View Full Version : Is altimeter setting an input to CADC?

3rd Jun 2002, 15:20

Is the altimeter setting an input to the CADC?



3rd Jun 2002, 21:55
Hi There,

By this question im presuming youre asking if the altimeter readings/data are fed TO the CADC ?

If I remember correctly the altimeter setting is NOT an input to the Central Air Data Computer (CADC).

If I recall correctly the Pitot, Static, Total Air Temp sensors and CADC power were the primary inputs to the CADC.

There are probably some more including the weight on wheels system etc but the above are the primary ones.

The CADC is fed by both the captains and FO's static and pitot primary and secondary lines. Using various calculations on the inputted data it can output to the various instruments which includes the altimeter. Im pretty sure the altimeter gets info *FROM* the CADC as opposed to feeding info *TO* the CADC.

Hope this helps .... and that my memory is correct !!!


4th Jun 2002, 01:35
Hi Teroc

my exact question is: do you feed de CADC with the information of the pressure reference you select on your altimeter: 1013, or 1020, etc.?

But your comment makes me believe that the CADC feedís the altimeter and changing the reference pressure on it, just moves a scale on the instrument. And that means: you do not feed the CADS with the pressure reference.



4th Jun 2002, 03:03
Hi Tango,

Yes thats my understanding too. The pressure reference is not fed to the CADC. The "raw" data is sent from the CADC to the altimeter and the pressure reference, for want of a better phrase, "re-aligns" the scale to present valid readings.

Its been awhile but I think thats pretty much as I remember it.


Pub User
4th Jun 2002, 03:21
I've never used one, but I can't believe that a CADC does not have to be told the QNH somehow. How can it do its wonderful calculations without such a fundamental input?

5th Jun 2002, 01:38
Just fished out the old manuals and dusted them off.

Nope the pressure setting is not an input to the CADC. Power, Pitot, static, TAT and weight on wheels are the only inputs.

It doesnt go into details apart from saying that it continually uses and analysis mathematical logarithms and formulae to compute everything else from these basic inputs.

P.F.M. I suppose (Pure Fecking Magic)


5th Jun 2002, 12:27
I have just had a look at the course notes for the 747-200.
The CADC's are referenced to 29.92 and the baro correction to the outputs is carried out by synchros in the Capts Altimeter when the baro is adjusted.

Flight Detent
5th Jun 2002, 15:58
Hi all,
Yes, you are quite right, the output of the CADC feeds info to the altimeter without QNH correction, the pilot makes that adjustment.
So then, you ask, can the pilot change the altitude indicated on the radar screen of the ATC controller by simply changing his QNH setting, mode C?
The answer is a definite NO, as we have already mentioned, the mode C altitude info, fed from the CADC, is uncorrected for pressure variations prior to transmission, the radar controller makes these corrections.
Logical really!
The B747 Classic does what you want, when you want it!

5th Jun 2002, 16:07
Thanks to all!

nice to belong to a community that can interact like this!


5th Jun 2002, 19:13
From the A300-600 MM, Chapter 34-12-00-0:

B. Principle of Operation
(1)Barometric pressure display
The barometric display module enables the barometric pressure correction value to be manually displayed on two counters, in millibars and in inches of mercury, through BARO set knob. This value is transmitted to both ADCs, in sine and cosine resolver voltage form, where it is used as reference pressure for Baro Corrected Altitude processing. When BARO set knob is pulled out, a blue flag marked STD covers the two counters and both ADCs process Standard Altitude using 1013.25 mb as reference pressure.

The input from the Altimeters to the ADCs is shown on the system schematics, also.

5th Jun 2002, 23:34
At risk of throwing a spanner in the syncro.:)
Both can be true. I have just looked at the schematics for the MD83 which used a late model DADC, from Honywell if I recall, that clearly show Baro input from Captís Altimeter to DADC #1 and Baro input from F/O Altimeter to DADC #2.
The A320 and A330 ADIRUs also have Baro input to the AD (Air Data) part. It is, after all, a simple software fiddle for the Altitude output to the ATC Transponder to be related to 1013 Hpa and for the output to the Altimeters to be related to Baro Set.
I recall that the previous posts about the Classic are also correct regarding itís, more primitive, system.
The same goes for CADC equipped 727s and 737-200s but I am not climbing into the roof space to verify that !
No offence to the Classic lovers Ė I liked them too

7th Jun 2002, 05:47
Have to agree with Woderick - there's two types. Older units did the correction in the Altimeter but more modern EFIS-equipped aircraft input the correction from the Air Data Reference Panel (ARP in Rockwell-Collins) to the DADC. Both 29.92 and Baro-Corrected Altitude is output on the 429 buses. A feature of these systems is that one push on the center of the Bar Correction knob turns the Altitude over to 29.92.