RNP Appch

[DLP084 5]

Gents,

 

The NGX canot fly any RNP approach which requires RF legs.  RF is not modeled in the NGX.

 

JW

 

Thats only partially true. When using NavdataPro, RF-Legs are interpolated just like AF-Legs. It is still not the real thing but better than nothing. 

 

PMDG mentioned in an earlier posting, that this is going to change in the future as a new data format is in development. 

 

 

 

Jan-Paul

[Jeffrey L. Whitaker 0]

@ Jan-Paul, it is wholly true.  RF is not AF.  Treating an RF leg as an AF leg is not RF.  Therefore, the NGX cannot fly an RNAV approach which requires RF legs. 
 

I agree, the NavdataPro solution is a step in the right direction- but it is not RF.

 

JW 

[scandinavian13 5,192]

 

 

If a GPS failure occurs would my only indication be a switch from "GPS" to "IRS" in the ND display?

 

I know there are situations in which the FMC will display GPS or IRU navigation only (I think it has something to do with polar operations, but I don't remember what it shows), so you should have an associated FMC message.  I believe an alert should show up on the EICAS as well.

 

 

 

Meaning a lateral or vertical error indication is displayed only because the primary, secondary, or even 3rd backup has been exhausted?

 

I don't believe it will show your error tolerance visually in any way, other than your ANP value increasing.

 

 

 

If so would this mean an out of limits caution would be advisory only and execution of the appch would still be allowed because you have an ANP value that is is still better than the required RNP value?

 

Correct.  As long as your ANP is below RNP, you can shoot the approach.

 

 

 

Or do you only get an error sequence when all this equipment has failed?

 

No, they are independent systems, for the most part.  Explained below.

 

 

 

Lastly, wouldn't the failure sequence actually be....GPS, Radio Nav, ADIRU?

 

There is no sequence, really.  It's not like they're built on top of each other, though each one of them can use data from another source to make it more accurate.

 

-Inertial reference units are generally the most precise because they are calculating difference in position at a higher rate than GPS, using a different method (inertial rates, not triangulation).  Their weakness is that they get less precise over time.

 

-GPS is slightly less precise because of its method of calculation (and years ago, it actually had error built into it to ensure civilians didn't have military precision), but its accuracy does not degrade over time.  It is only dependent on the number of satellites visible to the unit at a set time (which is a big deal, when using GPS alone for tighter RNP procedures - see RAIM).  For this reason, the GPS position is used to update the IRU position from time to time to maintain precision.  If the GPS fails, the IRU is as precise as its last update plus whatever time has elapsed since that update.  The IRU position can, however, be updated by radio (below).

 

-Radio navigation is the least precise of all of these (comparatively).  It can be used to navigate on its own (something that was very common until GPS and IRUs were commonplace - FDXs 727s were all /A, or radio nav), or it can also be used to update the IRU position.  An example of IRU updating using radio navigation (without GPS) is Concorde, and several of the 757s that United currently flies.  In the case of United, it's DME/DME/IRU (the use of two VORs and their associated DMEs gives a more accurate position calculation).

 

All of these together help to maintain ANP.  If the GPS fails, the FMC can still tune VORs in its database and triangulate positions because the coordinates of various VORs are stored in the database.  If the IRU fails, GPS and radio navigation can help to maintain nav performance.  GPS alone can be used to shoot many RNP approaches, provided the RAIM check passes (basically, the required number of satellites is available overhead).  Instead of thinking of it like a linear system - IRU fails to GPS, which would fail to radio nav - think of it like a collective system: IRU provides a constant calculation of position, GPS tosses in a position update for the IRU from time to time to keep it accurate, and radio nav is there to back the team up.

 

This site (managed by the FAA) allows you to see areas of GPS outages for the various realms - en route, terminal, and NPA.  En route has the highest tolerance, with the next being terminal, and lowest tolerance being NPA (stands for non-precision approach, but specifically refers to GPS non-precision approaches).  You can toggle them at the lower right hand corner.  The difficulty for dispatchers (and pilots, too) is that the area must be not be red when the approach is to be conducted in order to use GPS alone for the approach.

