Baro VNAV 4 more replies - updated

08.12.2010

Submission:

READ Baro VNAV 4 (entry 273) FIRST!

Firdovsi Jalilovv - R.I.S.K. - Azerbaijan

The assertion that VNAV is independent from LNAV can cause misuse of ARINC 424 rules of procedure coding, based on which onboard systems function. Let's assume that VNAV is independent from LNAV and there are two various points in the procedure LNAV FAF (7 nm from THR) and VNAV FAP (5 nm from THR). How is it possible to code correctly such a procedure according to ARINC 424? What stage does the segment between FAF and FAP belong to (intermediate approach or final approach)? For coding of one approach procedure ARINC 424 allows applying of only one final approach segment. Consequently, we come to a conclusion that for correct interpreting of Baro-VNAN procedure by onboard system it is necessary to overlap the point LNAV FAF and VNAV FAP. As to the statement that – “Some older Airbus systems were actually unable to compute the vertical guidance when the LNAV MAPt is not at the THR.” – we can give the following explanation: For all procedure types with vertical navigation ARINC 424 requires that the end point of final approach segment should be runway THR. But this does not at all mean that THR in this case plays the role of MAPt and missed approach should commence not later than THR. 15° splay angle of missed approach lateral area should be connected not with the landing point which is THR, but with the point where aircraft really begins changing of configuration for performing missed approach (point at VPA corresponding to DH height). That's why in my opinion it is more logical to overlap LNAV MAPt with corresponding DH at VPA.

Laurent Delétraz, OOTX, Head of Instrument Flight Procedures

Skyguide – Switzerland

I fully support the comment of Beat Zimmermann and why looking to mix LNAV with a LNAV/VNAV which does not fit. We cannot combine two procedures with different FAF or/and MAPt, even the publication is then not possible anymore in the same chart. But this means that developing a LNAV/VNAV requires to draw the LNAV protection area but with the same FAF and the threshold considered as the MAPt. Anyway according the new ICAO resolution 37-11 it is asked to publish LNAV minima only when publishing a LNAV/VNAV approach. In Switzerland the problem is solved as we define in our Instrument Flight Procedure manual that the MAPt shall be on the threshold for RNAV approaches. This is also the only way for the crew to know the distance to the threshold and avoid confusion.