Visual descent point

25.10.2004

Submission:

1. Following extensive research into Controlled-Flight-Into-Terrain (CFIT), primarily coordinated by Flight Safety Foundation, the stabilized approach has enjoyed, rightly so, a great deal of attention. One of the recommendations were to fix a 5% PDG for non-precision approaches. The idea being to do away with stepped descents on the final segment (terrain allowing).

2. As far as I can ascertain TERPS used a Visual Descent Point (VDP) for some time to place the aircraft (theoretically) at a position from where "...normal descent from the MDA? to the runway touchdown point may be commenced" : Appendix C to Doc 9365-AN/910. I would prefer OCA as terminology but agree with the principle. Some questions arise in the execution:

3. VDP Construction. The first question is not significant in context but could be for standardization. I have come across the following design principles:

15m above the threshold 5% towards the final approach segment.
Where the lowest VASI glide slope intersects the lowest MDA. Where no VASI is installed: a point from where a 300-400 ft per NM can be commenced to the threshold.
VDP Use. I am comfortable with the use as outlined in par 2. Some Airlines however require the VDP and MAPt to coincide: "so that there is no need to fly level close to the ground, thereby minimizing the chance of a CFIT accident occurring".
This could, and have resulted in aircraft descending below MDA as the approach followed is similar to a precision approach (these operators believe it to be the lesser of 2 evils).
It is my opinion that the VDP and MAPt should preferably not be the same point to force round out at MDA should visual reference not be established at an altitude above MDA as determined by individual operators. Furthermore it provides valuable time to obtain visual as this still remains a non-precision approach and positioning will not be as accurate as where vertical guidance is available, the risk is contained/minimized by the protection areas associated with a non-precision approach
I look forward to your reader's comments.

Gerhard Coetzee: Flight Procedure Specialist (South-Africa)