RNAV legs vs RF legs

[Holger Wende]

Hi all,

I read a lot in this forum about PBN, RNP, RNAV and RF legs.
And to be honest I am (still) a bit confused about RNAV legs vs RF legs 

To make it short a few questions:

1. In approach charts how can I differentiate "RNAV legs" from "RF legs"?
I have seen posts from e.g. Will about Quito and I found charts for Katmandu where the charts state "RF required".
But e.g. Queenstown chart for NZQN RNAV Z RW23 does not state "RF required". Does this mean these arcs are no RF legs but just RNAV legs/arcs? (e.g. http://www.aip.net.nz/NavWalk.aspx?section=CHARTS&tree=Queenstown and open RNAV (RNP) Z RWY 05 & 23)

2. When I see a procedure stating RF required, does this mean it must be flown by the autopilot? I assume a human cannot achieve the required navigation performance.

3. Is there any way to identify approaches that contain "RF legs" in the FMS arrivals list?
As far as I understood (only) aircraft equipped with the NG FMS can fly the RF legs. And aircraft unable to fly RF legs use slightly different approaches.
Therefore I assume 744 with the old FMS have another database on board or somehow filter the RF leg required approaches?

4. Is there a special reason why only one NZQN RNAV approaches appears in the PSX FMS arrivals list?
Of course I am able to build an approach based on the waypoints, but I assume this is not the preferred/professional procedure.

Thanks, Holger

EDIT: I had the old Nav database, the new NAV2003 contains only NZQN RNAV 05

[Jeroen Hoppenbrouwers]

This helps a bit, I think:
https://en.wikipedia.org/wiki/Performance-based_navigation

[Holger Wende]

Hi Hoppie,

Yes, that helped, thanks... and raised another question/observation:

How does RNP provide vertical guidance? Barometric VNAV?

I ask this because while reading this and related Wiki articles and looking more detailed into the NZQN RNAV (RNP) Z 05/23 charts I noticed that these procedures are not authorized below -10°C and above 35°C.
But the RNAV (GNSS) approaches do not seem to have these limitations.

It sounds a bit contradictory to me:
The apparently higher precision approach "RNP" has limitations which the less precise approach does not have.
But maybe the required precision is exactly the reason for this limitation?

Thanks, Holger

[Hardy Heinlin]

1. In approach charts how can I differentiate "RNAV legs" from "RF legs"?

RNAV legs are a collection of various leg types, the RF leg being one of them.
RNAV legs are pizzas; the RF leg is pizza salami.

RNAV is simply "area nav": https://en.wikipedia.org/wiki/Area_navigation

The arcs on the Queenstown chart for NZQN RNAV Z RW23 are RF legs. They are not DME arcs; they have no DME station at the arc center. And their start and exit courses are in line with their respective adjacent courses. They are clearly RF legs.

2. When I see a procedure stating RF required, does this mean it must be flown by the autopilot?

I think manual control with the F/D on is allowed, but A/P operation is recommended.

3. Is there any way to identify approaches that contain "RF legs" in the FMS arrivals list?

No.

As far as I understood (only) aircraft equipped with the NG FMS can fly the RF legs. And aircraft unable to fly RF legs use slightly different approaches.
Therefore I assume 744 with the old FMS have another database on board or somehow filter the RF leg required approaches?

Correct. When you switch to the legacy FMC in PSX, the DEP/ARR list will be refreshed and any procedure containing an RF leg will be removed from the list, and existing RF legs in the route will change to direct-to-fix legs (initial fly-over turn with groundspeed based turn radius, then direct straight line to next fix).

4. Is there a special reason why only one NZQN RNAV approaches appears in the PSX FMS arrivals list?

The 744 cannot fly GNSS procedures. This requires special GPS hardware which is installed on the 74-8 only.


Regards,

|-|ardy

[Hardy Heinlin]

How does RNP provide vertical guidance? Barometric VNAV?

