Compass deviation

[Vulkan 2]

I noticed the other day when flying my NGX that the compass in the front window said 208, the IRS said 205 and the DU said 203. Is this due to gyro drift? And is there anyway to reset the DU in the same way that you would reset the HSI or DG in a GA?

[Vagabondo 23]

I presume by DU you are referring to the PFD/ND. The ADIRUs(IRS) is TRUE, the PFD/ND MAG. The difference between TRUE and MAG is known as variation and is location dependent. The compass is MAG (obviously) and in real life may show differences on specific headings which is called deviation. But I very much doubt NGX/FSX models deviation as it is airframe specific.

 

The ADIRU is automatically aligned inflight with GPS, VOR/DME in the NG and automatically corrects for variation in accordance with your current location, therefore gyro drift is not a factor unless the control panel is set to ATT - in which case a compass reading is required every few minutes and the system updated via the CDU. Normally, there can be rounding differences of a maybe a degree compared to a perfect compass, but that is it.

 

However, the compass reading is always inaccurate by various degrees when accelerating/decelerating and turning. Therefore, only make a comparison when in stead wings level flight.

[Vulkan 2]

Great answer. Thanks!

[FBC57 1]

It could also be from magnetic declination.

 

My old mate, Iain from Flaps 2 Approach has a wonderful explanation of it and how to fix it in FS9 and FSX.

 

Here's his URL: http://www.flaps2approach.com/

 

 

 

 

Frank Cooper

[rsvit 16]

the IRS said 205 and the DU said 203

 

IRS shows true heading and ND shows magnetic track or magnetic heading (check what is written near heading redout on ND). If it's TRK then another possible difference between this two can be caused by the wind.

[Vagabondo 23]

Rsvit makes THE obvious point. Most NGs these days are configured to be TRK up on the ND and HDG up on the PFD. Is your ND TRK up or HDG up? Are you referencing the ND or PFD? (NGX is configurable as per actual customer options in this regard). The track line is white and the heading bug is magenta on both displays. If there is wind (as is usual) then the track will be displaced from the heading and may explain the differences you are seeing.

[rsvit 16]

Not obvious for everyone. There are +80 options on NGX including TRK/HDG up. I know some simmers that were randomly setting them, because they just don't what does each option means. It doesn't matter what do real airplanes have, people can set HDG up displays for themselves.

 

I've written it just in case, because some people mistaken kgs with lbs or something like that.

 

The track line is white and the heading bug is magenta on both displays.

 

Heading bug is magenta, but current heading pointer is white.

[Vagabondo 23]

By obvious I was referring to your reference of wind in the equation. I just wanted to mention the display options for info or in case it was causing confusion.

 

I was also referring to the track line, not the heading pointer. I primarily use the track line as a current direction reference on both the ND and PFD. For heading I primarily use the heading bug (the heading line disappears after 10 seconds). I only seem to use the heading pointer to eyeball wind correction and adjust the selected heading accordingly, except on approach when the selected heading is set to the desired track.