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In the latest FCOM revision under the preliminary cockpit preperation normal procedures chapter ADIRS alignment has been added. However most of the conditions remain the same except for a new line which has been added wherein a full ADIRS alignment has to be performed at airports where the latitude is within 2 degrees North and South.

Could anyone explain with a reference why Airbus has decided to add this ?

As per my company FCOM this is only applicable to certain MSNs (mainly our newest A320-251N's)enter image description here

Jai
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  • Airbus may cover it in a future FAST issue, but common sense says at/near the equator full alignment is very quick, so might as well do it. Are there other operational notes? –  Apr 04 '21 at 11:11
  • @ymb1 nothing has been attached as a note as this got added in Jan 2021. – Jai Apr 04 '21 at 11:20
  • It's an interesting question, but I doubt someone here will be able to answer it (unless someone from Airbus posts an answer). Can you ask your chief pilot about it? They can probably talk to Airbus directly and will likely get an answer... – Bianfable Apr 04 '21 at 11:59
  • This may be related to the fact that near equator the error in coordinates estimation has a larger geographical distance error than the same coordinate error near the poles. – ocirocir Apr 12 '21 at 09:36
  • When I look for info about this on the web I see that people interchangeably use high latitude with low latitude numbers, as in high when approaching the north or south pole. This makes no sense, as the poles are 0 degrees latitude. So what is this note referring to? 2 degrees from the poles or 2 degrees from the equator? If they are referring to 2 degrees from the poles, it's likely because it is almost impossible to do a fast IRS alignment that close to the poles. ?? – Juan Jimenez Apr 13 '21 at 01:03
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    @JuanJimenez, 0° is the equator, poles are 90° (+/- or N/S), always. – Jan Hudec Apr 13 '21 at 05:09
  • @JanHudec I was not asking you about that, I am simply pointing out the fact that some people use the numbers incorrectly. – Juan Jimenez Apr 14 '21 at 02:34
  • @JuanJimenez When talking about geographical coordinates, a latitude of 0° is always the equator, I've never seen anyone using a different convention. Maybe you have seen people using spherical coordinates, where $\theta=0$ is pointing along the z-axis (North pole) and $\theta=\pi/2$ is pointing in the x-y-plane (equator)? – Bianfable Apr 14 '21 at 09:24
  • @Bianfable "I've never seen anyone using a different convention." I have, threw me for a loop. – Juan Jimenez Apr 15 '21 at 16:20
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    I checked with Honeywell, the manufacturer of that IRS system. They told me that they have no clue why Airbus put that in there, as they have no such requirement in their operating handbook. They referred me back to Airbus to ask the question, but I have no contacts at Airbus. – Juan Jimenez Apr 20 '21 at 02:43
  • @JuanJimenez that’s very strange and adds further doubt. – Jai Apr 22 '21 at 14:07
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    .Alright everyone the answer I have posted is as received by my company on raising it with Airbus. Although I don't understand whatsoever how is there excess drift specifically at these co ordinates. Any further inputs to clarify will be much appreciated. – Jai Apr 28 '21 at 17:50

1 Answers1

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SOP is modified to introduce a new condition to perform a complete IRS alignment during cockpit preparation. When departure airport is located between latitudes 2° north and 2° south, a full alignment must be done. This is to avoid excessive drift of Honeywell ADIRU Block III during the subsequent flight.

Jai
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    What is it about being within 2 degrees of the equator that makes excessive drift of the ADIRU a problem? – Wayne Conrad Apr 28 '21 at 19:52
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    @WayneConrad unfortunately thats the answer I got when raised it with my company's flight operations department who in turn raised it with Airbus. This was their response. – Jai Apr 29 '21 at 17:56
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    It seems the answer is, "we made the change because we needed to make the change". – FreeMan May 06 '21 at 17:25