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  • #46
    Originally posted by Evan View Post
    But when have I ever said that? All I've been saying all along is that it's NOT just the one thing (basic airmanship).
    Probably at the same time that I said that it is only the one thing (basic airmanship): Never.

    --- 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 ---

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    • #47
      Originally posted by Gabriel View Post
      Evan, do you have any source for this? In particular, do you have access to an AFM, FCOM or training manual?
      I did but the link no longer works. I should have downloaded it.

      But I am sure the AFM listed reference speeds for a clean wing at various weights and configurations, and then specified, for icing conditions, to add a fixed 20kts to those figures. Reference speeds are increased by 20kts.
      The function of the reference speeds switch was described as the same. It simply takes the low-speed cue, i.e. stickshaker speed, calculated for a non-iced wing AoA, weight and config, and raises it by 20kts. That way, pilots watching their airspeed will never get a stickshaker as long as they have set the bugs +20kts.

      Keep digging I guess.

      Comment


      • #48
        Gabieee:

        Airspeed and AOA are strongly correlated...as is airspeed with stall prevention.

        I can see scientific engineers designing electrical stuff that works off of airspeed and kicks it up 20 knots with a switch flip...Remember it's a stall warning, not a stall indicator.

        I mean I totally endorse that other, somewhat contradicting, giant fundamental rule, but a good Airman (or Airwoman or Airperson) can reconcile all this into a very important gray area and intelligently manage it to not_fall out of the sky out of control. (without type specific training either).
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

        Comment


        • #49
          Originally posted by 3WE View Post
          Gabieee:

          Airspeed and AOA are strongly correlated...as is airspeed with stall prevention.
          Yes: L=nW=1/2*r*v^2*CL*S

          I can see scientific engineers designing electrical stuff that works off of airspeed and kicks it up 20 knots with a switch flip.
          .
          I cannot.

          If it kicks it up by 20 knots at the lowest operational weight at 1G, it has to kick it up at by more than 20 knots at MLW and 2.5G.
          If your stall at the minimum wight is say 100 kts at 1G, 20 extra kts is a 20% (1.2) in speed and a 44% in lift (1.2^2=1.4).
          If the same plane can fly at 1.5 times the minimum weight and is pulling up 2.5Gs (for which it has to be certified), the lift will be 1.5*2.5=3.75 times the minimum weight, the accelerated stall speed will be sqrt(3.75)=1.94 times that of minimum weight at 1G, or 194 knots. 20 extra knots is a 10% (1.1) in speed and 21% in lift (1.1^2=1.21).
          Then the location of the CG also affects the stall speed measurably (and 3WE, you can find this in the manual of the Cessna 172 too), because how much lift the wings needs to do depends on the weight and load factor but also on how much lift the tail is doing, which depends on the location of the CG.
          Now, for each configuration, the "no ice" stall will happen at the same AoA, no matter the weight, load factor or location of the CG, and the stall warning will be triggered by a specific AoA a fixed number of degrees below the stall AoA, and the AoA is measured by a single probe: the AoA vane. So the actual speed at which the stall warning / stick-shaker occurs at a given airspeed will depend on airplane weight, load factor and location of the CG, but it will be always at the same AoA that has always the same margin over the stall AoA.

          To add an additional margin to account for icing, and make that margin a fixed number of KNOTS instead of degrees of AoA, is feat that needs to take into account different values from different sensors and from information loaded by humans before take-off (weight and CG), and that is to obtain a safety margin that is consistent in knots but inconsistent in AoA and because you need an AoA margin (by regulations), a fixed number of knots will need to give you the correct margin in the worst condition which will mean a larger-than-needed margin in other conditions.

          This is BAD ENGINEERING from so many points of view.
          - It is complicated.
          - Has many failure modes (technical an human).
          - It is inefficient.

          Why on Earth would en engineer do that when they can simply kick it up (er, down) by x degrees instead of x knots, and have a simple design, with few failure modes (and non of them human), and give a consistent margin across the operational conditions? Because... the would not.

          It makes no sense from en engineering point of view nor from an operational point of view.

          And perhaps the most important part: IT IS NOT A MARGIN. How is the stall speed /AoA defined?
          For the no-ice condition, you need to perform a specific maneuver in certain conditions until the plane stalls.
          For the ice condition, you need to perform the same specific maneuver in the same conditions until the plane stalls, but with one additional condition: You have to artificially "contaminate" the wings with a specified geometry of ice accretion specified in the FARs. So, if in flight you have a similar ice accretion, you don't have any additional margin over what you would have with a clean wing.

          --- 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


          • #50
            Originally posted by Gabriel View Post
            I cannot.
            You put a reverse-printed compass in the back wall of a cockpit with a little bitty mirror on a DC-9.

            And I'm cool with carrying 20 extra knots if there's potential icing...

