Is an overlay where a GPS approach is overlayed on an existing NDB or VOR approach?

 

Also on an RNAV AR app how come you don't need the GPS if its not working or something.

Is an overlay where a GPS approach is overlayed on an existing NDB or VOR approach?

 

Also on an RNAV AR app how come you don't need the GPS if its not working or something.

 

Yes, an overlay approach is a GPS approach overlayed onto an existing procedure.  That's why it has "or GPS" in the approach title.  A VOR or GPS approach could be flown using GPS only if that's all you've got or using the VOR only if that's all you have.  For a non-overlay approach , VOR Rwy 36 for example, the aircraft has to have a working VOR receiver and the VOR must be operational to fly the approach regardless if the aircraft has GPS or not.

 

The FMC uses several different sensors to determine it's position.  The GPS is only one of those sensors.  If you are flying in an area with good coverage the FMC can fix it's position using DME/DME almost as good as it can with GPS.  Unless a RNAV approach specifically prohibits the use of DME/DME you could fly the approach without GPS.

Done some homework and found this

 

 

http://www.caa.co.uk/docs/33/CAP773.pdf

 

If you are employed in UK/Europe then your airline will probably be working to JAR-OPS or EU-OPS regs, in which case -

- If it says GPS (or GNSS, same thing but more correct) on the chart then you must be receiving valid GPS/GNSS data.

- If you are flying an overlay procedure and lose the GNSS, you may revert to the conventional 'reference aid' and continue, if you lose the reference aid (VOR or NDB) you must execute a missed approach.

- If it names a VOR or NDB aid then it is an overlay procedure and the RNAV system may only be used if the underlying 'reference aid' is available and agrees with the RNAV.

- An RNAV (GNSS) Approach is what ICAO (Doc 9613, the PBN Manual) calls a 'RNP APCH', and requires multi-sensor (ie, GNSS and DME at least) RNAV kit with a specific standard of 'on-board monitoring and alerting' (their phrase).

- For a GNSS (Only) Approach you need two independent GNSS receivers for starters, and, I believe, space-based augmentation system (SBAS).

Your '(GPS) VOR RwXX' I've not seen before. The principle in Europe at least is 'If it says GPS you must have GPS', if it says VOR (or NDB) you must have VOR (NDB).

 

We're talking about RNAV or FMS based approaches here, and an approach designed for VOR or NDB but coded for the FMS (ie, an overlay) may only be flown using the FMS if the underlying reference aid is available and confirming the FMS accuracy.

I may be getting the "overlay" and "non-overlay" wording mixed up.  I'd have to look it up.  In any case if the approach title contains GPS then you are good with GPS only.  Under FAA rules anyway, I couldn't tell you what the JAR regs are.

Hi Vernon.

 

This whole RNAV nomenclature sure is hard to get a grip of, and I am just coming to grips with it as well.

 

In the US you have tons of Rnav approaches that are either RNAV (GPS) or RNAV (RNP). From what I gather, the whole nomenclature is rather misleading, for they sound similar in nature but are in fact very different indeed. RNAV (GPS) (or GNSS in other parts of the world) is a type or area navigation non precision approach where your primary means of acertaining position comes from a certified GPS (GNSS) receiver. Its just one of many ways of doing area navigation. GPS is so widespread today that you could have a 737 and a small single-prop airplane being certified to shoot an RNAV (GPS) approach. And within the GPS world, there are all sorts of subsets of approaches based on WAAS to lower the minima, such as LPV. These are not our primary concern since we are not general and business aviation simmers. Boeings and Airbuses don´t belong in the WAAS world, as far as I know. 

 

However, RNAV (RNP) approaches are different in that the aircraft must have a flight computer capable of executing performance based navigation (PBN), and in so doing offer a minimum required navigational performance to guarantee the safety of the approach. This is the part that took me awhile to figure out. It doesn´t matter how the computer is getting its data and calculating its position and navigation performance. If you go to the FCOM, you´ll see the NG FMC uses GPS, radio (DME/DME, VOR, LOC and ILS), and IRS signals in a magical mathematical formula to derive its current position and tell you how well its performing. 

 

Thus, to shoot an RNAV (RNP) approach, you do not have to necessarily have GPS working. If you can maintain the minimum RNP of the approach without GPS updating the FMC, I believe you´ll still be eligible to continue the approach. If at any time the FMC spits out its unable to maintain the RNP you must execute a missed approach. So you´ll see modern airliners executing RNAV (RNP) approaches, but not Skylanes, even if they have WAAS enabled GPS receivers. Mainly because they have no FMC capable of performance based navigation. 

 

If you start studying the minima of the RNP and GPS charts, you might go crazy, like I am right now! And some charts don´t help at all. For example, try to find KMIA´s RWY 30 RNAV approach plates. There´s an RNAV (GPS) RWY 30 and an RNAV (RNP) RWY 30 approach. You´d expect aircraft with an FMS and no GPS might be eligible to shoot the RNP approach. But the chart states clearly that GPS is required to execute the approach. In other words, it´s an RNP approach where GPS must be used by the computer. Infuriating, isn´t it? I believe they might´ve  put both types of approaches there so that the maximum number of airplanes will be able to use the rwy with similar minima. The RNAV GPS approach would cater WAAS based  general aviation and business aviation while the RNP approach would service GPS equipped airliners. But I do beilieve there are many RNP approaches that do not require GPS.  

I found google and PPRUNE to be my best friend now

 

 

Yehh been looking at KMIA charts for rwy 30 i'll have a search about that and let me know if you find anything about that.