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B737 controlability-questions & surprises.

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B737 controlability-questions & surprises.

Old 5th Feb 2019, 17:02
  #41 (permalink)  
 
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roll to bring the nose down...

we were shown this in the sim more than once beginning quite a while ago, on a periodic rotation of non-normals in an airline with blue and white paint and a musical instrument on the vertical stab.
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Old 5th Feb 2019, 20:29
  #42 (permalink)  
 
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PEI - thanks for your clarification regarding Post #34. Below is my attempt to address your question with regard to 737. As always, if there is something I have missed please let me know.

737 pitch control involves an all-moving horizontal tail (stabilizer), a pair of elevator surfaces mounted on the trailing edges of the stabilizer (left and right), and a pair of elevator tabs attached to the trailing edge of each of the elevators. On some airplanes control tabs are used to drive a trailing edge control surface with the tab driven but the larger surface floating. That is not the case with the 737. The 737 elevator tabs are directly geared to the elevators themselves. Geared tabs that move in the opposite direction as the larger surface are called "balanced" as they generate hinge moments that subtract from those of the larger surface thus the total force required to move the elevator and tab is less. Conversely, tabs that move in the same direction as the larger surface are called "unbalanced" as they increase the force required to move the surface against the airflow past the horizontal tail. Interestingly, the 737 across its run of derivatives and in response to flap configuration and hydraulic power availability uses its elevator tabs as either fixed (no motion relative to the stabilizer), balanced, or unbalanced. The details of when the tab behaves in these three different manners is beyond what I want to get into here.

The 737 (as with all of Boeing's commercial transports) uses pilot controller position based control to command the associated control surfaces when flying manually. (I acknowledge that the autopilot does provide a Control Wheel Steering mode, but let's leave that beast out of the picture here.) The 737 elevator (and thus its geared elevator tab as well) are driven by the position of the elevator aft quadrant. If one 737 column is jammed, no amount of force on that column will result in any elevator motion provided that jammed column remains stuck and does not move. Motion of the other column will result in limited motion of the elevator aft quadrant and thus some elevator control. A key element in providing this mitigating control path is that the mechanical cables connecting the elevator aft quadrant to the jammed column are not infinitely stiff such that when subject to force applied by the crew on the column that is not jammed the cables on the side with the jam will stretch allowing elevator aft quadrant motion.

The 737 is in pitch trim if the net pitching moment for the full airplane is zero with no force applied to either control column. There will be only one stabilizer position that achieves pitch trim at the current flight condition including thrust setting, flap position, and all other variations that generate pitching moment. If the stabilizer is not at this trim position, the airplane will experience a pitch acceleration / rate unless the elevator is positioned so as to provide the needed pitching moment balance. The pilot task is to position the column as needed to control pitch attitude and through that other response parameters such as altitude or speed. Stabilizer trim is to be used (when available of course) to relieve steady column forces to allow the pilot to reduce workload and to preserve available control power in both the nose up and down directions for subsequent maneuvers or disturbance rejection.

I must admit that I get a little lost trying to follow the thought process of the last couple of paragraphs of Post #34. I don't agree with the notion that the pilot has to form a new model of how the airplane responds depending on the trim setting. The incremental response to incremental pitch control inputs from the point of pitch equilibrium (be that trimmed with zero column force or holding a steady column force) is essentially constant. There will be variations in incremental response with weight, CG, flap, and speed, but not with changes in pitch trim.

Hoping this helps,

FCeng84
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Old 5th Feb 2019, 22:43
  #43 (permalink)  
 
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FCeng84 thanks for the explanation.

Not to challenge your technical understanding, but to provide and share an alternative piloting view. Concentrating on an unjammed control system in normal operation, then the basis of stick-position-control mechanism is relative to the zero force ‘in trim’ position (pilots more often relate to force than position). i.e. the aircraft is manoeuvred by moving the stick, which is referenced to, or relative to the null point, more likely zero force. The specific piloting view depends on training and / or technical understanding.

However, with either mismanaged or unwarranted non-pilot trim inputs, the reference point is changed, the zero force ‘in trim’ position does not relate to the aircraft control system being in balance (the trimmed state), thus the pilot is required to hold a compensating force, opposing trim.

