I know I saw this addressed somewhere before but darn if I can find it now.  What is the significance of the * that occasionally appears on the NavDisp.

 

Thanks in advance.

 

Jeff Even

 

 

What is the significance of the * that occasionally appears on the NavDisp.

 

Where on the ND, specifically?

Usually somewhere 10 to 15 nm ahead of current position.  Sometimes offset by 10-20 deg either way.

 

J

 

 

Usually somewhere 10 to 15 nm ahead of current position. Sometimes offset by 10-20 deg either way.



J

The ND shows ADIRU position as a five pointed star if you have POS selected.

 

See FCOM vol 2, p 10.40.18

10 to 15 nm ahead, offset by 10 to 20 degrees... Sounds a bit far off to be an ADIRU position, I'd think (and hope).

The * indeed indicates the ADIRU positions, but 10-15nm ahead is way too much! Best way for the forum to help you is to post a screenshot.

This thread already answered one of my two questions regarding POS mode on the ND. In addition to ADIRU marker, there is another icon looks a bit like >()< and it floats out in front of the aircraft position indicator and bobbles around some. What is this indicator? I couldn't find a reference to it in the FCOM (or maybe I didn't know what to look for!)

This thread already answered one of my two questions regarding POS mode on the ND. In addition to ADIRU marker, there is another icon looks a bit like >()< and it floats out in front of the aircraft position indicator and bobbles around some. What is this indicator? I couldn't find a reference to it in the FCOM (or maybe I didn't know what to look for!)

That's the GPS position.

The aircraft actually combines different methods to ensure (and verify) the most accurate position. Over time, inertial reference accuracy degrades. Back in the day, this was updated from time to time by the crew with VOR and/or DME cross-references during the flight. Now, of course, we also have GPS in the mix to make that even more accurate.

 

The POS key allows you to display where each navigation method believes the aircraft to be.

The _* is the Inertial position, while the >-o-< is the GPS position. The green line is a bearing/distance to the ground-based navaid (VOR, VOR/DME).

FCOMv2 10.40.18.

Great info. Thanks!

Interesting discussion.

 

I call it the "snowflake" - rather than an asterisk (*) it's a three pointed symbol - I assume to reflect the triple mix IRS basis of the position calculation.

 

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10-15 miles is indeed a long way off. I haven't looked at ADIRU performance in the simulation - I'm going to do that now! Inertial position is typically most accurate just after alignment and deviates slowly thereafter. I have the formula somewhere for determining the guaranteed performance of the ADIRU ('ANP') - but it's ridiculously large compared with how accurate the device is. After a 14 hour flight the ADIRU is usually within 1 or 2 nm of the GPS/Aircraft position, even as the ANP for Inertial is 24 + miles.

 

 

I call it the "snowflake" - rather than an asterisk (*) it's a three pointed symbol - I assume to reflect the triple mix IRS basis of the position calculation.

Interesting theory, but it's definitely a five pointed symbol, just like an asterisk, not three pointed. Also, Boeing use the same MAP symbology on the 747 which shows the three IRS positions separately.

The amount of drift allowed was quite a bit.

 

http://i177.photobucket.com/albums/w205/B1900Mech/Drift.jpg

Good Lord - your exactly correctly. I'd clearly built and idee fixee in my mind. Next time I'll make sure I head into the FCOM before I write!