            As long as you aren't running around doing relentless pull ups every time some warning goes off as you sort through 15 different memory checklists (and try not to confuse them with the 15 memory checklists from a couple of previous aircraft).
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #51
              Originally posted by Evan View Post
              I did but the link no longer works. I should have downloaded it.

              But I am sure the AFM listed reference speeds for a clean wing at various weights and configurations, and then specified, for icing conditions, to add a fixed 20kts to those figures. Reference speeds are increased by 20kts.
              The function of the reference speeds switch was described as the same. It simply takes the low-speed cue, i.e. stickshaker speed, calculated for a non-iced wing AoA, weight and config, and raises it by 20kts. That way, pilots watching their airspeed will never get a stickshaker as long as they have set the bugs +20kts.

              Keep digging I guess.
              I don't know where else I can dig.

              This is what I found in the AAR some loose paragraph in different sections, and yes, I am pick-and-choosing them):

              The ice protection panel, which is located on the overhead panel on the captain’s side of
              the cockpit, includes a “REF SPEEDS” switch that can be set to the INCR (increase) or OFF
              positions, as shown in figure 2. During the May 2009 public hearing on this accident, an
              engineering manager from Bombardier stated that the ref speeds switch, when set to the increase
              position, advances the stall warning so that the stick shaker would activate at a lower AOA; as
              a result, the airplane would have the same performance margins relative to the stall speed during
              operations in icing conditions as it would have with a clean (no ice accretion) configuration (and
              the ref speeds switch set to the off position). The engineering manager also stated that, because
              the stick shaker would activate at a lower AOA with the ref speeds switch set to the increase
              position, the flight crew would need to increase landing airspeeds between 15 and 25 knots
              depending on the flap setting (to remain above the stall warning threshold).

              The stall protection modules use AOA, flap position, body axis attitudes,
              normal acceleration, true airspeed, Mach number, engine torque, and icing status data to
              calculate when the airplane is approaching a stall condition. The stick shaker activates when
              these parameters, in particular AOA, reach a specific threshold.

              Data from the FDR and a Bombardier Q400 stall protection system design document
              were used to calculate the fuselage AOA during the stall sequence. The calculated fuselage
              AOA at the time that the autopilot disengaged was about 8°. The calculated fuselage AOA at the
              time of stick shaker activation would have been about 7.6° with the ref speeds switch selected to
              the increase position and about 12° with the ref speeds switch selected to the off position.77
              These AOA calculations, combined with the available CVR and FDR evidence, showed that the
              ref speeds switch was in the increase position at the time of stick shaker activation.

              The evaluation of these alternate flight control input scenarios showed that pilot intervention was
              required to avoid stalling the airplane after autopilot disconnect; that is, holding the control
              column at a fixed position (the expected autopilot disengage position) and adding power would
              result in the airplane continuing to increase AOA up to an aerodynamic stall. However, the Q400
              simulation airplane with a conservative icing accumulation factor of 0.3 still had adequate
              performance capability, with appropriate pitch axis inputs, to maintain either a constant pitch
              attitude of 10° or constant altitude with engine torque values at or above the values for the
              accident airplane.

              In addition, the accident airplane’s fuselage AOA achieved at the peak normal load
              factor (13°) was compared with both the stick shaker and the stick pusher AOA schedules for
              the nominal stall protection system. According to calculations provided by the supplier of the
              Q400 stall protection system, the stick shaker for the accident flight condition was scheduled to
              activate at a fuselage AOA of about 11.9° with the ref speeds switch selected to the off position.
              The calculations also showed that the stick pusher for the accident flight condition was
              scheduled to activate (after an aerodynamic stall) at a fuselage AOA of 17.5°. Because the
              accident airplane achieved a peak normal load factor at a calculated fuselage AOA of about 13°,
              the airplane’s AOA for maximum lift was greater than the AOA for the Q400 clean-wing stick
              shaker onset (by about 1°) and was less than the AOA for the Q400 stick pusher (by about 4.5°).

              For the accident airplane, turning the ref speeds switch from the off to the increase
              position lowered the AOA reference for stick shaker activation and raised the position of the
              low-speed cue on the pilots’ IAS displays by about 15 knots.

              The CVR recorded the activation of the stick shaker about 2216:27, and FDR data
              showed that the activation occurred at an AOA of about 8°, a load factor of 1 G, and an airspeed
              of 131 knots, which was consistent with the AOA, airspeed, and low-speed cue during normal
              operations when the ref speeds switch was selected to the increase position.

              CVR and FDR data indicated that, when the stick shaker activated, the autopilot
              disconnected automatically. The captain responded by applying a 37-pound pull force to the
              control column, which resulted in an airplane-nose-up elevator deflection, and adding power. In
              response to the aft control column movement, the AOA increased to 13°, pitch attitude increased
              to about 18°, load factor increased from 1.0 to about 1.4 Gs, and airspeed slowed to 125 knots. In
              addition, the speed at which a stall would occur increased.

