SD&H...
I think TCAS works faster than you or I can... and the RA is only issued when collision is imminent. If you have given a climb instruction and the pilot has a TI he should act on your instruction... If its closer and the pilot has started to act on your instruction, modern systems should take that into account in the RA and issue a corresponding manoeuvre to the other aircraft (and probably an increase climb to yours)
If the pilot is still reacting to your instruction but not made any control input, the system might generate an opposite RA. Lerts have a look at an account of the Uberlingen accident immediately prior to collision...
Within seconds of getting their initial TCAS RA to climb, the Swiss Skyguide controller instructed them (the Tu-154 crew) to “descend Flight Level 350, expedite, I have crossing traffic.”
(They) did not respond and 14 seconds later Skyguide told them to “…descend Level 350, expedite descent” at which time (they) acknowledged the instruction and, contrary to the onboard RA, complied with the ATC instruction to descend. Thirty-one seconds later the Tu-154 hit the DHL aircraft
This was not the result of slow reaction... I believe the Tu-154 pilot's action was the result of lack of standardisation of response to RAs... (their national procedures was obey ATC instructions...) so they continued to descend against the RA advice. This loophole has since been resolved by Intl agreement.
The fitted equipment at Uberlingen was also early generation kit.
Later systems will monitor the pilots' actions and even if they act to the contrary, will continue to give RAs designed to avoid the collision, based on the updated positions of the aircraft.
In the account you'll note there is time after the initial RA for the controller to pass two sets of instructions to the Tu-154 crew before they initiated the manoeuvre... and that over 45secs elapsed from first trigger to collision. I don't think there is much chance of you issuing a simultaneous, conflicting action that cannot be countermanded by ACAS and that does not allow the pilot time to react?
A Google search comes up with many different sites that discuss the how and why of ACAS/TCAS that you may find interesting?
Last edited by Pierre Argh; 4th Aug 2005 at 16:11.