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B737 controlability-questions & surprises.

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B737 controlability-questions & surprises.

Old 21st Jan 2019, 22:52
  #21 (permalink)  
 
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Originally Posted by gums View Post
Salute!

@Hans My point is that the 'bus and several FBW planes have extremely simple control yokes/wheels/sticks and nothing close to the 737 "feel" system and the "speed stability" trim inputs to "help" the pilots and then recently make things worse with MCAS.

I wonder how much of this was present in the original plane. Any 737 dinosaurs here?

Gums sends..
Hi Gums,
This old dinosaur flew B707 then 737-200 for over 10 years. 707 stab trim runaway could continue despite control column application in the opposite direction.
737 design overcame this - application of control column force would prevent stab trim running in the opposite direction. (Both types handled beautifully).
737 Max's MCAS allows stab trim to run in opposite direction to control column force (by design) - but is apparently unknown to some pilots. Hence the aircraft can continue to trim against the pilot - and can do so until both stab trim switches on centre pedestal are turned off.
See https://theaircurrent.com/aviation-s...em-mcas-jt610/
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Old 21st Jan 2019, 23:17
  #22 (permalink)  
 
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Salute Sidestick !!

Yep, you are making the point. A well-designed system does not require adding one mod after another to get thru certification or just plain be a safe and basic airplane to fly.

I can prolly find some folks that have flown both the 320 and later, and were active Guard or Reserve Viper pilots. I just thot it would be better to get feedback here as most understand the forum protocols and such.

It appeared to me from the "other thread" that the 'bus was an excellent basic aero design without FBW, and that contributed to lack of the realization by the crew that the jet was in a stall. I flew at least two that did not "depart" violently and had very little buffet. But I am now getting worried about the evolution of the 737 from its original form. Flying an "electric jet" like the Viper or A320+ or Space Shuttle is not a big step as far as your stick and rudder "feel" goes. You know going in that you will have no feedback from your ailerons or elevators or rudder and as long as the thing goes where you want then you're happy.

Secondly, Sidestick, I don't think you or I could fly the Viper in the "direct law" like we have in the 'bus without using different stick transducers that have the input/output functions to keep you from ripping the wings off, heh heh.. Maybe the sfwe folks clould have a simple reversion that drastically reduced gains, but there are many other variables that Hal uses, just as in the ;bus. As I may have mentioned, the control forces are an order of magnitude less than what the 737 feel system presents. e.g. 4 pounds for each gee on a linear plot, and 17 pounds for a 300 deg/sec roll rate using another fairly straight plot. The rudder was "direct", but I only used rudder on the runway or to kill a few knots when flying close chase.

So my main point is why has Big B been adding these systems all along? I realize that a completely new wing design and other changes would require lottsa $$$ for certification, so that's where I am putting the excuse for the feel, STS and MCAS.

Gums sends...
P.S. Thanks, Golden. Good poop.

Last edited by gums; 21st Jan 2019 at 23:22. Reason: added thanks to Golden
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Old 22nd Jan 2019, 00:08
  #23 (permalink)  
 
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Originally Posted by eckhard View Post
I don’t know about the 737-max, but on the “classic” -300 and -400, we had a “Control-column mounted Stab Trim cut-out Override” switch. Its function was to enable continued use of the stab trim switches after a stuck or jammed elevator event.

The “Control-column cut-out” was a device to enable the pilot to stop a stab runaway by applying opposite elevator. The system would recognise the opposite input and remove power from the stab motors.

The downside of this system was that in the event of a stuck elevator, the cut-out would prevent the pilot from moving the stab. The “Override” switch was there to disable the system in this event, thus restoring stab control to the pilot, notwithstanding the stuck elevator.

The switch was mounted on the aft pedestal and its function became a favourite question of mine during route checks. Few pilots knew what it was for.

Anyway, I wonder if this system (or its malfunction) may have been a factor in these accidents?
eckhard - 737 column cutout switches prevent stab commands from pilot wheel mounted pitch trim switches or STS from moving the stabilizer in the direction opposite the column. As you mention, there are separate, manually controlled, cutout override switches that allow the crew to trim the stabilizer opposite the column to mitigate a jammed column. I don't see how these would play a role in an event where pitch control is compromised and the pilot is commanding stabilizer trim in the same direction as the column. No cutout function in that event and thus no need/function for the cutout override switch.

