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Dash 8 near crash when recovers from stall at 75ft

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  • #16
    Originally posted by Gabriel View Post
    Yes Evan, I have stopped answering because I am tired of repeating the same thing, but I will do it once more:

    Stick shaker: too much AoA
    Stall: too much AoA (even more than before).
    Recovery: Reduce AoA.

    There is no difference in the recovery procedure because:
    Both of them are too much AoA and hence both of them require an AoA reduction to recover.
    The zone between the stickshaker onset and the full stall is very unsafe and of vary bad performance. There is no gain not reducing the AoA to silence the stickshaker.
    You cannot tell the difference anyway, not until it is too late. And you cannot sense if the situation is worsening or improving until you are falling out of the sky or the stickshaker stopped.

    I could care not less if we are talking new procedure, old procedure, stickshaker, approach to stall, during take off, cruise, landing, power on, power off, accelerated, or whatever.

    The problem is too much AoA.
    Any procedure that doesn't involve reduction of AoA until all any signs of stall stop makes no sense whatsoever.
    Yes and gaining airspeed without reducing pitch at all = reduced AoA. But I'm only pointing out what I think the old t-prop procedure was (from the research I did back in the Colgan days, from distant memory). Without the underslung turbine issues and with the propwash going for you, I can see how they might have assumed only a slight reduction is pitch (forward pressure, not pushing down to give up altitude) + power might have been the safe course of action. Then. Obviously, we know now that the safest course of action is to first get the nose significantly down, then add power... unless that puts you at 75' AGL.

    And 3WE, no procedure ever called for pulling up.

    Comment


    • #17
      Originally posted by Evan View Post
      Yes and gaining airspeed without reducing pitch at all = reduced AoA. But I'm only pointing out what I think the old t-prop procedure was (from the research I did back in the Colgan days, from distant memory). Without the underslung turbine issues and with the propwash going for you, I can see how they might have assumed only a slight reduction is pitch (forward pressure, not pushing down to give up altitude) + power might have been the safe course of action. Then. Obviously, we know now that the safest course of action is to first get the nose significantly down, then add power... unless that puts you at 75' AGL.

      And 3WE, no procedure ever called for pulling up.
      I don't believe that all the procedures in all the airlines for all T-props shared the same procedure.

      I agree that increase airspeed without increasing the pitch = recuded AoA. But depending on the situation that might require a good bunch of forward pressure and forward displacement of the column.

      In fact I am perfectly ok with INCREASING pitch in the middle of the stall recovery IF that happens with a reduction of AoA that silence the stickshaker. You did well in the first sentence, but then mixed the two concepts saying "I can see how they might have assumed only a slight reduction is pitch".

      Forget pitch. Or better said, do whatever you want (or can) with pitch. As long as you stop the stickshaker. (3we would add here keep in mind that pulling up relentlessly can sometimes be counterproductive).

      The thing is to manage AoA. And while an AoA indicator would be ideal (and it's implementation is gaining a lot of momentum in the GA side, including the FAA removing the requirement for an FAA certification to speed up the development and lower the cost and price), ALL airplanes have at least one binary AoA indicator that says either "too much AoA, reduce" or "AoA ok".

      And a final word, again, don't worry about losing altitude unless ground clearance is an immediate concern, and almost never is.

      Even in this case, where the stickshaker started at 400ft, altitude was not of concern, or would not have been if the pilot would have reduced the AoA instead of just firewalling the throttles and letting the plane fully stall.

      Note that even when the FO took action, he could not think in "try to minimize altitude loss". Even when they were falling through just 250ft, the rapidly reduced the pitch 20 deg (from 10 up to 10 dn), immediately unstallling the plane and gaining enough airspeed to pull 2.7G arresting the descent with 75ft to spare (it took less than 200ft to recover from a fully developed stall when the sink rate was likely already high at the start of the recovery).

      Again, at which point exactly did they pass from being "just" in a sticksaker to fully stalled? It is impossible to know. So at what point should the pilot have "transitioned" from "keeping the pitch with forward pressure" to "pushing down"?

      So, stickshaker (or any stall warning, natural or artificial)? REDUCE AOA AT LEAST AS NEEDED TO MAKE IT STOP. If ground clearance is really really REALLY of concern, don't go beyond what minimally necessary to make it stop, and add full power at the same time.

