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  • Originally posted by brianw999 View Post
    You don't need a high angle of attack to stall. You need AIRSPEED as well as a proper angle of attack to avoid a stall.
    Nope. Of course if you lack airspeed you start to drop altitude fast and you need to point the nose down to keep the angle of attack in sane range. Otherwise you end up with airspeed=0 and AoA=90 degrees (if you somehow manage to maintain close to horizontal attitude).
    But what happens in a theoretical case if the plane maintains constant altitude (e.g. test conditions)? Does it stall below the normal "stall speed"? It doesn't.
    BTW the AoA only makes sense with at least some airspeed, no airspeed = no AoA.

    Comment


    • Originally posted by kris View Post
      I just meant they shouldn't have hit water unexpectedly. Since they were in one piece prior to hitting the ocean, and presumably in some sort of level flight... possibly with wrong altitude information, dark night without any visual cues - but there is radalt.
      And because there is radalt, there is EGPWS. Apparently, they were practically falling out of the sky when they impacted the water. A lot of good it would have done them at that point.

      Comment


      • Of course it's just speculation, but perhaps they never actually stalled, just sunk. try this out:

        1) They underestimate or misread the weather system, a fatal error, and fly into extreme turbulence, lightning, what have you. Unable to divert or otherwise avoid the system at this point, they command reduced speed and commit to traversing it. No big deal for the A330.

        2) Suddenly, the AP is out, the AT is out, the ADIRU's are shutting down, the yaw damper is out, TCAS is out, a lot of crazy sh*t is happening.

        3) The last thing they expect is pitot failure at that altitude, so they work the problem not as unreliable airspeed, but as a systems malfunction and place their focus there.

        4) They are in alternate (or abnormal alternate) law. They are in hard turbulence, and perhaps electrical discharge as well. Chaos essentially. There is not even attention to spare to issue a mayday call.

        5) Therefore, while assuming that there is no imminent need to address basic flight parameters at cruise level coming off, say M.82, they neglect to realize how much speed has decayed (as they have no reference for this) nor how much sink rate is rapidly increasing. The PF is focused on manually controlling attitude in the turbulence, and they are distracted by ECAM messages.

        6) At some point, they notice the altimeter or the vertical speed. They have forgotten in the confusion that they had been commanding a lower airspeed before things got hairy (The AT would have kicked off with thrust in a retarded position), and not wanting to overspeed in the turbulence, the PF initially adds pitch, not power, to regain lift. Possibly while not accustomed to the altered sidestick sensitivity.

        7) In actuality, the airspeed is much lower than they realize, and therefore, with the high sink rate the airflow is coming at a steeper upward angle than they expect. In a matter of seconds they reach critical angle of attack at a surprisingly low pitch angle and suddenly stick shaker kicks in.

        At wits end, they firewall the thrust levers and bring the nose down, but the engines are slow to respond and the lower pitch increases the sink rate without decreasing angle of attack, due to the steeper airflow vector. They are falling out.

        9) Perhaps they begin to recover by pitching down, maintain aileron authority, wings level, but can't slow the rate of descent in time... pitch up at the last moment and impact in that attitude.

        All this occurring in degraded control law and while descending through a violent storm.

        Comment


        • Let's just say that the attitude and power that the plane had before the airspeed indicators failed were perfectly Ok to keep the airplane cruising straight and level.
          True if you were in level, constant speed and constant altitude flight at the time. If you are going through turbulence, the A/T would be working to maintain speed, and at any given time could have been well below or well above that required for flight at that altitude. Same applies for attitude - it varies up and down to maintain altitude, and as I'm sure you know, even a half a degree pitch change will have a major effect on rate of climb or descent at those altitudes.

          Comment


          • Originally posted by Evan View Post
            Well, I guess I don't think basing your theory on all the known evidence and the findings of the BEA after one of the most exhaustive investigations ever conducted "ass-uming". The weather seems to have been the initiating factor in a chain of events.

            Now guess what happens to a chain of events when you remove the first link.

            except thousands of flights per year fly into and out of weather without disappearing...

            "one of the most exhaustive investigations ever..."??? what did they investigate? a few random pieces and some cryptic maintenance messages? all friggin speculation.

