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OrtegaS

RNP Appch

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Hello,

 

What is the lowest RNP cat appch the NGX is capable of?  I flew the RNAV (RNP) Z RWY 16L into KSEA and had to use the min's for RNP 0.30 DA since I was unable to input 0.12 into the FMC for the lower min's.  Is this even possible?  It seems that since the ANP is usually around .02 this shouldnt be an issue.

 

Thanks,

 

Steve

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I couldn't find anything specific in the manuals regarding such RNP limitations, so I guess you are correct: you can fly any RNP approach as long as the Actual Navigation Performance (ANP) stays within the limits established by the chart.

 

Also, I don't think the FMS will accept RNP values with more than one decimal place. Try entering 0.1 instead. That should work.

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Hello,

 

Thanks. I will try using just the single value after the decimal. I wasn't sure if its a limitation of NGX or an input "sequence." From what I have read the FMC always allows you to fly the RNP appch of the most restrictive min's without user input, but if you wish to actually fly the lower min's you must actually input the RNP DA value. I'm not 100% certain though.

 

Thanks.

 

Steve Ortega

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The only RNPs I have come across for RNAV approaches are 0.3 and 0.1 and these values can be input to the LEGS page or PROG page 4. You may see other values depending if you are in the airway, arrival, non RNAV approach, missed approach, etc.

 

A 0.1 RNP approach is known as RNP AR (Authorization Required) and required special certification and are usuall denoted as "RNAV RNP" on the chart. Most RNAV approaches are 0.3 and are usually called "RNAV GNSS" or "RNAV GPS" on the chart.

 

If the LEGS page or PROG page 4 doesn't show the correct RNP just change it through the CDU scratch pad.

 

The minimums are set usual as BARO on the PFD using the EFIS Control Panel according the procedure. Remember to add 50' to the minimums for a 3.0º slope as all RNAV approaches constant descent approach (CDAs) and perhaps more than 50' is the slope a steeper - some operators take account of steeper slopes some don't.

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Hello,

 

I reflew the same appch into KSEA and was successful in inputting 0.1 into the CDU.  I do believe though that I should be able to input 0.12 as this is on the appch chart.  I am not absolutely sure of this as I am not a real B737 pilot :(.

Also, some of the radius to fix dont look so RF.  Some are more straight line instead of curved.  Is this a NGX display or data quirk?  I am using the NavData Pro set as I understand that at this point Navigraph does not yet support this feature.

 

Thanks,

 

Steve Ortega

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What is the lowest RNP cat appch the NGX is capable of?

 

As long as the ANP is lower than the RNP, in the sim, you can shoot the approach.  The limitations in the real world are mainly that of a stupid amount of red tape, crew certification, aircraft certification, and training (to follow the same two [darned] dots on the PFD), but in the end, the only real limitation is the aircraft's ability to know where it actually is (ANP), versus what's required for that segment (RNP).

 

Sorry - touchy subject with me, as this is one of the projects a team I'm working with is working on (BEBS, if you're curious), and it's proving rather frustrating.

 

 

 


A 0.1 RNP approach is known as RNP AR (Authorization Required) and required special certification and are usuall denoted as "RNAV RNP" on the chart. Most RNAV approaches are 0.3 and are usually called "RNAV GNSS" or "RNAV GPS" on the chart.

 

This isn't entirely accurate.

The precision does not dictate the fact that it's AR or not.  The KJFK RNAV RNP 04L is a perfect example, with 0.3 as its required performance (yet is still authorization required).  While more precise approaches are often RNP-AR, the value isn't the trigger for the -AR.  The value may, however, trigger what DA you use (as in the case of KDCA RNAV RNP 19

 

There are different types of approaches:

RNAV GPS

RNAV RNP (which includes GNSS in RNP 0.3 and below)

RNAV RNP-AR (Authorization Required)

 

If you look around at approaches, you'll find that most all of the RNAV RNP approaches in the States are RNP-AR, though the two are meant to be independent.  The reason AR is supposed to exist is for special, non-normal procedures.  While many approaches merit that designator (KDCA RNAV RNP 19 would be an example, given its oddities and proximity to obstacles/P-airspace), there are several others that do not (KIAD RNAV RNP 01C is essentially an ILS overlay - clearly meriting special attention...right?).  This is one of my aforementioned beefs with the system, but I'll spare you that rant.



