I am currently designing a system that reads the baro altitude (on label 204) from an LRU. Ultimately in my software I would like to display the baro altitude reading in real-time using a graphical interface. In doing this, I'd like to have the units of measurement next to the reading. However I am having a bit of trouble in deciding how to display the units of measurement.

In the ARINC specification (attachment 2 "data standards"), label 204 (baro altitude) is listed as having units of knots. Is this an error, or can altitude be in Knots?

I am referencing ARINC specification 429 part 1-17, mark 33 digital information transfer system (DITS) published in May of 2004.


It's an error. Attachment 2B does show that label 204 from the GNLU (Equip ID 056) and GLU (Equip ID 060) as being in knots (highlighted below). That is not correct, it is a BNR in feet as shown for Air Data System (ID 006) and ADIRU (ID 038). Knots only applies to Label 204 from the FMS (ID 002).

Source: ARINC 429

It's still incorrect in A429 P1-18, March 2012.

ARINC 429 contact at ARINC Industry Activities, ARINC 429 Maintenance is Jose Godoy. email:jose.godoy@sae-itc.org

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    $\begingroup$ Nice!! Thanks for this, guess we'll need an updated spec. $\endgroup$ – Snoop Oct 6 '17 at 16:17
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    $\begingroup$ @ Gerry, @Snoop, Label 204 might refer to an airspeed as well to an altitude it depends on the emitting equipment, please read my answer to understand the matter. $\endgroup$ – user40476 Jun 4 '19 at 15:46
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    $\begingroup$ @user40476 While you are correct in your statement, it's irrelevant to the original question which asked why the Label 204 Baro Altitude outputs from the GNLU and GLU was defined in Knots instead of Feet. I've edited the answer to make the point clearer. $\endgroup$ – Gerry Jun 5 '19 at 12:58

Please read all the explanation to understand how the same label might refer to different parameters, therefore to different units.

As a start the label uses only 8 bits which gives a space for only 256 different labels ( 377 labels in octal base), this is obviously insufficient to cover a huge number of parameters of a modern aircraft.

To increase the number of possible parameters, and to cover the complexity of modern systems, the same label according to the emitting computer might refer to a different parameter; more precisely, different equipments might use the same label to send different parameters even though using the same label.

Please refer to the following Website


You will find that depending on the emitting equipment the same label might refer to : utility airspeed, or baro corrected altitude, or duct temperature, etc etc therefore the units will depend on the equipment that transmits the label. Label 204 could be send by the flight management, or by the air data computer, or by the fuel and quantity computer, etc, etc.

As a conclusion the same octal label do not refer always to the same parameter.

Is this confusing?

No because a sink computer knows on which bus he is reading and therefore knows the equipment that is sending the data.

  • $\begingroup$ It isn't an equipment ID or label decoding issue, the A429 standard does in fact have an error for Baro Altitude on equipment IDs 056 and 060. I expect it is a copy paste error from the 002 Utility Airspeed entry, as the table data is the same. I'm assuming that the equipment ID the OP is either 056 or 060 for this to be an issue. $\endgroup$ – selectstriker2 Jun 4 '19 at 17:02
  • $\begingroup$ @selectstriker2, please refer to the website I mentioned, and have a look at label 204, then you will understand that label 204 when emitted by the fuel system necessarily has different unit than when emitted by the Air Data, or by the flight management etc, because it concern different parameters eve though having the same label. So label 204 could be in knots or in feet or in pounds, it is your job to use the proper unit as a function of the equipment IDentifier $\endgroup$ – user40476 Jun 4 '19 at 18:11
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    $\begingroup$ I understand that, but that isn't what this question is about. The OP is getting label 204 from a transmitter with the equipment ID of either 056 or 060, and the entries in ARINC 429 for those IDs and label 204 are incorrect. $\endgroup$ – selectstriker2 Jun 4 '19 at 18:16

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