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Old 22nd Jan 2019, 10:19
  #24 (permalink)  
 
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Ok, thanks for the information.
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Old 24th Jan 2019, 11:29
  #25 (permalink)  
 
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FCeng84,

I have acquired the following information regarding "cable stretch".
Using a 3/16th, 7x19 strand Carbon Steel Tin-Zinc Coated cable for example. Upon manufacture, the cable is "proof loaded" to 2520 +125/-0lbs. The load must be applied within 3secs and held for a minimum of 5secs.
Before installation into, for example, the elevator system, to ensure you have the correct cable tension, permit a min of 1hr at constant ambient temp (+/- 5F or +/- 3C) for aircraft temp to stabilise. Install the cables and operate the system for several cycles at TWICE the working tension per the applicable system tension/temp chart. Then back off tension (via the turnbuckles) to the correct operating tension and cycle the system again. If all rig pin checks are good then lock the applicable turnbuckles. This must definitely prove the cable "stretch/spring" theory.

Machinbird,

The answer to your question "What keeps the PCU input torque tube from back driving the jammed control column side and therefore jamming the free side. How is the necessary free motion generated?" is the "Elevator Breakout Mechanism" located between the control columns under the cockpit floor.
The breakout mechanism is attached to the forward input torque tube between the control columns. The mechanism separates the left and right sides of the input torque tube. This allows elevator control if one column has a jam.
The breakout mechanism is a cam-roller type. The cam connects to the Capts torque tube and the roller connects to an arm on the F/O's torque tube. 2 springs hold the roller in the cam detent position (both control columns move together). The pilot must apply 31lbs of additional force to "breakout" of the cam detent and extend the springs during a jam. When 100lbs is applied, the elevator moves4 degrees. (hence FCeng84's cable stretch, albeit minimally)
During normal operation, a control column moves and operates both torque tubes together. If one column has a jam, the other column can still move after the pilot overcomes the breakout force of the 2 springs.

Rgds McHale.


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Old 24th Jan 2019, 18:40
  #26 (permalink)  
 
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Capt McHale,

Thank you for providing such details regarding mechanical cable testing, installation, and stretch when flying with a jammed column. I was aware of this scenario being evaluated by test pilots and determined to be acceptable for continued safe flight and landing, but did not know how much elevator motion it is able to provide. I find it quite sobering that 4 degrees is all you get. The flare prior to touchdown will be compromised, but everyone walks away for a landing with a 10 ft/sec sink rate. It would be a firm one, but acceptable given the very low probability of such a failure. I'm not aware of any 737 encountering this failure in flight.

Regards,

FCeng84
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Old 3rd Feb 2019, 13:18
  #27 (permalink)  
 
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An old & retired 100-hr 'bus driver here. Was tipped off the type (with pleasure) from the 2nd intake of the second '320 new to the second airline to use them, and was horrified at the control interface & design assumptions. On the other hand the Porsche designed cockpit layout and side-stick made it a dream machine to ride around in, and the things are built like Swiss watches.

For my previous airline & other legs I always hand-flew aircraft to TOC, accelerated, trimmed and cruise inserted before plugging george in. This habit keeps you tightly in the loop re mach effects as you transition from asi to mach, while manually or electrically trimming so you are only ever controlling with your fingertips, locking in zeroing-out elevator loads out constantly. Nothing is then under stress.

Reverting to type on the 'bus I quickly found it a useless occupation. You are basically flying in permanent control wheel steering. You point it to up and it goes up. But doesn’t stay like that.

I found that if you took hand off in CWS, the pitch would very slowly increase, at approx 1° in 12-15 seconds. Autothrottle would mask speed loss. You are playing a computer game in 'buses. I established that it took a fwd pressure with one finger at the top of the side stick (which I lurved) of 8-10 grams, from a spring balance at home with skin deflection comparisons.

So if an autopilot dropped off-line unannouced (of *course it couldn'd happen, could it, coff) the aircraft would slowly begin a long upward arc.

I twice wrote to Airbus, attn. flight testing, at Toulouse, first time as a query, second time more tersely, but was never answered AFAIK. The airline dispute of '89 interupted my hours on type, and I was very happy to revert to actually controlling other birds again, elsewhere.

So in (a long) answer to your Q, Gums. It uses springs, and to make it worse of course, there is zero interconnection between sticks. There is a dinky little dial down there somewhere that shows you who’s doing what with which side-stick. I’m told.

