILS glidepath mindset.

At the same time, continuous descent final approaches on non-precision approaches offer very real safety benefits and are working their way into general aviation, as evidenced by LNAV+V capabilities in WAAS GPS navigators.  How can we take advantage of the benefits of CDFA while avoiding obstacles if following an advisory glidepath below MDA is currently a hot topic within the FAA and industry; expect to hear more once the dust settles.

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