View Full Version : TCAS Co-ordination?

9th Jul 2002, 08:20
My Airbus maintenance manual says that only one TCAS aircraft involved in conflict will be issued a maneouver order. The other TCAS aircraft will be ordered to 'stay put'. It depends on which aircraft identifies the conflict first. This makes sense, one guy getting DESCEND and the other guy getting MAINTAIN VERTICAL SPEED. Recent reports in the press imply otherwise. Is there a TCAS 'insider' out there who can confirm the RA co-ordination operation for me?

9th Jul 2002, 09:21
Jetboy - your manual is being economical with the truth!

TCAS equipments communicate with each other when determining what RAs to post to their flight crews. When it is agreed that one TCAS will post 'Descend', for example, the other will post a manoeuvre that does not conflict: this will not necessarily be a 'Maintain Rate' style of advisory unless this is appropriate. In such circumstances, a 'Climb' advisory would double the difference in altitude between both aeroplanes at the Closest Point of Approach.

For more information see, the 'Tcas' thread in this Forum, and read also the excellent briefings given by Cpt PitBull both in the ATC Forum, 'Nasty Incident' and in the Reporting Points Forum, 'Accident in Southern Germany'.

10th Jul 2002, 02:58
My expeience with Airbus manuals is that they are a 'cut and paste' from the manufacturer's books. As only one TCAS computer calculates the avoidance manoeuver for both aircraft, I see merit in having one move while one stays put. Is there anyone working for Honeywell who can confirm the TCAS RA co-ordination principles? Searching for the truth......

Crash & Burn
10th Jul 2002, 09:17
From the Honeywell TCAS II PIlots Handbook:

"Both aircraft issue RAs, if TCAS II equipped. The aircraft first initializing the RA is the controlling TCAS. However, if both aircraft issue the RA simultaneously, the aircraft with the numberically lower Mode-S address is the controlling TCAS."

working boy
22nd Jul 2002, 19:15
Anyone please care to remind me (having forgotten & now unable to find the relevant notes) the improvements TCAS II give over I?

Capt Pit Bull
23rd Jul 2002, 15:07
Maintain Vertical Speed is a 'preventative' RA, i.e. it stops the aircraft in question from getting any closer to the intruder.

Corrective RAs (e.g. 'Climb') change the flight path to move it further away.

There is different vertical thresholds for each type, based on Altitude Layer.

e.g. at low level:

Corrective threshold: 300'
Preventative threshold: 600'

So, if TCAS predicts you will pass >600' vertically at the time that you are closest horizontally, you get no RA.

If prediction <600', but >300' then you get a preventative RA stoping you manoeuvering any closer.

If prediction <300', you get a corrective RA to move you away.

Generally, since both boxes work to the same thresholds, if you have a preventative, the other aircraft will as well. Similarly, if you have a corrective, so will the other aircraft.

Generally, but not always.

Alternate scenario 1.
If one aircraft has high vertical rate, greater than the magnitude of an RA that can be issued, then it obviously can not be moved any further in that sense. It which case it has to be 'prevented' whilst the other aircraft is corrected out of its way.

Ultimately, if both aircraft have very high rates, in the opposite sense, the system can get painted into a corner which is why it is only certified for 10,000 fpm rates of vertical closure ('only' - hah!)

Alternate scenario 2.
If both aircraft are inhibited in the same sense (e.g. both at low rad alt, so descent impossible) one aircraft gets a monitor vertical speed RA, with red arc saying not to climb. Other aircraft get the corrective RA.

TCAS Version

TCAS I is a TA only system.

TCAS II is a vertical RA system, with coordination if other aircraft is TCAS II.


Think you might be referring to major software revisions within TCAS II. Specifically, v6.04 versus v7.0.

Incidentally, one of the big 'behind the scenes' changes from 6.04 to 7.0 is how coordinated RAs are handled. See other threads.


4th Sep 2003, 22:48
How does AIRBUS display their TCAS II - Resolution advisories ?

7th Sep 2003, 10:11
A320 RAs. On the Primary Flight Display the vertical speed scale on the right side of the display area will be overlaid with coloured sectors. Red band for vertical speed range to be avoided (Preventative RA). Green band for vertical speed range to fly into for Corrective RA.
On the Navigational Display the Threat aircraft for which an RA is generated is a standard red square symbol with relative altitude and vertical movement displayed alongside. This is in most modes and ranges selected on the ND. If an appropriate mode is not selected a red message TCAS:CHANGE MODE or RANGE will be bannered across the display to advise the pilot to switch.
Also standard trajectory orders by voice announcements.