Vectored RNAV/RNP Approaches

[MFarhadi]

Hi,


In the part of the world where I live and work, we use RNAV approaches left, right, and center. Although I am not currently flying a Boeing product, I have spent time in the cockpit of 737s (commuting life is not fun—try to avoid it, seriously) while being vectored for RNAV (ICAO: RNP APCH) approaches.

I have observed crews using the extended centerline from the IF or FAF, arming LNAV, and capturing the final lateral path without difficulty. So far, so good. My main question concerns VNAV PTH in this scenario. I have seen 737 crews at my company arm VNAV and, almost magically, have an extension of the glide path (most of the time the 3.00º) appear backward toward infinity. However, despite trying to replicate this in PSX, I have not had any success. It seems that the 747-400 only recognizes the final approach glide path from the FAF onward.

How do you typically fly a vectored RNAV approach? Is this practice common anywhere outside Canada?


Best,

[Mariano]

If doing a course-intercept from the FAF doesn't work, have you tried deleting all altitude constraints between the point from where you get approach clearance and the FAF? It's just a theory.

It's a good question. I can't recall how (the 767) behaves in this area when we get approach clearance via vectors. We never delete anything once we get clearance (we do LAVS and let it be - KISS ;-).

Try the method above and see if VNAV PTH (actually the extended glidepath) respects the deleted charted waypoint altitude restrictions while flying the FAS glidepath angle (if it actually does fly the correct angle).

Best regards,

Mariano

[jawomba]

...

How do you typically fly a vectored RNAV approach? Is this practice common anywhere outside Canada?


Best,

Hi,

related to the RNAV technique you can check out the following promo video for Captain Al's Subsonic Flight Training
B747-400 FTD-PC FS 7 BRIEF PROMO.

regards,
Jan

[MFarhadi]

Thank you all for pointing out the LAVS technique.

From my experiments, being vecotred for RNAV with extentions made from FAF outwards does not necessarily extend out the approach glide path angle, already defined between FAF and runway threshould (or Missed Approach Point). It is usually much more shallow than the extended glide path.

[Hardy Heinlin]

On the FMC ARRIVALS page, when a VFR runway is selected, the default FPA of 3.0 is displayed in large font, but you can overwrite it with a lower angle. The lowest valid entry is 2.4. Did you try that or do I misunderstand your question? I don't know why the FMC designers decided to display the default FPA in large font instead of small font. Small font would make it clear that a manual entry is possible.


Regards,

|-|ardy

[MFarhadi]

Hi Hardy,

For example, let's reference EDDF RNP Y Rwy 07R. When ATC starts vectoring, you'll go to CDU DEP ARR page, select RNPY07R, no transition, DF650 INTC> on LSK 6R. Between DF650 and RW07R is now a 3.20º Glide path which VNAV PTH will hapilly follow. But if ATC vectors you to intercept the final at somewhere near DF652 with an altitude of 6000 feet, the vertical path the FMC takes you is not an extention of that final 3.20º, but a "vertical direct" towards DF650 and then 3.2º down to the runway.

edit: From what I can see from the instructor's window, the V/B in this scenario is lower than the final approach glide path angle and the DES path type is "non-idle path" until you cross the FAF. Then V/B turns into 3.2º and the descent type turns into "Final 3.2º".

My question is that how does the V/DEV diamond actually behave before reaching FAF (in our example, DF650 at 4000 feet). Is it an extention of final glide path?

Best,

[Hardy Heinlin]

Hi Mohammadreza,

I think the VNAV path deviation indicator behaves like a radio glideslope system; i.e. when you're approaching the target glidepath from below, the pointer will be above you. While you're flying along the platform the pointer gets lower and lower, so you can anticipate the T/D point. If the pointer would be centered along the platform flight, you couldn't anticipate the T/D; the pointer would just suddenly start to drop and you would be surprised :-) – To control your platform altitude you have the altitude bug; so there's no need to use the VNAV pointer for that job.


Regards,

|-|ardy