Ok, those tables are still somewhat simplified and I cannot vouch the values are truly identical with 7.1 or whatever is the latest. Secondly, I prefer not to hijack this ALT* V/S = 0 stuff, but as I already broke the first glass...
1) The right column shows the absolute (shortest) distance limit. Normally ACAS calculates the "closest point of approach" from the vector geometry and then warns at a pre-determined
time threshold. That does not work at all for closely parallel trajectories, where CPA is minutes ahead but you'd be getting physically too close. DMOD (Distance-MODification) works in the lateral plane, imagine same-level traffic converging far ahead. Similar distance threshold exists in vertical plane (IIRC @ FL200+: TA=850 ft, RA=650 ft).
3) Agreed. Higher up the logic is more sensitive, asking for larger protective bubbles around respective traffic.
2) It's easy to calculate.
FL160, one traffic at level, the other descending.
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With 2000 to go before the cleared level, aiming to systematically avoid
TA which is enough of a distraction.
Limit V/S = 4000 fpm (rate) = 3000 ft (to-go distance + separation) / 45 sec (TAU TA) * 60 sec (per minute)
-->> Descending with 4000 fpm gets you TRAFFIC, TRAFFIC audio call 2000 ft before clearance limit.
Assuming pilots reduced the rate and continued with 2000 fpm instead of 4k,
DIST = 1500 feet = 2000 fpm (rate) / 60 sec (per min) * 45 sec (TAU TA)
-->> TA "traffic, traffic" re-activates when 500 feet above the cleared level = 1500 feet from CPA (other traffic). Assuming the other plane is still there.
DIST = 1000 feet = 2000 fpm (rate) / 60 sec (per min) * 30 sec (TAU RA)
-->> a hard RA fires at 1000 before the CPA itself!. Exactly at your 1000 ft separation clearance limit = you need a level bust for it to sound - any sort of altitude capture will prevent it.
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Play around with the numbers, the final discovery is far from surprising. The original guidance provided from Airbus to its pilots (different from, or badly misquoted in table at post #35) is optimal.
- when actually there is intruding traffic
- if the V/S is excessive call "Check V/S" 2000 before target alt
- do something so that
- within the last 1000 ft before level off the V/S is not greater than 1500 fpm.
- but do not change anything unless there was actual traffic.
Is it not overly conservative then? No, keep good room when the other plane is not level but manoeuvring against you (in line with ATC clearance).