Kalium Chloride - that's a valid point, i.e. about the 50' - 100' being as good as a mile.
Indeed it's all the more relevant here because the controller actually cleared the TU154M down to FL350 - indeed this occured some seconds prior to the RA.
To make sense of what follows, see appendices
1&3 and
2,4,5,6,7,8,9.10
If one examines the superimposed ‘Altitude + V/S’ charts for the B757 & T154 ( in Appendix 6 ) in conjunction with the other FDR extracts versus timeline one sees the following:
From the altitude graph it would appear that, when both aircraft were ostensibly at 36000’, the T154 was actually cruising approx 50’ higher than the B757 - i.e. the former was at approx 36070’ and the latter at 36020’.
Given that both should be using 1013 ( Qff ) then one might put this variance down to the T154 using metres ( converted from feet ) plus some ‘altimeter error’ in either aircraft's altimeter.
21:34:42 – TCAS TA onboard both aircraft announces “Traffic Traffic”
21:34:49 - ATC - tells the T154 “Descend FL350”
21:34:56 – TCAS RA onboard both aircraft tells T154 “Climb” & B757 “Descend” respectively.
Nb. Inspection of the V/S graph shows:
21:34:58 - B757 - commences descent from FL360 ( i.e. within 2" of the RA ).
21:35:02 - T154 - commences descent from FL360 ( i.e. as per the ATC instruction and contradicting the TCAS RA saying that they should "Climb" )
Nb. In the full report ( the ACAS/TCAS section ) it suggest that the T154 crew commenced descent immediately when instructed by ATC ( i.e. at 21:34:49 ), however the graph on appendix 5 ( and others ) seems to contradict this wherein it shows the T154 leaving FL360 no earlier than 21:35:02, i.e. 13" after being instructed to do so by ATC.
From the gradients on the V/S graph it would appear that the initial ROD of the T154 was twice that of the B757 and accordingly at 21:35:04 – whilst having started out at a slightly higher altitude than the B757 - the T154 crosses the level of the also descending B757 ( this occurs at approx 35940’ ) and, from this point on, the T154 is always lower than the B757 ( that is until just prior to impact ).
21:35:02 – ATC - tells T154 to “Expedite Descent” ( possibly due to their delay in having commenced it in the first place ? )
21:35:07 – T154 - crew acknowledge the request to “Expedite Descent”
21:35:10 – B757 - TCAS “Increase Descent”
21:35:19 – B757 - informs ATC of "TCAS Descent”
21:35:24 – T154 - TCAS “Increase Climb”
21:35:27 – T154 - F/O “Climb it says”
21:35:28 – T154 – commences a climb, having ducked below FL350 by approx 120’.
21:35:31 – Both aircraft collide.
Whilst it’s admittedly something of conjecture, had the TU154 levelled at its assigned FL350 ( which is, after all, what the T154 crew were prioritising ) it would appear, according to the graphs, that the B757 would have passed overhead by approx 50’ ( a miss & a mile and all that ). However, as it happened, the TU154's attempt at climbing back to its assigned level made matters worse - the collision occuring at approx 34890'.
A lot of people need to do a lot a soul searching about this whole episode, but wherein lets remember that the TU154 not only failed to comply with the TCAS RA but also failed to level-off at its assigned altitude - there's a lot to be learnt from this, unfortunately.
