Cozumel (MMCZ) approaches

[ontheair]

Bonjour,

 

Something rather strange with some Cozumel approach charts :

 

VOR/DME rwy 11 (CZM) 112,6 apch course 259° !! Instead I would say 102° as charted (offset 14°)

 

VOR/DME rwy 29 (CZM) 112,6 apch course 259° !! Would say 305° (offset 9°)

 

Please tell me if I'm wrong

 

Guess it's not on Aivlasoft side but Navigraph's

 

Jean-Louis

[GG_Flyer]

Bonjour Jean-Louis,

 

Those approaches do appear to be highly difficult to fly.

 

I can't find r/w charts for MMCZ, but just by comparing the VORDME 29 with the VOR 29 approach I would assume that the final fixes on both of the VORDME approaches (that is, the fixes that place CZM just before both runways....) should not be present.

 

And if you think the approaches are a bear, wait till you try one of the SIDs - every departure from 11 or 29 will send you over CZM before leaving the area.

 

I can't say Navigraph's data is wrong, but I can't understand what it's trying to tell me.

 

Still, I don't know everything. Does anyone reading have any real experience with this airport? If so, please chime in....

[ontheair]

Hi dear Travis,

 

Thanks for contributing once again.

 

I did fly the VOR-DME rwy 11 approach with Concorde X (track 102° CZM VOR - 2000' ASL) coming from Panama/MPTO.

 

That's true the proc. turn involved (302° radial outbd CZM/102° track inbd ) places the CZM before the rwy . That's the way I understood the EFB/Navigraph chart. As you know what seems to be incorrect is the mentionned approach course 259° (for both rwys).

 

Amicalement

 

Jean-Louis

[Tetra]

I think the values displayed as 'Approach Crs' in EFB are calculated as the direction from CZM VOR to the runway threshold.

 

Actually CZM VOR should not be a waypoint on the approach, as it is in the SIDSTAR data from Navigraph (MMCZ.txt):

APPROACH VORD29 FIX CF29 2000 FIX FF29 1200 FIX CZM 420 RNW 29 ...

 

If you remove the FIX CZM 420 :

APPROACH VORD29 FIX CF29 2000 FIX FF29 1200 RNW 29 ...

 

The calculated 'Approach Crs' is 301°

 

Which comes closer to the published charts. IVAO provides Cozumel Charts.

 

Paul

[ontheair]

Thanks "Tetra" for the Cozumel charts link you just provided.

 

As we all can see the apch courses are :

 

099° / rwy 11

308°/ rwy 29

 

Regards

[aivlasoft]

Hi all,

 

thanks for contributing all these information. From my point of view there are only some small additions:

 

- the IVAO Cozumel chart shows a magnetic variation of 1° E and is from March 2008, whereas FSX (I guess) is from around 2005 with a magnetic variation of 0.9 W. This is a difference of about 2 degrees.

 

- the chart shows a final course of 099° whereas the Navigraph procedure is not giving you a course but two fixes. So EFB has to calculate the course considering the magnetic variation. As a result of this EFB shows a final course of 102°.

 

- Real charts are representing the data from a certain time, EFB is representing the data from the time where the data for FSX has been collected.

 

... just my two cents

[GG_Flyer]

Hi Paul,

 

Welcome, and many thanks for that information!

 

It seems (to me) that the intent of the approaches is similar to an offset localizer - fly towards the beam... align with the runway... land.

 

Now that I've updated to AIRAC 1010 I see where I have to update my own edit to the SIDSTAR data again. If only there were a way to have these specific edits be remembered....

[lonewulf47]

Hi all,

 

the official values - derived from the actual worldwide Lido FMS database - are:

 

VOR DME one RWY 11 -> 102°

VOR DME one RWY 29 -> 305°

 

There are two things to observe:

 

1) Mexico is among the worst (sad to say...) countries when it comes to official sources. Believe me, I know exactly what I'm talking about Database work is my daily business.

 

2) Variation (declination) is a subject that has two key features in navigation: first the actual variation which influences any earth magnetic derived compass system (icluding all GPS/INS based systems who use a tabulated variation value) Second - and especially important for VOR approaches - is the STATION DECLINATION of the VOR station. Even if the local variation has - say a value of 2°E - it does not necessarily mean that the VOR is aligned to the same variation. Adjusting the VOR's declination to the actual value is a time consuming and therefore costly process. This means that - even if a track can be accurately calculated to have 100° M it might be on a different radial e.g. 102° from the VOR if the station declination is not properly adjusted. And don't think that only "third world" coutries have a somewhat sloppy way of coping with Station Declination: in the US there are many MAJOR airport VORs with a Station Declination Error as big as 4-5°!! (ORD, JFK etc.)

 

p.s. Don't worry about the two namings: variation and declination are the same. In aviation we use "variation" when it comes to on-board navigation however when speaking about fix-based systems we stick to the old nautical and terrestrial term "declination". Don't blame me for that

[ontheair]

Thanks Oskar for the information provided related to déclinaison/declination/variation.

 

"in the US there are many MAJOR airport VORs with a Station Declination Error as big as 4-5°!! (ORD, JFK etc.)" Is such an error a real danger in terms of navigation ?

 

"the actual variation which influences any earth magnetic derived compass system (icluding all GPS/INS based systems who use a tabulated variation value)"

 

I know what is INS and how it works but I don't know about "GPS/INS using tabulated variation value"

 

AFAIK, an INS system gives information with respect to the TRUE north. In other words INS doesn't care about the current declination wherever the flight takes place.

 

Regards

[lonewulf47]

Jean -Louis,

 

Sorry for the late answer. I was quite busy outside Flight Simulation You are right with your remark that INS is working with true bearings only. Every modern Aera Nav System does so - even GPS. but after all our Aviation Navigation is still based on Magnetic North (except beyond 70N/65S or so). That's why the Heading Output of such sytems needs to be shown based on Magnetic North. This is done unsing an internal table within the Naviagtion System itself - be it INS/IRS/AHRS or GPS. So there is another source of discrepancies. We have found - don't tell it anybody - and A/C within a B737 fleet with an old table (they usually are updated through Service Bulletins of the respective Manufacturer) which led to complaints of the Crew that on a certain RWY the A/C HDG was misaligned by 3° ...

[ontheair]

Thanks very much Oskar,

 

Can you please confirm (or not) the triple-mix INS nav system (CIVA) we are using with Concorde includes the internal table you are talking about ?

 

And for my information, how do they proceed in real life beyond 70N/65S ? I would be glad to know a little bit more about that.

 

Best regards

 

Jean-Louis

[lonewulf47]

Well, I'm not sure about the CIVA INS - btw one of the best freeware designs I have ever come across! I haven't been digging too deep into that system. My guess is however that for simplicity reasons it uses the native FSX variation - but as I said -only a guess.

 

As for the navigation beyond those limits - not too sure about the correct figures anymore - might also be 68N - in the A330 (as this one was certified for polar navigation) the whole system simply reverted to TRUE NORTH navigation, of course accompanied with the relevant warning. You would also be able to use TRUE NORTH all the time. There's a simple pusbutton on the panel, however it's not SOP in any airline - yet In a non-polar certified A/C you would of course lose all heading information - not a pleasant outlook... however on the IRS Control Panel you will have a True track output in plain numbers all the time.

 

Edit: I rechecked on the Airbus' 320 FCOM. The limit is 73°N (on a certain latitude sector even 82°). So it confirms that my brain is not 100% reliable anymore