              As I said, I pick-and-chose, and you can find other paragraph that put more focus in speed than AoA, for example:

              The Bombardier Q400 AFM, section 5, dated August 4, 1999, provided reference and
              approach speed information for various configurations of the airplane. The information showed
              the following for the accident airplane at its estimated landing weight:
              • With flaps set at 0°, the 1.23 reference stall speed (VSR) was 145 knots and a
              minimum of 170 knots (1.23 VSR plus 25 knots) during icing conditions.
              • With flaps set at 5°, the approach speed was 133 knots and a minimum of 153 knots
              (flaps 5 approach speed plus 20 knots) during icing conditions.
              • With flaps set at 10°, the approach speed was 124 knots and a minimum of 144 knots
              (flaps 10 approach speed plus 20 knots) during icing conditions.
              • With flaps set at 15°, the Vref for the planned landing was 118 knots and a minimum
              of 138 knots (flaps 15 Vref speed plus 20 knots) during icing conditions.

              Note the "at its estimated landing weight" part, which is then missing in the following paragraph:

              Bombardier specified procedures for the use of the ref speeds switch in the Q400 AFM,
              which is an FAA-required document. The AFM’s procedures for use of the ref speeds switch
              were included in a section about ice protection features. As discussed in sections 1.17.2.2 and
              2.2.2, the procedures indicated that the ref speeds switch was to be turned to the increase
              position before entering icing conditions or upon initial detection of icing and was to be turned
              to the off position when the airplane was aerodynamically clean (that is, all ice was removed
              from the visible leading edges of the wing and wing tips). For a flaps 15 landing with the ref
              speeds switch in the increase position, pilots needed to increase airspeeds by 20 knots.

              Other than the AAR, I found loose information which was also not conclusive.

              I found an AFM for an add-on Q400 for a flight sim. It said it was based on the real manual. And it had many tables for different weights with reference take-off and approach speeds for different flaps settings. This AFM also contained a section on ice protection. There was diagram of the ice protection panel that showed the REF SPEED switch and it said "Toggle to activate (not fully simulated)". I aslo found a comment in a fora where a (supposedly) pilot said that the AFM includes 2 sets of reference speeds: one for normal and one for icing conditions. But I couldn't find actual reference speed tables.

              --- 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


              • #52
                Originally posted by 3WE View Post
                You put a reverse-printed compass in the back wall of a cockpit with a little bitty mirror on a DC-9.

                And I'm cool with carrying 20 extra knots if there's potential icing...

                As long as you aren't running around doing relentless pull ups every time some warning goes off as you sort through 15 different memory checklists (and try not to confuse them with the 15 memory checklists from a couple of previous aircraft).
                I doesn't need to be that complicated. You do need to lookup the reference speeds in the manual (or look-up tables based on the manual), and you do have to use the airplane weight and flaps setting as input to get the reference speed, so it is a matter of having 2 values for each reference speed for each weight and flap setting: clean and ice.

                --- 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


                • #53
                  Originally posted by Gabriel View Post
                  Why on Earth would en engineer do that when they can simply kick it up (er, down) by x degrees instead of x knots, and have a simple design, with few failure modes (and non of them human), and give a consistent margin across the operational conditions?
                  I expect this is because pilots don't fly by AoA, they don't set an AoA bug and they don't have an AoA reference angle to fly by. They don't even have an AoA indicator AFAIK. Airspeed is the guide value. 20kts has been deemed the safe margin to account for ice contamination. It might not be the most efficient solution, but it might be the most reliable one.

                  I'm afraid many pilots (not you, of course) could not be trusted with L=nW=1/2*r*v^2*CL*S calculations. You could always keep the switch off and just increase your airspeed by 16.23446892076kts, or whatever your superior vulcan mind deems efficient.

                  This is BAD ENGINEERING from so many points of view.
                  Compared to what? The other aircraft that don't have ANY provision for adjusting the low-energy warning speed to account for icing?

                  I think this is just an added safety provision, not a mission-critical function. I'll bet it can be MEL'd for a week. Too bad it wasn't on this flight...

                  Comment


                  • #54
                    Originally posted by Evan View Post
                    I expect this is because pilots don't fly by AoA, they don't set an AoA bug and they don't have an AoA reference angle to fly by. They don't even have an AoA indicator AFAIK. Airspeed is the guide value. 20kts has been deemed the safe margin to account for ice contamination. It might not be the most efficient solution, but it might be the most reliable one.

                    I'm afraid many pilots (not you, of course) could not be trusted with L=nW=1/2*r*v^2*CL*S calculations. You could always keep the switch off and just increase your airspeed by 16.23446892076kts, or whatever your superior vulcan mind deems efficient.



