Three related topics were linked that IMO adequately address the question, but OP said:

However, I don't think this post explains [...] in detail.

I left a comment but removed it after deciding to check here, but my VTC remains.

Barring actual algorithms in paywalled standards (e.g. DO-185B), what level of detail is being requested exactly that isn't covered by the other posts, one of which includes a link to the FAA TCAS guide with multiple pages of details?

The help center says:

Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much.

I don't know why it wasn't voted as dupe to begin with, but given OP's interaction, I feel my VTC for narrowing down the scope is correct. Or is it not?

So far after my vote there's one leave open vote.

Update 1

The question is now answered with two quotations from two of the linked topics. IMO that does not constitute additional missing details.

  • $\begingroup$ "What level of detail"??? You're expecting the OP to provide information that he doesn't have because he doesn't have the answer. Nor is specifying such necessary: the lower Mode S address "wins" as the final tie-breaker. The fact that one can, with enough time & careful reading, dig that answer out of the linked threads doesn't mean that the answer is already there... it wasn't! What if the other threads had linked extensive articles & it took a greater research effort than mine to synthesize an answer... is that a "dupe" question? No! This question wasn't answered, until the OP asked it. $\endgroup$
    – Ralph J Mod
    Apr 21, 2022 at 4:26
  • $\begingroup$ @RalphJ: "'What level of detail'???"??? :-) The rest of the paragraph in the body explains. If you ask a question, and someone gives a link that may have the answer, you as OP need to say what is missing, based on the problem *you're* solving, as is SOP here. And when that first post has a whole FAA book linked, the problem being solved *does* need narrowing down, but it's been over 3 weeks, so it doesn't matter... PS the current answer doesn't fully cover all scenarios of which one goes up/down (OP neither specified level, nor conflicting RAs), i.e. there's more than those two. $\endgroup$
    – user14897
    Apr 28, 2022 at 7:20

1 Answer 1


When I got the question in the close votes review queue as needs details or clarity I voted to leave open because the question itself is quite clear. IMHO, the OP doesn't have to specify what level of detail they expect for the question to remain open. That would help to get better answers of course, but not specifying the level of detail should not be a close reason.

I also don't think that the topic is too broad as in "an entire book that answers your question". There are a few cases to discuss (maximize altitude difference at CPA, prefer no crossing, prefer descent when both are level, lower mode S wins) and that's it.

That leaves us with close as duplicate. I think the question is on a knife-edge of being a duplicate of the 3 linked questions. On the one hand, it would be nice to have all the knowledge of the sense selection in a single place rather than have it scattered in multiple Q&As, therefore justifying a new question for it. On the other hand, Ralph's answer is only based on quotes from the existing answers, therefore justifying a close as duplicate.

While it's possible to list multiple duplicates in the closed question box, I wouldn't even know how to do that. When I vote to close as duplicate, I can only choose one other question. I know ◆ moderators can edit the duplicate list, but can normal users do that?

  • $\begingroup$ no, you can't edit the duplicate list. but you can flag for mod attention $\endgroup$
    – Federico
    Apr 6, 2022 at 6:55
  • 1
    $\begingroup$ RE "the OP doesn't have to specify what level of detail they expect for the question to remain open" – they do per the help center, more so when possible dupes exist and they claim more details are needed without limiting the scope or specifying what is missing, hence the book remark. Thanks anyways. $\endgroup$
    – user14897
    Apr 7, 2022 at 9:13
  • $\begingroup$ I thought the question, as asked, was a really good one: when everything is equal, one plane will climb, but which one? Do they flip a coin over the datalink, or what? Neither of the other threads in isolation answers that question, though if you read them both then you can piece together an answer. If that piecing together process is sufficient to close as dupe, then there are a lot of things that you could close that way even without a clear answer to be found in any 1 place. The question got a concise, clear answer that you couldn't find elsewhere - not a dupe answer, nor a dupe question. $\endgroup$
    – Ralph J Mod
    Apr 21, 2022 at 4:20

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