If they had just run some model aeroplane wire-in-tube across the top of the cockpit between the sticks, so that the p’lots could feel what’s going on, those hundreds of people flying across the Western Atlantic in a ‘bus would still be alive.
__

One slightly off topic question: does anyone know why aeronautical engineers went off flying-tabs, a.k.a. servo-tabs. I flew them on the Vickers Viscounts 700, 800 & 900, and they are soo logical. (What’s that? What’s a Viscount 900 then? Well Vickers went bust, and a raft of their designers were grabbed by Douglas, then designing the DC-9. Servo tabs came with those blokes, & they knew their stuff.) Those type of tabs mean that airspeed-induced control-loads are dealt with by well designed servo-tabs sitting in that same airflow. Fabulous control feel & feedback. Light and progressive stick loads, light gauge & light-weight control runs, and all self powered to boot. Lose every service and you don’t have to *think of unpowered controls problems. (I’m looking at your flying hefferlumps, Mr Boeing.)

Any ideas why they’re no longer designed in?
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Old 3rd Feb 2019, 13:28
  #28 (permalink)  
 
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I'm with you, Mr/s Git.
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Old 3rd Feb 2019, 13:57
  #29 (permalink)  
 
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There is also another issue that may be at play (I don’t have any FDR plots for the mentioned accidents to hand so supposition only) - once pointing at the ground and seeing the IAS winding up the instinctive response is to bring the thrust levers to idle. In an aircraft already out of trim nose down with high pitch up force from the engines, the aggravation of the out of trim condition by removing the thrust could be what rendered the aircraft uncontrollable.
Thread drift coming up. Re above quote. During a Singapore court case concerning the Silk Air B737 MIA 185 crash (deliberate although unproven) FDR evidence indicated the pilot rolled the aircraft inverted and pulled through to the vertical while simultaneously selecting full forward stab trim. The company defence claimed the first officer probably forgot to close the thrust levers or select speed brake while performing an emergency descent due perceived pressurisation failure, while the captain was absent from the cockpit. In other words the first officer stuffed up the emergency descent. At the time, indications were the defence seemed intent on shifting any blame away from the captain for the event.

Asked how come therefore the 737 did a purported emergency descent and went in with high power still applied and no speed brake use, the expert witness for the defence said if you apply high thrust in underslung engines the nose will tend to rise. It could therefore be argued the pilot was using high power to try and raise the nose after completing the emergency descent. The aircraft passed Mach One in the dive and broke up before hitting the ground.
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Old 3rd Feb 2019, 15:54
  #30 (permalink)  
 
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Salute Centaurus and Jen from OZ!

I would not worry about thread drift, too much. There's another thread about the Flight Director and we still see many references to "children of the magenta line".

Although I haven't flown for decades, a blog/forum like this would have been invaluable 40 or 50 years ago. Lessons-learned are part and parcel of our profession, and many times a glimmer of recollection concerring some other pilot's problem and actions will save the day. II used to sit in the waiting line for a haircut, or actually in the chair, and go thru every bad thing I could imagine and what I could do. Many reactions/procedures were spelled out for us, but many were not. i also "flew" my next mission in my head. If something happened that I had not done in my 'perfect" mission, I noticed it very quickly.

The biggest difference between the controls in the 'bus and the Viper and Raptor and Lightning is using pressure versus the physical movement of the stick/wheel/yoke. Then there's the control laws that are dictated by the plane's mission. The Viper and 'bus are both a gee command for pitch. But the fighters lack corrections for pitch and roll attitudes, as you can imagine. And BTW, I flew one "light" that had a great control stick steering mode and was used frequently due to the "pitch up" problem ( the VooDoo).

The insidious effect of the 'bus control laws has to do with the attitude and trim corrections. At any attitude other than st-and-level, Hal cranks in both elevator, aileron and trim. And the nominal one gee "neutral" command is biased by pitch and roll. For example, at 30 degrees of pitch the gee command from Hal is about 0.87 gee. If it remained at one gee the plane would continue to increase pitch. This was easy to demo in the Viper because we didn't corret for pitch angle. In fact, at extremely high pitch this attribute helped us to enter a deep stall when we ran outt speed before the elevator could get the nose down. In the 'bus, we saw some of this in the AF fiasco. The stab kept trimming to allow a neutral stick, and the normal reduction in gee command wasn't there due to the control law reversion once air data was deemed unrealiable by Hal.

I really like Jen's comment about the servo tabs. In fact, they can even be used with full FBW systems. I flew one light with the things on the ailerons to enhance roll authority when carrying heavy munitions. A simple torque tube would command the tab to move when control pressure reached a certain value. Our system was mechanical, but with FBW actuators it still works the same. When the ciontrol surface actuator reaches a certain pressure or torque, the tab noves to reduce the pressure. Good ideas here most of the time.

