RNAV (GNSS)
 

Hi there

Can someone please advise that an RNAV (GNSS) approach requires a working GPS in all cases? I.e. with failed GPS receivers the approach cannot be flown with reference to RADIO UPDATING, INERTIAL pos etc. Silly question I know. Just trying to confirm my assumption.

Thanks

Correct.

What is in the brackets is the basis of your approach. No GPS no RNAV (GNSS) approach. If it was an RNAV (RNP) you could still continue an approach as long as the ANP was within the RNP.

Point of order there.
If it says RNAV (RNP) you certainly cannot continue without dual GPS receivers as that's an RNP AR approach or SAAR as our American friends call it and GPS is a required item.

8che:

Absolutely correct. IRU position is for extraction in the event of a loss of GPS.

We stopped using SAAR a couple years ago. They are now RNP AR.

And this is where all the confusion starts, as in all aviation "it depends" on how ahead or behind your certifying authority are in the PBN world.

RNAV (RNP) is not equal to RNP APCH, it is similar in many respects but it is certainly not SAAAR or RNAV (RNP AR) or RNP AR APCH (three names for the same approach by different states).

RNAV (RNP) is not part of the ICAO PBN structure. It was an attempt to allow such equipped aircraft to operate using the current aircraft navigation capability. As such in some states RNAV (RNP) was able to be flown without GNSS, but required Radar monitoring, however many states decided (quite rightly in my view) that GNSS should be part of the aircraft system requirements. RNAV (RNP) did not have RF legs or an RNP less that 0.3:

RNAV (RNP AR), SAAAR or RNP AR APCH can have RF legs, could be below 0.3 and could be either public or tailored (private) for specific requirements.

The whole PBN debacle of nomenclature, sensor requirements etc was the result of the US and Europe heading for similar outcomes via different pathways and the slow cumbersome nature of ICAO to develop a standard.

As the ICAO contracting states start towards PBN standardisation most of the confusion "should" disappear.:ugh::ugh::ugh:

The USA name for RNP AR APCH was SAAAR (three A's) Aircraft, Aircrew Authorisation.

My regulator: you cannot start an RNP AR APCH approach without Dual GNSS but after the FF you can continue with a single GNSS.

Well from our newly updated operations manual :


RNAV (GNSS) APPROACHES


Definition : non-precision approaches with LNAV or LNAV/VNAV minima.


- "RNAV (GNSS) approaches" corresponds to "RNP APCH operations"


- "RNAV (RNP) approaches" corresponds to "RNP AR APCH operations"


RNP AR OPERATIONS


General


Required navigation performance instrument approach procedures with authorisation required (RNP AR) are charted as RNAV (RNP) RWY XX " and contain a special note on the chart to aircrew and operators. These procedures are commonly referred to as "RNP AR" or in the USA "RNP SAAAR".

8che:

Old or incorrect source.

The U.S. changed to RNP AR at least two years ago.

There are approaches which are labelled as RNAV (VOR DME) or RNAV (DME DME), although they seem to be very rare.
There are some of them in Egypt like in Sharm El Sheikh (HESH): RNAV (VOR DME) 22L and 22R.
(Other airports that have them are HELX, HEAX, HESN, ...)

This RNAV approach is based on an RNAV system that doesn't need GPS, but only VOR DME position updating. I believe you still need the 0.3 RNP.
However, in the FCOM of my aircraft, it is written that for any RNAV approach which doesn't require a GPS (like the RNAV(VOR DME)), you need to apply the procedures for a RNAV(GPS) approach. Hence you need a working GPS. (However, that might be a company requirement.)

The HESH 22L/R RNAV IAPs state "VOR/DME or GNSS."

So, the folks with GNSS follow standard procedure.

c100:

That's fine for some circumstances in more conventional instrument flying, but entering the final segment in some RNP AR approaches (say RNP of .2 or less with an RNP required missed approach) is literally going into an area where extraction should be the rule when any significant part of the avionics fails.

I have never come across a confusing topic such as the Rnav & rnp stuff

A single GPS "unit" failure (in a dual GPS system supporting a dual FMS) should not affect the position solution just the redundancy. As long as ANP is less than RNP and there is no trending of the ANP increasing then risk has not increased appreciably. A single GPS "unit" failure was the only failure that we could continue with after the Final Fix. If it failed prior to the Final Fix the approach could not be continued.

It has been a few years since my airline and the regulator went down the RNP AR journey of discovery but I recall that the extraction was based on, dual GPS sensor loss, FMC failure and Engine failure at missed approach point i.e. Single FMC, IRS updating only.

This was for a RNP of 0.11 for Boeing and 0.10 for Airbus into an airport that the RNP AR approach was really the only option due to the terrain and has multiple RF legs to position into a valley.

Our company SOP's have us continue with a single GPS failure or a single. Rad Alt failure if it occurs after the IAF. Prior to that the approach can't be commenced. That is for RNP-AR.
For RNP App LNAV / VNAV you can start the approach with only one GPS and only one Rad Alt but if you lose either you must go around.
Very very confusing evolution of terminology.

RNP-AR with a single GPS.?

