Baro VNAV 4 even more replies
Firdovsi Jalilovv - R.I.S.K. - Azerbaijan
There is a significant difference between the designer's thinking and real functioning of onboard system. Of course the designer remembers that he has designed and implemented LNAV procedure with designated FAF location at distance 7 nm from THR and MAPT at THR. At the next step he designed Baro-VNAV procedure with FAP height 600 meters, with nominal position located at 5 nm distance from THR where lateral boundaries are based on already existing LNAV procedure created a step earlier. But how can all these information be passed to onboard system memory? For managing the approach with Baro-VNAV onboard system load only one procedure from memory. And this procedure should store on one hand FAF LNAV which will allow to construct lateral area configuration correctly, and on the other hand contain FAP VNAV for the correct performing of procedure vertical profile. There is only one field in the LNAV/VNAV procedure retrieved from onboard system for storing two different FAF and FAP - either at distance 7nm, or 5nm from THR. So there appears a dilemma - how to simultaneously provide correct lateral boundaries and vertical profile taking into account the capabilities of onboard system? There is only one possible way-out for solving the collision considering the functionalities of onboard system - To combine FAF and FAP.
Beat Zimmermann - Air Navigation Institute - Switzerland
You need to show me a case where the LNAV FAF is at 7NM and the LNA/VNAV FAP at 5NM... Let's get that straight: Baro-VNAV only introduces the vertical component to an RNP APCH. That means you will calculate the FAP based on the distance THR-FAF, the intermediate altitude and the nominal descend gradient of the NPA. Y0u cannot have a gradient less then 5.2% on the NPA therefore when you chose the nominal VPA for the Baro it will be at a similar distance. It will not match exactly but the difference of 2NM like in your example is not going to happen. Furthermore the VNAV guidance will only be used when the VNAV mode is armed. In this case the FAP overrules the FAF. From an obstacle assessment aspect there is no concern, as the accountable obstacles laterally are defined by the LNAV procedure and the VNAV FAS will take care of the vertical component, immaterial of the FAF location.
Regarding the 15% rule The 15% rule is applicable in 3 different ways:
1. For conventional NPA only within the fix tolerance, maximum 5NM.
2. In the ILS case a surface starting at the early fix, sloping down until connecting to the W surface
3. In the RNAV case starting at the N-N-N line, ALWAYS until 5NM thereafter. This is no contradiction. Case 1) is general criteria, the other two cases fall under the "general criteria applies, as amplified or modified in this chapter" option. Just to remind designers out there: The case 2) in ILS should only be applied when there is a need to eliminate penetrations right before or right after the FAP. I have seen tendencies to establish a FAF at the FAP on a general basis. This should not be done for various reasons.
Answer or Commentary:P.S.
I.W. was stuck in Heathrow airport for days
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