threemiles
S. Dumont, excellent post, but here's were it hangs:
My sources (indeed I have sources) indicate (indicate is different than state) that the Legacy XPDR turned C mode off 15 minutes before the collision.
If that is true: until 100 NM before the collision, i.e. 300 NM or 45 minutes after BRS and the planned level change ATC would have seen Legacy 600XL still being at FL370 without taking any protective action. And: knowing opposite same level traffic coming in from Manaus.
…. Most interesting posts!
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The odd shaped piece in this jigsaw may well be summarised in this one musing (assuming it is even remotely accurate)
XPDR turned C mode off
… a mode change rather than a TXPDR U/S is a very different scenario … perhaps one of our technical experts can advise us if the Honeywell TXPDR in question could fail C only, and what would that system then do i.e. shut it all down or just the alt; and would TCAS continue to function in the Embraer??? … obviously without Mode C the B738 might only get a TA … anyone?? ….. it might explain why the B738 was ‘apparently’ banking …. Crew may have been looking as a result of a TA …. only to realise too late that there was indeed traffic closing at 900+kts at the same level .. remembering TCAS rules .. on track and vertical resolution!!
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….. what Embraer Flight deck indications would result from Mode C fail??
…. 15 minutes before the collision
Squarking Mode C and FL370 in Radar airspace
until 100 NM before the collision
… hmmm
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.. didn’t someone earlier suggest the B738 was FL350 climbing FL390
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.. from what I have seen of the H4 chart and the supposed information on MAC position, and the timings of loss of TXPDR (mode C apparently) … I am lead to ponder what ATC instructions were likely to have occurred, read backs that would likely have been required and perhaps received, and how they (time) correlate to the loss of Mode C??
… from the information (accurate or not) posted here I would think (logically based on normal ATC distance buffers for achieving vert in radar airspace) they could be almost coincidental …..very very interesting!!
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…. to me the only thing that would make this thing ‘inexplicable’ is if ATC had some reason to believe N600XL was at FL360 and the Mode C on the climbing B738 had reached FL370 .. all well prior (distance) to conflict proximity
in radar airspace nose to nose yet only a short period of time before passing due closing speed!!
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.. radio fail procedures
may be academic in this case, and of little significance to ATC reasoning and/or decision making!!
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.. the other issue that
may be relevant is the ATC boundary proximity! Brasilia may have coordinated FL360 based on the intended level of N600XL at the boundary, Manaus may have coordinated FL390 for the B738 based on expected time to climb prior to the boundary ....
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.. from memory (do not have the H4 with me) the distance from the Brasilia VOR to the ATC boundary was around 157nm, then another 125nm to abeam the military base that N600XL landed at! ... I might be wrong, but given the NW'ly track of N600XL, the MAC would have occured somewhere within 120nm NW of the boundary waypoint NABOL (between the mil base and the ATC boundary) .. which if the '15min prior' is correct, mode C loss occured just prior to NABOL .. stand to be corrected though
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.. neither sector may have had initial 'trigger' prompts to consider all was not as it should be ... it depends largely where the mode C loss occured in relation to the boundary .. if you follow my reasoning ..... and by saying that I am not for one moment suggesting that an ATC error occured .....if you get my drift!!
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Here is where it hangs indeed! ….. the plot thickens or thins as the case may be!!
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Tis late, so I hope you all can see what I am getting at!!
