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Check Airman
6th Feb 2017, 04:51
Hi all,

Does anyone know why on RNP approaches, the final approach segment begins with a FAP, as opposed to a FAF on "conventional" RNAV approaches?

peekay4
6th Feb 2017, 06:06
I think...

ICAO:

RNAV / GNSS (RNP APCH) approaches may be flown as a Non-Precision Approach (e.g., LNAV only) with no vertical guidance, hence the final approach start is indicated as a FAF like on any other NPA
RNP AR APCH final approaches requires vertical guidance (APV) -- by definition -- hence the beginning of the final segment is indicated as a FAP


FAA:

We don't care... both are FAFs

Check Airman
6th Feb 2017, 06:40
Thanks. I take your point on APV- we've got a ton of them here in the US- usually as LPV approaches. However, even the FAA RNP approaches show "FAP", so they don't really consider them fixes, as you've suggested.

Going back to your point on the vertical guidance though, isn't the start of the final segment a defined lat/long point anyway? Why not just call it a FAF instead of FAP?

peekay4
6th Feb 2017, 08:03
However, even the FAA RNP approaches show "FAP", so they don't really consider them fixes, as you've suggested.

Here are excerpts from three FAA RNP approaches, with the start of their final segments charted as "FAF":

http://i.imgur.com/l5HlNB3.png

isnt the start of the final segment a defined lat/long point anyway? Why not just call it a FAF instead of FAP?
Yes I believe that's what the FAA does (per the above examples).

Technically a FAP isn't defined as only lat/long (2D) but it is a 3D position with an altitude as well. The FAP is the intersection between the final approach path and the intermediate segment altitude.

And operationally the FAP is where the glideslope is intercepted, which geographically may shift a little bit depending on the actual altitude flown.

underfire
6th Feb 2017, 20:58
FAF is for non-precision instrument approach, FAP is for precision instrument approach.

the FAA calls RNP non-precision.

peekay4
6th Feb 2017, 22:52
the FAA calls RNP non-precision

Nope, both FAA and ICAO call them APV approaches. Hence they are flown to a DA(H). A Non-Precision Approach would be flown to an MDA & MAP instead.

Check Airman
7th Feb 2017, 00:49
Here are excerpts from three FAA RNP approaches, with the start of their final segments charted as "FAF":

http://i.imgur.com/l5HlNB3.png


Yes I believe that's what the FAA does (per the above examples).

Technically a FAP isn't defined as only lat/long (2D) but it is a 3D position with an altitude as well. The FAP is the intersection between the final approach path and the intermediate segment altitude.

And operationally the FAP is where the glideslope is intercepted, which geographically may shift a little bit depending on the actual altitude flown.

Thanks. I understand the concept of a FAP on an ILS, but on any RNAV procedure, the point will always be referenced by a lat and long, correct, so why not FAF? It doesn't really affect the way we fly the approach. More of a curiosity really.

Thanks for those chart excerpts. Those are the FAA charts- which I hadn't checked before my initial post. The Jepp charts for those approaches use the FAP terminology. I find that interesting because my understanding is that Jepp simply converts the government AIP format to their own format- they don't modify the procesures.

peekay4
7th Feb 2017, 04:24
but on any RNAV procedure, the point will always be referenced by a lat and long ...

Not necessarily. E.g., if flying an RNP procedure using Baro-VNAV then the final approach is commenced at the GS intercept -- which might not precisely match the waypoint lat/long due to altitude variation (with pressure, temperature, ATC instructions, etc.)

This is analogous to an ILS approach where the FAP is defined as the GS intercept point, which on any given day is not always directly above the marker beacon depending on the actual altitude flown.

Check Airman
7th Feb 2017, 05:23
I've never flown with Baro VNAV, so I hadn't considered it. That makes sense though. I'm planning to email Jepp about an error on a chart, I guess I'll ask why they used the FAP terminology instead of FAF, as the government charts use, at that time. Cheers!

underfire
7th Feb 2017, 05:47
the FAA calls RNP non-precision
Nope, both FAA and ICAO call them APV approaches. Hence they are flown to a DA(H). A Non-Precision Approach would be flown to an MDA & MAP instead.