Again with the assumption that pilots generally, based on overwhelming normal experience, reference the aircraft control to the zero force trim position, then all control inputs are relative to this position.
Then a failed trim condition is like flying ‘a new aircraft’, all control inputs are relative to an erroneous null position . This requires significant mental compensation of how to fly, get the feel of, and trim the aircraft - the latter being unachievable.
Furthermore if the failed trim condition also changes (AoA - MCAS) then Pilots will have further difficulty in revising their understanding. Add to which all of the consequential changes in force, feel, and speed.

Thus the apparent inability to control the aircraft (#1) involves poor understanding of aircraft control and trim interaction (172 driver #4), and similar, greater difficulties with trim failures; all of which is exacerbated by surprise and high mental workload.
A further and generally untenable view, is that pilots ‘inadvertently’ fly the aircraft with the trim - stick and trim together; yet there are examples of just that behaviour.
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Old 5th Feb 2019, 22:58
  #44 (permalink)  
 
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Salute!

@ FCeng, remember that PEI speaks from a test pilot point of view ( Pax River and then back in Britain). Gums speaks from having learned in WW2 observation planes and then flying 2 Century series jets and then a throwback straight wing beast that had its own aero problems, and then two more bent wing planes. One of those stretched the envelope for planes and we human operators more than any platform to that time. And so...

We operators expect certain things to happen when we command attitude/AoA changes and "trim" to keep from holding a force, pressure, position, etc
The 737 implementation of several modifications to the flight control system appears to many of us as kludge solutions to basic areodynamic problems. And to be brutally honest, a complete FBW system seems to me to be a better way to correct the plane's aero deficiencies ( THERE! I said it.) than all the gears, levers, cables, pulleys and then two electronic systems with connections to the controls that put reverse trim inputs than a pilot would! GASP!

Gums opines....
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Old 5th Feb 2019, 23:35
  #45 (permalink)  
 
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All of the features of the 737 stabilizer control system that are active when flying manually (i.e., when the autopilot is not engaged) are designed to present the crew with speed and maneuver stable characteristics. An increase in speed results in airplane tendency to pull the nose up (thus to slow down) in a speed stable manner. An increase in AOA results in airplane tendency to push the nose down (thus seeking a lower AOA) in a maneuver stable manner. When the pilot has trimmed the pitch axis via repositioning the stabilizer to allow steady flight at the current condition with zero column force, the system should not interfere by moving the stabilizer such that the pitch axis is no longer trimmed. Instances of the system running the stabilizer turning a steady, trimmed condition into one that is no longer trimmed should be recognized as improper operation of the automatic stabilizer control system. The procedure for improper stabilizer control is to the follow the checklist actions that include disabling electric stabilizer control via placing the stabilizer cutout switches in the cutout position.

It seems clear to me with regard to the Lion Air events of late October that the crew on the second to last flight recognized that the automatic stabilizer control was not functioning properly. They took the correct action of disabling further electric stabilizer control and flying the remainder of the flight employing manual, trim wheel control of the stabilizer as needed. It also seems clear to me that the crew on the last flight repeatedly recognized the need to apply pilot commanded electric stabilizer trim via the wheel mounted pitch trim switches to return to a trimmed condition when the malfunctioning system ran the stabilizer away from the proper trim position. To me there are a number of mysteries surrounding this tragedy:
1. Why the crew on the second to last flight did not mention in their post flight write-up that they flew the entire flight with the stick shaker rattling and that they found it necessary to activate the stabilizer cutout switches to stop the system from repeatedly taking them away from trim. If the crew of the last flight had taken off with that mitigation action in mind we would probably not be having this PPRuNe discussion today.
2. Why the crew of the last flight went through so many cycles of trimming manually only to have the system take them away from trim each time without suspecting that there was something wrong with the automatic stabilizer control and thus it should be shut down.
3. Why when faced with difficulty managing pitch control the crew of the last flight chose to stay at only 5000 feet altitude and chose to fly at such a high speed. Climbing higher and at a slower speed would have been prudent to give them more room and also preserve more elevator pitch control. 737 pilots should know that at higher speeds elevator travel is hinge moment limited and thus their control authority reduces with increased speed.
4. Why the crew of the last flight were successful for several minutes in their correct actions to counter the errant system stabilizer motions with pilot initiated stabilizer motions that re-trimmed the airplane, but did not continue with that during the final 30 seconds to a minute of the flight.

