Thanks for the responses. I know AOA is relative to airflow NOT the horizon, it is the -ve aspect I cannot get my head around. Thanks M(F)S for the answers so far but its just not gelling.
The maintenance manual says if the reading is +ve then the vane is pointing downwards, ie a downward AOA.
If you assume the AOA is 0 degrees for level flight. If the speed decreases the AOA should increase so shouldnt the trailing edge of the vane go below the horizontal and become negative, to indicate its pointing upwards and visa versa?