From ICAO:

APPROACH CLASSIFICATION. Approach classifications have traditionally comprised NPAs and PAs. A third classification of APVs is now included. APVs are instrument approach procedures which utilise lateral and vertical guidance but do not meet the accuracy requirements for PAs.

"PANS-OPS Volume I defines an APV as ‘An instrument approach procedure which utilises lateral and vertical guidance but does not meet the requirements established for precision approach and landing operations"

"An APV is characterised by a DA and does not have a MAPt. It may be:
 an addition to a GNSS-based NPA (RNAV(GNSS)), in which case the minima line will be indicated by the term LNAV/VNAV (for Baro-VNAV approaches);
 an augmented GNSS based approach in which case the minima line will be indicated by the term LPV; or
 a procedure designed on RNP principles, in which case the minima line will be indicated by the term RNP XX (where XX will be a number from 0.3 to 0.1 to indicate the accuracy to
which the procedure is designed). Any instrument procedure with an RNP value less than 0.3 will require special authorisation from CASA. These procedures are called RNP AR but
are currently charted as RNAV (RNP)"

https://www.casa.gov.au/file/104796/download?token=iTG4MIcl

Not necessarily. E.g., if flying an RNP procedure using Baro-VNAV then the final approach is commenced at the GS intercept

That is incorrect.

peekay4
7th Feb 2017, 23:12
That is incorrect.

Oops you're right. Or rather, the intercept will always be at same location as the waypoint if flown at the indicated altitude. I was looking at a Jepp RNP chart (noting the GS intercept) and got mixed up when I made that comment.

http://i.imgur.com/q8usSnj.png

aterpster
8th Feb 2017, 19:26
Jepp labels RNP AR in the U.S. and elsewhere as FAP. The FAA can pound sand.

LH777
9th Feb 2017, 21:13
The RNP APCH LNAV/VNAV (basically a Baro VNAV) altitudes (hence glideslope position) are based upon the aircraft altimeter subscale (hence temp limits) whereas the ILS glideslope comes from the ground aid.

underfire
10th Feb 2017, 04:50
I was looking at a Jepp RNP chart (noting the GS intercept) and got mixed up when I made that comment.

Curious what Jepp RNP chart lists a waypoint with glideslope intercept?

JABGO is for SFO, right?

http://i.imgur.com/coNl4vv.jpg

peekay4
10th Feb 2017, 06:42
I believe they are transitioning to FAP as they slowly revise the charts over time. So, a plate will show FAP or GP intercept depending on its last amendment date. A current cycle example showing GP intercept would be the RNP Y 19L approach at Tampa.

http://i.imgur.com/UirogwP.png

aterpster
10th Feb 2017, 14:56
Here is the current Jepp chart for the RNP AR to 19R at KSFO:

underfire
12th Feb 2017, 06:13
It does seem to be a transition from FAF to FAP, I was thinking this was something the FAA was doing to better coordinate with ICAO.

trerpster, get your glasses on, that is 10, not 19!

The GP Intcpt, ...oi vey

peekay4
12th Feb 2017, 08:07
It does seem to be a transition from FAF to FAP, I was thinking this was something the FAA was doing to better coordinate with ICAO.

The transition is Jepp only, not FAA.

FAP in FAA parlance means something else entirely. On a non-precision approach, FAP is the point where a procedure turn intersects the final approach course. The final approach may be commenced from this point unless a FAF is charted elsewhere.

The glideslope intercept point (ICAO FAP) is called "Precision FAF" (PFAF) in FAA lingo. On an ILS approach the PFAF is charted as the "lightning bolt" symbol on FAA plates, and is almost always at a different location than the non-precision FAF (if any) on the same plate.

For APV approaches, instead of having both a FAF and a PFAF at different locations, the FAA will use a single FAF. And on FAA charts this location will always be charted as FAF, not FAP. There are too many legal rules / regulations which reference "FAF" so it would be a lot of effort to rename it now.

aterpster
12th Feb 2017, 16:48
Underfire:

trerpster, get your glasses on, that is 10, not 19!

