In order to offer a contribution, I would like to first contextualize and otherwise summarize/recapture the dialogue between
machinebird, TheShadow and
CONF iture regarding the notion that the THS responded to a failing speed and therefore trimmed nose-up while the elevators became increasingly nose-down, keeping the airplane from departing the baro-altitude until the autoflight system "could no longer hold on" and "de-clutched".
I do not believe this scenario is plausible, (though anything may be possible).
First, a summary of the quotes I find relevant to the discussion, with a caveat that if I has mis-captured, misunderstood or otherwise missed an important point by the author(s) in these quotes which has taken my understanding off in the "wrong" direction, that I wish correction, and we can take it from there. Also, for others such as PBL or HN39 who may be reading this and who are qualified for more than I in such discussions, I welcome clarifications/comments. This is a sense of the theory I have from the available information, and not a refutation of the theory per se.
Originally Posted by TheShadow 1st July 2010, 11:15 #1663 in response to machinebird
It's obviously the auto-trimming to a false premise (i.e. incorrect airspeed) that the BEA is discussing quite obliquely there. But when you (Machinbird) talk about bleeding off 35,000ft worth of kinetic and potential energy in around 4.2 minutes, I'd agree that you need either very high speed/descent rate or a very high drag scenario/descent rate. Looking for continuity from my early Vmo/Mmo mach tuck scenario, I could amplify it thusly in a different direction. If you can't agree with an acceleration all the way into mach tuck, how about an acceleration to the point where the increasingly out-of-trim horizontal stabilizer (THS) finally exceeded the autopilot's force gradient holding ability to maintain FL350 (i.e. it just couldn't hold the increasingly deflected elevator loads beyond a certain point and declutched itself). Try to imagine what would've happened next.
The THS is trimmed for a lower speed and so the baro-hold is causing the autopilot (at the higher actual airspeed) to hold stick forward/elevator down pressures. At the point where the autopilot disconnects, there'll be a quite significant instant pitchup . . .
Originally Posted by Confiture 2nd July 2010, 12:00 #1668
TheShadow,
Originally Posted by TheShadow
The BEA has conceded that the automation can be duped by false airspeeds to misposition flight control surfaces
I don't see it as a question of "misposition" but more as an inapropriate amplitude of movement. Also, for the 330, the outboard ailerons could be displaced when actually only the inboard ailerons should move for higher speeds.
Originally Posted by TheShadow
The THS is trimmed for a lower speed and so the baro-hold is causing the autopilot (at the higher actual airspeed) to hold stick forward/elevator down pressures.
I don't think it is the way it works. The THS is not set in function of the indicated airspeed.
Originally Posted by TheShadow 4th July 2010, 02:20 #1672 in response to Confiture
,
1. Are you able to expand upon this and supply/propose what the alternative input factors might be that normally cause the Trimmable Horizontal Stabilizer (THS) to auto-trim?
.
2. Are you placing a different interpretation upon the BEA statement (which is admittedly a mite obscure)?
.
3. Do you not subscribe to the theory that the autopilot could have kicked out due to holding a limiting force gradient in the pitching plane? (and therefore that the surprise factor that may have made all the difference to any subsequent loss of control could have been a sudden high-rate pitch-up)
Originally Posted by Confiture Today, 07:37 #1676 in response to TheShadow
1.The pilot, or the AP, set the trim in order to relieve the effort on the flight control command once the desirable attitude is established. That effort is related to the real airspeed, not the indicated one.
I imagine the hydraulic jacks have the capacity to measure the effort applied on them the same way the pilot would feel it through his arm (?)
2.I don't see it as a question of "misposition" but more as an inappropriate amplitude of movement (the Rudder Travel Limiter Unit would be duped by a false indicated airspeed - See the graph posted by mm43 earlier)
Also, for the 330, the outboard ailerons could be displaced when actually only the inboard ailerons should move for higher speeds.
