Overlay Approaches

[Kefuddle]

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]

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-Sym...s/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.gou...NSS__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]

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]

Not in my company, Kefuddle. We need the raw data backup, unless specifically flying a GNSS/GPS approach. European operator.

[latetonite]

To JeroenC: and what is your approved way of doing this monitoring?

[Elephant and Castle]

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]

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]

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]

Even in Airbus it is permitted to do NPA without the aid being serviceable provided it is permitted by DCA.

[mikedreamer787]

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]

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]

PappyJ..

There is no CARs reference. Here is the AIC on GPS use: http://www.navcanada.ca/EN/products-...008_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]

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]

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]

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]

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]

@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]

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]

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]

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?