PEBLE3 SID being flown incorrectly (magnetic vs true)

[Keith Smith]

I'd like some clarification on whether headings and radials are interpreted to be in true or magnetic degrees in the FMS procedure database that's included with the CRJ.

 

The reason I ask is because the PEBLE3 SID out of KSAN is not being flown correctly. The procedure calls for a right turn to heading 290 until crossing the OCN R-170, then heading 315 to intercept the MZB 393 radial to MELDY, then PEBLE.

 

I noticed the 315 heading was entered in the KSAN.xml file as 305 (if memory serves), otherwise, the radial appeared to be correct. However, even after correcting the 315 heading in the database, I noticed that it was turning PRIOR to the OCN R-170 radial.  When I looked closely, I noticed it was turning at approximately the 155 radial (15 degrees off, which matches the magnetic variation for the area).

 

So, when I updated the OCN 170 radial to 185 (the TRUE heading, rather than magnetic), AND updated then 113 radial (reciprocal of 293) to read 128, it flew the procedure PERFECTLY.

 

Here's the updated snippet for the procedure, updating the revision to PEBLE4. It covers everything up to PEBLE intersection:

 

    <Sid Name="PEBLE4" Runways="27">

      <Sid_Waypoint ID="1">

        <Name>(414)</Name>

        <Type>ConstHdgtoAlt</Type>

        <Latitude>0.000000</Latitude>

        <Longitude>0.000000</Longitude>

        <Speed>0</Speed>

        <Altitude>414</Altitude>

        <AltitudeCons>0</AltitudeCons>

        <AltitudeRestriction>above</AltitudeRestriction>

        <Hdg_Crs>1</Hdg_Crs>

        <Hdg_Crs_value>273</Hdg_Crs_value>

        <Flytype>Fly-by</Flytype>

        <BankLimit>25</BankLimit>

        <Sp_Turn>Auto</Sp_Turn>

      </Sid_Waypoint>

      <Sid_Waypoint ID="2">

        <Name>OCN</Name>

        <Type>VorRadialIntc</Type>

        <Latitude>33.240639</Latitude>

        <Longitude>-117.417711</Longitude>

        <Speed>0</Speed>

        <Altitude>0</Altitude>

        <AltitudeCons>0</AltitudeCons>

        <AltitudeRestriction>at</AltitudeRestriction>

        <Hdg_Crs>1</Hdg_Crs>

        <Hdg_Crs_value>290</Hdg_Crs_value>

        <Flytype>Fly-by</Flytype>

        <BankLimit>25</BankLimit>

        <RadialtoIntercept>185</RadialtoIntercept>

        <Sp_Turn>Auto</Sp_Turn>

      </Sid_Waypoint>

      <Sid_Waypoint ID="3">

        <Name>(INTC)</Name>

        <Type>Intc</Type>

        <Latitude>33.089400</Latitude>

        <Longitude>-117.693856</Longitude>

        <Speed>0</Speed>

        <Altitude>0</Altitude>

        <AltitudeCons>0</AltitudeCons>

        <AltitudeRestriction>at</AltitudeRestriction>

        <Hdg_Crs>1</Hdg_Crs>

        <Hdg_Crs_value>315</Hdg_Crs_value>

        <Flytype>Fly-by</Flytype>

        <BankLimit>25</BankLimit>

        <RadialtoIntercept>126</RadialtoIntercept>

        <Sp_Turn>Auto</Sp_Turn>

      </Sid_Waypoint>

      <Sid_Waypoint ID="4">

        <Name>MELDY</Name>

        <Type>Normal</Type>

        <Latitude>33.089400</Latitude>

        <Longitude>-117.693856</Longitude>

        <Speed>0</Speed>

        <Altitude>14000</Altitude>

        <AltitudeCons>0</AltitudeCons>

        <AltitudeRestriction>above</AltitudeRestriction>

        <Flytype>Fly-by</Flytype>

        <BankLimit>25</BankLimit>

        <Sp_Turn>Auto</Sp_Turn>

      </Sid_Waypoint>

      <Sid_Waypoint ID="5">

        <Name>PEBLE</Name>

        <Type>Normal</Type>

        <Latitude>33.181950</Latitude>

        <Longitude>-117.836167</Longitude>

        <Speed>0</Speed>

        <Altitude>0</Altitude>

        <AltitudeCons>0</AltitudeCons>

        <AltitudeRestriction>at</AltitudeRestriction>

        <Flytype>Fly-by</Flytype>

        <BankLimit>25</BankLimit>

        <Sp_Turn>Auto</Sp_Turn>

      </Sid_Waypoint>

 

