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.