      --- 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


      • #18
        Originally posted by Gabriel View Post
        Forget pitch. Or better said, do whatever you want (or can) with pitch. As long as you stop the stickshaker. (3we would add here keep in mind that pulling up relentlessly can sometimes be counterproductive).
        Ok, while I respect your authority as Chief Instructor as Sweet Monkey River, I think you miss the point I am trying to make. What matters at stickshaker? AoA, of course. And what means do you have to control it? Pitch and power. In a turboprop, non-swept aircraft, at stickshaker, you should be able to add full power and manage pitch (preventing the natural pitch-up response to adding power/airspeed and ideally reducing it enough to silence the alarm without sacrificing significant altitude and just fly out of the situation. In this instance it appears that the pilot allowed the pitch to increase to 10° ANU, resulting in a fully-developed stall and necessitating a steep reduction in pitch to recover. I submit to you that he was just plain lucky to have recovered at 75' but I also concur that this was the only correct action after the stall. The thing I am addressing is approach-to-stall procedure and I am only pointing out that the older procedures had merit if applied correctly but they did not consider a margin for error. It is far safer to instruct pilots to first significantly reduce pitch and then add power for that reason and also becuase t-prop pilots often transition to turbines and bring their habits with them. We're not arguing about current stall procedure Gabe.

        Comment


        • #19
          Evan, I understand that you are talking about the old procedure and that now things have changed. But what I do not agree with the concepts you are tossing around. And I will not be humble here. This is not just my opinion. This is Physics says. And I consider myself to have an excellent understanding of the Physics involved here. So let's review:

          Originally posted by Evan View Post
          Ok, while I respect your authority as Chief Instructor as Sweet Monkey River, I think you miss the point I am trying to make. What matters at stickshaker? AoA, of course. And what means do you have to control it? Pitch and power.
          There are several factors that affect AoA. But to CONTROL it you use ELEVATOR (and pitch trim, especially in airplanes with a movable horizontal stabilizer, now on whenever I say elevator I mean elevator and trim). There is a very strong relationship between elevator position and AoA. But the airplane can oscillate a bit around the trim AoA in the short-period mode of longitudinal motion (basically, a weathervane effect in pitch). This mode is of a very short period and highly damped. Other things that affect AoA are thrust and drag not passing through the CG and hence having a pitch moment, as well as flaps, slats and spoilers config (in part due to the drag they create, in part because they change the airfoil's camber and pitching moment, and in part because they change the relative AoA between the wing and the elevator, which is equivalent to moving the elevator). Thrust can have an additional effect by changing the speed and local AoA in the zone washed by the propwash and jetwash (although in most cases jets are designed so as jetwash is not a factor). AoA is also affected by changes in CG (like fuel moving back in the tank when you pitch up) and, finally, by turbulence, wind gusts, and the like.

          Finally, there is one more dynamic effect that affects AoA, which is quite strong and is normally overlooked. You will have hardly anybody explain this very important effect: Pitching motion affects AoA. Pitching up reduces the AoA and pitching down increases the AoA ("up" and "down" are relative to the airplane frame of reference here, not to the Earth´s up and down, so if you are inverted you lower the nose by pitching up). No, I am not crazy: Pitching UP REDUCES the AoA, and pitching DOWN INCREASES the AoA. Now a big warning here. We are talking about the EFFECT the pitching MOTION, not about what CAUSED this pitching motion in the first place. For example, if you are flying straight and level at a good speed (i.e. somehow low AoA) and pull up (without stalling) the AoA will increase and the plane will pitch up, but the AoA will not increase as much as if you slowly reduced speed and pulled up to keep the pitch constant (reaching the same elevator position than in the first "pull up" case). This is the reason why, during the phugoid (long-mode longitudinal motion) the AoA is not exactly constant, but it oscillates around the trim AoA (normally just a little when the amplitude of the motion is not very big), lower than trim AoA in the half period of pitching-up motion and higher than trim AoA in the half period of pitching down motion. This also the never explained part of the reason why you have to pull back in turns to prevent the nose from going down, since a turn involves pitching motion as can be easily visualized in a steep-bank turn (the other part, the usually explained one, is that you need more lift in a turn, hence you need more AoA, hence you need to pull back). The reason for this effect is that, being the elevator at a distance back of the CG, the pitching motion generates a vertical motion of the stabilizer (vertical in the airplane´s frame) that affects the AoA. For example, when the airplane pitches up the stabilizer is going down. This downward motion affects the local AoA of the stabilizer: it combines with the forward motion of the airplane generating that the air hits the stabilizer a bit more from below "pushing it up" (i.e. reducing the AoA). This effect is the very reason why "recover an approach to stall with power alone (hands off the yoke)" can work in the short term: You increase power, the speed increases, the plane starts to pitch up (this is the first half of the phugoid), the pitch-up motion reduces the AoA, and that alone may be enough to stop the stickshaker (again, hands off the yoke). However, this effect will disappear as soon as the plane stops pitching up, and will worsen (increasing the AoA past the original value) when the plane starts pitches down in the second half of the phugoid.

