Imho some of you are on the wrong path concerning the stability issue.
Fact is, that BEA brings in the "neutral stability" issue and they wouldnīt have done it for no reason.
This neutral stability is not a built in by design into the airframe or caused by shift of CG (trimtank), it is done by managing the position of aerodynamic pressure by the computers with the elevators in the short term and the THS in the long term. Therefore i did use the term "natural" and "artificial".
Gums described it in his words.
Gums
If we are talking about little or no speed stability because HAL is trimming for a gee versus an AoA ( just like the Viper), then you may think you have "neutral" static stability, but you don't. The 'bus has positive static stability throughout a good combination of cee gee versus center of aero pressure, but never has inherent aerodynamic negative static stability.
While the new generation of combat jets are "natural unstable" and are made stable by the computers, it is quite different in the A330. It is natural stable with positive static stability (See the graph posted by TurbineD) even when computers change CG a bit by transfering fuel to or from the trimtank. With the automatic THS trim besides the elevators the computers have a formidable tool to change the aerodynamic pressure, hence the airframe is kept in neutral stability all the time.
In a conventional aircraft it is the same situation when you have the aircraft perfectly trimmed, with one big difference. The trimmed conventional aircraft is speed stable and will return to equilibrium if disturbed out of that trim condition, the discussed A330 is not speed stable but flightpath stable within limits defined by the built in flight envelope protections.
JT
However, this latter consideration may be tied in with the present mishap - inextricably. Are we not all concerned with the possibility that this crash may have been involved with events so out of left field (but only because of presumptions regarding training and competence standards) that the original certification program may not have considered them at all ?
At least BEA mentions this neutral stability, wether as excuse for the problems the crew faced, as a hint to the manufacturer that the reversion sequence and the documentation needs a look up, or as a hint to better training and understanding of the resulting flight behaviour when vital inputs are lost? Or any other reason we can think of.
JT
neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable
As described, here with the A330 itīs vice versa. The aircraft has a natural positive stability, it needs the computers to keep it in the neutral region.
JT
However, some of the posts appear to be suggesting that the degradation can extend to nil box assistance and a statically unstable aeroplane.. that's my problem at the moment.
Imho the "Nil Box Assistance" is not the problem, as the aircraft would then (with manual THS trim) be a natural positiv stable airframe.
The problem here is, that with all protections except load factor protection lost the aircraft was kept in this neutral stability by the computers and the crew could not cope with the responsiblity to keep the aircraft within the flight envelope.
See RRīs post below.
rudderrudderrat
The aircraft is only statically neutral with the aid of the FBW computers. In Direct Law (stick deflection proportional to control surface deflection) they are (beautifully) dynamically stable.
Exactly
JT
This is the bit with which I'm having difficulty although my reading of the various links (and thanks to those who PMd/emailed other suggestions) may answer my questions ..
Neutral static stability infers a more aft CG than "normal". The box then can give the pilot an impression of stability (and, at the end of the day, it is this impression that is required).
Imho not necessarily, as explained before. The aft CG helps in saving fuel, but is not necessary for creating the neutral stability. The continuous autotrim is creating this permanent neutral stability with the help of the box.
If, now, the box departs controlled operation .. the aircraft CG is still where it was (until it can be moved) and the pilot potentially now is stuck with neutral static stability .. OK if you recognise it and know what to do .. but, otherwise, a recipe for rapid disaster ...
The CG doesnīt change except for the fuel in the fin, the location of the center of aerodynamic pressure is constantly changing and keeps the aircraft in neutral stability until the degradation drops into direct law. Then you might have an aircraft completely out of trim, but it is basically now an aircraft with positive static stability. As in direct law only the trim wheel is available for longitudonal control, you wind it where it needs to be and the aircraft would be no longer flightpath stable, but speed stable.
Whereas in ALt law2 the aircraft was still kept in neutral stability by the box and was flightpath stable, but the necessary protections to make such a system safe (including the backup system "aircrew" ) were not working, thus leading to the flight-envelope excursion.
Bubbers44
John, I agree, neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable. A pilot would have a hard time to keep it at a constant altitude.
True for a conventional aircraft, ts different here.
I hope Owain Glyndwr can cut in and correct where iīm wrong or to add where necessary.