If I understand it corectly, the airbus FBW family have neutral APPARENT lateral stability, because the flight computers ensure the aircraft does not generate a roll rate unless the pilot tells it to by moving the stick left or right.
However, the airframe DOES have inherent lateral stability, due to wing sweep and dihedral among other factors. Therefore any sideslip induced by asymmetric thrust will cause a laterally stable response by the airframe, which the FBW computers have to cancel out using aileron and spoiler inputs. These cause additional drag, so you need to find the sideslip angle that gives the lowest combination of rudder drag and aileron/spoiler drag while maintaining a steady heading sideslip sufficient to counter the asymmetric thrust. This gives you the highest possible L/D ratio and thus the highest possible climb rate. This happy balance of control inputs is what the beta target is helping you to achieve.
As I see it, the key to a successful and not-too-scary OEI climb out, once speed is pegged to the optimum climb speed for the configuration, is to avoid unecessary sideslip as much as possible.
Regarding the flex issue, not entirely sure selecting TOGA rapidly is a good idea, it depends on the airspeed you have at the time and how good your climb rate is. The last thing you want to do in a low-speed OEI situation is rapidly exacerbate the thrust asymmetry and sideslip (which increases drag hugely) if you are only a few kts above the minimum control speed. IMHO any thrust increase on the remaining engine should be smooth and carefully controlled with matching rudder input so as to minimise sideslip excursions, which, if mismanaged, would certainly hurt your rate of climb and could even cause a speed reduction in the worst case.
Having said that, if selecting TOGA causes the beta taget to appear, then at least the aircraft will be helping you manage the sideslip. I can see Airbus's thnking on this, and it seems sensible. What is the usual SOP regarding TOGA use in this situation?
Last edited by WeekendFlyer; 21st Jun 2013 at 12:37.