@Quagmire
There are many points in your postings with which I disagree, but here are just a few:
As a general point, making fundamental system changes on the basis of a single scenario is poor airworthiness. If you want your proposals to be considered seriously you must discuss their effects in a variety of possible situations.
“Since the phugoid is a roller coaster up-down slow cycle, why are phugoid damping terms allowed to pitch the aircraft down close to the ground when the ground is right there, close up, and the next event is touchdown, so the phugoid can't be excited anyway?”
You are missing the point that although the pitch rate and speed variation feedbacks stabilise the phugoid they also have an effect on the short period motion.
As a control engineer you will appreciate that the more rapidly you approach a system limit the more likely you are to overshoot it and the bigger the probable overshoot dimension. If the system limit is a critical safety boundary like a stall you would take steps to ensure that you did not overshoot. This is a short period phenomenon. The pitch rate feedback in particular does this job by limiting the rate at which you can approach the limit (but does not prevent you getting to the limit slowly).
“I think the solution would be to ONLY have a little pitch rate damping, and do away with any other anti-phugoid mode feedback in alpha-protect below 100' AGL near flare”
In effect your proposal would remove dynamic stall protection below 100 ft or thereabout. One can argue whether the feedback gains are set too high and unduly limit the rate at which pitch can be increased, but not I think concede the principle that dynamic stall protection must be retained even at low altitudes.
“To clarify, you can't go into a roller coaster phugoid excitation since your altitude apogee is already close to the ground. Enter the edge of stall at 30 feet off the ground while descending, no problem”.
WOW! – go to the edge of a stall at 30 ft RA with no stall protection – I don’t think you will find many takers on this forum.
You are assuming that pitch attitude is the only important parameter in water landings, which is not correct. The probability of structural damage depends on the overpressure generated on the fuselage undersurface by the impact (and let us not forget that despite the fact that Flight 1549 ended well, the pressures were twice the design value).
The pressure generated depends on the square of the total (forward plus downward) velocity times the sine of the angle between water surface and fuselage at impact. This latter is the sum of pitch attitude and flight path angle with a correction for the shape of the fuselage at the point of impact. Pitch plus FPA of course equals angle of attack; angle of attack dictates lift coefficient and lift coefficient times speed squared equals lift. Flaring takes time! So if you flare late and hit the water before the flight path has had time to change and the speed to bleed off you risk generating more impact pressure not less because you have increased the impact angle. Piloting technique matters!
“To me the perfect landing would have been 11 degrees pitch, 1 degree stall margin remaining, and the slow airspeed that corresponds to, while hitting the water at 500 ft/min. Let's call perfect an "11/1/500". Instead, Sully did it as an "9.5/3.5/750" and I blame that on some unwanted feedback terms in alpha-protect below 50' AGL.”
Let’s dissect that proposal:- alphamax was 17.5 deg, so with 1 degree stall margin alpha would be 16.5 deg, and with 11 degree pitch that means a flight path angle of -5.5 degrees. The speed would drop to 115 kts, making the rate of descent 1100 fpm not 500 – far from perfect! If you want to be taken seriously you need to get your facts right.
Airbus have said that below 8 degrees pitch and above 15 degrees there is a much increased risk of major damage. They have also said that the optimum attitude for ditching varies with circumstances such as sea state. 11 degrees, sitting just about the middle of the usable range should I think be considered as an acceptable compromise rather than an ideal. We do know, do we not, that 9.8 degrees at 128 kts onto calm water is a viable combination.
“Originally, the early NTSB reports mentioned it all. Page 194 of the NTSB Report (the BEA French section) says:
"However, this accident demonstrates that, by offsetting the pilot's ANU sidestick inputs, the phugoid-damping feedback function of the alpha-protection mode could make flaring the airplane to attain the recommended ditching touchdown parameters more difficult." --- That statement didn't make it into the final report, but was in the appendix as a reported document change”
But the final report did say:-
“Deliberately or inadvertently slowing the airplane into the alpha-protection mode may result in an attenuation of pilot nose-up stick inputs, making it more difficult to flare the airplane, even if AOA margin to alpha maximum exists.”
“On this forum, we do need the Airbus/BEA apologists to present better arguments, if possible. Too weak.”
Sounds like a Trumpism!