 

If you want to stab around with the tool to check it out, make sure to select the required level (NPA for approaches, at the bottom right).  You'll also want playback (bottom left).  In the summary view, the red areas just mean that there will be an outage at some point within the selected time period (18 hours by default), and the outages are normally pretty brief.  The yellow areas do not mean degraded performance, they're just alerting you to the fact that the area will go red at some point in the time period.  Pretty nifty.

[Spin737 102]

 

 

If a GPS failure occurs would my only indication be a switch from "GPS" to "IRS" in the ND display and maybe an FMC msg?

You'd get an "GPS - L INVALID" or "GPS - R INVALID."

 

For some time when taking off from Gimpo or Incheon, we'd get those messages because the DPRK was jamming the signals. Ahhh, good times. . .

[Copper. 78]

... should also have GPS annunciated on the IRS mode select unit, and both MC's illuminated and IRS annunciated on the System Status Annunciator (6 pack) on recall (for single GPS failure - with dual (2) GPSs installed).

 

Brian Nellis.

[DLP084 5]

@ Jan-Paul, it is wholly true.  RF is not AF.  Treating an RF leg as an AF leg is not RF.  Therefore, the NGX cannot fly an RNAV approach which requires RF legs. 

 

I agree, the NavdataPro solution is a step in the right direction- but it is not RF.

 

JW 

 

Technically there is not much difference in ARINC 424. Both are ARCs on a specific radius around a center fix. On AF-Legs its the VOR and on RF-Legs its a database fix. 

 

So if the current PMDG Format would support AF-Legs it would be absolutely no problem to implement RF-Legs. 

 

 

Jan-Paul

[Jeffrey L. Whitaker 0]

Jan-Paul,

 

Can the NGX currently fly RNP aproaches which require RF legs?

 

JW

[johnsturm 22]

Jan-Paul,

 

Can the NGX currently fly RNP aproaches which require RF legs?

 

JW

 

Umm, why are you asking a question that you already answered in a previous post?

 

Gents,

 

The NGX canot fly any RNP approach which requires RF legs.  RF is not modeled in the NGX.

 

JW

[Jeffrey L. Whitaker 0]

John,

 

My question is directed at Jan Paul.  The purpose of my question is to clarify my understanding of one of his previous posts.  I think language might be getting in the way of my understanding.  If the NGX is able to replicate RF legs using AF in NavDataPro i'd sure like to know about it.  Would you?

 

JW

[DLP084 5]

Basically the NGX can fly it. But it is not the same as it would be if AF and RF-Legs would be supported directly without creating additional fixes that are necessary to interpolate the ARCs. 

 

With NavdataPro additional waypoints are created between the start and the ending fix of an arc to interpolate it. The distance of this fixes is approximately 3nm between each other. 

 

The arc is then not flown perfectly with a constant bank angle like in reality because of the straight segments. But it is working quite nicely when considering the limitation of the current PMDG data format. 

 

Just try out KPSP for example. 

 

 

Jan-Paul

[Jeffrey L. Whitaker 0]

Jan-Paul,

 

Thanks for the quick reply.  I'll grab a cycle of NavDataPro tonight and give it a whirl.  I'll be a happy camper if the NGX can replicate the RNAV (RNP) Rwy 34L into KRNO.

 

Thanks!

 

JW

[vonmar 124]

Gents,

 

The NGX canot fly any RNP approach which requires RF legs.  RF is not modeled in the NGX.

 

JW

Is that in the aircraft or the Navdata or both?

Wondering if it will it be modeled/available ... ??

[johnsturm 22]

When the NGX was developed the information was not available in the NAVADATA so it was not implemented. I have seen it mentioned in the 777 forum that they are giving it another look. If it gets implemented there it will probably be added to the NGX for the next update.