 

 


Also, some of the radius to fix dont look so RF.  Some are more straight line instead of curved.  Is this a NGX display or data quirk?

 

Radius-to-Fix is not implemented because of a data access issue (at the time, at least).

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The NGX doesn't recognise the second decimal place. I.e. the FMCS won't recognise a lateral RNP value of 0.10, but it'll accept 0.1. So Lateral RNP values like 0.12, 0.15, 0.18 etc won't work.

Aircraft that are capable and approved for RNAV (RNP) AR operations, are capable of accepting those values no problems.

If the weather is good, consider leaving the default value in (0.30) so you don't have to delete your manual entry in the event of a missed approach.

Brian Nellis.

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Hello,

 

Really good information. Scandinavian13 thanks for the link. Interesting ready though slow. In the sim ANP is always good. I cant think of any reason why I wouldn't be able to shoot an RNP appch using the lowest min's or why my tolerances would ever go yellow inside the FAF, Unless mag dec changed so much between FSX inception and current data base, but even then I'm not sure.

Thanks copper. NGX can only do one decimal place. Figured that one out. But as mentioned above in FSX it doesn't really matter.

Also, when is it appropriate to change the MCP altitude to your missed appch alt? Doing so with LNAV and VNAV engaged and auto pilot still active seems to effect the NGX. Is it always done after disabling auto pilot? Got to love this stuff.... :-)

 

Thanks,

 

Steve Ortega

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Hello,

 

Really good information. Scandinavian13 thanks for the link. Interesting ready though slow. In the sim ANP is always good. I cant think of any reason why I wouldn't be able to shoot an RNP appch using the lowest min's or why my tolerances would ever go yellow inside the FAF, Unless mag dec changed so much between FSX inception and current data base, but even then I'm not sure.

Thanks copper. NGX can only do one decimal place. Figured that one out. But as mentioned above in FSX it doesn't really matter.

Also, when is it appropriate to change the MCP altitude to your missed appch alt? Doing so with LNAV and VNAV engaged and auto pilot still active seems to effect the NGX. Is it always done after disabling auto pilot? Got to love this stuff.... :-)

 

Thanks,

 

Steve Ortega

I believe the earliest you should reset the MCP to missed alt is when you're 300' below that altitude on the approach. The latest would be on the missed approach itself. Your SOP should fall somewhere on that spectrum.

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This isn't entirely accurate.

The precision does not dictate the fact that it's AR or not.

We don't do RNP AR and I am not specifically trained and thus I confidently assumed...incorrectly :) Good info and thanks for the correction Kyle.

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Good info and thanks for the correction Kyle

 

Welcome!  To be honest, it would all still be foreign to me unless I was working on a project for it with my company.

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

 

See post #6...

 

 

 

I cant think of any reason why I wouldn't be able to shoot an RNP appch using the lowest min's or why my tolerances would ever go yellow inside the FAF, Unless mag dec changed so much between FSX inception and current data base, but even then I'm not sure.

 

Think failures  :wink:

 

If I lose GPS, the IRS stays accurate but its error in position calculation increases over time.  If the GPS fails relatively close to arrival and the ANP is still within what's needed, I can still shoot the approach.  If it failed early in the cruise, I might not be able to.

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ANP predictions can also be an inhibiting factor as to what RNP is used for the approach.

 

Brian Nellis.

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Hello,

 

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? Meaning a lateral or vertical error indication is displayed only because either the primary, secondary, or even 3rd backup has gone OTS? 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 still better than the required RNP value? Or do you only get an error when all this equipment has failed all together(outside factors not withstanding) forcing a discontinuation of the appch? Lastly, wouldn't the failure backup sequence actually be....GPS, Radio Nav, ADIRU? Still learning......... :-)

 

Thanks

 

Steve Ortega

 

Wondering if PMDG has plans to update the NGX for RF.

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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

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@ 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 

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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.

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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. . .

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... 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.

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@ 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

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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

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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

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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

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