Baro VNAV 4
APV/BAROMETRIC VERTICAL NAVIGATION
4.1.3 The LNAV-only FAF and MAPt are used to define the areas but are not part of the VNAV procedure.
There are two schools of thought concerning this sentence in Pans-Ops.
“The LNAV approach protection areas define the BVNAV protection areas.”
“The BVNAV obstacle protection areas are determined by applying the LNAV criteria.”
There is a subtle but important difference between the two interpretations. Take the case of an LNAV approach that has a MAPt a 1 nm before the threshold. Group 1 would design the BVNAV protection areas with the missed approach area splay commencing at the LNAV MAPt (see diagram below).
While group 2 would argue that in a BVNAV approach the onboard system is flying not to the LNAV MAPt but to the threshold. So LNAV criteria apply as if the MAPt was at the threshold immaterial of where the LNAV MAPt is.
Construction method 1 or 2 could have a significant impact on the BVNAV OCA/H values as can be seen in the previous diagrams. This is like saying that the LOC only approach can affect the ILS minimums.
Now let us look at the beginning of the BVNAV approach. For example, the LNAV approach has a FAF at 7nm. This could have been done to enable the use of the 15% descent criteria to eliminate obstacles in the fix tolerance area. However the BVNAV approach has an intercept point (FAP) at 5nm. Group 1 would design the BVNAV approach protection areas starting from the 7nm LNAV position. This would bring into play the ILS “W” surface with all it’s restrictions as the final approach is longer than 5nm (see discussion page entry 256). Group 2 on the other hand would argue that the BVNAV segment of the approach did not start until the aircraft establishes on the pseudo glide path at 5nm. This would have the benefit of eliminating the ILS “W” surface from the equation and possibly significantly lowering minimums.
Once again this is a situation where the LNAV approach could significantly raise the minimums of the BVNAV approach.
Are you a member of group 1 or group 2?
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