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Airmann
9th Oct 2014, 13:10
Flying into an airfield in Africa the other day

Was a DME Arc onto the ILS

Approaching the field I noticed the DME was U/S. Pointed this out to the captain, he acknowledged it, mentioned it to ATC, no further discussion.

GPS was Primary and the aircraft few the arc perfectly (as it was programmed to do) intercepted the ILS landed, went to the hotel. Didn't think twice about what we had just done.

A while later I realized that we had just done a DME Arc with no DME! Which led me to a discussion with another captain; our reliance and expectation of our FMS is such these days that we forget that except for RNAV approaches all other NPA are not meant to be flown by the FMS alone. So long as the chart says, VOR, DME, or NDB the said ground aid should be fully serviceable, even if we are going to let the box manage. Technically and legally speaking that is. I think.

Would anyone like to comment on this, i.e. the permissibility or impermisibility of flying approaches, or parts of approaches without the required ground aids working. Pros and Cons, the reality vs. what the book says. :hmm:

ItsMeFromEarth
9th Oct 2014, 13:37
FMS/FMC fly ARINC 424-XX leg type, it doesnot not use ground stations, the only prerequisite is that the stations (DME,VOR,NDB) have to be in the NavDataBase.

Airmann
9th Oct 2014, 13:51
Yes, I know, that's how we managed to pull off the approach without a hitch, but legally speaking, can we fly the approach if the ground aid is U/S? I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.

aterpster
9th Oct 2014, 14:35
Airmann:

Yes, I know, that's how we managed to pull off the approach without a hitch, but legally speaking, can we fly the approach if the ground aid is U/S? I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.

If a VOR or NDB approach is not coded in the database as an approach, the FMS will remain in terminal mode sensitivity, alerting, and monitoring. You can force the sensitivity to 0.30, but you will still be in terminal mode.

Amadis of Gaul
9th Oct 2014, 14:52
Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?

italia458
9th Oct 2014, 15:09
I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.

Reality should match the book but it doesn't always.

I'll give an example of an NDB approach in Canada that is NOT approved for a GPS overlay.

Example approach is the NDB RWY 01 into CZTM. For final approach track guidance you're required to use the NDB raw data as your primary navigation source. This airport is located where there are magnetic disturbances in the area. If you relied entirely on your RMI, you could be way off your inbound track which is not a problem from an obstacle standpoint because they've taken that into account and given you a very high MDA. It can throw you off though when you're expecting the runway to be a certain way in the windscreen when you break out and it's not.

Anyways... I would fly that approach using GPS as my primary navigation and have the NDB as a back up. It's silly to think that the NDB is going to provide you better navigational accuracy... it will most certainly not!! I'll OBS the final approach course on the NDB and use that to fly the approach.

If the NDB is NOTAMd to be u/s then I won't do the approach. Following the rules makes your life a lot simpler. There is no decision making here - you're not doing the approach. Even though we all know that it's much safer to fly it based off the GPS anyways - even with en route RAIM and CDI sensitivity!

I should clarify that I do still monitor the morse code and do timing for approaches where an overlay is not approved.

As far as why you can't do an overlay approach without it being published has to do with the approach needing to be in the database.

To add to what aterpster said:

If a VOR or NDB approach is not coded in the database as an approach, the FMS will remain in terminal mode sensitivity, alerting, and monitoring. You can force the sensitivity to 0.30, but you will still be in terminal mode.

You can force the CDI to 0.3 but you can't force the RAIM to go to 0.3 which is what you require to descend to altitudes in the final approach segment.

When an overlay approach is made, sometimes they'll add a GPS waypoint to act as a FAF when there originally wasn't one.

Example approach is the NDB RWY 31 (GNSS) at CYAV. There is no intermediate segment and no FAF. MUVRI has been added so that the approach will work on the GPS as it requires a FAF. Within two miles of MUVRI, the approach will go ACTV when the automatic check of approach-level RAIM (0.3) at the FAF and MAP says approach-level RAIM will be available. This is where CDI will transition from 1.0 to 0.3 at the FAF so that you fly the entire final segment with 0.3.