                    Compared to what? The other aircraft that don't have ANY provision for adjusting the low-energy warning speed to account for icing?

                    I think this is just an added safety provision, not a mission-critical function. I'll bet it can be MEL'd for a week. Too bad it wasn't on this flight...
                    Compared to my previous post in response to 3WE.
                    I mean, yes, we can aslo take a single Vref for any weight and flaps setting. But we don't. And we don't ask the pilots to "calculate it" based on L=nW=1/2*r*v^2*CL*S. We calculate it for them and ask them to look them up at a table. The same table that can contain the Vref for icing conditions.

                    --- 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


                    • #55
                      Originally posted by Evan View Post
                      Too bad [the icing stall-indicator trigger speed/Aoa] wasn't [MEL'd] on this flight
                      Too bad the airspeed wasn't monitored after leveling AND dropping flaps AND dropping gear AND flattening the props. (Good thing to do in a 182RGs and a 777s on beautiful sunny evenings)

                      Too bad there was a relentless pull up. (Good thing to avoid in 172s with fixed gear and prop and A-300s)
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                      Comment


                      • #56
                        Originally posted by Evan View Post
                        They don't even have an AoA indicator AFAIK.
                        I have to grab this for posterity.

                        Guess what, almost all aircraft have AoA indicators.

                        Some are pretty gross and make noise and do other things when you start getting close to 16 degrees.

                        Others show you an angle and green yellow and red regions. Gabieee really likes those. (I have no objection to them, but most 172's only have the basic type).
                        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                        Comment


                        • #57
                          Originally posted by 3WE View Post
                          Too bad the airspeed wasn't monitored after leveling AND dropping flaps AND dropping gear AND flattening the props.
                          Maybe it was. They seem to have been aiming for 118 knots. The stickshaker activated at 131 knots. By then, they still had 13 knots and 6 or 7 seconds of intended slow down.
                          Maybe they "just" missed barber pole stripe raising from below.

                          --- 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


                          • #58
                            Originally posted by Gabriel View Post
                            Maybe it was. They seem to have been aiming for 118 knots. The stickshaker activated at 131 knots. By then, they still had 13 knots and 6 or 7 seconds of intended slow down.
                            Maybe they "just" missed barber pole stripe raising from below.
                            No argument- but when you flip it around, I do NOT agree with wishing that the 'give-a-an-earlier-stall-warning switch was MEL'd' . Wishing that is right up there with wishing that all the passengers had stayed in bed that AM.

                            (Look, I give you the tail stall theory as plausible...You need to give me that they dirtied the hell out of the aircraft and sat there (admittedly tired), made no power inputs, and waited while the speed dropped at a very nice pace to a stall warning as similarly plausible...)

                            Edit: And to keep you happy, the stall warning was probably triggered by an AOA indication, all the big aeroplanies seem to have an AoA vane, and you CAN stall at “fast” speeds.
                            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                            Comment


                            • #59
                              Originally posted by Gabriel View Post
                              Compared to my previous post in response to 3WE.
                              I mean, yes, we can aslo take a single Vref for any weight and flaps setting. But we don't. And we don't ask the pilots to "calculate it" based on L=nW=1/2*r*v^2*CL*S. We calculate it for them and ask them to look them up at a table. The same table that can contain the Vref for icing conditions.
                              The design philosophy for the Q400 seems to be to automate and simplify the procedure as much as is practical.

                              1) Ice probes automatically detect icing.
                              2) Flashing ICE DETECTED message appears on the ED.
                              3) Flashing message continues until the PROPS switches are turned on (to energize prop heating).
                              4) As a result of turning the PROPS selectors on the "Stall Protection System (SPS) is modified for icing conditions".
                              5) A command [REF SPEEDS INCR] message on the ED comes on, prompting the crew to switch the reference speeds switch to INCR.
                              6) The crew reset the bugs to +20kts (there are 2 bugs shown on the speed tape during approach, so I assume they can set one of them for icing and the other for normal ops).

                              Maybe they "just" missed barber pole stripe raising from below.
                              And there is no amber zone, which seems a bit like BAD ENGINEERING.

                              Comment


                              • #60
                                Originally posted by 3WE View Post
                                (Look, I give you the tail stall theory as plausible...You need to give me that they dirtied the hell out of the aircraft and sat there (admittedly tired) while the speed dropped at a very nice pace to a stall warning as similarly plausible...)
                                I give you that, even without a tail stall theory. They might have been aware (or not) of the speed as it decayed and still reacted terribly wrong for no apparent reason. As I mentioned earlier, a few months after Colgan we had AF 447. The pilot reacted very similar to the Colgan crew and I don't beleive that the AF pilot was thinking "tail stall".

                                --- 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 ---

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