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Air Data Computer

Old 16th Apr 2010, 01:06
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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?
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Old 16th Apr 2010, 14:01
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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.
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Old 16th Apr 2010, 23:56
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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.
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Old 17th Apr 2010, 08:29
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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.
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Old 18th Apr 2010, 01:42
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Info

HSpilot23

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)
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Old 22nd Apr 2010, 16:15
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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.
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Old 22nd Apr 2010, 17:08
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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.
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Old 22nd Apr 2010, 17:14
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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.
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Old 22nd Apr 2010, 17:16
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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.
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