I know that I am only rehashing questions that many of us have been struggling with over the last 3+ months. I sure wish that the CVR information that has now been recovered were available to the public to better understand what the thinking was among the flight deck crew on that last flight. While the cockpit recording will probably not answer all of our outstanding questions, it will likely move us to greater clarity. As I contemplate pressing send on this post I hope that I am not triggering a re-run of PPRuNe volleys that have already been lofted on this topic. In the end, however, I find it helpful to share my lingering wonderments. Lot's of questions and not enough answers. Bottom line is that 189 who should not have died did and it is our collective job to minimize the risk of adding to that number.

My several cents worth,

FCeng84
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Old 5th Feb 2019, 23:46
  #46 (permalink)  
 
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gums - thanks for your response. I sent my last post before seeing yours. I find no fault what so ever with what you have presented. Having been fortunate to be involved in design of fully FBW systems I fully agree that when handled properly, that is a better direction. Use of a very slow control surface to make up for a phugoid related aerodynamic short coming is one thing. Use of that same very slow control surface to make up for a short period related aerodynamic short coming is something else. One of my goals is to see to it that enough of the design community learns these expensive lessons so that future editions of control systems on current and yet to come models behave both on their own and in conjunction with pilots in a robustly predictable and safe manner.

FCeng84
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Old 6th Feb 2019, 07:07
  #47 (permalink)  
 
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Originally Posted by FCeng84 View Post
All of the features of the 737 stabilizer control system that are active when flying manually (i.e., when the autopilot is not engaged) are designed to present the crew with speed and maneuver stable characteristics. An increase in speed results in airplane tendency to pull the nose up (thus to slow down) in a speed stable manner. An increase in AOA results in airplane tendency to push the nose down (thus seeking a lower AOA) in a maneuver stable manner. When the pilot has trimmed the pitch axis via repositioning the stabilizer to allow steady flight at the current condition with zero column force, the system should not interfere by moving the stabilizer such that the pitch axis is no longer trimmed. Instances of the system running the stabilizer turning a steady, trimmed condition into one that is no longer trimmed should be recognized as improper operation of the automatic stabilizer control system. The procedure for improper stabilizer control is to the follow the checklist actions that include disabling electric stabilizer control via placing the stabilizer cutout switches in the cutout position.

It seems clear to me with regard to the Lion Air events of late October that the crew on the second to last flight recognized that the automatic stabilizer control was not functioning properly. They took the correct action of disabling further electric stabilizer control and flying the remainder of the flight employing manual, trim wheel control of the stabilizer as needed. It also seems clear to me that the crew on the last flight repeatedly recognized the need to apply pilot commanded electric stabilizer trim via the wheel mounted pitch trim switches to return to a trimmed condition when the malfunctioning system ran the stabilizer away from the proper trim position. To me there are a number of mysteries surrounding this tragedy:
1. Why the crew on the second to last flight did not mention in their post flight write-up that they flew the entire flight with the stick shaker rattling and that they found it necessary to activate the stabilizer cutout switches to stop the system from repeatedly taking them away from trim. If the crew of the last flight had taken off with that mitigation action in mind we would probably not be having this PPRuNe discussion today.
2. Why the crew of the last flight went through so many cycles of trimming manually only to have the system take them away from trim each time without suspecting that there was something wrong with the automatic stabilizer control and thus it should be shut down.
3. Why when faced with difficulty managing pitch control the crew of the last flight chose to stay at only 5000 feet altitude and chose to fly at such a high speed. Climbing higher and at a slower speed would have been prudent to give them more room and also preserve more elevator pitch control. 737 pilots should know that at higher speeds elevator travel is hinge moment limited and thus their control authority reduces with increased speed.
4. Why the crew of the last flight were successful for several minutes in their correct actions to counter the errant system stabilizer motions with pilot initiated stabilizer motions that re-trimmed the airplane, but did not continue with that during the final 30 seconds to a minute of the flight.

I know that I am only rehashing questions that many of us have been struggling with over the last 3+ months. I sure wish that the CVR information that has now been recovered were available to the public to better understand what the thinking was among the flight deck crew on that last flight. While the cockpit recording will probably not answer all of our outstanding questions, it will likely move us to greater clarity. As I contemplate pressing send on this post I hope that I am not triggering a re-run of PPRuNe volleys that have already been lofted on this topic. In the end, however, I find it helpful to share my lingering wonderments. Lot's of questions and not enough answers. Bottom line is that 189 who should not have died did and it is our collective job to minimize the risk of adding to that number.

