I believe it's the ICAO 24-bit address that's used as a tiebreaker, rather than the Mode A squawk.

Hi john_smith,
See page 13:
http://www.skybrary.aero/bookshelf/books/106.pdf
"2000 ft/min.
1000 ft/min.
500 ft/min.
0 ft/min.
RA requires one of
these vertical speeds"

All of my TCAS warnings have been due to other traffic with big ROD / ROC approaching their cleared levels. If only they observed ICAO Recommendation:
"Unless otherwise specified in an air traffic control instruction, to avoid unnecessary airborne collision avoidance system (ACAS II) resolution advisories in aircraft at or approaching adjacent altitudes or flight levels, operators should specify procedures by which an aeroplane climbing or descending to an assigned altitude or flight level, especially with an autopilot engaged, may do so at a rate less than 8 m/sec or 1 500 ft/min (depending on the instrumentation available) throughout the last 300 m (1 000 ft) of climb or descent to the assigned level when the pilot is made aware of another aircraft at or approaching an adjacent altitude or flight level."
See http://www.skybrary.aero/bookshelf/books/1804.pdf

So with say 3,000 ft per min ROC, pilots will have to have started to change their VS towards 1500 ft per min before 2,000 feet to go - else it is too late and the AP mode will have changed to ALT*.

Hi DaveReid,

You might well be right, I was told it was the squawk. Anyway, the two TCAS involved do have a way of nominating which becomes the master.


Hi Rudderrat,

Yes, I personally think it's high time there was an autopilot or altitude capture software update installed to all aircraft which softened the rate of climb/descent as aircraft approached their cleared level to avoid the sort of nuisance TAs you refer to.

TCAS RA
 

Or pilots use the VS mode- same like a turn indicator in your car: it's there for a reason, without surcharge.

Can's say about other types, Jeroen, but the 737 require complete manual flying, and I suspect all Boeings are the same. It sounds like Airbus advocate manual flight controls with autothrust. Certainly, using autopilot V/S modes is a no-no; it's too slow to respond and too likely to result in a SNAFU.

I have had two RAs (and plenty of TAs sorted by adjusting v/s pre-emptively). It really isn't a big deal, and requires just a very small attitude change. Like others said, if the cabin crew or pax notice it, then it's not been done correctly. But it is a message that needs training and sim experience to iterate.

One airplane heading north. One heading south. TCAS goes off. One a/c does a 90 degree. There is no airway change of 90 degrees for their route of flight.


Comment about the TCAS 'turn' mode was tongue in cheek.