RNAV (RNP) Procedures Clarification

[downscc 1,327]

FCOM SP4.6 provides a good overview of procedures for the RNP approach; however, I am old school and habitually try to use any resource available when shooting an instrument approach including monitoring the ILS freq if available when shooting the nonprecision approaches. Is this possible in the NGX?

 

Shooting the RNP per procedures works like a charm, but if I dare turn NAV to and ILS the computers respond as if I am flying the ILS... so far. Next time I will turn VOR updates off on the NAV Options page... is that the secret or should I not be monitoring the ILS?

[markadeane 272]

Hi Dan

 

If I'm thinking of the same page in the box as you are, VOR updates has to do with position verification. The box determines it's position using a mix of sources, including IRS position, DME and VOR triangulation as well as the standard GNSS receiver(s). 

 

In NZ we have a relatively sparse number of VORs and DMEs. In some locations, DME and VOR updating is inhibited during RNAV approaches to avoid spurious position information being calculated as varios navaids are 'lost' during the descent. 

 

As for the system logic that the autopilot uses, I'll defer to wiser heads!

[TheHorsley 3]

When flying an approach the safest way to determine the type of approach is through the order of precision, ILS, Rnav,VOR,NDB. So if the runway has an ILS, you would use that over an Rnav. If you're doing an Rnav for practice, when the same rwy has an ILS, having the ILS up is in fact not aiding in your practice of the Rnav, because as pilot's were trained to centre the Loc/G/S, so you'll end up following the ILS anyway. If you're going to go to the effort of flying an Rnav, might as well do it properly that way your profile management gets a good workout!

[Tabs 2,790]

FCOM SP4.6 provides a good overview of procedures for the RNP approach; however, I am old school and habitually try to use any resource available when shooting an instrument approach including monitoring the ILS freq if available when shooting the nonprecision approaches. Is this possible in the NGX?

 

Shooting the RNP per procedures works like a charm, but if I dare turn NAV to and ILS the computers respond as if I am flying the ILS... so far. Next time I will turn VOR updates off on the NAV Options page... is that the secret or should I not be monitoring the ILS?

 

Dan,

 

It should stay in LNAV / VNAV even with the ILS tuned provided you don't actually press LOC or APP. Doing this sort of thing where you actually fly the RNAV but monitor the ILS raw data is called an "overlay" approach. That said though, you don't technically need to monitor the ILS at all - RNAV RNP stuff is certified to be flown with the FMC/GPS as the sole source of navigation as long as ANP < RNP - it just has higher minimums than the ILS does.

 

VOR update and all of that has nothing to do with this - that page is controlling what sources feed into determining and updating the FMC's internal position. There's various sources of that data that it combines to arrive at the actual value it uses - GPS, the IRUs, external VOR signals etc. That's all handled automatically as long as all the sources are enabled.

[Cai Zicheng 171]

Dan,

 

It should stay in LNAV / VNAV even with the ILS tuned provided you don't actually press LOC or APP. Doing this sort of thing where you actually fly the RNAV but monitor the ILS raw data is called an "overlay" approach. That said though, you don't technically need to monitor the ILS at all - RNAV RNP stuff is certified to be flown with the FMC/GPS as the sole source of navigation as long as ANP < RNP - it just has higher minimums than the ILS does.

 

VOR update and all of that has nothing to do with this - that page is controlling what sources feed into determining and updating the FMC's internal position. There's various sources of that data that it combines to arrive at the actual value it uses - GPS, the IRUs, external VOR signals etc. That's all handled automatically as long as all the sources are enabled.

Hi Ryan,

If I have ILS frequency tuned, can I still activate FAC/GP?

 

Thanks,

Zicheng

[markadeane 272]

Hi Zicheng,

 

I believe the answer is no.

 

LOC / GS and FAC / GP modes are mutually exclusive (i.e. you can have one or the other but not both)

 

It would appear from comments here that the system logic looks for the highest precision guidance it has available to it. By tuning an ILS (making it available) the system would follow that in favour of the glidepath it has built for itself.

 

That said, I would encourage you to review the manuals, as I'm recalling this all from memory here and may not have the whole story!

[Cai Zicheng 171]

Hi Zicheng,

 

I believe the answer is no.

 

LOC / GS and FAC / GP modes are mutually exclusive (i.e. you can have one or the other but not both)

 

It would appear from comments here that the system logic looks for the highest precision guidance it has available to it. By tuning an ILS (making it available) the system would follow that in favour of the glidepath it has built for itself.

 

That said, I would encourage you to review the manuals, as I'm recalling this all from memory here and may not have the whole story!

Thanks.

[downscc 1,327]

"It should stay in LNAV / VNAV even with the ILS tuned provided you don't actually press LOC or APP"

 

Yup, the procedures DON'T say press APP so I've earned a headslap. Works better now, thanks. OBTW the book says to turn OFF VOR Updates and I'm sure there's a reason for that. My guess is that the TERPS folks designed the approach to be flown with GPS (with WAAS if applicable) within its performance envelope and the FMS would combine a VOR position to the mix thus adding a factor not included in the design of the approach. With that in mind, wonder why some fixes on some RNP approaches have the note "RF REQUIRED?"

[Fabo 213]

 

 

With that in mind, wonder why some fixes on some RNP approaches have the note "RF REQUIRED?"

 

would it be radius-to-fix waypoint type capability? The exact thing the NGX does not have, owing to the ancient datafile design

 

(btw, Rob, Ryan, you were talking about changing the format to ARINC 429 compliant one, is that still on the table? Possibly with 777 release?)

[downscc 1,327]

Don't think so, lots of RNP approaches with RF legs don't have this note (e.g., KPSP RNAV (RNP) Z 13R). I've seen RADAR REQUIRED notes but to me RF means radio (I'm a communications engineer).

[Steven_Caffey 23]

Don't think so, lots of RNP approaches with RF legs don't have this note (e.g., KPSP RNAV (RNP) Z 13R). I've seen RADAR REQUIRED notes but to me RF means radio (I'm a communications engineer).

 

I thought this example seemed odd, look closely at the notes in the top left (if looking at FAA chart, may be in a different place on Jepps) It says "RF and GPS required" And yes, the "RF Required" does in fact refer to Radius to Fix capability.

[Porthos 9]

 

 

VOR update and all of that has nothing to do with this - that page is controlling what sources feed into determining and updating the FMC's internal position. There's various sources of that data that it combines to arrive at the actual value it uses - GPS, the IRUs, external VOR signals etc. That's all handled automatically as long as all the sources are enabled.
 

 

 

also when flying an RNP AR approach VOR and DME updating is generally disabled as it pollutes the nav solution from the GPS, IRU, and MMR. 

[Contact409 0]

https://www.youtube.com/watch?v=cpySigq3XxI   

 

Take a look of this video.  

[Contact409 0]

Don't think so, lots of RNP approaches with RF legs don't have this note (e.g., KPSP RNAV (RNP) Z 13R). I've seen RADAR REQUIRED notes but to me RF means radio (I'm a communications engineer).

 

Briefing strip of KPSP RNP Z 13R says 1: Authorization Required. 2 RF and GPS required.