Turin - the essence is that a change from a certain negative AoA to a lesser negative AoA is, de facto, an increase in AoA towards a positive val;ue in real terms. That is why it is important to understand our 'framework'. It is basic mathematics. Think for one minute how you would describe the situation IF the AoA of the tail 'changed' so as to be 'positive' (ie nose above tail) to the airflow, and then the a/c pitched further nose up - would you still maintain the tailplane AoA decreased? You would find a mathematical approach very difficult if you did. Every angle and movement/rotation must have a reference to the a/c frame. EG Start at -10 degrees and finish at -5 degrees - increase or decrease? Once you start saying the negative angle 'decreases' you are in 'double negative land'. Zero is a problematical number!