Gums sends...
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Old 3rd Feb 2019, 23:09
  #31 (permalink)  
 
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Those type of tabs mean that airspeed-induced control-loads are dealt with by well designed servo-tabs sitting in that same airflow. Fabulous control feel & feedback. Light and progressive stick loads, light gauge & light-weight control runs, and all self powered to boot. Lose every service and you don’t have to *think of unpowered controls problems. (I’m looking at your flying hefferlumps, Mr Boeing.)
Bring back the 707 and its balance tabs.
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Old 4th Feb 2019, 13:12
  #32 (permalink)  
 
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BAe146 has servo tabs, and yes they were lovely to fly with very good feel. Always looks really bizarre when you see one taxiing and just one elevator moves from full up to full down or vice versa, due to a wind gust affecting just one side. You can overpower servo tabs when making quick control inputs though. In gusty approaches, you can feel that you’re hitting the servo tab end stops.

Flew the 737 Classic and didn’t like the fact that when taking out the AP on approach, it often gives you the aircraft out of trim - hence the column snatch referred to earlier in the thread. So the first thing that happens when you disconnect the AP is you go unstable! I have to say I shuddered every time I stood in the main gear well during my walkarounds, and looked at the mechanical horror of the flight control system. All those cables, and cams, and levers and all over the place. It looks as if each part was added onto what was already there. Oh, we need a spoiler mixer? OK let’s put that........ here. Oh, we need an autopilot? OK let’s have it control another hydraulic jack to push and pull the existing set of rods and levers, and we’ll bolt that....over here. Then all those cables and pulley wheels which all need greasing and tensioning. Shudder......

I can quite understand that it is possible to stretch a 3/16” cable with your own strength: Imagine a 20m length fixed to a point and you have a 3’ lever on the other end. For sure you will be able to stretch it.

Someone mentioned rolling the Boeing to drop the nose? OK, fine, but most of us are not fighter pilots, nor test pilots. If you want us to do this stuff, you are gonna have to train us properly to do it. Telling us to read a book about it is a cop out.

Am quite happy with the simple spring ‘feel’ of the Airbus sidestick, and the FBW system. Lovely !
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Old 4th Feb 2019, 14:11
  #33 (permalink)  
 
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Someone mentioned rolling the Boeing to drop the nose? OK, fine, but most of us are not fighter pilots, nor test pilots. If you want us to do this stuff, you are gonna have to train us properly to do it.
What an extraordinary admission! It is basic stuff used even on Tiger Moths. There is no mystery to it. 200 hour cadets undergoing 737 type ratings are taught in the simulator how to recover from an UA.

Extract from B737 Classic FCTM.

Quote: If normal pitch control inputs do not stop an increasing pitch rate, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45°, up to a maximum of 60°, could be needed. Unloading the wing by maintaining continuous nose-down elevator pressure keeps the wing angle of attack as low as possible, making the normal roll controls as effective as possible.

With airspeed as low as stick shaker onset, normal roll controls - up to full deflection of ailerons and spoilers - may be used. The rolling maneuver changes the pitch rate into a turning maneuver, allowing the pitch to decrease. Finally, if normal pitch control then roll control is ineffective, careful rudder input in the direction of the desired roll may be required to induce a rolling maneuver for
recovery.
Only a small amount of rudder is needed. Too much rudder applied too quickly or held too long may result in loss of lateral and directional control. Because of the low energy condition, pilots should exercise caution when applying rudder. The reduced pitch attitude allows airspeed to increase, thereby improving elevator and aileron control effectiveness. After the pitch attitude and airspeed return to a desired range the pilot can reduce angle of bank with normal lateral flight controls and return the airplane to normal flight.
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Old 4th Feb 2019, 15:55
  #34 (permalink)  
 
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An alternative view of flight control in conventional aircraft is that stick position via cables / rods moves the elevator / tab. Force is a necessary feedback for structural limiting (g) and speed stability (offset trim position). There are many compensating mechanisms - temp / cable stretch, springs for neutral position / stability, alt-speed for structure (q feel).

Offsetting the pitch trim in level, constant speed flight will change the stick force, but does not change the stick position (depends on aircraft type, also ignoring inevitable ‘small’ changes due to mechanisation - trim / elevator).
The aircraft flightpath does not deviate. It can be controlled quite adequately, but requires a pilot compensating force for a given stick position - having to fly out-of-trim.
Without force compensation the aircraft will deviate because the corresponding stick position - position of zero force has changed.

With a trim malfunction pilots are required to apply a compensating force for an ‘apparent’ correct stick position; but the aircraft has a different, and on occasion variable stick position / force relationship.
This requires considerable mental compensation - change from the normal relationship, and applying ‘unnatural’ force / direction. The perception is that aircraft is not controllable according to previous experience and training, and by reversion to the norm the pilot inadvertently seeks to fly zero stick force, allowing the malfunctioning trim to fly the aircraft.
Auto thrust / speed can further complicate the pitch control relationship.