Technically, if you're doing the overlay, you can't descend to the MDA until past MUVRI as you require the CDI and RAIM to be at 0.3 and the CDI won't reach 0.3 until at the FAF. In that case you could look at your RMI and see that you're within 5 degrees of your final approach course and descend to the MDA. It's a bit awkward when there isn't a FAF to begin with.

Specifically as far as your approach goes, it sounds like you should have discontinued the approach and flown a different one. Possibly request vectors to intercept the ILS.

Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?


There was one. Dunno what happened to it. There was a lot of good discussion in it.

mcdhu
9th Oct 2014, 15:39
Surely an overlay approach without the raw data becomes an RNAV approach?
My understanding of an overlay is simply that you are saying to the AC that you can't be bothered to manage it yourself (selected/selected) so please will you do it. But the primary source of NAV data is the raw stuff. So if it's not there, you can't do it.
Unless you know different!!

BOAC
9th Oct 2014, 15:45
I know it was Egypt, but ATC clearing you for a DME procedure with no DME....? I recall Tripoli giving me an NDB09 with no NDB once ---yes, we cracked it:ok:

italia458
9th Oct 2014, 16:33
Surely an overlay approach without the raw data becomes an RNAV approach?

No, an overlay approach generally doesn't require the underlying NAVAID (NDB or VOR) to be serviceable. That's the case in Canada. But we're talking about a lot of countries here so you'd have to check your country's regulations.

My understanding of an overlay is simply that you are saying to the AC that you can't be bothered to manage it yourself (selected/selected) so please will you do it.

You're thinking of an FMS doing the overlay approach. And possibly calculating the 3 degree path for you. You only need a GPS to do a GPS overlay approach... no FMS required.

But the primary source of NAV data is the raw stuff. So if it's not there, you can't do it.

Not true! Again, speaking from Canada but I think most countries have the same rules. When an overlay approach is allowed, you do not require the underlying NAVAID (raw stuff) to be operational. The whole point of an overlay is to make the primary source of navigation a GPS instead of a VOR or NDB!

When there is no overlay published then your primary source is the raw data and you must follow it even though you really should be setting it up on the GPS or FMS, as I detailed in my post above.

Airmann
10th Oct 2014, 10:51
There's a bit of confusion here regarding the term overlay. In my case I am using it to refer to the fact that the FMS is flying what is coded in the box, and that is the DME arc in my case or whatever other procedure you are supposed to be flying. In reality my chart simply says ILS VOR DME it is a traditional approach. I did my flight training Canada so I know that there are certain procedures known as overlays. I am not referring to that here. What I mean is that we are essentially flying our own overlay, and we have become so confident in the FMS and with GPS navigation that we happily let the computer fly the procedure oblivious to the NAVAIDS we should really be monitoring.

The airfield we were going into had no radar. They could not have given us radar vectors, all their approaches are based off using the VOR and the DME to set yourself up for final. In reality the airport should really not have been accepting any IFR approaches. When we informed approach of the issue their response was just OK, we'll let someone know about it and we just happily continued along, confident that the FMS would get us to the IAF.

The other issue is that a DME arc is a curve, and an FMS is not meant to fly a curved path based on its own internal position estimates except during RNP-AR (SAAAR) approaches, all other RNAV legs whether terminal or enroute are straight lines.

Ultimately I believe that legally speaking what we did was wrong, and that it speaks of our age that we continued oblivious to the obvious.

Elephant and Castle
10th Oct 2014, 11:15
For us if you are flying a conventional non preciccion approach you can allow the FMS to guide the airplane but you must have the raw data displayed to confirm that a valid path is being flown.

In your case the legality would depend on wether the dme arc was flown in IMC or VMC and wether it was above or below the MSA.

What you must ask your self is what would you do if while flying below the MSA you got a "position disagree" type message. RNAV approaches take this into account while a conventinal non precission approach doesn't. For expample it may ask you to get to a dme point before turning on the missed approach. If the DME is not working and your FMS spits the dummy you are left with limited options.

Rocket3837
10th Oct 2014, 16:24
Hello all,

If the navaid on which the letdown procedure is based on is not working, then it is illegal to perform the approach even if the app is coded in the nav db.

