ferris: Quoting from the report:
C421 ... was en route ... at FL175
Then:
At 23:05:23 the controller instructed the crew of the 737 to maintain FL180
So the point is (a n d n o w i a m t y p i n g s l o w l y y o u i n s u l t i n g i g n o r a m u s . . .) that the descent of the 737 had been stopped at FL180, while the C421 was level at FL175. The 737 was no longer on descent, so there was never any "countdown" to impact. The reason for this is the 500 ft separation established by the controller - the controller did a good job: he was in contact with both aircraft for over 7 minutes before issuing the instruction which provided this separation. Then, after the vertical separation had been achieved:
When there was approximately 5 NM between the aircraft, the crew of the 737 identified the C421 on their TCAS and subsequently saw the aircraft.
And then, about two minutes later (still level at FL180):
At 23:07:06, the crew of the 737 reported to ATC that they had received a resolution advisory on their TCAS on the C421 as it passed in front of, and beneath their aircraft, that they were clear of the C421, and were requesting further descent.
So tell me, oh great one: where in that sequence of events was the "20 seconds to impact"? The VFR aircraft was at a VFR level, the IFR aircraft was at an IFR level, 500' nominal separation was provided.
The RA went off because when both aircraft are LEVEL and if the aircraft are within 750 feet or so (see below) then the alert is based on time to CPA - Closest Point of
Approach. Note: that does equate to "time to
collision". This is why some people think that Civil Air's widely reported claim that the aircraft were "20 seconds from collision" was unduly sensationalist. (And this, in turn, may be why people think that Civil Air may have some other agenda...).
(N.B. "For encounter geometries involving low vertical closure rates ... the vertical dimensions for RAs vary from 750ft to 900ft, depending on the TCAS aircraft’s altitude." Ref.
http://www.casa.gov.au/avreg/fsa/dow...r/apr_tcas.pdf)
(By the way, are you sure that 20 seconds is the correct "tau" figure?: above 10,000', I think a higher "SL" may apply, hence the RA would trigger at 30 seconds time to CPA not 20... not that this changes anything).
Furthermore, the aircraft were never "1NM apart"... you are about 50% out as the report says the closest they came was as follows:
vertical spacing between the 737 and the C421 was 400 ft when there was approximately 1.5 NM laterally between the two aircraft. The minimum vertical spacing reached was 300 ft when there was 2.74 NM between the two aircraft.
As for Civil Air's latest press release: yet again it has an overly sensational headline which shouts that the aircraft were "300 feet apart". No, they were not... see above: the VERTICAL SPACING was 300 feet "when there was 2.74 NM between the two aircraft", according to the report. Hardly the same, is it?
Now, you can call me any names you like - if that's what float's your boat!
Have a great day!
Andy