Could you report some examples of airport with RNAV (RNP) and RNAV (AR RNP) ? To see differences in charting, title notes and minima.

RNAV (RNP) implies AR.

To be more precise, in the context of PBN there are two types of approach specifications: RNP APCH, and RNP AR APCH.

1. RNP APCH. Generally charted as RNAV (GPS) in the US or RNAV (GNSS) internationally. This type of approach mainly consists of straight legs. Obstacle clearance and approach minimums do not require specific RNP capabilities. (Instead, minimums depend on approach type being flown, e.g., LPV, LNAV/VNAV, etc.) Example: KSFO RNAV (GPS) Z RWY 28R.

2. RNP AR APCH. Generally charted as RNAV (RNP). This type of approach may include curved (RF) legs. Obstacle clearance and approach minimums developed against specific RNP capabilities (e.g., RNP 0.3 or RNP 0.1). Special authorization is required. Minimums not necessarily lower than RNP APCH (might even be higher). Example: KSFO RNAV (RNP) Y RWY 28R.

Here is a link to a full tailored type approach for RNP AR APCH even though the landing plate is is called RNAV (RNP) Y RWY 23 at Queenstown NZ

AIP New Zealand

The second link is a Public RNP AR APCH for Auckland NZ

AIP New Zealand

The third is for a straight RNAV (GNSS) Z approach at Auckland NZ

AIP New Zealand

Here are approaches to Rifle (KRIL), Colorado, Runway 8. For folks with biz jets that can do RNP AR, look at the huge improvement with RNP AR, especially RNP 0.10.

The smart folks with the equipage divert to Rifle when Aspen weather is marginal.

http://i201.photobucket.com/albums/a...ps255e96e0.jpg

http://i201.photobucket.com/albums/a...ps2bb039af.jpg

Nice approach that RNAV (RNP) Z RWY 8. Interesting the difference in GP angle achieved with the RNP by being able to reduce the altitude further out on the approach than the RNAV (GPS)

RNAV (GPS) Y 08
Minima in only LNAV, not LNAV/VNAV: is consider anyway to be a APV approach?

No. It is considered to be a NPA. Because the visual segment area does not have significant obstacle penetrations, it does have what the FAA calls a vertical descent angle (VDA). But, the VDA is advisory; does not meet APV criteria. If it were an APV, there would be an LNAV/VNAV line of minimums.

Thanks, an other question:
RNAV (GPS) do not require a specific RNP (eg. 0.3). On my AOC I' m authorized to operate down to RNP 0.3 but we are not authorized for RNP approaches ( coded RNAV(RNP) ).
Where can I use my RNP 0.3 ?? :ugh:

Topper80:

I can only speak for the US. LNAV and LNAV/VNAV are RNP 0.3, but the lateral obstacle clearance areas are larger than RNP AR 0.30, and don't require the avionics redundancy required for RNP AR.

Then, we also have LPV and LP, which are predicated on SBAS augmentation. Western Europe is also doing SBAS augmentation.

The latest ICAO concept that has reached us, 2D and 3D approaches, has NPAs that are flown with an FMS-derived approach path may be classed as a 3D approach. It won't have a DA, so it's up to the operator to not go below the MDA.

Captain Bloggs:

Be that as it may be, FAA flight inspection is continuing to remove or not accept VDAs on many NPAs in the U.S. If the runway doesn't have an APV there are obstacle issues in the visual segment. If flight inspection judges them significant the VDA is removed (or disapproved on new NPAs, such as LP and LNAV), thus there will be no vertical path such NPAs in the FMS.

Underfire:

As you know the missed approach segment is often not critical. The only clue the pilot has is when the chart states RNP of less than 1.0 is required for the missed approach.

Since Rifle, Colorado Runway 8 has been an example of low minimums but not a critical missed approach (other than climb performance) here is a more "terpsy" look at RNAV Z Runway 26. The missed approach is critical. By continuing past the FAF with one GPS receiver, the odds of having to conduct an IRU coasting missed approach are certainly increased significantly; probably reducing the target level of safety below the presumed value for this IAP.

I have pasted the missed approach "telescoping" values on the Jepp chart, followed by the actual FAA TERPS map. Not a friendly environment for coasting on IRU(s).

http://i201.photobucket.com/albums/a...ps20afa7e1.jpg

http://i201.photobucket.com/albums/a...ps412d9adb.jpg

An APV approach will always list a DA instead of an MDA, so that's an easy way to tell if the approach is considered an APV or an NPA.

Currently the only APV approaches are: LNAV/VNAV, LPV and RNP.

That's not exactly right. Some countries are operating with a special waivers / ops specs / procedures allowing approved operators flying certain NPA approaches using Continuous Descent Final Approach (CDFA) to treat MDA as DA. Under this procedure the operator may actually descend below the published MDA during a missed approach.

However such an approach is still not considered a "precision approach" under ICAO definition. Currently only ILS, MLS and GLS approaches meet the criteria for "precision approach".

This surprises me and I find it hard to believe. Why don't they just add 50ft like they do in NZ and Australia if they are going to treat an MDA as a DA?

