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  • #31
    Originally posted by 3WE View Post
    I've read lots of other theoretical stuff (books and Gabe both), but what else do they need to know to make a difference in not stalling an airliner?
    That a fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed.

    As we know, most airliners do not display angle of attack to the pilot and pilots are simply trained to react to stall warnings via procedure with pitch and power. Mostly, if they follow procedure, this works. But in situations such as the one we are discussing, if they lack a strong Gabrielian understanding of AoA vs CL and aerodynamics (fluid dynamics), it often ends with a wrong instinct reaction based on a (subconscious?) association with pulling back and climbing. If you can teach the wrong instinct you should be able to teach the right one, that's my thinking. Since 99.9999% of the time pulling back will result in climbing out, it would take an intensive fundamental education effort to assure that pilots understand that this never works during stall warning. If aspiring pilots already had the physics down, it might be easier for them to understand this intuitively. It would become 'common-sense'. It should be required.

    I took photography classes in high-school. We spent the first semester just learning the physics of optics and chemistry (i.e. the elemental chemical reactions involved). I just wanted to take pictures and thought this was just an academic waste of time. Later, when it came time to make exposures, the correct settings came from an understanding of these things. That is how pilot schooling should be. Basic science, then applied skills.

    I get the impression that this pilot, despite the jackass stunt, may have had this critical understanding.

    Comment


    • #32
      Originally posted by Evan View Post
      That a fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed.
      Kind of captured in #1 above?
      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

      Comment


      • #33
        After receiving 72 PMs asking me what happened with the long reply that I had threatened you with, here it goes:

        Evan:
        I think IF TREE PROXIMITY IS OF GRAVE CONCERN: Maintain but do not increase AoA and be prepared to lower it as needed to extinguish the onset of an actual stall. Stall warning is entry into the dangerous AoA regime but beneath you is the dangerous regime of trees. If you are still flying (and not decelerating) at that point, planting it there might be your only way out.

        Gabe:
        Forget about it. If the onset of the stall warning doesn't work, nothing will.
        You'll see, the lift vs AoA curve starts to flatten-out as it approaches the critical AoA. There is very little lift to gain between the AoA of stall warning and the critical AoA.

        On the other hand, drag increases a lot between the stall warning AoA and the critical AoA. Reducing the AoA from the critical one to the stall warning one will minimally reduce lift for a second or two, but the reduction of drag will quickly come back as increased climb performance.

        On the other hand (four and counting), the same flattening of the lift curve greatly reduces aileron authority, greatly reduces roll damping, and greatly increases adverse yaw. The combination of the above highly degrades the airplane's lateral-directional handling qualities.

        Fourth and last hand, how do you tell the onset of the "real stall" to begin with?
        The stall warning will sound from the stall warning AoA to any imaginable AoA well past the citical AoA.
        If the airplane has such a great mood that it will stall straight ahead, you will probably not notice the onset (a point barely past the critical AoA has the same lift than a point barely before, with the maximum lift point being just barely higher lift between these two points).
        If the plane is more typical, the nose or the wing will drop and at that point, if you were skimming the trees, lowering the AoA will not help you much.

        Note, additionally, that saying "fly a bit above the stall warning but below the stall AoA" is not attainable simply because, once the stall warning is sounding, you just don't know how far past its onset you are.

        So the realistic choices are:
        1- Keep the plane at the onset of the stall warning and either fly away or crash with that (like the famous AF A320 flying into the trees but not stalling), or
        2- Try to keep the AoA past the stall warning but short of the actual stall, have a wing drop (because loss of lateral control or actual stall), and crash at a high bank angle and nose down attitude (like in the video where you proposed banning turns after take-off, remember?)

        I choose 1. Again, in a real-life imminent-death situation, easy to say but perhaps not so easy to do.
        As I said in a previous post, wile I feel very confident that I would not stall in an AF or Colgan scenario (or even turning to final), situations where you can push down with confidence, I have to be honest and say I don't know how well I would perform in a situation where even perfect performance can kill me, but pulling one inch too much or too little is more likely to do so.

        Evan:
        I think this is also due to the Gabrielian theory that stall is taught only as an upset procedure without a strong understanding of the aerodynamics involved. I theorize that a pilot can be taught intuitively not to stall if he intuitively equates pulling up at stall warning with going down like a rock. I do not think many flight schools consider it necessary to foster this basic intuition and I think it should be a requirement for even a PPL.

