RNAV Approach and Platform Altitude

[Zenj]

I have a question on RNAV approach with full guidance that's LNAV and VNAV, why do we have to select platform altitude ( that's a final approach fix altitude ) when we are cleared for non precision RNAV approach ?

[jmmoric]

"Non-precision approaches" only contain LNAV and LP.

LNAV/VNAV is defined as "Approach procedure with vertical guidance" or APV.

So if you were cleared "non precision RNAV approach", you cannot be flying something with vertical guidance.

But generally you'll be cleared "RNP approach", and select the minimum yourself.

Your question.... I don't know :)

[rudestuff]

I have a question on RNAV approach with full guidance that's LNAV and VNAV, why do we have to select platform altitude ( that's a final approach fix altitude ) when we are cleared for non precision RNAV approach ?

Why WOULD you? If you're cleared for the approach you would set the first altitude below MDA (Or it won't go down in VNAV!)

[Check Airman]

Kinda depends on what airplane we’re discussing, doesn’t it?

[Zenj]

Well I see it being done on A320, E190 and B787
When cleared for approach you set altitude to whats shown on the chart as FAF altitude, so IAF and AF altitudes will always be respected due to VNAV.

Why we set this platform altitude or altitude at FAF? This is my question.

[FlightDetent]

What position and altitude was this cleared for APCH ?

[wiggy]

I think on at least one of those types what you do depends on the vertical mode you are using prior to the FAF.

According to one set of SOPs If you have fully checked the FMC etc, are descending in VNAV path ( not for example V/S or FLCH) on the arrival and then when cleared for the approach you can immediately set the MDA..

(I see whilst I was typing Alpine Flyer has covered the "trap")

[Alpine Flyer]

Once you're on VNAV you could also set the missed approach altitude (which you could also do once established on the final altitude while waiting for GS intercept). One reason not to to that could be maintaining uniform procedures over different approach types and/or better protection in case you decide for whatever reason to change from VNAV to VS or FLCH (e.g. in response to an "increase rate of descent" command) before being established at the FAF altitude.

[Zenj]

Usually you set missed approach altitude when you get a green indication of GP or VGP depending on the type, this is exactly like ILS where you set missed approach after getting G/S captured.

So like ILS approach where you also set platform altitude ( FAF altitude ) before the approach, we also set the same for RNAV approaches with VNAV guidance, why do we do this ?

[FlightDetent]

Rephrase and ask again, lost in the translation.

[sonicbum]

I have a question on RNAV approach with full guidance that's LNAV and VNAV, why do we have to select platform altitude ( that's a final approach fix altitude ) when we are cleared for non precision RNAV approach ?

How You will fly an APV is very much type/company specific.
What type to start with ?

[vilas]

I have a question on RNAV approach with full guidance that's LNAV and VNAV, why do we have to select platform altitude ( that's a final approach fix altitude ) when we are cleared for non precision RNAV approach ?
I take it that you're higher than FAF and you are cleared for approch. You will have to descend to FAF or platform altitude or FINAL APP will not engage from above. It can intercept the path from below but not from above.

[Check Airman]

I take it that you're higher than FAF and you are cleared for approch. You will have to descend to FAF or platform altitude or FINAL APP will not engage from above. It can intercept the path from below but not from above.

I’ve always wondered why it had this limitation. Do you know if FLS is the same?

[FlightDetent]

On the A320 when intercepting from above, the FINAL APP will engage. Of course only if the waypoints are correctly sequenced and once you position
sufficiently close to the geometrical profile.

[vilas]

There's only very small margin from above and in an Airbus presentation it is not recommended. Main thing is FINAL APP mode is designed to fly only between Final Descent Point down to applicable minima. It is not capable of flying any level off segments with altitude constraints before final descent point. That must be done in Des mode.
The To way point must be the FDP is one of the conditions for engagement of APP mode. So if vectored sequencing to FDP(Not beyond) is mandatory.

[vilas]

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/2000x1653/img_20200627_210326_d3b8acf4f2bfa9639c9c785a91a770efcf9f04a2 .jpg
The last condition VIP stands for vertical intercept point same as FDP.