12th Jun 2012, 14:13
Greeting to all!
I have a few questions on TCAS system (TACS 2 with Mode S)
1. is there any minima for TCAS to be ignored ? - lets say you are on your final approach and your Radio Altimeter is alive! if you get a TCAS RA, how would you behave?
2. Is there any maximum traffic load TACS2 can handle?
3. When TCAS2 RA is given to climb, what if a Mode A or C airplane is in the path? TCAS2 is blind for Mode A or C!
Sorry, too many things asked! just that I am trying to find the logic behind this. And if there is any place if I can find the "Logical Standard" of TCAS . . would be ideal or your valuable experience would really quench my thirst!
Thank you all!
12th Jun 2012, 16:15
TCAS has its own limits for RAs during approach and take off.
Below 1500ft AGL no INCREASE DESCENT RA's will be issued, below 1100ft AGL no DESCENT RA's will be issued and RA's alltogether are inhibited below around 1000ft AGL as the TCAS system switches to TA ONLY status.
My current TCAS manual doesn't have any information about this, however an older version told us that TCAS is able to track up to 127 targets at once, which does not mean that they are displayed.
TCAS cannot see true Mode A only transponder equipped targets, however it can see Mode C both with and without altitude reporting enabled. There are not many Mode A transponders around, most are at least Mode C and Mode S is becoming quite common now that it is required in many european airspaces, that might differ outside of europe of course.
If a Mode C target with altitude reporting is in the way TCAS will calculate that into its solution taking into account that no coordinated solution is possible. It will generate a proximity alert and a TRAFFIC TRAFFIC callout for Mode C targets without altitude reporting, but cannot take them into account for any RA solution as it doesn't know where exactly they are.
12th Jun 2012, 16:27
1. The ICAO recommendation is that ALL RAs should be followed. TCAS II uses the radio altimeter to estimate the aircraft's height above the ground. All RAs are inhibited below 1,000 ft AGL. Descend RAs are inhibited below 1,100 ft AGL. The cockpit avionics give priority to wind-shear and GPWS warnings over TCAS RAs.
2. TCAS II is required to be able to track at least 30 targets, and to be able to display at least 8 targets on the traffic display. TCAS II has specific algorithms to resolve conflicts with multiple threats.
3. TCAS II detects Mode C aircraft, Mode S aircraft, and other TCAS II aircraft (which are necessarily Mode S equipped). If the other aircraft's transponder is reporting altitude then an RA can be issued. If the other aircraft's transponder is not reporting altitude then only a TA can be issued. TCAS II can not detect aircraft with only a Mode A transponder.
The vertical sense of RAs in encounters between two TCAS II aircraft are coordinated.
TCAS is specified in ICAO SARPS (Annex 10, Vol. 4) and RTCA MOPS (DO-185B, also published as EUROCAE ED-143). A useful introduction is the EUROCONTROL ACAS II Guide, available at
13th Jun 2012, 10:55
I can't find any guidance on actions for an RA whilst in a turn.
Level the wings or continue turn as projected by TCAS?
13th Jun 2012, 13:01
yeah the TCAS2 is based on pitching axis only. But TCAS3 which is still in the conceptual stage will be based on rolling as well. Thanks for your reply
13th Jun 2012, 13:05
Thank you Denti and Fellman, your feedback was such enlightenment!
A small question, for TCAS2 operational logic, do different manufactures have different standards or is there any ICAO or any competent authority based Logic that all manufactures have to follow?
thanks in advance!
13th Jun 2012, 14:16
The official advice is that a turn should be continued IF the required vertical speed/pitch angle can also be achieved - if not then level the wings and execute the required climb/descent.
(Ref. ICAO Doc 9863 - ACAS Manual, 188.8.131.52.d "if possible, comply with the controller's clearance, e.g. turn to intercept an airway or localizer, at the same time as responding to an RA")
13th Jun 2012, 14:28
ACAS II (Airborne Collision Avoidance System) is specified in ICAO SARPs (Standards And Recommended Practices) - Annex 10, Volume IV.
TCAS II (Traffic alert and Collision Avoidance System) is a specific implementation of the ACAS standard (and as far as I know the only implementation). It is specified in MOPS (Minimum Operational Performance Standards) - RTCA DO-185B = EUROCAE ED-143.
The MOPS contain a state diagram description of the collision avoidance logic. They also contain a pseudocode version of the logic which has been verified as a valid represntation of the state diagrams. Manufacturers implement either the state diagrams or the pseudocode as software in their TCAS equipment.
14th Jun 2012, 13:18
and this is Fellman what I was searching for! Thank you a trillion for sharing your information and sources!