There has been a small amount of unhelpful and misleading comment on this thread regarding the reliability of TCAS as a one and only last chance of avoiding mid-air collision. For the benefit of those who have not yet been introduced to the system, to avoid you being mislead, may I say a few words?
We train using TCAS in the simulator. We can generate single and multiple targets to try and confuse the TCAS. We can even make it command manoeuvres to “thread the needle” for traffic both above and below. TCAS works; it has already prevented many close and possible midairs in our airline, and I speak from personal experience.
The principle of TCAS is very simple. If we were out flying in the same airspace, our transponders chat to each other if we are not too far apart vertically and laterally.
If you approach my safety-bubble, my TCAS will compare our Altitudes and Separation Distance and Closing Speeds with your TCAS. In you come inside my safety-bubble our TCAS’s will agree on a coordinated manouevre for each of us pilots to perform. That will be something like, for example, “If you tell your pilot to climb, I will tell my pilot to descend”. The TCAS commanded manouevre would only use the vertical plane due to limitations of antenna lateral resolution.
TCAS Resolution Advisory relies only and absolutely on the pilots to follow the manouevres commanded.
For the benefit of all professional pilots reading this, if you come into my safety bubble, please do what your TCAS commands and I hereby pledge that I will also do you the honour of precisely obeying what my TCAS commands.
If you don’t have a TCAS, please squawk Mode C and that will allow me to see you. And my TCAS and I will guarantee that we won’t hit you.