How about typing too fast? The chart does make the point about Jeppesen's transition.

underfire
13th Feb 2017, 08:45
terpster, no worries...

FAP in FAA parlance means something else entirely.

Yes, note back in post #5. Still wondering about peekay showing gp intcpt on JABGO in that procedure profile... the Jepp plate provided by terpster shows FAP....

Am curious what the RNAV visual shows on the FAF/FAP/whatever....

I remember on some of the RNP plates, we were asked to show it as a FAP instead of FAF, but it went back and forth, and most of the plates were RNP AR anyways, so not public.

peekay4
13th Feb 2017, 19:31
Yes, note back in post #5.

Not quite. FAA FAP is for a non-precision approach.

Still wondering about peekay showing gp intcpt on JABGO in that procedure profile... the Jepp plate provided by terpster shows FAP

Different approaches, RNP Y vs. RNP Z. (RNP Y has since been removed.)

aterpster
14th Feb 2017, 14:21
peekay4:

Not quite. FAA FAP is for a non-precision approach.

Are you certain about that?

peekay4
15th Feb 2017, 01:21
Yeah. Definition in AIM:

FINAL APPROACH POINT− The point, applicable
only to a nonprecision approach with no depicted
FAF (such as an on airport VOR), where the aircraft
is established inbound on the final approach course
from the procedure turn and where the final approach
descent may be commenced. The FAP serves as the
FAF and identifies the beginning of the final
approach segment.

From the Instrument Procedures Handbook:

For a non-precision approach,
the final approach segment begins either at a designated
FAF, [...]. When a FAF is not designated, such
as on an approach that incorporates an on-airport VOR
or NDB, this point is typically where the procedure turn
intersects the final approach course inbound. This point
is referred to as the final approach point (FAP).

However, TERPS was completely revised last year (for the first time since the 1970s?) The new direction is to use "Precise FAF" (PFAF) for all approach types -- NPA, APV and PA. FAFs are now designated PFAFs if meeting the "precise" criteria. FAP is no longer used for approaches without a (P)FAF.

underfire
15th Feb 2017, 02:03
A rose by any other name is still a rose....Bill Shakespeare.

Well, here we go...while the FAA has a FAF, FAP, and a PFAF, a "precise FAF,", do you see PFAF on a chart? Hell, at one time there was an FEP! GP intrcpt, given baro VNAV, which GP are you intercepting at a fix?

When there is no FAF, it is a FAP??! When in doubt "look to the box, look to the box..."

I can call it MFAF, ZFAF, LCHANCE, UR5NMFRMT, Uneed2BonFinalGP, whatever I want on the chart...... guess, what, bottom line, ARINC 424 code in the box understands FAF on a waypoint.

I really dont care what you call it, ICAO, FAA, precision, nonprecision, whatever....

ARINC 424: Fix Function in Coding: F = FAF, M = Missed Approach Fix

F=FAF.... this designates the beginning of the final approach segment.

go ahead, fight it, call it whatever you want...do you really want me to go into all of the other coding and sequencing in the aircraft FMS that is related to a FAF designation? (ie, what is the ROC required at FAP?)

peekay4
15th Feb 2017, 02:32
do you really want me to go into all of the other coding and sequencing in the aircraft FMS that is related to a FAF designation?

Perhaps subject to another thread, but if you'd like I would be fully happy to discuss finer points of ARINC 424 coding including Attachment 5, which defines rules translating PFAF into FAF. :E

underfire
15th Feb 2017, 02:51
you missed the point...why a translation, why just not use FAF, that is what the ac understands?

peekay4
15th Feb 2017, 05:06
An aircraft doesn't really understand what a "FAF" is. It can't: there isn't a single definition of FAF that's consistent across all approach types past, present and future.

ARINC 424 only describes a database format based on standards first formulated over 40 years ago! Consequently, codings such as FAF have been reused and redefined many times over the years.

And many procedures don't even have a FAF. Yet ARINC 424 requires a FAF entry thus we have to define some rules on what to stuff into this field, even though the published procedure doesn't have one. The onboard avionics might not actually do anything with this FAF entry other than to display it on the map, while for other procedures, the FAF is used to actually initiate descent.