        That was not exactly my theory. My theory is first that the stall warning is something a pilot hardy ever hear out of training (I'd argue that fresh PPLs are the best pilots at stalls because you practices it 100 times in the last months). But that you did that in the PPL course, intentionally, and at 2000ft, doesn't preapre you for the unexpected unintentional stall at 200ft. I think that pilots should practice flight and maneuvering at the onset of the stall warning. This practice is of no operative value by itself (we pilot's don't fly at the onset of the stall warning), but gives the pilot a feeling of how the airplane behaves when it's close to the stall and the practice to modulate the elevator to keep that stall warning "intermittent", which can be useful if you ever need to do a max performance escape maneuver, but also is like practicing 100 "micro-approaches-to-stall" within a few minutes, where you immediately lower the AoA a bit every time the stall warning sounds.
        I agree that a really good understanding of what's going on near and into a stall helps reinforce the practice with the understanding of what's going on, then the practice makes sense and it's easier for the brain to buy the idea.

        I have the luck of having a very very deep understanding of the stall (a result more of my studies of Aerodynamics in the university than the flight training), as well as the mechanics of fight in general, to the point that the concepts involved become very natural and many times I don't need to think a lot to understand the situation.

        There was a case where I was flying with an instructor (I was already a PPL and I had also finished my Aeronautical Engineering degree). I took off with a quite strong headwind, and after climbing to 500ft and making a left turn the instructor cut the throttle to simulate an engine failure. I established best glide speed and after assessing my options for a few seconds I decided to return to the runway for a tailwind landing. Normally that would not have been my option (and. in hindsight, neither should it have been this time), but the strong headwind had left us very close to the runway. So I did a sharp 180 left (first clue of AoA management, I didn't keep the best glide speed because I didn't pull up to hold the nose or airspeed during the turn, instead, I left the elevator -and hence the AoA- more or less where it was for best glide and let the nose go down and speed go up during the turn, so while a sharp turn at low attitude seems like a big risk of stall, that's when the pilots pull up like we usually do in usual turns, but I knew that my AoA was safe, plus, I was already mentally ready to relentlessly lower the nose at the first buffet or beep of the stall warning). In the base leg it was clear that we were very high, so I put full flaps, turned to final and did a full-rudder slideslip. Yet, the strong tailwind pushed us way down the not-so-long and grassy runway and we were past half of it when we touched down at a much-faster-than-usual groundspeed. So the idea to return to the runway had been a bad idea, or at least turning the short way was (perhaps my first turn should have been a right-hand 180, away from the runway). Immediately after touchdown I started to apply brakes, second mistake, and the instructor with very good criteria called "go around", which I complied with (although I believe that we would have been able to stop within the remaining runway, "believe" is not enough, and going around was undoubtedly the safest mode). Shortly past the end of the runway there was a line of pines crossing our flight path, and we were rushing towards it with the increases groundspeed and reduced climb gradient due to the strong tailwind. I established max climb speed of 70 kts and it became immediately apparent that we would climb past the trees with no problems. However, to the dismay of my instructor, when we reached the trees height I leveled the plane, let the plane accelerate to 80 kts, and started to climb again at that speed. The pause of the climb to accelerate and the new "sub optimal" climb speed meant that we were still going to safely avoid the trees, but with quite less altitude than if I had kept 70 kts.

        "Gabriel, what on Earth are you doing!!!???" the instructor yelled, without intervening with the controls (while my maneuver might have seem odd, it was clear that we were already higher than the trees). I knew very well what I was doing, even when I had barely thought of it (it came naturally), and I needed a few seconds to rationalize an answer. In the meantime, we flew above the trees and were hit by a strong updraft that made the stall warning beep literally for a split second (I didn't even had time to react lowering the nose).

        "We were flying towards the tree line with a strong tailwind. I expected that the wind would hit the trees and kick us from below. So I preferred to cross the trees with a bit less of altitude and more airspeed, and hence lower AoA. Even with 80 knots the stall warning activated, I am happy that we didn't have 70".

        The instructor ruminated my answer for a few seconds and just said "Okay", followed by like 15 minutes of silence.

        elaw:
        My understanding is that in the past, say 20-30 years ago, in primary flight training pilots were taught to *never* stall the aircraft and that was that. If you stalled you were a horrible awful person and deserved to die.