The point is to reduce minimums when safe practices allow it.

Examples:

1. FAA OpSpec C073 VNAV MDA as DA:

"The certificate holder is authorized to use minimum descent altitude (MDA) as a decision altitude (DA)/decision height (DH) with vertical navigation (VNAV) on a Nonprecision Approach (NPA)."

8900.183 - OpSpec/MSpec/LOA C073, Vertical Navigation (VNAV) Instrument Approach Procedure (IAP) Using Minimum Descent Altitude (MDA) as a Decision Altitude DA)/Decision Height (DH) ? Document Information

2. Transport Canada Special Exemption from 602.128(2)(b)

"The purpose of this exemption is to permit pilots-in-command of IFR aircraft operated by holders of an air operator certificate or a temporary private operator certificate to descend below the minimum descent altitude (MDA), when conducting a non-precision approach ... following a stabilized constant descent angle (SCDA) non-precision approach."

EXEMPTION FROM PARAGRAPH 602.128(2)(b) OF THE CANADIAN AVIATION REGULATIONS - Transport Canada

(SCDA is the Canadian term for CDFA).

Interesting stuff, PK. I've amended my earlier post re "precision".

Peekay4,
Thanks for posting that.
It's interesting how different regulators around the world view different subjects and what they are willing to provide exemptions for.
I understand that the lower the minima the better commercially but can't ever imagine our regulators " down under" being willing to sign off on certain operators descending below MDA even if it is just 50 feet or so.

mel question
 

I found that this is in my opinion is the best place to ask the question because it seems that the real experts are here.

Two weeks ago during a ramp check the only complaint they had was on the RNAV subject and the complaint was that we didnt specific which systems (DME, VOR and GPS) affect the navigation and any combination in between. Is this the way your MEL written/deigned ?

Short answer is "within half-scale deflection" of the CDI.

Generally, to be established on a segment implies all of the below:

1. You've passed the segment's defining fix
2. You are receiving "positive course guidance" (PCG) from the navigation system
3. You are within half-scale deflection on the CDI **
4. You are "stable" in the segment (i.e., with a positive trend to track the centerline)

** The "+/- 5 degrees" is mostly for NDBs (since there's no CDI).

Have you met all four criteria listed above? (Including #1)

If not, you're not established.

You have to be within half-scale of the centerline defining the segment your are joining, not your route to the segment.

Hmm, sorry, I have to take back some of I wrote above.

For RNAV and RNP:

1. Instead of "half-scale deflection", the guidance for established is: "An aircraft is considered to be established on-course during RNAV and RNP operations anytime it is within 1 times the required accuracy for the segment being flown." (The scale will depend on your FMS but this seems to imply full-scale deflection).

2. Instead of waiting to "pass" the segment's defining fix, the exact standard is: "(a) Pilots flying FMS equipped aircraft with barometric vertical navigation (Baro-VNAV) may descend when the aircraft is established on-course following FMS leg transition [to the next segment]. Leg transition normally occurs at the turn bisector for a fly-by waypoint [...] (b) Pilots flying TSO-C129 navigation system equipped aircraft without full automation should use normal lead points to begin the turn. Pilots may descend when established on-course on the next segment of the approach."

The italicized quotations are from revised FAA AIM 5-5-16. RNAV and RNP Operations:

https://www.faa.gov/air_traffic/publ...m/aim0505.html

There is another part of the AIM which discusses passing the bisector of a course change to be considered in the succeeding segment of an IFP.

That's how the avionics work.

What do they achieve by doing this?

The whole point of providing VDA's is to give those without FMS's a flighting chance of flying a stable approach, and those with FMS's the ability to use vertical guidance...which according to ICAO is more safe.

Have they not just increased the risk of instability on the approach?:confused:

There are other ways to deal with obstacles in the visual segment. Removing the VDA is a poor option.

Alpha

I review every FAA IFP as they are posted for public review. It appears that it is flight inspection that removes them, not the initial designers.

The FAA position is that the airport either remove the 20:1 penetration, or mitigation (sometimes) with a VGSI. But, when flight inspection doesn't like what they see for daytime operations the VDAs go.

I feel they are more concerned about FAA liability than safety.

KRIL RNP based minima could safely still be even lower
 

The KRIL RNP based approach minima at RNP .1 could still safely be even lower, if the present obsolete and unnecessary limitations were removed that are still being placed on RNP procedure design. The fully allocated real VEB could easily provide for DA(H) down to and even somewhat below 200' HAT compared to the higher DA(H) shown for the RNAV(RNP) Z Rwy 8. The present 250' HAT floor limit was only put there for largely political reasons, not due to any technical limitation of engineering or physics. That potential additional advantage for RNP is particularly valid considering that any aircraft with RA available could also use an RA floor to additionally bound improbable to extremely improbable VNAV non-normal events. Unlike with TERPS or PANS-Ops, with [real] RNP, particular non-normal as well as rare-normal events are already suitably addressed and accommodated.

Aterpster, I wasn't criticising you. I was just questioning the FAA logic

It would be interesting to know the logic and what they think they are achieving.