DC-ATE;
This is likely in the thread earlier on but I'm just lighting up the computer after a few days away.
As Dysag says, the 320 sidestick inputs are summed but the important point is, they are summed algebraically.
Double sidestick input is a serious no-no in 320-type aircraft. It continues to occur however, at least as shown in flight data analysis. Usually it's a training-line indoc situation. That said, CONF iture, you are absolutely right in what really happens.
What some transitioning to fbw (Airbus style) initially may not appreciate because it may not be emphasizd in training is, the stick does not control ailerons/elevators; - it "requests" a roll-rate or a g-force. You may still recall a CWS mode - Control Wheel Steering - where, when engaged the airplane "stayed" in the last attitude set by the controls before engaging. That is essentially what fbw/sidestick control is. "No input, no change".
The autoflight-autothrust-flight director-flight management guidance computer systems are deeply intertwined with one another, all talking to each other. The non-moving thrust levers is a non-issue in such a system as the various flight modes look after thrust when the levers are set in their normal flying position, called the "CLB" or climb position. In this mode depending upon climb, cruise or descent, the engines will respond to airspeed, rate of climb or cruise speed commands from the FMGC, (Flight Management Guidance Computer).
For example, in a climb, with autothrust engaged but the a/c being manually flown, power is "on the elevator". That means power is adjusted by FADEC to match the thrust required to maintain an airspeed which is set by the cost index. Usually that's the old max continuous, about 92% N1. If one pulls back, the airspeed will bleed off only because 92% isn't sufficient thrust to keep both the climb and the speed, but power is still governed "by the elevator" in manual flight. To understand this better, take the opposite case where the rate of climb is reduced by gently pushing forward on the stick - the rate of climb will decrease, but without touching the thrust levers, the power will come back to match the speed being commanded by the FMGC.
In descent, the usual auto-flight modes are either "IDLE, OPEN DESCENT" or VNAV. VNAV is a calculated path that is managed by both the autothrust and autopilot - the speed is permitted to vary to take advantage of energy before bringing up the power from Idle.
"OPEN DESCENT" means the engine thrust is commanded to idle, (about 35% N1). The autothrust will not bring the power up until the selected altitude is captured. In this case, it is airspeed, not power, that is "on the elevators". To better understand this, if the sidestick is pulled back slightly, the airspeed will reduce and continue to do so until a
minimum speed calculated by the flight augmentation computers using weight, angle-of-attack and CAS is reached, at which point the autothrust will revert from "IDLE" to "SPEED" and the "OPEN DESCENT" mode to "V/S", (to prevent a huge power increase to "chase" the originally targeted speed).
For this reason, in manual flight the flight directors must either be followed or turned off as they are indicating the "thinking" behind the autothrust system. If turned off, the autothrust reverts to the "SPEED" mode to again protect the airplane.
The other vertical modes are either variations on this theme or protections which guard against speed gain/loss or v/s excursions higher than certain values.
The sidestick is a very powerful little handle and is best flown gently, usually with the hand near the base to avoid large inputs.
As an aside, while the US Airways flight will have had "Alpha-prot", (stall protection), the fbw system would not have protected this airplane from stalling in this case because the engines were incapable of delivering TOGA (Takeoff/Go Around) power. In "Alpha-prot", the airplane is not pitched down to reduce the AoA - the only airplane response is to increase power, so Captain Sullenberger had to fly the speed indicated on the PFD, (Primary Flight Display) just above the alpha-max stall speed and by the initial indications, this is just what he did. See the graphic below - likely he would have flown between Vls and Alpha Prot but we'll know more after the reports.
