Early transition to QNH vs TCAS
We have a procedure to select QNH the moment we are cleared to descend to an altitude. It works well in respect that it's hard to forget to select QNH but we are worried that it my cause problems with tcas ta and ra. With big QNE/QNH difference two aircraft my be closer vertically than calculated by the tcas computers since one can be on QNH above TL while descending and other in cruise by QNE. Or there can be unnecessary warnings when difference is in different direction.
I would like to know if your SOPs allow such procedure (maybe under some conditions) or explicitly forbids it.
Thanks