My several cents worth,

FCeng84
As to point 3, I think they had the stick shaker going of, and they thought the airspeed indication was unreliable. I guess that made them reluctant to reduce power.
As to point 4, I read that control was handed to the FO about 30 sec before the end, and the FO made much smaller ANU trim inputs than the PIC before him, and each of those small inputs allowed MCAS to add 2.5deg AND, leading to loss of control.
Not suggesting I know more about this than you, as I am sure I don't, just wanted to add some possible relevant info. Also agree very much with everything in your post, just think the MCAS system Boeing implemented should have been more failure resistant, and better explained to the crew.
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Old 6th Feb 2019, 08:26
  #48 (permalink)  
 
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FCeng84,

I have a question. With AoA sensing being such an important element in the flight control systems of modern aircraft, why did Boeing not fit three AoA sensors so that a simple voting protocol could be used to eliminate the faulty signal? Was this to keep the design simple? Or was it to remain within the logic of the previous 737 variants so as to be able to retain the orginal type certificate and thus save the expense of re-certifying the aircraft?

I am still astonished that the failure of one component should have had such deleterious consequences.
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Old 6th Feb 2019, 09:29
  #49 (permalink)  
 
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In the simulator I have trained recovery from just about any upset possible with a 737. The upset is generally set up by one pilot while the other has his eyes closed. Great training!
What is very difficult to simulate is the startle effect or the confusion of an unexpected upset. A nose down, high thrust, trimmed down situation at low altitude is difficult but possible to save if you take the correct action right away. If you are condused and need to analyze the situation for a few seconds, you are dead.
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Old 6th Feb 2019, 13:26
  #50 (permalink)  
 
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The upset is generally set up by one pilot while the other has his eyes closed. Great training!
Why close your eyes? In real life UA your eyes wouldn't be closed unless you were asleep. Both pilots would have their eyes wide open if they got themselves into a UA. I have seen some simulator instructors tell the pilot to look down at their knees and close their eyes while the instructor stuffs around tipping the simulator like a drunken sailor, deliberately missing trimming and even pulling back one engine. Quite nonsensical and impractical. Great training? WTF??

The closing of eyes was done in a Cessna 172 where a instrument flying hood is worn and in theory prevents the student from peeking out from the side of the hood to cheat and see the horizon. The simulator instructor can simply select IMC on his instructor panel so there is no chance of cheating.
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Old 6th Feb 2019, 16:55
  #51 (permalink)  
 
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Bergerie1 - All Boeing commercial transports have been fitted with two AOA vanes. More recent FBW models have used both vanes in conjunction with an AOA estimate to perform signal selection / fault detection to arrive at a selected and validated AOA signal to be used by the control laws. Measuring AOA in the traditional means using a vane that sticks out from the fuselage no more than a few inches is complicated by the local flow distortion. One degree of change in airplane AOA results in close to two degrees of change to the flow as measured by these vanes. Boeing has chosen to fit its airplanes with a pair of vanes mounted on symmetrically identical locations so that they are equally impacted by symmetric flow over the airplane. Vane location selection has also taken into account the distortion caused by sideslip trying to choose positions that minimize this.

As for 737 signal selection logic the historic approach has been to have the right flight computer use right side sensors while the left uses those from the left. The approach has been to flag differences between left and right with reliance on the crew to compare their flight deck instrumentation readings to sort out which is to be trusted. We will have to see if control law logic changes that are under consideration by Boeing include cross checking between the two AOA vanes to make the MCAS logic more robust to what appears to have been a single signal failure.
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Old 7th Feb 2019, 08:11
  #52 (permalink)  
 
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FCeng84,
Thank you for making that so clear.
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Old 7th Feb 2019, 11:17
  #53 (permalink)  
 
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FCeng84,

Another very simple question. The 747's lack of positive longitudinal stability when approaching the stall in the clean configuration was compensated (for aircraft on the British register) by fitting a stick nudger when the stick shaker activated. Clearly, this was a 'rough and ready' fix, but it worked and was very simple both to install and for the pilot to understand. Would it not have been possible to introduce a similar device on the 737, designed to activate only when the critical conditions are met, before the stall is identified, and only during the period during which it is necesary to ensure the correct longitudinal stability? Or was the decision to install the MCAS, using software and acting upon the stabiliser, made because the STS was already a feature of the aircraft and it was considered simpler (or more elegant) to use a similar method of solving the problem?

I very much appreciate your precise and clear explanations of these things.
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