Yes, it refers to barometric altitude.

But if GNSS equipment is involved (additional GPS signals from the ground), the altitude is GPS based as well -- i.e. fully geometric, not barometric. The 744 is not fitted with this special GPS equipment.


|-|ardy

[dhob]

1. In approach charts how can I differentiate "RNAV legs" from "RF legs"?
I have seen posts from e.g. Will about Quito and I found charts for Katmandu where the charts state "RF required".
But e.g. Queenstown chart for NZQN RNAV Z RW23 does not state "RF required". Does this mean these arcs are no RF legs but just RNAV legs/arcs? (e.g. http://www.aip.net.nz/NavWalk.aspx?section=CHARTS&tree=Queenstown and open RNAV (RNP) Z RWY 05 & 23)

For RNAV (RNP) or RNP AR approaches, RF Legs are implicit in the design. In other words, the statement "RF Legs Required" is redundant.

2. When I see a procedure stating RF required, does this mean it must be flown by the autopilot? I assume a human cannot achieve the required navigation performance.

Not necessarily. RF Legs are contained in RNP STARS and SIDS, as well as approaches. RNP procedures typically require an A/P or F/D. RNAV (RNP) approaches require an autopilot.

3. Is there any way to identify approaches that contain "RF legs" in the FMS arrivals list?
As far as I understood (only) aircraft equipped with the NG FMS can fly the RF legs. And aircraft unable to fly RF legs use slightly different approaches.
Therefore I assume 744 with the old FMS have another database on board or somehow filter the RF leg required approaches?

No, nothing identifies if a RNP procedure contains an RF Leg. The Legacy FMS can do RF Legs. We had OpSpec C063 approval for RF legs for the since 2014. However, it was for RNP 1 SIDS and STARs only. The Legacy FMS was not certified to do RF Legs in the terminal (approach) segment. As such, the Legacy FMC was not able to do RNAV (RNP) / RNP AR approaches.

Currently all our 747-400's are now equipped with the NG FMC. However, the RNP certification document Boeing provides for 747-400/747-8 RNP capabilities does not contain approval for RNAV RNP approaches as of yet. Boeing will use one of our airplanes in June to gather data and certify the NG FMC for RNAV RNP approaches.

4. Is there a special reason why only one NZQN RNAV approaches appears in the PSX FMS arrivals list?
Of course I am able to build an approach based on the waypoints, but I assume this is not the preferred/professional procedure.

Legacy FMC could only have one approach per runway in the NDB, and was limited to 5 character names. NG FMC can have up to 8 character names, as such, the NDB can have multiple approaches per runway.

[dhob]

Hi Hoppie,

Yes, that helped, thanks... and raised another question/observation:

How does RNP provide vertical guidance? Barometric VNAV?

I ask this because while reading this and related Wiki articles and looking more detailed into the NZQN RNAV (RNP) Z 05/23 charts I noticed that these procedures are not authorized below -10°C and above 35°C.
But the RNAV (GNSS) approaches do not seem to have these limitations.

It sounds a bit contradictory to me:
The apparently higher precision approach "RNP" has limitations which the less precise approach does not have.
But maybe the required precision is exactly the reason for this limitation?

Thanks, Holger

The RNAV (GNSS) approach you linked is an RNAV to circling minimums. Only RNAV (GNSS) to LNAV/VNAV minimums or RNAV (RNP) approaches will have the temperature requirement. Reference FAA AC 90-105A, appendix B. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_90-105a.pdf

This document will answer many questions about RNP procedures, from RNP 10 to RNAV (GPS) approaches. Nomenclature can be cumbersome. Realize RNAV (GPS), RNAV (GNSS), and RNP approach are all the same. Just different name based on FAA, ICAO or EASA.

RNAV (RNP) or RNP AR (Authorization Required) are covered under FAA AC 90-101A, and require separate certification and OpSpec approval (C384).