It is ironic to say that performing the app with the ref navaid bieng u/s or deselected is, sometimes, safer than having the navaids selected and monitored.

italia458
10th Oct 2014, 19:16
Airmann,

If the traditional NAVAID is down for the approach, what data is your FMS using to continue providing guidance to you?

The FMS has multiple inputs including INS, GPS, WAAS, VOR, NDB, etc. It processes all inputs through a Kalman filter to provide one position solution. It uses this to plot your position relative to the FMS database points. That's the basics of an FMS.

So when you lose your traditional (VOR or NDB) then your FMS continues to provide a position solution to you using the other inputs from the system. However, if you are required to use the VOR or NDB for "final approach course guidance" then you must discontinue the approach - regardless of how accurate your position solution from the FMS currently is.

Now, with regard to your original question, I don't believe you did anything illegal or unsafe - provided the FMS was giving a position solution for the loaded DME arc without any warnings, and you used the ILS once you intercepted to fly to the runway.

The ILS RWY 08 approach into CYBR I think is a good example of the approach that you are asking about.

http://imageserver.fltplan.com/merge/Canada/merge1410/Single/CYBR_ILS_RWY_08.pdf

The title of "ILS RWY 08" indicates that you only require the ILS to fly the final approach course. Prior to the final approach course you may use another means of navigation (provided it's set up correctly and you adhere to the limitations for that navigation source [FMS]) to fly the procedure. In this case, I could fly FMS direct to LISTU and then start the DMC arc as programmed by the FMS all the way to the final approach course, at which time I'd intercept and fly the ILS. Or I could go FMS direct to KESVA for the straight-in ILS if I was coming from the west. Prior to flying the 085 degree course inside of KESVA, you may use another nav source to navigate... which seems quite obvious in this case since the ILS only works when lined up on the final approach course!

ItsMeFromEarth
11th Oct 2014, 01:26
There's a bit of confusion here regarding the term overlay.
In the FMS/FMC world, Overlay means that the procedure legs are copied from an NPA approach, with the same bearing, distances, height as the original NPA.

underfire
12th Oct 2014, 04:14
Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?

Yes, what did happen to that thread?

BOAC
12th Oct 2014, 07:52
This one? http://www.pprune.org/questions/543235-gnss-conventional-procedures.html

underfire
12th Oct 2014, 12:05
No, it was the thread where we were all talking about overlays, overlays by Country, et al....

Elephant and Castle
13th Oct 2014, 15:38
The ILS RWY 08 approach into CYBR I think is a good example of the approach that you are asking about.

you may use another nav source to navigate... which seems quite obvious in this case since the ILS only works when lined up on the final approach course!

I cannot say that I agree. At least where I am we are not allowed to fly below the MSA unless under radar control or visual with the ground or flying an approved instrument procedure. The OP said there was no radar so the only way to fly the approach , starting at the dme arc, is either if you are visual or you just keep above the msa (4300 in your plate) and then intercept the localiser at that altitude and descend with the GS. A RNAV approach would specify the required navigation (GNSS, IRS...) and might also specify a minimum temperature. An RNAV approach also has specific procedures in case of a nav accuracy downgrade. You cannot invent a RNAV appoach out of a normal approach just because one of the aids is not working.

All an overlay apprach does is allow the FMS to guide the airplane while you check it complies with the raw data. You cannot fly an overlay approach if the raw data is missing. In that case what you need is a RNAV approach and that is a slightly different animal for the reasons mentioned.

The only reason your plate does not say ILS DME is because arriving from the west you dont need the DME. That does not mean that you can fly a DME arc below the MSA without the DME.

italia458
13th Oct 2014, 17:41
Elephant..

At least where I am we are not allowed to fly below the MSA unless under radar control or visual with the ground or flying an approved instrument procedure.

I agree. And if you are visual with the ground, you need to request either a contact or visual approach before you're allowed to deviate from the instrument approach that you've been previously cleared for.

When you are on the DME arc, you are flying an approved instrument procedure so you may descend to the altitude depicted on the arc, and any other altitude for the procedure. The key is that you must have your avionics correctly setup for the phase of flight you're in.

I'm going to specifically talk about GPS use in Canada, however, these procedures are exactly the same or very similar for FMS or RNP use around the world.