So a FAF entry for a VOR approach may be treated differently than one for an ILS approach or (back to the topic of this thread) an RNP AR approach -- because technically they mean different things, even though they've all been shoehorned into the same ARINC 424 FAF coding.

Hence for every approach technology (from NDB to GBAS) we have to define translation rules on what to shove into the various ARINC 424 fields, and the corresponding changes to the aircraft avionics to interpret what those fields mean in the context of that specific technology.

aterpster
15th Feb 2017, 14:49
Gentlemen, I believe this thread was about charting, not ARINC coding. Pilots don't care about coding. The use of final approach point for on-airport, no FAF, VOR and NDBs has been around since the original issuance of TERPs in 1967. But, it was never charted until RNAV came along, then we got the sensor FAF. FAA charts still use the Maltese Cross for all procedures that have non-precision minimums.

Picking at nits, I believe P-FAF is the precision final approach fix.

EDIT: I stand corrected on P-FAF in 8260.3C. It is precise FAF, whatever that means. But, in the Pilot Controller Glossary it is still the Precision Final Approach Point. The FAA is overloading the pin's head with angels, so to speak.

peekay4
15th Feb 2017, 15:22
Yes PFAF was redefined last year with 8260.3C so now it is used for all approach types as mentioned earlier. You wont find plain old FAF or FAP anywhere in TERPS anymore.

I believe PFAF will continue to be charted as FAF on FAA plates. A PFAF is a FAF meeting certain accuracy constraints. With this redefinition, I'm curious to see if Jepp will follow FAA nomenclature for US plates going forward.

vilas
15th Feb 2017, 16:22
If you google Jepp's Briefing The chart Clinic Twenty First in a Series you will get the genesis of the term PFAF and FAP. Basically it had to do with the legality of the approach. On a LOC approach the Maltese cross designates the FAF. Once the aircraft passes the FAF you are allowed to continue to minima even if weather goes below minima. So they needed to define it also for precision approach. This precision FAF is the intersection the glide slope intercept altitude and the glideslope.

peekay4
15th Feb 2017, 18:31
Yup Vilas that was the original intent. However, the specific rule mentioned in the Jepp briefing has been amended since then, so a published FAF (or PFAF) is no longer required.

Then last year PFAF was redefined entirely (as above). It is no longer the glideslope intercept point.

underfire
15th Feb 2017, 21:37
Technically, it is the Precise Final Approach Point, not Precision.....

peekay4
16th Feb 2017, 03:27
Again, it was redefined. Summary:

Until 2016: "Precision Final Approach Fix". Old meaning: glideslope intercept point on a Precision Approach (hence the name). It is not defined on NPA procedures. Operationally, the location would vary with actual altitude flown.

2016 redefinition: "Precise Final Approach Fix". New meaning: the calculated position (fixed) where the vertical path or glideslope intersects the intermediate segment altitude. It may be defined for all approach types (PA, APV, NPA). A "Precise" FAF must be locatable within +/- 1 NM using intended navaids (e.g., VORs) or up to +/- 2 NM in certain other circumstances.

aterpster
16th Feb 2017, 16:53
A historical perspective:

The original addition of TERPs became effective in November, 1967. I don't recall how many revisions it had.

The second edition came out sometime in 1977 and we know it had many revision, up until last year.

Also, now we not only have the third edition, we have adjunct criteria in the RNAV Order 8260.58A. For a few years we had the RNP AR standalone Order 8260.52, which was subsequently rolled into 8260.58.

Garmin is pushing for RF legs to LPV, which presently exists in Switzerland. (LSMD)

underfire
17th Feb 2017, 00:51
ahhh...history!

Looking back, I seem to remember that many of the procedures were virtual ILS overlays (due to public outcry on any new procedure)
ATC was complaining that the FAF was at different locations than the FAF on the ILS procedures, and that ATC does not vector to a PFAF (or gs intcp).

From FAA guidance: "If the LOC FAF is defined by a facility such as a LOM, which cannot be moved, the present facility name will be assigned to the ILS PFAF. The RNAV FAF will be colocated with the ILS PFAF and share the common name. "

If that fails!

