Quote: Will Fraser
Histrionics aside, may I ask for clarification on one of your recent posts re: 447, flight computers, and hand flying? With possible corruption of sensors, 330 flight computers seem to produce a reasonably consistent chain of output that may 'confuse', or inhibit manual flying which results as a trip out of the a/p and consequent degradation to Alternate Law or Alternate Law 2. Reference Qantas and NWA, the computer displays 'Stall Stall'. Whether or not that cue is to be taken seriously seems to depend on decisions made by the PF. Do you consider that a hindrance in recovery? Or, could the PF, acting on cue, push the nose down, even add power, though the a/c is anything but stalled?
Will,
When a failure of an element of the flight control system occurs or data from a source becomes unreliable most of the "consistent chain of output" is intended to, and for the most part does, reduce rather create confusion or inhibit manual flying of the aircraft. The basic purpose of a reversion to Alternate Law and if necessary (the two are not always coincident) the disconnection of the autopilot is to simplify the pilot's means of controlling the aircraft by removing the autopilot and/or some of the more refined protections of Normal Law that might be adversely affected by the failure while the crew ascertains the source of the problem and configues the systems to best minimize the effect.
Some failures by their nature create greater problems than others. In any airplane a loss of valid airspeed or angle of attack data creates the potential for spurious stall warnings as have been recorded in the QANTAS, Air Caraibes and NW reports. This does carry with it the potential for some confusion, and it's precisely for that reason that diagnosis of validity and flight control response to such warnings following such a corruption of data are handed back to the pilot. In all the instances that I'm aware of with Airbus aircraft the crew was able to effectively determine whether the warning was valid and maintain positive control of the aircraft. This is what you would expect from a competent crew. In simple terms, if one moment you are flying along "fat, dumb & happy" and the next your airspeed starts to rapidly fluctuate or becomes grossly inaccurate without equivalent changes in pitch and power, it's a reasonable diagnosis that a stall warning occuring at the same time should not be considered valid unless other basic aircraft parameters have undergone very significant change. So, yes an invalid stall warning can be a source of some confusion which the crew needs to resolve, but no, it should not be a serious hinderance to maintaining basic aircraft stability as long fundamentals such as attitude, power, and trim are observed and managed by the pilot.
Also, re: Rudder Travel Limiter. If due to corrupt AS data the ship loses Rudder protection, and possibly all limit to Rudder travel, would you think that a Rudder input might put beyond-fail stress on the VS? To wit, as AS data is lost, could the Rudder Limit reach a status of Low Speed restrictions as determined by the computer? In other words, could the Rudder's travel be deflected enough to convince the sensors in the tail in concert with corrupt a/s that low speed had actually been obtained allowing this deflection to be the new parameter, though the a/c is still travelling at M.80?
Actually, as has been pointed out here several times in discussions regarding AA587 it is possible to create stresses beyond the designed limits even with the Rudder Travel Limiter fully functioning, so yes it's certainly possible for this to occur with RTL failed regardless of the deflection limit at the time of failure.
I think the question your driving at is: Since the RTL system on the A330 autmatically freezes the maximum rudder deflection value based on airspeed at time of failure is it possilbe for it to be conned into setting an erroneous max. deflection value prior to failure if the airspeed information becomes inaccurate? The simple answer is that I don't know for sure. The more refined answer is that such a situation, if it is possible at all, would have to be considered extremely remote. For it to occur there would have to be not just a simultaneous failure of the three different airspeed inputs, but those failures would have to occur at near identical rates and yet still allow the measured airspeed values to remain sufficiently consistent with the other air data inputs so as to not cause a diagnosed failure of each affected ADR. Any substantial differentials between the airspeeds or failures of the ADRs would cause the system to consider itself failed and lock the max. rudder deflection value at that point.
Even then, based on my experience with the aircraft I suspect that the designers may have considered such a case and included a test for it in the limiter's logic. Though I'm sure there are some very remote failure cases that likely haven't been considered, I think many here with a layman's or even a pilot's general understanding of the flight control systems fail to give sufficent credit to the designers/engineers for the degree to which they have gone to try and analyze and account for each possible failure mode. It doesn't make the system perfect, but it is a lot more so than you'd judge from the theories you are reading here, and in most cases it is a significant improvement on the previous generation's accepted standards of safety (i.e. no limit freeze function, just a warning to use rudder with care).
Quote: Will Fraser
PJ2 quoted P,R,Y limits a/p; ELAC quoted 3 Degrees deflection per side; I had thought 4 as well, I believe 6 total is correct.
This is not correct. My statement to you in response to your question was:
That value is dependent on speed, which according to my manuals at M.80 and FL350 (272 KIAS) is approx. 6 degrees (not 8).
You had stated 8 degrees of deflection which was incorrect. I referred to the same chart that PJ2 has recently posted and provided you with the figure corresponding to M.80 at FL350. You can make your own estimate if you like. If the aircraft's speed had been reduced to 260 KIAS (the turbulent air penetration speed below transition) then the limit might have been slightly, but not significantly, higher.
ELAC