3.I personally don't subscribe to such theory, but it doesn't mean I'm correct, I could learn something I'm not yet aware of.
CONF iture;
I don't subscribe to the theory either. So readers will understand where I am coming from, I will re-state up front that I am not an engineer or a mathematician or an AME. My speciality is flight safety work, specifically flight data analysis (A320, B777) and I have flown the A340/A330/A320 series aircraft since 1992.
Here is why I don't think the theory is plausible:
First, the scenario described is reminiscient of much earlier aircraft like the B727. Sometimes the B727 would be slightly out of trim to the point where we would actually roll the trim-wheel forward (ND) about a half-turn or so to avoid the inevitible "bump" when disconnecting the a/p to start the descent. I think this is relevant because aircraft flight control systems have clearly changed vastly since then.
Being FBW, the A330 flight control system is therefore much more sophisticated. From a bit of research in the AMM and other documents including the paper below, I think this is a much more sophisticated system which would not behave as has been suggested.
There is at least one ECAM Abnormal which will occur if there is a difference, or rather a disagreement between the FMS CG/THS calculated trim position, and the actual THS trim position, "
F/CTL PITCH TRIM/MCDU/CG DISAGREE).
Also, in the,
F/CTL STAB CTL FAULT ECAM message, the second dot-point states that "
IF trim is locked above 8deg NU, pitch down authority may be insufficent for speed above 180 kts". 180kts is a long way from the cruise speed of 272kts, (approximately); any degradation of control as a result of a degrading CAS and a THS trimming NU would have been gradually apparent through an increasing tendency of the aircraft to pitch up and, as the limits outlined in this ECAM Abnormal were approached, may perhaps begin climbing before either the autopilot disconnected, or the authority of the elevator to hold the altitude, was lost.
In other words, the aircraft would likely not "break away", but would gently (assuming smooth air!) begin climbing away in spite of the attempt by the flight controls to maintain altitude, when the limits of controllability, (maintaining the altitude against increased THS NU and limited elevator authority) were reached.
The following is from the
A340 AMM, (I have no reason to believe it would be materially different than the A330 system). The A330 pitch control system description discusses a number of factors and input data which have not been taken into account in the previous posts.
Briefly, the AMM states:
"Electrical Flight Control System (EFCS)
Pitch
The aircraft pitch control is achieved from the side sticks and in certain cases, from the pitch trim control wheels, which act on the elevators and on the THS, depending on the different laws.
1. Nz Law (Nz is 'normal' acceleration, Ny is lateral acceleration)
This law, elaborated in the FCPCs, (Flight Control Primary Computers), is the normal pitch law engaged in the flight phase."
Through a pitch action on the side stick, the pilot commands a load factor; the Nz law achieves this command, depending on the aircraft feedbacks, so that:
- The short-term orders are achieved by the elevator servo controls.
- The long-term orders are achieved by the THS actuator (Autotrim function).
The gains depend on the Vc, on the flap and slat position and on the CG location.
In addition, the Nz law permits to achieve:
- A load factor limiation, depending on the flap and slat position.
- A bank angle compensation, for bank angles lower than 33deg.
- A deflection limitation of the THS in the nose-up direcion in the event of the activation of the high angle-of-attack protection, the excessive load factor and the excessive bank angle exceeding.
The Nz law is such that the aircraft response is quasi-independent of the aircraft speed, weight, and CG location.If both ADIRUs are failed, the Nz law is kept, but with limited pitch rate and gains. A consolidation of the vertical acceleration and pitch attitude rate is then performed via the two accelerometer units."
There is nothing in this section of the AMM which indicates that the Nz law and specified responses will not function in Alternate law.