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            • Air France has become a Jeopardy answer, thanks to this incident.

              Comment


              • Originally posted by Evan View Post
                1-10.
                Yeah, sure why not.

                As someone once said, we're pretty certain that a plane crashed.

                Then again, maybe maybe the FDR had a problem and didn't catch it that the engines went into reverse.

                I like my theory too.
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                Comment


                • Originally posted by Brian paraphrased
                  Stalls happen when you go too slow
                  Originally posted by Gabriel View Post
                  Now that's "total bollocks"...
                  Well, not total BS....going slow is a good way to stall a plane.

                  But....Brian needs to spend some more time on Wikipedia.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                  Comment


                  • Originally posted by 3WE View Post
                    Well, not total BS....going slow is a good way to stall a plane.
                    Originally posted by Gabriel
                    The airplane by itself won't stall if you just choke the throttles and let it go.
                    Let's get past this stupid argument. Stall is all about the relationship between the direction of airflow and the airfoil, stall being expressed as an angle (i.e. critical angle of attack). However, if speed decreases and pitch remains the same, the angle of attack also increases due to diminished lift, i.e. the downward travel of the sinking airfoil redirects the airstream to a steeper angle. Eventually the airfoil will be sinking a rate great enough to achieve critical angle of attack, even though the pitch has not changed in any way, and the wing will stall. By reducing speed without increasing pitch and thereby lift coefficient (and thus descending), you are redirecting the other side of the stall equation, the airstream.

                    Therefore, it is possible to stall simply by 'choking the throttles and letting it go'.

                    Just because stall is expressed as an angle of attack, does not mean it has more to do with pitch than with thrust. It simply has to do with the angle of the airfoil in relation to the airstream. (Gabriel has used the word 'freestream')

                    Am I right?

                    Comment


                    • Originally posted by Evan
                      Let's get past this stupid argument.
                      Just to clarify about the stupid argument.

                      You know 3WE likes to play games and he's being funny by taking quotes out of context.

                      If you go back you'll see that my "Now that's total bollocks" aimed at Brian was not related to Brian's "Stall happens when you go too slow" (as 3WE with willful evil implied), but to Brian's "You don't need a high angle of attack to stall".

                      Stall is all about the relationship between the direction of airflow and the airfoil, stall being expressed as an angle (i.e. critical angle of attack). However, if speed decreases and pitch remains the same, the angle of attack also increases due to diminished lift, i.e. the downward travel of the sinking airfoil redirects the airstream to a steeper angle. Eventually the airfoil will be sinking a rate great enough to achieve critical angle of attack, even though the pitch has not changed in any way, and the wing will stall. By reducing speed without increasing pitch and thereby lift coefficient (and thus descending), you are redirecting the other side of the stall equation, the airstream.

                      Therefore, it is possible to stall simply by 'choking the throttles and letting it go'.
                      Your explanation (long paragraph) is perfect. The logic to reach the conclusion (last line) is not only flawed, but contradictory with the explanation above.

                      You've said "if pitch remains the same", which is not what happens if you "chock the throttles and let it go".

                      Instead, the plane will enter a phugoid oscillation where, after some cycles that happen at an almost constant angle of attack, the plane will end descending at the same speed and same AoA that is was before chocking the throttles.

                      Of course you understand that, because the AoA is the same and the plane's trajectory is a descending slope, the attitude will be more nose-down than before chocking the throttles and hence the pitch will not remain the same.

                      Hence the contradiction.

                      As I've said, no, The airplane by itself won't stall if you just choke the throttles and let it go (unless you have the autopilot set to keep the altitude or vertical speed a'la Turkish, but remember in this case the AP disengaged and the control law reverted to alternate).

                      Just because stall is expressed as an angle of attack, does not mean it has more to do with pitch than with thrust. It simply has to do with the angle of the airfoil in relation to the airstream. (Gabriel has used the word 'freestream')

                      Am I right?
                      Yes, but...

                      If you put "pitch and trajectory" (that's what I've said) then you have AoA (which is pitch minus trajectory). As I've said, if the pitch is more or less horizontal and the trajectory too, you won't stall.