        Gabe:
        Not so. Approach to stall and actual stall training has always been part of the PPL training. Power on, power off, straight and turning. Even more, maybe 40 years ago even spin recovery was part of the PPL training (now you just study the procedure but you don't practice it).

        elaw:
        Re teaching the aerodynamics of the stall, I think a *basic* understanding is helpful but in-depth knowledge of Bernoulli's and Newton's laws and how lift is generated is not necessarily of much value. IMHO what a pilot needs to know is that above a certain AoA the wing stops functioning as a wing, and that to prevent a stall you should never exceed that AoA, and to recover from a stall you must reduce the AoA below the critical level.

        Gabe:
        I stronlgy disagree. Not that a pilot should not know what you said, but they should also have a very good understanding of the why that happens, why this procedure works, why this one doesn't. And that means having a very good understanding of aerodynamics and the laws of Newton.

        Now, I don't expect a pilot to be able to derive the Navier-Stokes equations or be able to calculate the distribution of flight loads, but to have a very good conceptual understanding of the laws and mechanics involved, of the different variables, and the relationship between them.

        Most pilots think that the danger of a stall resides in the loss of speed. Yes, it does, but no so much as in the loss of lift damp and roll damp. These are things normally not known or understood by pilots. Read what I said regarding the degradation of the lateral handling qualities. That is way more dangerous than the loss of lift. Or read my real story of the close-encounter with the pines. Do you think that a pilot only knowing the "instant pudding" procedures will have the clarity of mind to naturally, without even thinking of it, came out with the reasoning process I did?

        I said it before and insist, I think that a good understanding of what's really going on helps the brain naturally "buy" that hing that you are otherwise taught and assimilate as a matter of faith towards your teacher.

        Evan:
        Maybe a good fundamental lesson is to tell pilots to think of the aircraft as a helium balloon and the yoke as a valve lever. When you pull back, you open the valve and inflate the balloon. When you push forward you close it and when you push a bit more you deflate it. Obviously, more gas in the balloon will produce more lift but too much will burst it. Stall warning is the 'balloon about to pop' warning. Sinking your (fast moving) balloon into terrain is not something you want to do, but popping it should be your greatest fear.

        Gabe:
        Are you serious? Why not better spending this precious training time explaining what really happens in a stall?

        3WE:
        1) At about 16 degrees AOA, a stall occurs on most all wings because the smooth airflow separates from the top of the wing from the sharp curve.
        2) This means a significant loss in lift and oftentimes a loss of roll control.
        3) Pulling up relentlessly is bad,
        4) a. airspeed control is a big help (b. but not everything),
        5) stalls should be avoided and 6) in extreme cases, you might need some nose-down input...

        I've read lots of other theoretical stuff (books and Gabe both), but what else do they need to know to make a difference in not stalling an airliner?

        Gabe:
        Why in a stall or near stall situation pushing DOWN can help you increase the climb speed, the airspeed, the climb gradient and even the pitch, all at the same time.
        Effects of very high AoA on lateral-directional handling qualities.
        Effects of the unavoidable large spoiler deflections in that scenario.
        Different kinds of trim designs, how each of them can cause a stall and help in the recovery.
        Effect of power (thrust).
        Use of roll and rudder as last resource.
        Don't be afraid to lower the nose if ground contact is not of immediate concern.
        How to fly a max performance escape maneuver when ground contact is of immediate concern.
        Effect of the stall on the flight controls (elevator, rudder)
        Variables that affect the AoA (main = elevator + trim, others, flaps, slats, pitch rate, turn rate, thrust, Mach number, speed offset vs trim speed, ground effect).

        Do I continue?

        --- 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


        • #34
          Originally posted by Gabriel View Post
          So the realistic choices are:
          1- Keep the plane at the onset of the stall warning and either fly away or crash with that (like the famous AF A320 flying into the trees but not stalling), or
          2- Try to keep the AoA past the stall warning but short of the actual stall, have a wing drop (because loss of lateral control or actual stall), and crash at a high bank angle and nose down attitude (like in the video where you proposed banning turns after take-off, remember?)
          Hold up... Your original post read:
          IF GROUND PROXIMITY IS OF IMMEDIATE CONCERN:
          Well, these hair splitting details become important.
          Reduce AoA AT LEAST AND AT MOST as needed to extinguish all the signs of stall.
          I countered:
          Maintain but do not increase AoA and be prepared to lower it as needed to extinguish the onset of an actual stall.
          Again, we are talking about flying through trees, where there is NO altitude to give up.