When you're within 30 NM of an airport and have an approach procedure loaded, the GPS will automatically change to terminal mode RAIM and CDI sensitivity - both being 1.0 NM. You are required to be in terminal mode when flying the initial segment, intermediate segment, and missed approach segment. The only time you are required to have approach mode RAIM and CDI sensitivity is during the final segment - between the FAF and MAP.

So as long as you are in terminal mode (1.0 NM), you may fly the entire approach up to the FAF. From the FAF to the MAP you must use the equipment specified as the title of the approach plate for lateral guidance. See this: http://i.imgur.com/r4lyK1f.png

As long as you load the approach from the database, everything else will be done automatically. If there is any issue, you will get a warning from the GPS telling you something along the lines of "RAIM NOT AVAILABLE" and that means that you need to discontinue using the GPS for your primary means of navigation for that phase of flight. The FMS should tell you the same thing but I won't guarantee it - you need to read your FMS's manual or AFM supplement for your airplane and equipment. Nothing you ever read on the internet will ever supersede what's written in your AFM!

So, as long as the FMS Airmann was using had the approach loaded from the database and it was not giving any warnings as to its performance, then they could use it to fly the approach all the way up to the final approach segment - transition to flying the ILS from that point (in real world you intercept and track the ILS before the final approach segment) - then transition back to the FMS navigation for the missed approach if they went missed. Most FMSs will automatically transition to using the ILS without you having to push any buttons. So, after loading the approach into the FMS you'd only have to press one button for the entire procedure - (the G/A button) to get it to sequence to the missed approach.

Considering Airmann said this:

GPS was Primary and the aircraft few the arc perfectly (as it was programmed to do) intercepted the ILS landed, went to the hotel. Didn't think twice about what we had just done.

I don't believe he violated any rules. If the GPS didn't have the proper integrity for the airspace he was in it would have given him a message.

Elephant and Castle
13th Oct 2014, 22:22
I cannot speak for Canada. In the UK that is not allowed.

Kefuddle
14th Oct 2014, 04:52
A while later I realized that we had just done a DME Arc with no DME! Which led me to a discussion with another captain; our reliance and expectation of our FMS is such these days that we forget that except for RNAV approaches all other NPA are not meant to be flown by the FMS alone. So long as the chart says, VOR, DME, or NDB the said ground aid should be fully serviceable, even if we are going to let the box manage. Technically and legally speaking that is. I think.
Going by my manuals, an overlay approach is a specific technique to aid approach navigation when the the specific procedure being flown is NOT in the FMC database. The overlay technique permits the use of an alternative approach that matches the course and profile. For example, loading up an ILS approach that has the same flight path as the VOR/DME procedure when the VOR/DME approach is not available in the FMC. When using an overlay approach, the actual raw data is the primary reference and must be servicable.

However, for non-ILS work and transitions, when loading up the specific VOR/DME approach in the FMC to fly an the appropriate VOR/DME procedure raw data is not required (but is recommended) unless unless following a localiser. The FMC is the primary means of navigation. So it would have been legal for me to fly the approach with the DME unservicable.

Just take the Boeing ILS approach procedure for a ILS VOR DME approach. The normal Approach Procedure and the FMS expects you to have the ILS tuned at the IAF, even if you are flying a procedure transition based on a VOR/DME (arc, racetrack, base turn, etc). The Boeing FMC is intended to be used as the primary means of navigation when the required procedure is contained in the database.

italia458
14th Oct 2014, 05:33
Kefuddle..

That doesn't sound right. Which country are you talking about? Edit: I see you're in London.

For example, loading up an ILS approach that has the same flight path as the VOR/DME procedure when the VOR/DME approach is not available in the FMC. When using an overlay approach, the actual raw data is the primary reference and must be servicable.

Why would you fly an ILS on top of a VOR/DME and monitor the VOR/DME? Why not just fly the ILS?

This is not an overlay approach.

GPS overlay approaches are traditional VOR- or NDB-based approaches that have been approved to be flown using the guidance of IFR approach-certified GNSS avionics. Because of approach design criteria, LOC-based approaches cannot be overlaid.

