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Evan
Evan
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Last Activity: Yesterday, 21:16
Joined: 2008-01-19
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  • Updated report out on Southwest WN-1380

    CNN has reported that "Investigators of a fatal accident on a Southwest Airlines plane last year recommend that Boeing retrofit the [engine cowlings] of nearly 7,000 jets to prevent a repeat of the accident."

    The NTSB report concludes this:


    I'm not sure that a practical redesign capable of withstanding such forces is possible, but if it is, there is no associated AD requiring Boeing to redesign it....
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  • Evan
    replied to New web format
    One thing: there is no longer any link to the main site from the forums pages. The logo really should have the link to take you there.
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  • Lexicon:

    Human factors: Accidents attributed to the limits of human performance, involving such things as fatigue, panic, disorientation, mental bias, stress, etc.

    Stoogery: Stupid, negligent, intentional behavior, such as blasting high-pressure jets of water at sensitive air-data probes, using any old lube lying around instead of referring to the manuals, repeatedly punching faulty ELAC's off and on while continuing to fly multiple cycles, pulling multiple FCC circuit breakers in flight because you saw someone do it on the ground and just generally being a person who should never be allowed near a commercial airliner.

    Yes, there's some grey area there, but really, you know what I'm talking about. In 30 years of service, how many Airbus or Boeing FBW aircraft have experienced an accident caused by a flight control system failure that didn't require remarkable levels of unforeseeable stoogery?

    The point is that FBW is waaaaaaaaaay safer...
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  • Rarely but yes. However, these failure modes require stoogery that wasn't forseen when those systems were created. So where where does the real problem lie?



    That is quite the understatement. Can you list the accidents created by the new technology and not the blatant abuse of the new technology?...
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  • Yes, and how many people have died as a result of that? Look, this argument has been over for along time. We're not going back to cables and bellcranks. It's computers baby. The trick now is to do it right. It will require systems that have the highest possible confidence in their situational 'awareness'. That REQUIRES at least triple redundancy.



    The direct mode is a deflection of control surfaces proportional to stick input, just like a control column. That is essentially eliminating the computer 'interpretations'. But not the safeguards against things like runaways in a system with no physical feedback to otherwise alert you.

    The bottom line to this thread is that, if you want to break the airplane, you are going to break the airplane. Idiot proof is probably further off than AI. Airbus did a bang-up job of finding the discretionary middle ground. But if you want to survive airline stoogery on this level, you have to be discerning about who you...
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  • I am disputing that 'principal'. Risk only increases when redundant systems are not very carefully engineered, whereas risk decreases with redundancy, so when the trade-off for increased risk due to complexity vs. decreased risk due to redundancy is overwhelmingly in favor of complexity and redundancy, as it was here, then we are doing the right thing.



    I'm not implying that. I'm implying what I just wrote above. Carelessly designed redundancy, such as MCAS, is a separate issue and will always be detrimental.



    I don't think you understand the sequence of events that led to this incident. First of all, it could never have occurred without THE COMBINATION of shoddy, reckless maintenance and careless piloting following a non-standard company procedure. It required the wrong viscosity oil in the OVM, pilots repeatedly resetting ELAC faults over a five-hour period involving multiple touch-and-goes, an ELAC left inoperable (not reset) between...
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  • Two things. Firstly, I said this was phenomenal because of the very rare and complex combination of failures and pilot errors that made it possible. Secondly, that combination is not going to present itself on a revenue flight.

    In order for this to occur, the SEC's must be in control of pitch (both ELAC faulted) on the ground AND the LGCIU's have to be sending opposing air/ground signals to them (the left and right MLG have to be in a very different state of compression for at least one second.)

    In other words, this requires the aircraft to begin the take-off roll with zero ELAC's functioning and then experience an asymmetrical "bounce" large enough to make that possible BEFORE rotation.

    How could that ever happen in a revenue flight?

    1) You could dispatch a flight with a single functional ELAC. Then during the takeoff roll, the remaining ELAC would have to suddenly fault (why?) AND then you need to lift off, drop back hard...
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  • The COM signals on both elevators would have been the same. The issue is the discrepency between the COM and MON lanes on each SEC, not between the left and right elevator commands. So I don't see anything that would create a split elevator condition.

