I thought in the (737NG) there was a sensor in the wheels, so as the moment the wheel started to spin spoiler deployed if in the armed position. I was riding jump seat on a 738 NG and on touch down on a contaminated runway the spoiler didn't deploy so the captain who was the pilot flying, manually deployed the spoilers.
after we vacated the runway and were taxing to the gate he asked both myself and the F/O if we noticed that we were aquaplanning and that's why the spoilers didn't deploy.
I could be wrong so if tha's the case please correct me.
Cheers Red
P.S. I'm not sure if the initial question was for automatic deployment of spoilers when armed ........ or just when can they be deploed.
please let me know if my post above is correct for auto deployment.