That definition is from the Canadian documents. If you're ICAO you can read the same thing here: http://www.icao.int/Meetings/PBN-Symposium/Documents/9849_cons_en[1].pdf

See paragraph 2.2.6.

In some States, pilots are authorized to fly suitable VHF omnidirectional radio range (VOR), VOR/distance measuring equipment (DME), non-directional beacon (NDB) and NDB/DME non-precision approach (NPA) procedures using GPS guidance. These are termed “GPS overlay” approaches and allow operators to benefit from better accuracy and situational awareness without the need for the service provider to design a new approach.

Here's yet another definition of an overlay approach: http://www.developpement-durable.gouv.fr/IMG/pdf/ST-GuideO1-_PBN-RNAV_GNSS__ENGv3.pdf

See paragraph 7.2.

B. Overlay approaches: publication example   (mainly found in the United States)
The primary navigation means is either the VOR DME or the GNSS.
To perform them with the RNAV/GNSS system, the operator must comply with the requirements of EASA AMC 20-27

Based on these definitions, it seems ICAO, Canada, the United States, and France all agree on what an overlay approach is. Where are you flying that this is not the case? RNP 0.3 is virtually identical to the specifications for GPS overlay approaches so I'm not surprised regarding Australia.

Elephant..

Do you have a reference for overlay approaches not being allowed in the UK? I'd be interested to read it.

Kefuddle
14th Oct 2014, 09:41
Why would you fly an ILS on top of a VOR/DME and monitor the VOR/DME? Why not just fly the ILS?

I guess taking into account the possibility that the ILS is u/s. Anyway, this is from the Boeing FCTM, so I am working to their and hence my company's definition of an overlay approach, which is the caveat I introduced at the beginning of my post. I should add that the phrase "overlay" in the Boeing documentation is informally introduced with quotes within the descriptive text, so I don't think they are intending to align nor contradict any other definition that exists outside of the Boeing manuals.

So, if we just move terminology to one side for the moment. Flying a procedure with the applicable procedure loaded in the FMC, then raw data need not be, but is recommended to be, monitored. That much I think is established.

JeroenC
15th Oct 2014, 21:50
Not in my company, Kefuddle. We need the raw data backup, unless specifically flying a GNSS/GPS approach. European operator.

latetonite
16th Oct 2014, 02:06
To JeroenC: and what is your approved way of doing this monitoring?

Elephant and Castle
16th Oct 2014, 09:10
Overlay approaches are allowed provided the raw data is displayed. What is not allowed is turning a normal approach into a GPS approach because the relevant aids are not available (or you do not wish to display them), for that you need a specifically published GPS approach.

PappyJ
16th Oct 2014, 09:56
Surely an overlay approach without the raw data becomes an RNAV approach?

No, an overlay approach generally doesn't require the underlying NAVAID (NDB or VOR) to be serviceable. That's the case in Canada. But we're talking about a lot of countries here so you'd have to check your country's regulations.

Do you have the CARS reference for this readily handy?

Denti
16th Oct 2014, 10:50
European operator, on our boeing fleet we can fly all non precision approaches including LOC, VOR, NDB out of the database and with the exception of the LOC approach raw data monitoring is recommendet, but not required. However the relevant navaid has to be operational for all approaches. For a LOC approach the localizer has to be used as guidance and is therefore monitored, it is displayed as lateral deviation on the PFD and ND (approach), the vertical deviation scale is a computed glide path.

On the airbus fleet raw data monitoring is still mandatory though, even on managed approaches.

vilas
16th Oct 2014, 16:13
Even in Airbus it is permitted to do NPA without the aid being serviceable provided it is permitted by DCA.

mikedreamer787
16th Oct 2014, 16:15
You can't do a NPA without the required published
aids, whether those aids are VOR, VOR/DME, NDB,
or GPS with or without the box.

The "what if" principle resolves the question in a
lot of cases.

Vilas - if say the Tawau VOR and DME is out and
you get a NAV ACCUR DOWNGRAD while still in a
rebut putir what you gonna do?

vilas
16th Oct 2014, 17:21
It is not my opinion but aircraft manufacturer's statement. Why does Canada permit it and UK does not? How does your what if applies to selected VOR approach with NAV accuracy low and it goes off? You go around isn't it?

italia458
16th Oct 2014, 18:38
PappyJ..

