Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Air Data Computer

Tech Log The very best in practical technical discussion on the web

Air Data Computer

Old 16th Apr 2010, 01:06
  #1 (permalink)  
Thread Starter
Join Date: Dec 2009
Location: Wyoming, USA
Posts: 13
Air Data Computer

Can anyone tell me why an Air Data Computer cannot compensate for extreme cold temperatures (-30C to -50C) at the surface? Does it have something to do with the compression rates of less dense air at altitude versus air at the surface?
Hspilot23 is offline  
Old 16th Apr 2010, 14:01
  #2 (permalink)  
Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
I guess we could have such thing as a TA indicator, but this should not substitute the conventional altimeter.
Same as EAS, CAS, IAS... We could have CAS, or even EAS, an indicated speed much closer to EAS, anyway, with some additional computing. I guess that instruments have to meet certification requirements, and these have not changed much, and probably are ok as they are now. The simpler, the better.
Microburst2002 is offline  
Old 16th Apr 2010, 23:56
  #3 (permalink)  
Thread Starter
Join Date: Dec 2009
Location: Wyoming, USA
Posts: 13
Cold temperature conversion charts exist, so why can't the ADC simply compute the correction the same way I can in my head? It mystifies me.
Hspilot23 is offline  
Old 17th Apr 2010, 08:29
  #4 (permalink)  
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 3,951
Dunno about your operation, but we are not allowed to use temperature correction on ATC advised altitudes as ATC does their own temperature correction. If the ADC uses temperature correction on all altitudes, which would have to rely on a correct entry of the ground temperature, you have a problem with ATC there.

What you can and should temperature correct are procedural altitudes and minima. There are some FMC programs allready that can do automatic temperature correction to approach procedure crossing altitudes which would cover the first thing and as for a minimum you need to use the table at the moment.
Denti is offline  
Old 18th Apr 2010, 01:42
  #5 (permalink)  
Join Date: Aug 2007
Location: The Netherlands
Age: 62
Posts: 287


The ADC merely has inputs from pitot and static sources and Total Air temperature.

The corrections that you want depend on such things as your height above the altimeter setting source. As you can read in the first line, that is something unknown to the ADC.

Remember that Air Data are there, not only to keep you free from contact with the ground, but also to keep you separate from the other guys flying around - and for that last task, every Air Data unit has to work according to the principle numbers one, STANDARD, atmosphere.
The flip side of that airborne separation requirement is that you have to make corrections for ground separation related altitudes in vastly non-standard conditions.

(edit for a typo)
EMIT is offline  
Old 22nd Apr 2010, 16:15
  #6 (permalink)  
Thread Starter
Join Date: Dec 2009
Location: Wyoming, USA
Posts: 13
Thumbs up

So in order for the ADC to correctly adjust the DH on your ILS for extreme cold temperature it would have to get altitude from your WAAS GPS (or a manual input) and OAT from a temp gauge... Gotcha. Somebody should design that.
Hspilot23 is offline  
Old 22nd Apr 2010, 17:08
  #7 (permalink)  
Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,342
All you have to do is to change the minimum altitudes and DA according to the table. No need to know what is your TA at all times.
Microburst2002 is offline  
Old 22nd Apr 2010, 17:14
  #8 (permalink)  
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 4,543
It would need the height above sea level for the QNH station you are using. i.e. If you are flying to an airport, and have that airport's QNH set up - the airport adjusts the QNH figure they pass you so that your bog-standard altimeter reads the correct level at the airport. The aircraft would need to know the height of the airport above sea level in order to apply the same correction.
Checkboard is offline  
Old 22nd Apr 2010, 17:16
  #9 (permalink)  
Join Date: Oct 2006
Location: Ankh Morpork, DW
Posts: 592
Ugg. I wish there was an easier way to do this. KAL does cold-wx corrections for 0C and less.

I think they're introducing more chance for error than they are preventing.
ImbracableCrunk is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.