                      To put it in more practical terms, if you keep pitch at reasonable cruise values (say anything between 0 and 5 deg nose up) and the vertical speed within 3 digits in fpm, you won't stall. (you might overspeed, though).

                      Of course this needs the power to be set within certain limits. If the throttles are idled this is not going to happen. Either the pitch will go down and the sink rate will increase without risk of stalling (for example if you "let it go") or speed will go down and the sink rate will increase too if you keep the pitch. In either case, the sink rate will increase past 3 digits. In the second case, you risk a stall too, as you very well explained, but again "keep the pitch" is not "let it go". You'll need to apply increasing amounts of nose-up stick input to keep the pitch (remember we are in alternate law).

                      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                      --- Defend what you say with arguments, not by imposing your credentials ---

                      Comment


                      • Originally posted by Gabriel View Post
                        You've said "if pitch remains the same", which is not what happens if you "chock the throttles and let it go"....the plane will end descending at the same speed and same AoA that is was before chocking the throttles.
                        I'm no so sure I trust French Composite Crackerbox Flight Routines to behave like the real airplanes you describe above.

                        [Pedandic mode]WRONG! YOU DO NOT END UP AT THE SAME SPEED! Power adjustments usually affect trim and you wind up at a different airspeed than before[/pedant]
                        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                        Comment


                        • Originally posted by Gabriel View Post
                          Of course this needs the power to be set within certain limits. If the throttles are idled this is not going to happen. Either the pitch will go down and the sink rate will increase without risk of stalling (for example if you "let it go") or speed will go down and the sink rate will increase too if you keep the pitch. In either case, the sink rate will increase past 3 digits. In the second case, you risk a stall too, as you very well explained, but again "keep the pitch" is not "let it go". You'll need to apply increasing amounts of nose-up stick input to keep the pitch (remember we are in alternate law).
                          Ok, so I understand from you that if left unattended, the pitch will naturally decrease with diminished lift, but what I am suggesting is that the PF 447 was focused on the attitude, thus maintaining, lets say, a 3-5° pitch, while assuming the airspeed was much higher than it was. So, as the sink rate was increasing, he was holding the pitch, and therefore approaching stall. Once the stickshaker kicked in, there was no longer an option to hold pitch, and to lower the nose would only increase the sink rate. Until thrust could build up to overcome the upward angle of the airflow, there was no direction available but down. And maybe that didn't happen in time.

                          This assumes that the crew was not noticing the vertical speed or the altimeter, which is a stretch, but under the circumstances, not so hard to imagine either.

                          Comment


                          • Evan, now we have a deal: that's a possible scenario. But that's all: possible, meaning not impossible. We don't know if they stalled or if they were even close to that.

                            They could have lost situational awareness and descend in any: your description, a spiral dive, an uncontrolled and erratical descent with the plane visiting a varietty of odd attitudes, speeds, AoAs, etc.

                            They could have also stalled high and descended in a stall (happened before)

                            In any event, the way the plane hit the water (flat) might be a coincidence, a consequence of a late recovery, or of a continued flat fall (which doesn't necesarily means stalled).

                            Without the recordes I seriusly doubt that we'll ever know what heppende between the initial loss of control to the crash. I seriusly doubt we'll ever learn how they lost control in the first place. Probably a number of imaginable modes will be explored during the investigation.

                            What's pretty clear by now is that there are two thing to focus in terms of prevention of a recurrence: the recognition and management of an unreliable speed situation and the prevention of the unreliable speed situation itself. Weather avoidance can be a third. I think that discussing anything related to what happened after the loss of control is awaste of time, because we will never ever know if our speculations are anything close to the real thing.

                            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                            --- Defend what you say with arguments, not by imposing your credentials ---

                            Comment


                            • Originally posted by 3WE View Post
                              I'm no so sure I trust French Composite Crackerbox Flight Routines to behave like the real airplanes you describe above.
                              Remember: AP off and alternate law. That's a "flight routines" close to a Piper Cub.

                              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                              --- Defend what you say with arguments, not by imposing your credentials ---

                              Comment


                              • Has someone answered what happens when you have a stiff headwind and then the airspeed sensors fail?

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