HDG reference: descent in polar latitudes

[Markus Vitzethum]

Hi,

while reading some manuals, I came across a section that I was not aware of before. It refers to "heading reference" being TRUE when flying in polar latitudes, that is, N of 82°N or S of 82°S (with some exceptions).

The manual says "Because descents are normally not planned at these latitudes, if the heading indication is referenced to true north and the airplane descends 2000 feet at more than 800 feet/minute,the heading reference box changes to amber and flashes for 10 seconds. The box returns to white when the airplane climbs 2000 feet at more than 500 feet per minute." (for PFD, there is a similar comment for the hdg reference box on the ND).

I just tried in PSX and it seems that this is not modelled. Is that correct?

Markus

[John H Watson]

Interesting... It was discussed during Alpha testing (back in August, 2010), but I can't remember if it was implemented.

[Hardy Heinlin]

Hi,

it hasn't been implemented because the ADC or IRS data sources were unknown and because it might have been an unusual airline option.

Maybe I'll implement it in the next PSX version.


Regards,

|-|ardy

[G-CIVA]

because it might have been an unusual airline option

I have just checked my references & it appears in all the FCOM v2 I have save UAL - who do not mention it; although since they have totally rewritten their document in their own prose & style there are lots of missing items.

I would hazard an assumption that it is included in their fleet too since their a/c configuration is very basic.

[Markus Vitzethum]

Slightly late in following this up ... but I still have a question.

> it hasn't been implemented because the ADC or IRS data sources were unknown

I understand ADC data sources but why would you need to consider IRS data sources? My understanding would be that the reference needed is whether heading reference is TRUE or NORM. That should be a single bit. Does that come from the FMC? Can it be different across both FMCs?

Markus

[Hardy Heinlin]

You need the 2000 fpm data.

V/S data is a mix of barometric data for long term stabilization and IRS data for short term velocities.

The V/S indicator blanks when IRS vertical velocity data is not available.

Secondly, if this feature is FMC independent, the IRS is needed to determine whether the aicraft is beyond polar latitudes.


|-|ardy

[Markus Vitzethum]

Understood, thanks.

Markus

[Hardy Heinlin]

Boeing says: "if the heading indication is referenced to true north and the airplane descends ..."

Two things are not clear to me.

1. By "true" they probably mean "when in polar area", not "when TRU is manually selected anywhere on the planet".

2. By "polar area" they probably mean the 82° latitude only, excluding the additional sectors shown on page 289 of the Aerowinx manual. There are airports in those additional sectors.


Cheers,

|-|ardy

[Britjet]

Quote:
"If the ND is referenced to true north and the airplane descends 2,000 feet at more than 800 feet per minute, the heading reference box changes color to amber and flashes for 10 seconds. The box returns to white when the airplane climbs 2,000 feet at more than 500 feet per minute."

This text does refer to any latitude. I've seen it done - for example if the pilots leave it in TRUE after an Atlantic crossing, and then are made to descend by ATC. I don't think it necessarily happens on all aircraft though. It's a mod...

Peter.

[Hardy Heinlin]

So the warning really means to ask the crew: "Are you sure you want to descend in TRUE mode?"

Because for non-polar airports you need MAG, and because in polar regions there is not even an airport.


|-|ardy

[Britjet]

Indeed!

[Jeroen Hoppenbrouwers]

Which suggests to me that it may really be "when the switch is in TRUE". The switch won't flip out of TRUE by itself, so it warrants a warning of some sort, and the descent is a nice hint.


Hoppie

[Hardy Heinlin]

This "TRU" alert is now implemented in PSX version 10.1.3:

http://aerowinx.com/board/index.php?topic=4191.0

(Refer to item 1.3.020.)


|-|ardy