    Tha attachment shows this more clearly. See the COM and MON lanes at the bottom....
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  • I warned you this was complicated. The SEC's 'gave up' on elevators because the aircraft experienced a L+R ELEVATOR FAULT. That results in pitch reversion to mechanical back-up (for obvious reasons). It went into this fault mode because the system assumed there had been a 'spool valve runaway':



    Both the ELAC's and the SEC's have three servo loops in pitch: L elevator, R elevator and THS. If any of those three loops becomes invalid, the ELAC's go into pitch fault mode. However the SEC's can remain engaged in pitch with only one of those servo loops valid, so, if a THS loop becomes invalid, the elevators loops can still be valid and the elevators will remain in direct law. However, in this rare scenario, both SEC's detected an invalid condition with the ELEVATORS, not just an invalid THS position. The system reached this assumption because the opposing air/ground signals from the LGCIU's resulted in the COM and MON lanes for the ELEVATOR position on both SEC's...
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  • Does it? Do you think this waaaaaaaaay out there scenario is too easy? Do you realize that this incident required not only a component botched by maintenance but also a touch and go sequence with uneven gear contact and a manually arrested trim wheel (why would you ever do that on a revenue flight?) and a single ELAC operation? So, in terms of revenue service, we are talking about what... a MEL'd ELAC, a very late go-around with bounced runway contact on one MLG followed by manual pitch trim input and a sabotaged OVM unit. Do you really think that's giving up too easily? The very cautious minds that went into designing this didn't miss much although it sometimes seems that way to avforum parlour talkers. I think they could explain their reasoning, but on the face of it, it seems that the philosophy is to remove flight control from a component that has detected a flight control fault. WIth four levels of redundancy, that's not a problem. So much went wrong here that it broke the airplan...
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  • A shorter answer is that, in the FBW loop, in any control law, it must be able to verify that the commanded position results in the expected position. The chain-of-events described above resulted in the COM and MON lanes of the SEC's diverging beyond a safe threshold, signaling an apparent fault and causing the system to lock the elevators at neutral and hand over pitch to the mechanical backup (THS)....
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  • Obviously, MEL was not considered between these touch-and-go flights. Nor was there any instruction to do so:





    Warning: This is a VERY complex scenario and will be Gabriellian in length. It may contain acronyms. As simple as I can state it: ELAC 1, which normally takes over on the ground, was not reset from a fault on a previous cycle, thus ELAC 2, (which is normally in control in the air) remained in control on the ground. When the pilot manually stopped the trim wheel, with the OVM unit malfunctioning (explanation to come), the system detected a discrepancy between the computer-commanded position of the THS (COM) and the actual position (MON) and faulted ELAC 2. At this point SEC2 took over pitch control. However, due to a VERY complex relationship between the SEC trim runaway detection logic and the LGCIU's, which are receiving data from the compression sensors on the MLG, and the bounced landing due to the failure to arm ground-spoilers (as directe...
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  • On the NG:...
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  • Depends on the aircraft of course, but on the Helios 737-300, the initial warning was only a horn (and the overhead gauge and the green lighted overhead indication that the outflow valve was open in manual mode). After the findings of the crash, warning lights were added to distinguish a cabin altitude warning from a takeoff configuration warning.

    However, at about 17,000ft, before hypoxia set in and when the passenger masks automatically deploy, there was a master caution light and an 'overhead' light on the glareshield. The 'overhead' light directs pilots to find the source of the problem on the overhead panel, where the green manual indication should have been noticed (as it is never otherwise seen in flight).

    The human factors involved (including a little gem known as 'expectation bias") overwhelmed these alerts. Better ones are needed, but for cabin altitude, not blood oygen levels....
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  • Well, to begin with, there is this requirement:



    Kinda doubt that was done......
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  • The manuals have apparently been updated to point out the very obvious limit on resetting FCC's, and hopefully pointing out that the procedure is NOT intended to keep the a/c in service indefinitely. Also, the incident would not have happened if the ground maintenance hadn't used whatever random bunker oil they had lying about on a precision component of a passenger airliner. I doubt there will ever be an aircraft that can stand up to improvisation like that. So, fouled component, multiple fair warnings ignored and an ELAC left faulted between cycles, this is one rotten stack of swiss cheese that hopefully future crew will smell before it ever goes this far.

    I have to dig up the MMEL and see what the conditions are for single ELAC dispatch....
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  • I'm 100% for synthetic voice alerts over ding dongs (On the 737CL it's a "horn" actually, you know the one). Helios was a 737, so not only primitive aural alerts, also no ECAM. But the central concern in your story there is pilots who clearly don't understand the rapid onset nature of hypoxia. Masks firmly fitted as the very first action should be drilled into every pilots head the way it is done to us passengers. Perhaps the FA should do the same routine in the cockpit before every flight and also show them where the whistle is on their life vests....
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  • 3WE, I think your account has been hacked by someone who thinks systems are better than pilots. Helios 522 did trigger the cabin altitude warning, but the crew dismissed it as a nuisance warning, the same crew that failed to pre-flight check the pressurization switch prior to takeoff.

    So, since no alarm would work there, what do we do, ban all pilots? A system that prevents the aircraft from continuing a climb without pressurization or above a given cabin altitude would be alright by me, as long as it has triple modular redundancy......
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  • How would an O2 alarm be any different? O2 concentration is a known factor of cabin altitude. Regardless of which measurement triggers it, either the alert works or it doesn't....
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  • We have cabin altitude warnings, which do the same thing....
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