My question, then, is if the CRJ FMS interprets the radials as TRUE degrees. If so, that should probably be changed, no?

 

Keith

[cessna729]

Hi Keith,

Real World navdata includes station declination, most "parsed" simulator natdata databases do not include this information. VOR station declination of record, is what the declination (Mag Var) was when the VOR antenna was originally bolted to the ground (or last "swung", in some parts of the world this could have been a very long time ago (VOR's on your PEBLE SID, have dates in 1965 and 1975), and in some areas can now be very different from the current local Mag Var, though only an approx 3 deg difference for your SID). In your example station declination for most of the VOR's in the PEBLE 4 SID were approx 15E, the local Mag Var epoc 2010 is 12.3E moving W at only 0.1 deg/yr). For more info try these links: www.studentpilot.com, www.touringmachine.com, www.faa.gov, ww1.jeppesen.com

 

Real World FMS's have Mag Var tables or computer model in them (which is updated every 5 or so years).

 

On the simulator, FMS's can use X-planes table? or their own table/model which works fine for GPS type FIXES but can lead to problems for VOR derived FIXES.

 

Most FMS's use inertial/GPS FIXES and do all there calculation in True and only convert at the last minute to display to the pilot in Mag (as in the-good-ol-days, he was used to wisky compasses). Some FMS's have an option to display in True, Mag or Grid. (checkout Philipp's 777, Aerosoft NDP pic in following post, "Track True").

 

cessna729.

Update: added text.

Edited December 2, 2012 by cessna729

[philipp]

This, Gentleman, is an excellent example of one of the many many reasons why I decided to drop the XML-Format for the Navdata and use a much more robust and more coherently specified format when I started development of the 777. Of course, hindsight is always easier than foresight.

 

CRJ, Navigraph XML

vs

777, Aerosoft NDP

 

Philipp

[Keith Smith]

Cessna, this is true. I should have stated that it was a declination issue, rather than strictly a magnetic variation issue.

 

Philipp, I'm glad you have a solution for the 777. It seems to me, though, that this is not a file format issue, but one of data interpretation in the CRJ200 code, no? Even if you don't have specific declination info for each VOR, could you not apply the derived magnetic variation from X-Plane, and use that in lieu of the declination?

 

At least that way, it'll only be a few degrees off, rather than whatever the difference betwee the declination and true heading is.

 

When the aircraft is being flown in an online environment, it's not fun being queried by the controller, "SKW4305.....what are you doing?"

 

I suppose I can just edit the procedures as I come across them for now. For a more robust solution, if you'd like to expand the navaid DB to include an OPTION for declination, I'd be happy to enter the data in that file as I run across these situations.

[philipp]

There is no easy fix for the CRJ (other than editing each file by hand).

The problem is, we have three different opinions on magnetic variation here:

-X-Plane has a built-in model for magnetic vatiation

-the procedures were encoded with a variation at that airport some point in time

-the navdata XML file is derived from the procedures using yet another model and the magnetic variation used here is NOT encoded anywhere in the file

The CRJ uses WMM2005 to guess the magnetic variation at other places and uses X-Plane's magnetic variation for the position where it is right now. This works out well in the average case, and not so well in special cases like yours. 

The new navdata encode a field called "recommended navaid" and that in turn has the magnetic variation used to encode this procedure.

Edited December 3, 2012 by philipp