There is no CARs reference. Here is the AIC on GPS use: http://www.navcanada.ca/EN/products-and-services/Documents/AIP/Current/part_5_aic/5aic_eng_2008_16.pdf#top

Look at paragraph 6.0(b).

Second reference is paragraph 3.15.5.2.2 in the TC AIM which you can search for on the internet:

"Unless required by the aircraft flight manual (AFM) or AFM Supplement, it is not necessary to monitor the underlying navigation aid, and it is even permissible to fly a GPS overlay approach when the underlying navigation aid is temporarily out of service."

Elephant..

Overlay approaches are allowed provided the raw data is displayed. What is not allowed is turning a normal approach into a GPS approach because the relevant aids are not available (or you do not wish to display them), for that you need a specifically published GPS approach.

You can't just turn an approach into a GPS approach. It either isn't a GPS approach or it is - you, as the pilot, cannot make a GPS approach.

The GPS approaches include RNAV approaches, overlay approaches, and RNP approaches. I've provided two references for GPS overlay approaches in Canada not requiring the underlying NAVAID to be serviceable or monitored. Do you have a reference for what you're saying?

Elephant and Castle
16th Oct 2014, 21:28
My reference comes from our company manuals. I don't have access to the Airbus Master copy but with us it is pretty much all from airbus.



Part B 2.3.18.1.2

Non Precision Approaches
For Non Precision Approach, the final approach shall be flown as a Constant Descent Final Approach (CDFA) to the appropriate minima.

Lateral managed guidance (NAV) can be used for overlay non-precision approaches provided the following conditions are met:

The Navigation Database must be current.

The approach is stored in the navigation database.

GPS PRIMARY must be available.

Conventional aids must be displayed.

Arctaurus
16th Oct 2014, 21:44
As a general principle, overlay approaches do not require the underlying NAV AID to be serviceable at the time of the approach. However, the FMS database must be checked against the paper chart and the FMS accuracy of the nav aid position established.

Approvals will also generally require regulator involvement. Regarding raw data, if it's available, it should be monitored. The primary position reference though is GPS Primary with a fixed RNP value (for example 0.3 nm) on the PROG page.

So if (for example) the XYZ VOR approach is selected in the database with the XYZ VOR notamed as unserviceable, the approach can indeed be flown in managed NAV with an approved RNP value (ie 0.3 nm).

If the NAV AID is unserviceable and dual GPS/dual FMS/FMS position difference errors occur, then the approach must be discontinued (IMC).

latetonite
17th Oct 2014, 06:34
If you want to monitor a NPA approach in the conventional way, why not fly it conventional in the first place, if you think this results in a better job?
And what is raw data monitoring? Displaying positions? Flipping the 'rabbit ears'?
Selecting full rose VOR mode on the ND?

Kefuddle
17th Oct 2014, 08:21
I know that was a flippant question, but raw data monitoring is flying the FMC and checking that the needle is on the number (by whatever means). But raw data monitoring is more or less pointless if you're not prepared to go around on the basis of it, even if the ANP is still good and the FMC is on its calculated track. Not many will go-around on that basis alone and will continue the approach full of excuses.

Denti
17th Oct 2014, 17:08
@vilas, im sure if the correct documentation is provided and the authority agrees that even on our airbus fleet we could do away with the need to monitor the underlying navaid. Currently that is not the case. In general i was kinda surprised upon switching from boeing to airbus how oldfashioned its avionics and especially its PFD and ND are. But that is a discussion for another thread.

@Kefuddle, dunno, with the big allowable nav aid error for conventional nav aids compared to constant monitoring of your onboard navigation performance (which is not only based on GPS), i'd rather take the latter. Even better would be GLS of course, cheap to have and serving precision approaches to all runways in the vicinity with one installation.

vilas
19th Oct 2014, 13:02
Denti
This is from Airbus.