An underlying assumption of is that the ILS glidepath angle/TCH will coincide with the RNAV VDA/TCH. If this can't be done, a RNAV procedure, completely independent of the ILS procedure, will be developed."

peekay4
17th Feb 2017, 03:19
That wasn't for overlays, but for RNAV APV approaches on runways served by ILS. There was potential confusion because there could be two FAFs in close proximity to each other: one for the LOC and another for the RNAV.

aterpster might remember better but I believe the FAA never had any virtual ILS overlays. Overlays were on top of NPA procedures only (NDB, VOR, etc.)

aterpster
17th Feb 2017, 13:29
Peekay4:

aterpster might remember better but I believe the FAA never had any virtual ILS overlays. Overlays were on top of NPA procedures only (NDB, VOR, etc.)

Correct. The overlay program was to jump start the use of GPS when it became usable for IFR approach operations in the early 1990s. It was limited to VOR and NDB approaches, both on and off airport, with or without FAF. (thus the inception of the sensor FAF). There were only TSO C129 avionics at the time. Accuracy and sensitivity was inadequate for ILS, LOC, LDA, or SDF. Also, selective availability was still in use at the time the program was initiated.

As I recall over 5,000 IAPs were run through the regulatory mill in just a few months. Now, as a given runway end gets an RNAV IAP the overlay approach to that runway is cancelled.

underfire
18th Feb 2017, 08:35
I was talking in context about RNP AR procedures, which were overlays on the existing ILS to avoid environmental review.

ILS overlays were very popular because the obstacle clearance area, and the similar 200' HAT/250'HAT

aterpster
18th Feb 2017, 15:42
underfire:

My view is the Reno ILS 16R is the way to go. Too few operators can do RNP AR. RF to LPV is also coming along once Garmin, et al, can convince the FAA. As I posted earlier the Swiss are already doing RF to LPV.

underfire
20th Feb 2017, 00:15
terpster,

Yes, overtime, there has been the transition to simplify, and the whole AR requirement, while perhaps good at the beginning, should have settled down a little more to reality by now....turns are not unheard of close to final!

In the grand scheme of things, the ac holds the track pretty close, and the missed, well, while we kept the same 0.3RNP through the missed, the outcry when it was suggested to ICAO. Somehow, on the missed, the ac navigation falls apart and you need 1 RNP to contain it.

Currently, the RNP to GBAS final works very nicely, and while unofficial of course, the RNP to visual final.

On Reno ILS 16R, why is this a way to go, the minima at 500 is a bit tough...
http://i66.tinypic.com/x416s8.jpg
In reality, the RNAV/RNP visual has the best chance, as it works well with the avionics, this would be similar to the RNP Visual (with turns!) that we had been working on. Hopefully, having turns in a procedure becomes more comfortable, and the AR requirement is reduced accordingly.

aterpster
20th Feb 2017, 01:53
Wrong Reno ILS. Look at the "Z" approaches.

underfire
20th Feb 2017, 21:30
Okay, I see this looks just like those other overlays the FAA was doing, what 5000 of them, to try to show some sort of progress.

Now there are RNAV/RNP to ILS procedures (which work so well in the box), yet another attempt by the FAA to claim NextGen progress?

I thought the meeting with Trump and the airlines was entertaining, I cant believe they let the cameras in there, and kept them going when the FAA was getting trashed. Great PR for the FAA and great information for the public to know.
Look for a separate FAA and ATO....

aterpster
21st Feb 2017, 18:26
Reno 16R Z with RF legs was driven by Southwest Airlines, not the FAA.

underfire
23rd Feb 2017, 03:07
of course, everyone has given up on any progress, such as GBAS, so the airlines have to run with what the FAA has the ability to approve, ie 1950's technology.

RNAV RNP turns to an ILS final, certainly not any disconnect in the FMS systems?

Desperate times call for desperate measures.

aterpster
23rd Feb 2017, 15:08
underfire:

RNAV RNP turns to an ILS final, certainly not any disconnect in the FMS systems?
It's been done for years: LNAV to ILS to LNAV missed approach. The only difference here are RF legs in the LNAV portion.