          Now, normally, the elevator (+trim) has enough power to compensate for ALL these effects and still keep enough authority to put the AoA where you want it. I don't know of a single case where elevator (including trim) would not have been enough to manage the AoA, but theoretically, because the aerodynamic forces go with the square of speed and the thrust is either nearly unaffected by speed (jets) or increases the slower you go (props), if going extremely slow (much slower than the "official" stall speed) the pitching moment of the the thrust may overpower the elevator, so it may be necessary to reduce power to control the AoA.

          So, with this theoretical exception for which I have never seen a case, we must say that ELEVATOR CONTROLS AOA (again, including trim). Not pitch and power (which can affect AoA, but are not a direct means to control it).

          In a turboprop, non-swept aircraft, at stickshaker, you should be able to add full power and manage pitch (preventing the natural pitch-up response to adding power/airspeed and ideally reducing it enough to silence the alarm without sacrificing significant altitude and just fly out of the situation.
          Also in a piston non-swept or in a jet swept. If spool time is what you are thinking about, some turboprops can have significant spool-up times too, since big props have a lot of inertia.

          For the rest of that, I tend to agree, but I strongly prefer to say to manage AoA than to manage pitch, which is not the same. For example, when you say "ideally reducing it enough to silence the alarm", what is it? You didn't mention AoA in this sentence, and the only thing that you can reduce to silence the alarm is AoA, not pitch.

          In this instance it appears that the pilot allowed the pitch to increase to 10° ANU, resulting in a fully-developed stall and necessitating a steep reduction in pitch to recover.
          OK THIS IS THE CRUX OF IT. IF YOU ARE GOING TO READ ONLY ONE PART OF MY POST; READ THIS.

          The problem is not that the pilot allowed the pitch to increase to 10° ANU. I can 100% assure you that, with 120% of torque applied, the pilot can have safely allowed the pitch to increase WELL PAST 10° by REDUCING THE AOA (i.e. pushing down, pushing forward, relieving back pressure or doing what it was needed with the elevator to silence the stickshaker!!!!!)

          I submit to you that he was just plain lucky to have recovered at 75' but I also concur that this was the only correct action after the stall.
          I submit to you that the FO overreacted and he can have recovered the plane much sooner. It doesn't look that the AoA, while fully stalled, was so much past the stickshaker limit like in AF or Colgan, or the plane would have lost control. Lowering the nose 20° was an exaggeration, and then he still let it accelerate more than needed as proved by the fact that he was able to pull 2.7G (it takes 60% more than the 1G stall speed to do so).

          AND STILL, that was much better than not lowering the nose enough. With all that, since he recovered in 175 ft, 75 ft is a considerable margin: he had 43% more altitude available than what he used to recover.

          Try to visualize this: He recovered a fully stalled and already sinking plane in just 175ft, and that overreacting and could have been done in less altitude. Now, imagine how much altitude you need to swiftly recover from a stickshaker as soon as it starts by reducing the AoA, when the plane is not stalled yet, and not sinking yet. You only need to reduce the AoA 1 or 2°. Even if you don't increase the thrust the loss of altitude will be negligible.

          The thing I am addressing is approach-to-stall procedure and I am only pointing out that the older procedures had merit if applied correctly but they did not consider a margin for error.
          The big problem with the old approach to stall procedure is that, in a condition where the problem is too much AoA and the recovery requires necessarily a reduction in AoA, AoA was not even mentioned as a footnote.

          It is far safer to instruct pilots to first significantly reduce pitch and then add power for that reason and also because t-prop pilots often transition to turbines and bring their habits with them.
          NOOOOO!!!!!! It's safer to instruct pilots to reduce AoA, AoA, AoA!!!!!! Saying pitch causes confusion (as you are a living example).

          And it doesn't need to be a significant reduction. The stickshaker can be on because the AoA is just a fraction of a single degree past its threshold. Reduce the AoA at least as needed to silence the stickshaker, 1° can be enough, apply full power and up we go (and with up I mean pitch -but not AoA-, altitude, vertical speed, flight path and airspeed).

          Now, if ground clearance is not of super-immediate concern, don't play with fire and be more liberal with the AoA reduction (as this pilot did when he was falling fully stalled through 250ft, and although I would advise to be more careful with the AoA reduction in a case like this, it worked).

          --- 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


          • #20
            Phew. Ok, I guess I asked for it. I have to set aside some time to read this.

            In the meantime, let me just say....
            Originally posted by Gabriel View Post
            NOOOOO!!!!!! It's safer to instruct pilots to reduce AoA, AoA, AoA!!!!!! Saying pitch causes confusion (as you are a living example).
            It's safer in a procedure to instruct pilots on what control inputs to make, saying AoA seems to cause confusion. The first action on current procedures in to push the yoke to lower pitch, right? I can't envision a scenario in which lowering pitch does not lower AoA, can you?

            (I promise I'll read the rest of it later)

            Comment


            • #21
              Originally posted by Evan View Post
              Phew. Ok, I guess I asked for it. I have to set aside some time to read this.

