Gents,
If you will allow me to chip in ....
therefore i try to summarize the issue about stability in my own understanding (feel free to correct).
The airframe itself is, no, must be natural positive stable, otherwise it would not be flyable in direct law.
Quite so - but when people say that they are usually talking about the short period motion. CG ahead of aerodynamic centre in the classic sense for stability - that qualification says nothing about the long period (phugoid) motion
In Nz-Law, which applies in all Laws except Direct Law Ground Law and Flare Law (did i forget one?) the flight control system creates an artificial neutral stability by use of elevators and THS trim, leading to a stable flightpath without SS input. Not the SS drives the elevators, but the flight control system using present flightpath and the demanded change from that flightpath.
C*, by itself, is a system to shape the short period response into a form that pilots will like. When implemented as Airbus do it acts, in the absence of any further pilot input, to maintain the flight path constant as Franzl says.
But if the flight path is constrained, whether that be level flight or an approach path, the natural phugoid is suppressed because you have taken away one of its degrees of freedom. What you are left with (for the 'natural aircraft at least and apart from a stable short period) is a simple subsidence or divergence in airspeed, i.e.it becomes a speed stability issue and one where what matters is how (Thrust - Drag) varies with airspeed
with throttles held constant.
Something like the A330 is speed stable for excursions above the (trimmed) cruise speed because you very soon run into drag rise. Below trimmed speed however it becomes a bit more problematical.
When one takes Mach number effects as well as the classical drag 'bucket' into account modern airliners have a drag vs speed curve that looks more like a flat-bottomed 'bathtub' than the classic parabolic shape. Depending on how thrust varies with airspeed (again at constant throttle) the speed stability is just about neutral over a fairly wide range of airspeed. It varies a bit with altitude, being slightly less stable at say FL370 than at FL350.
It is because of this neutral stability on the low speed side that 'buses' use A/T in cruise to sharpen up the aircraft response to disturbances.
Before anyone starts to yell "I told you so - the aircraft is unstable without A/T" just remember that if the aircraft is only slightly speed unstable (as seems to be the case) it will take a very long time for small initial diversions in speed to develop into serious excursions. Typically, from cruise down to Vs1g might take anything from 5 to 10 minutes but the rate of divergence escalates rapidly as one gets near that point and most of the speed divergence will be in the last say 30 secs. Once you get beyond 'stall' the rate of divergence rockets up. Hence the need for prompt recovery action.
It must also be apparent that modest speed instability, by itself, could not explain this accident. To reach the timescale recorded the aircraft divergence would have to have been 'helped' by pilot action - as it was. But once the aircraft was down to speeds at or below Vs1g the aircraft's natural speed instability would have made a significant contribution to the rate at which speed was lost.
When those protections are degraded or lost, we still have an airframe with artificial neutral stability, as the flight control system still works along the demands of Nz Law, but the protections to keep the aircraft within the flight envelope are gone and have to be replaced by the skill of the crew.
Agreed!
This NU SS had not much influence after the stall happened, as the flight control system was beyond itīs capability to maintain the flightpath (descending already instead of continuing on the ordered climbing trajectory).
I'm not so sure about this. The flight control system would still have been trying to maintain the flight path, but the flight path it was trying to maintain was that being demanded by the pilot as seen by SS movements - an increased 'g' command. So far as I can see, the continued NU SS and accompanying THS movement would have made things worse for recovery because they trimmed the aircraft into a higher AoA than it would otherwise have achieved.
Neutral SS wouldnīt have cured the problem, only ND SS would have changed the trajectory.
Agreed.