KNOWLEDGE - POSSIBILITY TO FLY SOME NPAS WHEN THEIR REFERENCE NAVAID IS UNSERVICEABLE. (On behalf of STL). In this case, the approach (VOR, VOR/DME, NDB or NDB/DME) is flown in a similar way as an RNAV approach.
However, as mentioned in the FCOM, the operator has to submit for an approval to their National Authorities.
The prerequisite from our point of view is that the airline fulfills all the operational requirements to fly RNAV(GNSS) approaches and it is even better if the airline is already approved to fly RNAV(GNSS) approaches. Therefore, the first step is for the airline to contact their national authorities to discuss this possibility.
From an operational point of view, we would recommend that the following elements are fulfilled:
• The operator is approved to fly RNAV approaches.
• The approach procedure is coded in WGS 84 in the Navigation Database.
• The protection areas of an RNAV procedure are always inside the protection areas considered in the original procedure (NDB, VOR), or there is no obstacle.
• FCOM limitations and SOP for RNAV approaches are observed.
• Specific requirements as imposed by the authorities

Kefuddle
19th Oct 2014, 16:16
Denti,

Yes, the FMC position must be a far superior navigation aid than any single beacon (assuming RNP is correct and ANP is within limits!). Which is probably why the Normal Amplified Procedures don't involve tuning the navaid for the transition from IAF to FAF/FAP and the FCTM declares that approach raw data monitoring is not required, except for an ILS or a localiser approach.

But as I mentioned before, Boeing's mention of "overlay" approaches is something else entirely and not generally what is meant by when other posters here are using the term "overlay".

vilas
20th Oct 2014, 03:18
When a VOR approach is not coded in data base you have to do selected approach and if that VOR fails you go around. When the approach is also coded in data base you have possibility of doing managed approach which is flown from data base and monitor on the VOR. If NAV ACC becomes low you changed to VOR approach. Similarly if the VOR fails and NAV ACC is high it is possible to do the data base approach treating it like RNAV. But this requires regulatory approval and that means certain conditions have to be fulfilled as stated by airbus in my earlier post. VOR/NDB approaches have their own limitations of accuracy as long as FMS accuracy is not beyond that what's the problem?

underfire
20th Oct 2014, 08:33
Currently that is not the case. In general i was kinda surprised upon switching from boeing to airbus how oldfashioned its avionics and especially its PFD and ND are. But that is a discussion for another thread.

Denti, I know for another thread, but which do you consider old fashioned?

acroguy
23rd Oct 2014, 00:17
I'm coming late to this discussion, but let me add a couple of points.

In the US, all so-called "overlay" approaches are being discontinued. Here, an overlay is an approach that was previously labeled GPS/NDB-A, for example. When in effect, you could substitute the GPS for the NDB, even if the NDB was out of service. With the discontinuation of these approaches, the FAA has commissioned separate RNAV and, in the previous example, NDB approaches. There was an interval during which the NDB approach to my local airport was separated but the RNAV was not yet commissioned. At that point, if the NDB was OTS, one couldn't fly the approach even with GPS terminal guidance and approach wouldn't approve it. GPS is allowed to substitute for an NDB or VOR but not if it is the primary navaid for the approach.

Now, with respect to DME, GPS is allowed to substitute for DME in virtually all circumstances, at least in the US. There is a specific page in the front of the Jepps that deals with this. However, on an ILS one can only use GPS up to the FAF and must use the localizer/glideslope inside the FAF. In fact, the box will warn of this.

I think that the discussion of FMS has confused the matter, as there are multiple sensors involved. However, inasmuch as the FMS includes GPS guidance, I see no problem with using it to intercept the localizer outside the FAF on an arc with DME inop as long as the actual localizer/glideslope is tuned and used as primary guidance inside the FAF.

At least that is my understanding of the FAA's stance on the matter...I realize that individual operators have opspecs that are specific and may differ.

Avefenix
13th Nov 2014, 19:21
But the reality is, you were in Africa, are you going to divert to the alternate just because the DME was us, knowing that probably the accuracy of you navigation capability is grater than the ground equipment ... use the airmanship ....I would landed as you, and I can understand that you were worried about the legality, as you know the real world is not perfect, so you have to balance between pros and conts

latetonite
13th Nov 2014, 22:16
Did once an LNAV/VNAV pproach on the out of service ILS in Brazzaville. Looked better than the offset VOR approach on the same rwy. As long you know what you are doing, and who is responsible..