          Now you've modified your answer to:
          1- Keep the plane at the onset of the stall warning and either fly away or crash with that (like the famous AF A320 flying into the trees but not stalling)
          Which is exactly what I am saying above. I didn't say increase AoA to the onset of stall. I just say that you must be prepared to lower pitch if a bit of windshear or drop in engine performance alters the currently flyable AoA and controllability is threatened. (I also didn't say this sort of 'razors-edge' flying would be easy.)

          If you are accelerating, however slowly, and not stalled at stall warning, and literally skimming trees, you can only reduce AoA by gaining airspeed. I assume when you wrote 'reduce AoA' you meant reduce pitch. This is why I questioned you.

          Evan:
          Maybe a good fundamental lesson is to tell pilots to think of the aircraft as a helium balloon...
          Gabe:
          Are you serious? Why not better spending this precious training time explaining what really happens in a stall?
          Obviously I would teach the aerodynamics first, but this second lesson would serve to reinforce a mental image, helping to establish an intuitive response.

          I enjoyed your story though. A bit of Gimli Glider forward slip technique there at the end. You could have been a smart-ass when the instructor told you to go around and ask how the hell you are going to do that since you've lost the engine (assuming you felt you could get it stopped in time).

          Comment


          • #35
            Originally posted by Gabriel View Post
            3WE:
            1) At about 16 degrees AOA, a stall occurs on most all wings because the smooth airflow separates from the top of the wing from the sharp curve.
            2) This means a significant loss in lift and oftentimes a loss of roll control.
            3) Pulling up relentlessly is bad,
            4) a. airspeed control is a big help (b. but not everything),
            5) stalls should be avoided and 6) in extreme cases, you might need some nose-down input...

            I've read lots of other theoretical stuff (books and Gabe both), but what else do they need to know to make a difference in not stalling an airliner?

            Gabe:
            Why in a stall or near stall situation pushing DOWN can help you increase the climb speed, the airspeed, the climb gradient and even the pitch, all at the same time.
            Effects of very high AoA on lateral-directional handling qualities.
            Effects of the unavoidable large spoiler deflections in that scenario.
            Different kinds of trim designs, how each of them can cause a stall and help in the recovery.
            Effect of power (thrust).
            Use of roll and rudder as last resource.
            Don't be afraid to lower the nose if ground contact is not of immediate concern.
            How to fly a max performance escape maneuver when ground contact is of immediate concern.
            Effect of the stall on the flight controls (elevator, rudder)
            Variables that affect the AoA (main = elevator + trim, others, flaps, slats, pitch rate, turn rate, thrust, Mach number, speed offset vs trim speed, ground effect).

            Do I continue?
            No objections to your comments- BUT you missed the context there.

            Evan says that pilots need to take Aerodynamics 301 and 401, and maybe even the 513 topics discussion class so that they will know not to stall planes.

            As much as I like background and genuine understanding of "how things work", I'm thinking that the stall stuff can be summarized pretty succinctly including the basics of how things work.

            Given that I whip out a list of 5 things (after 5 min of thought) and that after WEEKS, you feel that the short summary should actually be 15 things, that's what I'd call totally normal for the way you and I operate.

            So, I ask you again- do pilots really need full-bore aerodynamics? or

            do they just need a little more of the basic reminder that "A plane can be stalled at any attitude or airspeed (though going slow and pulling up relentlessly is a really good way to stall) (and our 14 other items)...

            ...and a little less of the memory checklist: "1. Power levers TOGA", 2) Attitude: 10 degrees ANU 3) Adjust attitude to maintain Vx, 4) Climb 300 ft straight ahead, 4) Execute missed approach, 5) Establish proper altitude..."

            ...What do you think?

            IMPORTANT EDIT: Ok, you answered the question to Evan/Eric which reverts all of this back to 3WE is overly brief and has a 5 critical items after 5 min of ass-hat thinking and Gabe has 15 items after weeks of analysis...Kudos to you for your brevity!

            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #36
              Originally posted by Evan View Post
              I enjoyed your story though. A bit of Gimli Glider forward slip technique there at the end. You could have been a smart-ass when the instructor told you to go around and ask how the hell you are going to do that since you've lost the engine (assuming you felt you could get it stopped in time).
              Interesting response.

              Here's Gabriel in a fairly critical flight mode- not 100% sure he can stop, confident he can go around, exercising solid CRM, and accepting the recommendation of a higher trained, higher hour pilot with more experience in the particular aircraft...

              ...and you suggest using bad CRM and violating sterile cockpit guidelines with smart ass comments all during a critical, low-error-margin maneuver.

              But since there isn't a QRHPOHFCOM for this and since basic fundamentals don't count, it's ok.
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment

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