Modern Transport Aircraft Stability Question
Simple one.
With the advent of modern FBW control systems, together with the (significant) fuel saving advantage of flying with an aft CofG, why are modern FBW transport aircraft not designed to be longitudinally unstable in the cruise with an aft co of G? I hear some of the modern airbus aircraft schedule fuel to push the CofG rearwards, but are they fully longitudinally unstable? As I understand it, efficiency savings from tailplane up force are similar to the savings from winglets (on my type, we are talking 4% of cruise burn for most rearward C of G).
Would FARs/JARs prevent it from happening?
Or are there concerns with fallback modes (in particular 4 eng flame out/complete electrical failure case, although FADEC engines typically hold independent PMAs for this case)?
I would imagine there would be a maximum limit to the benefit from this, when the alpha at the tailplane reaches a stage that it starts generating more drag.
Any insight from People in the industry?