              In the meantime, let me just say....

              It's safer in a procedure to instruct pilots on what control inputs to make, saying AoA seems to cause confusion.
              Yes. Elevator is the main AoA control (as, when you read my "brief" post, you'll see I've said). However, pilots need to understand (theoretically and practically) other things that affect AoA, so they know how to use the elevator to control AoA when other factors come into play.

              The first action on current procedures in to push the yoke to lower pitch, right?
              I am not sure what the current procedures say exactly, but "push" isn't the right word. Perhaps "move forward" (what can be not push but let go a bit on the back pressure).

              PITCH IS NOT A CONTROL. So "reduce pitch" or "hold pitch" or things like that are not an improvement vs "reduce AoA".

              A side comment is that while the current procedure is a huge improvement over the old one, it is not necessarily what my version of the procedure would say.

              I can't envision a scenario in which lowering pitch does not lower AoA, can you?
              Yes, I absolutely can. It just takes the flight path angle go down faster than the pitch does, which is not that uncommon around the stall.

              But most important, I can envision a scenario where reducing the AoA does not lower the pitch. You too, you mentioned one. Pitch can perfectly increase while the AoA is being reduced.

              --- 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


              • #22
                Points are good Gabe but you start stepping on your schlong trying to be too "technically correct" ... "pushing the stick" .... has worked for an endless number of folks and gets them to do what (for the most part), they need to do.
                Live, from a grassy knoll somewhere near you.

                Comment


                • #23
                  Originally posted by guamainiac View Post
                  Points are good Gabe but you start stepping on your schlong trying to be too "technically correct" ... "pushing the stick" .... has worked for an endless number of folks and gets them to do what (for the most part), they need to do.
                  And I agree. I made the distinction because there are persons that think that "push the stick" (in the sense you used it) means aiming down and sacrificing altitude that could be otherwise saved.

                  So it is important to know that in the pilot´s argot "push down" doesn't mean necessarily "apply a forward force on the stick" or "make the nose go down".

                  Depending on several factors, "push down" may involve anything between "just relieve a little bit of back pressure" to "shove the stick fully forward to the stop and then add a good bunch nose-down trim".

                  --- 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


                  • #24
                    ...all this to address a fairly straight forward basic fundamental cowboy concept I learned in my first flying lesson?
                    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                    Comment


                    • #25
                      Originally posted by 3WE View Post
                      ...all this to address a fairly straight forward basic fundamental cowboy concept I learned in my first flying lesson?
                      Exactly.

                      That said, while you might have learned it very well, GA pilots at great are not doing it precisely exemplary well at this.

                      The accident rate is comparable to motorcycle riding, the main type of fatal accidents is loss of control, and the main subtype within loss of control is stalls.

                      That's why the FAA made an exception to the rules and authorized the development, manufacturing, sell, installation and use of non-FAA-certified AoA indicators for general aviation.

                      --- 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


                      • #26
                        Originally posted by guamainiac View Post
                        Points are good Gabe but you start stepping on your schlong trying to be too "technically correct" ... "pushing the stick" .... has worked for an endless number of folks and gets them to do what (for the most part), they need to do.
                        [Hangar talk, redundant, beer-drinking mode]

                        Indeed, but a long time ago in a thread far far away (before several recent stall-involved airliner crashes), I was semi-flamed with the accusation that I was out of line for suggesting steep dives as the proper reaction for a stall warning.

                        Gabe tends to be verbose and hit all the details, but I too started adding all sorts of caveats...if you hear a stall warning, you might (emphasis on might) want to think (albeit briefly) about an ever so slight nose-down input...maybe just ease up on the back pressure...slightly...just think about it...briefly...maybe...only if you seem to be losing altitude going slow with the nose up and a stall warning...just a tiny bit less nose up...

                        [/redundancy]...and indeed I tripped over my schlong...
                        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                        Comment


                        • #27
                          Originally posted by 3WE View Post
                          Gabe tends to be verbose and hit all the details
                          Thank you?

                          --- 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


                          • #28
                            Now gents, I wonder just how many details a fellow can process between 75' and the ground?
                            Live, from a grassy knoll somewhere near you.

                            Comment


                            • #29
                              Originally posted by guamainiac View Post
                              Now gents, I wonder just how many details a fellow can process between 75' and the ground?
                              Not many. That´s why it is important to process them BEFORE you find yourself at 75' (for example, when you are still on the ground), and then again after you recovered so you understand what happened and why.

                              As an AA instructor said: Don't ask "what is it doing now". Click click, clack clack, and after you recovered ask "why was it doing that".

                              --- 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


                              • #30
                                ^^^
                                Aaaah, back to "Children of the Magenta" ? I've been looking for one of his videos that might help here but couldn't find the best one for this situation.
                                If it 'ain't broken........ Don't try to mend it !

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