I must admit to a bit of misunderstanding regarding AT and bumping it into MCT.
While enroute, I bumped the levers out of CLB and into MCT thinking I'd lose AT as well and I didn't. The EPR started climbing, obviously, and putting them back into the CLB detent returned everything to normal.
It was as dramatic a shift in automation levels - more of a hybrid change! but it was easily returned to the previous level.
Yes, the point is when the thrust levers are moved forward out of the CLB detent, 2 things happen:
1. The thrust goes that commanded by the actual thrust lever position,*
2. Autothrust disengages - but REMAINS ARMED, (unless you are below 100'). So if you then return the thrust levers to the CLB gate, autothrust will re-engage. (This was how phase advance worked, but it was very poorly taught and therefore very poorly understood by most pilots, who then got it wrong).
*A slightly fuller description: With the thrust levers in the CLB gate and auto-thrust engaged, the auto thrust will vary engine power as required between idle and CLB power. It does not backdrive the thrust levers, which do not therefore move, but one can easily imagine the 'virtual' thrust levers moving between idle and CLB and see it on the EWD gauges. When one pushes the thrust levers forward of the CLB gate, the auto-thrust dis-engages and the FADECs increase power from what might have only been idle, to the thrust commanded by the actual lever position. This could be a very large increase if it got all the way to above CLB power from idle, so phase advance was just intended to be a second or so out of the CLB gate and back in again. This would start accelerating the engines towards CLB, giving the speed increase required, but after that second, putting the levers back to the CLB gate to re-engage autothrust, which would then return the engines to idle. 'Click out' - 'click back' in, in about the time it takes to say that, is all that was needed.
PS
If you pull the thrust levers
back from CLB, I think the auto-thrust stays engaged, but the engine thrust would then limited between idle and the new thrust lever position. I guess this would be so one could instinctively reduce power in the event of an overspeed I suppose?, but I've never done it that way. (The ECAM would ask you to return the levers to CLB soon after - giving it control of the full range again).