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"The chopper was equipped with a TCAS (collision avoidance system) but apparently didn't have it switched on".
Why this is optional?
I have no knowledge on the subject, but I would speculate that military aircraft need the ability to turn it off if, in a combat situation, it would broadcast their location. Not good if the enemy can see where you are.
That said, I can think of no reason it should have been off in this situation.