Carnage,
Yes the thread contains an excellent explanation, however i disagree that the system worked as designed. TCAS is there to back up ATC when we get it wrong as we do sometimes, in this situation ATC got it entirely correct..ie Embraer descended 1000 on top of DH8-panic over.
For the Embraer to then descend through the DH8 level in response to TCAS RA as discussed is a TCAS induced loss of separation. Imagine the poor ATCO, 2 days after the DHL collision he watches an a/c descend through another 2 miles head on working on 100 mile range, labels garbled, blips merge, Embraer appears out the other side 300ft below DH8. If he is not off work with stress for some days i will be surprised.
We need to continue to develop TCAS and use all incidents to make aviation safer for all.