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cknipe

NAV display question (all PMDG aircraft)

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Hi,

I've asked a couple of people (incl. real world pilots) and either they haven't noticed, or don't know.  What is the little white star (to the left of DAN intersection) on the NAV display?  They come and go, sometimes there's only 1, sometimes as many as 3.  No one (that I found so far) seems to know what this is?

https://ibb.co/c6zCjT

Many thanks,

Chris Knipe.

 

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I'm not sure, but it isn't that (the star) an indicator of where the individual IRS units think they are? So in the pic, one unit thinks its 20 miles ahead. Can someone confirm?

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19 minutes ago, SKEWR said:

I'm not sure, but it isn't that (the star) an indicator of where the individual IRS units think they are? So in the pic, one unit thinks its 20 miles ahead. Can someone confirm?

Does make sense, as the star remains static to the plane...  Always the same location / distance.  In other words, it would be indicative of one of the three IRS units being out of alignment (assuming you are correct)?  I'm just not sure if I've ever seen more than 3 stars...

This is also on a return long haul, with no IRS realignment in between.  On the ground for an hour or so at JHB, then JHB to HKG, then about 3 hours on the ground in HKG, and now busy with the return to JHB.  In total this would easily be over 24 hours of P3D and the 777 without an IRS realignment.

Would be very appreciative if someone can confirm yes.  I have a lot of very curious people about this one.

 

Regards,

Chris Knipe

 

Edited by cknipe

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Press POS on the EFIS and it will appear, POS again and it's gone.  The 3 IRS individual positions are combined and smoothed, and that is where the a/c believes it is @ the barbell.  Add in the GPS POS for extra accuracy.  Without/before GPS, the pos refs. were usually closer to the barbell, as there was no definitive position fix, just the 3 IRS positions to mean average.

HTH- C

Edited by cavaricooper

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Carl beat me, it took 2 min to find it in the NGX FCOM on pg 10.41.13.  Every symbol used on all displays is in the FCOM... first place to look when you have one of those "I wonder what that is" questions.

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21 minutes ago, downscc said:

Carl beat me, it took 2 min to find it in the NGX FCOM on pg 10.41.13.  Every symbol used on all displays is in the FCOM... first place to look when you have one of those "I wonder what that is" questions.

Thanks Dan.  Will look through it.  As I've mentioned, I asked a couple of real life pilots and none of them knew... So ya.  Excuse me for not RTFMing in the 737 manuals, considering I'm flying a 777,

For anyone interested... 

ADIRU position (W)MAP,MAP CTR When the EFIS control panel POS map switch is selected on, the star indicates ADIRU position relative to FMC position

Doesn't say jack about why it's 20nm away from the plane, or why there's 3 separate stars some times.  I am for the sake of my own sanity rather just going to assume it's the IRS that drifted.  

I'd prefer to rather refrain from making any additional unnecessary posts on this forum. Thank you for the constructive, and not so constructive feedback however.

 

Regards,

Chris Knipe.

 

 

Edited by cknipe

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49 minutes ago, cknipe said:

Doesn't say jack about why it's 20nm away from the plane, or why there's 3 separate stars some times.  I am for the sake of my own sanity rather just going to assume it's the IRS that drifted.

 

The three IRU's calculate their own position relative to the the FMC position and, as already stated, when you press POS each one will be displayed on the ND with a Star. On a longhaul flight the three stars will gradually drift apart, although it is very rare to see one significantly different to the other two unless the IRU is developing a fault (e.g. 3+3xT as a max error, where 3 is nms and T is time in hours).  To determine which of the three IRU positions has the largest 'shift' relative to the other two all you need to do is move the IRS Source Selector switch away from AUTO on the pilot's panel until the relevant Star moves to the tip of the aircraft triangle on the ND and note which IRU the switch is pointing to.     

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Here's a 747-400 display after a 4 hour flight...

IRSDriftANZAklSyd.jpg

Note that the display (from the FMC) heavily favours the GPS position ">-O-<" and nav radio updating, not the IRU "weighted average".

 

If one IRU wanders too far off, it will be ignored, and not used as part of the weighted average.

Edited by Qavion2
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18 hours ago, cknipe said:

Excuse me for not RTFMing in the 737 manuals, considering I'm flying a 777,

And how was I supposed to know that?  Had I known that it would have taken half the time to find it because the TOC links in the 777 FCOM work.  I don't see where you asked the question you wanted an answer to until your last post, I'm just trying to help.  The FCOM is the place to look for information such as "what is this"  and then you have guys like John who can answer the "why."  Hope you got Jack.

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You're all wrong.  Those are points of interest. 😉

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