TCAS one RA, one TA

[ferris]

Is it possible to have one aircraft in a perceived conflict receive an RA, and the other receive only a TA? (ACAS Ver II 7.1)
If so, what would be the conditions?

[FlyingStone]

1. One TCAS in TA/RA mode, the other one in TA.
2. One aircraft with enabled altitude reporting (will get TA) and the other one with the feature disabled (will get RA).

[ferris]

This event was at FL300/310, so assumed they would not be in traffic mode. Thanks.

[underfire]

if one ac was in level flight, and the other was climbing

[Anvaldra]

if one ac was in level flight, and the other was climbing
____________________________________________________

If TCAS features are the same, no matter what aircraft attitude are at these FLs

[Chesty Morgan]

I've had an RA off a climbing Airbus as we were level. They only had a TA (apparently).

[FlightDetent]

ferris / chesty:

That's a feature, not a bug. Introduced in version 7.0 together with a number of RVSM driven enhancements over the previous 6.4.
Yet this function is fact not RVSM related.

There are several triggers that will set off the TA and RA warnings, the most commonly observed would be time-to-CPA*. In a vertical plane if the CPA falls inside 700 feet of vertical separation, the RA is "armed". The alarm activates when the aircraft are 35 seconds from the CPA. ... and now ...

To reduce the number of false positives, commonly referred to as "nuisance" warning during 1000 ft level-off manoeuvres, the above described logic is somewhat modified:

For the aircraft in level flight, the start of RA indication by on-board system is delayed until 25 seconds to CPA. The idea is that if somebody's climb vector penetrates your proximity zone, both of you will get a TA at -45 seconds, but at -35 seconds the RA is generated only by the intruder's computer. The RA on your aircraft - as long as you maintain level - is delayed until 25 seconds to CPA. This gives a chance for the shirts to stay white, coffee to remain in cups and ceiling panels live unbroken.

The 10 seconds delay is designed to be enough

• a) for normal level-off geometry to take over (alt capture) when the CPA moves outside the "close proximity" zone
• b) for the crew to correctly follow REDUCE CLIMB** RA

Google should be capable of digging an Eurocontrol ACAS II / RVSM implementation presentations, one picture is really better than all of my description. Try "Project ACTOR".

I do not think 7.1 changed in this respect, but did really not check.

---
*CPA: closest point of approach
**still speaking of v 7.0

[FlightDetent]

a) Chesty, I read your post properly and realized you described exactly an opposite result. In that case, could it be you were equipped with 6.x version in RVSM levels during a 1000 ft level off encounter?

b) found the picture:

time threshold value is further reduced from 30 s to 25 s in order to provide a few more seconds to detect a possible leveloff manoeuvre of the intruder and thus to avoid the triggering of the RA. This should decrease the number of RAs for level aircraft triggered by aircraft levelling off 1000 ft above or below.

[Chesty Morgan]

FD, it was a long time ago now but we were in a Q400 (no idea what version of TCAS we had) and somewhere at or below FL250 so not RVSM but maintaining level. The Airbus was climbing well to 1000' below us and we had the RA as they approached their level.

We reported the RA and ATC asked them if they'd had one as well but they said they'd only had a TA.

[FlightDetent]

My books offer more possible behaviours which I left out on purpouse. But it is only your italics that offer an explanation.

In low twenties, modern jet twin is certainly capable of rather impressive climb rates.

[ferris]

FD- thanks very much. Riddle solved.