Second, I began looking for research concerning flight control laws, (initially trying to find out what "Nz" meant!), and found, among many papers and books, the following which seemed to fit the bill in terms of describing the nature, depth and intent of electronic flight control system design; the focus of this (and many papers) was "flight control clearance", which, contrary to perhaps an intuited notion, has to do with "clearing" flight control law design with the appropriate authorities. The process is, I find, extremely complex, requiring much in-depth, expensive and thorough testing in many flight regimes. The reasons are clear and obvious - one cannot "see" electrons as one can see cables and pulleys! That aside, the point being made here is, the posited scenario is well within the realm of experience, (which is the reason I provided the example of the B727 'bump' upon disconnection), and not something that is an outlying matter or even a QF72-type issue. I'm not saying it's not possible - QF72 showed us that it is possible for FCPCs and FCSCs to misbehave, but that is not what is being claimed here.
For these two reasons, I don't think the suggestion is plausible within what may reasonably be known about the EFCS that the THS was somehow responding to a degrading, false airspeed and trimming nose-up, until either the autoflight system or the autopilot "declutched" and the aircraft zoomed upwards.
I welcome informed disagreement. Here is the paper:
Link to this paper
Nationaal Lucht- en Ruimtevaartlaboratorium
National Aerospace Laboratory NLR
NLR-TP-2004-147
New Analysis Techniques for Clearance of Flight Control Laws
An overview of GARTEUR Flight Mechanics Action Group 11
M. Selier,
C. Fielding (BAE Systems),
U. Korte (EADS),
R. Luckner (Airbus)
The design process for modern FCS is a complex, multi-disciplinary activity, which has to be transparent, correct and well documented, in order to allow certification of the aircraft. The design and validation of the flight control laws (FCLs) is an important part of the FCS design and certification, as the safety of aircraft operations is primarily dependent on them. The FCL development process has a highly iterative nature of design and analysis activities. Basically, five phases can be identified:
1. Off-line phase, using desktop design, analysis and simulation,
2. Pilot-in-the-loop tests, using manned, real-time simulation,
3. Iron-bird tests, with hardware in the loop,
4. Formal Clearance of the control laws,
5. Flight tests.
This paper focuses on the formal clearance in the fourth phase where prior to flight tests, it must be proven to the clearance authorities that the flight controller is functioning correctly.
Exhaustive analysis results from phases 1 until 3 are used to demonstrate that all certification criteria are fulfilled. As the costs of analysis runs increase exponentially each phase. Improved identification of the weak spots of the controller and worst case parameter combinations in phase 1, can lead to less required analysis in phases 2 and 3, thereby reducing the overall costs of the clearance process. Also, the chance of missing worst cases can be reduced by more effective analysis in phase 1.
Typically, for the purpose of clearance, criteria are employed that cover both linear and nonlinear stability, as well as various handling and performance requirements. For each point of the flight envelope, for all possible configurations and for all combinations of parameter variations and uncertainties1, all violations of clearance criteria and the worst-case result for each criterion, have to be found. Based on these clearance results, flight restrictions may be necessary.
The number of cases that have to be checked is huge, especially for fighter aircraft. Many different store configurations have to be investigated, involving large variations of mass, moments of inertia and centre of gravity location. It must also be proven that the controller can cope with error tolerances on air data signals that are used for control law scheduling. The aerodynamic data that is used in the mathematical models for design and analysis can only be determined within given bounds. These uncertainty bounds have to be taken into account in the clearance. The models usually use linear approximations for nonlinear effects that are not fully known, or for nonlinearities that would make the model unacceptably complex. Since flying the aircraft in the presence of failures might involve the use of alternative control laws (e.g., by switching to a backup control law after the loss of a certain sensor or a control surface failure), the additional cases that have to be investigated can be significant.
(The rest of the paper may be found at
http://www.nlr.nl/smartsite.dws?id=2821)
I think that the speed with which these events occurred precludes such a scenario. I do not think that the airplane suddenly pitched up, nor, for different reasons, do I agree with the "mach tuck" suggestion, (mainly because, the water entry was at low forward speed - unless I've missed something in the theory, we've been over this many times)
PJ2