[dhob]

How does RNP provide vertical guidance? Barometric VNAV?

Yes, it refers to barometric altitude.

But if GNSS equipment is involved (additional GPS signals from the ground), the altitude is GPS based as well -- i.e. fully geometric, not barometric. The 744 is not fitted with this special GPS equipment.


|-|ardy

As stated in my previous post, the 747-400 can do RNAV (GNSS) approaches (they are the same as RNAV (GPS) approaches). It does not have GLS (Ground Based Augmentation System (GBAS) Landing System) approach capability. GLS approaches use GBAS combined with a Local Area Augmentation System (LAAS). The GLS displays just like an ILS, to include autoland. The 747-8 is GLS approach capable and is certified to GLS Category II approach capability.

Vertical RNP/ANP is displayed on the 747-8 NG FMC. It requires the NPS and IAN OPC options to be enabled, which is not possible on the 747-400 w/NGFMC.

[J D ADAM]

Hi
I understand that NZQN approach is  RNP AR (Authorization Required). The actual charts are hard to come by.

Derek

[cagarini]

Also some interesting reading from here:

https://www.skybrary.aero/index.php/Required_Navigation_Performance_(RNP)

https://aviation.stackexchange.com/questions/55180/is-gls-gbas-landing-system-an-rnav-approach

[Hardy Heinlin]

The Legacy FMS can do RF Legs. We had OpSpec C063 approval for RF legs for the since 2014. However, it was for RNP 1 SIDS and STARs only. The Legacy FMS was not certified to do RF Legs in the terminal (approach) segment.

Thanks for the info. Interesting. Did this approval include a "minor" software update within the legacy version history? As fas as I understand it, the legacy software is basically the same as the original version from 1988. The ARINC 424 standard introduced the RF path terminator definition many years later. I guess the legacy FMC needs to know that definition in order to be able to compute the RF geometry.

Re vertical RNP: I remember you wrote earlier that it's not displayed on the 744 NG. I will consider making this feature an option so that PSX NG users can blank this display.

[dhob]

There were no software updates to my knowledge for the Legacy FMC, but there were different OPC loads. Our Legacy FMCs were all OPC load 16 I believe.

We retrofitted the NG FMC onto our 744s as we took delivery of our first 747-8. We used the -8 differences training to also train the 744 NG FMC, to include the subtle differences between the 744 NG FMC and 747-8 NG FMC (744 had no Quiet Climb, Vertical RNP/ANP etc).

[Hardy Heinlin]

Hi Dhob, regarding the blank vertical RNP display on your 744: Could you tell me what text section exactly is blank?

When it is shown, the text in 2R contains this (for example):

VERT RNP/ANP   <== line title in small font (should this blank as well?)
250/ 69FT         <== RNP and ANP data


And in 6R:

---VERT RNP   <== line title (should be replaced by dashes probably)
         250FT   <== data


Thank you!

|-|ardy

[dhob]

Hi Dhob, regarding the blank vertical RNP display on your 744: Could you tell me what text section exactly is blank?

When it is shown, the text in 2R contains this (for example):

VERT RNP/ANP   <== line title in small font (should this blank as well?)
250/ 69FT         <== RNP and ANP data
Both these lines are blank.

And in 6R:

---VERT RNP   <== line title (should be replaced by dashes probably) Just dashes
         250FT   <== data [img]This is blank
Thank you!

|-|ardy

[Hardy Heinlin]

Thanks :-)

[Will]

dhob,

Are you still following this thread? If so, did I read you correctly, that your 744s which were retrofitted with the NG FMC did not have the Q-CLB feature? I'm just wondering if Q-CLB is implemented on any 744 that anyone knows about. Thanks!

[dhob]

No, Quiet Climb is not enabled on the 747-400. I don't even think it's possible to enable that OPC option for the 747-400.

[Will]

Good to know. Thanks.