This is a difficult dynamic interaction to describe.
Simulator flight with an offset trim provides a good representation of the revised piloting task (but without surprise), similar to flight with trim failed fixed.
Repeating the demonstration with varying trim position (737 MCAS not yet in simulators?); then the piloting task is to generate a new model of how the aircraft should be controlled for each errant trim setting. In addition the stick force / datum position will change with speed, which also changes because of inadequate aircraft control, which also changes with pilot trim input, and further errant trim input, and … , …
There is no specific anchoring point for reference.

An alleviation is to reference a constant attitude, move the stick to control the aircraft, ignore any force reference. This requires considerable mental effort, which could be already challenged by the circumstances leading to the trim failure (is the aircraft really changing state or just the instrument indications - AoA error generating an UAS like situation, MCAS trim only active with flap up).
Thereafter determine the source of apparent control problem, and if trim related, isolate it.
The speed (altitude, config) will generally determine the maximum trim force which can be held and still manoeuvre the aircraft - depends on specific aircraft design / configuration. If excessive then certification would require an alternative trim mechanism.





Last edited by PEI_3721; 5th Feb 2019 at 10:28. Reason: Typo; misplaced quote removed
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Old 4th Feb 2019, 18:13
  #35 (permalink)  
 
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Originally Posted by Centaurus View Post
What an extraordinary admission! It is basic stuff used even on Tiger Moths. There is no mystery to it. 200 hour cadets undergoing 737 type ratings are taught in the simulator how to recover from an UA.

Extract from B737 Classic FCTM.

Quote: If normal pitch control inputs do not stop an increasing pitch rate, rolling the airplane to a bank angle that starts the nose down should work.
Not at all. I went to a professional CAA approved flying school, and did a full time integrated approved CAP509 course.

I have since done 7 full type ratings on large passenger types and been flying commercially for 17 years. I have studied and flown actual unusual attitude recoveries in PA28 and Zlin aircraft and A320/321/330 and B737 in the SIM, but at no time has anyone mentioned, demonstrated or allowed me to practice rolling from a nose high attitude to allow a nose to drop. This is my point.

If you find it extraordinary, have a go at my instructors and flight training school.

I also notice that the section of FCTM you quote says ‘should’, not ‘will’. I am not sure if normal line pilots ought to be trying fighter jet or test pilot procedures they have never practised?

(I have never yet got a big passenger jet into an unusual attitude, but I will remember this procedure if I ever do in the future )


.







Last edited by Uplinker; 4th Feb 2019 at 19:03.
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Old 5th Feb 2019, 04:11
  #36 (permalink)  
 
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PEI,

I have read (several times) through your input a couple posts back that starts with reference to one of my earlier submissions. I'm having a challenge following the direction of your message. In particular the line from me that you start with spoke to 737 pitch control when one column is jammed and the flight crew is making inputs to the other that is not jammed. If there is something in particular you would like me to comment on I'd be glad to if I was provided a clear question to address.

Respectfully,

FCeng84
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Old 5th Feb 2019, 04:34
  #37 (permalink)  
 
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Uplinker, megan, gums, et al.,

Given the title of this thread and your combined references to balanced surfaces and/or control tabs I feel the need to comment on what balance/tabs can do and how that would not be sufficient for the issues faced by the 737 control system designers. Balanced surfaces and/or tabs can help address control forces when they could otherwise be too heavy (or even too light for that matter). These control system design tools cannot, however, compensate for aerodynamic characteristics that yield control forces with the wrong polarity. For instance, if an airplane has a tendency to pitch down with increased airspeed such that the pilot has to pull to maintain the desired speed the inherent speed instability cannot be solved via elevator surface balancing or design of an elevator tab. Similarly if an airplane exhibits pitch up characteristics with increased AOA that pitch maneuver instability cannot be addressed by balancing or tab design. For both of these characteristics there needs to be compensation through one or more surfaces capable of generating pitching moment to yield net pitch response in the desired, stabilizing direction given a variation in speed or AOA.

The undesirable pitch characteristics described above (speed instability and AOA instability) are both present in some parts of the 737 flight envelope. Insufficient speed stability has been a 737 issue reaching back over a number of minor models. Need to mitigate speed stability issues gave rise to the 737 Speed Trim System (STS). When the 737MAX was found to also have issues with AOA stability the control system features that implemented STS were expanded in the form of MCAS.

Please excuse my going over ground already covered within PPRuNe over the past three months. I just felt the need to make the point that elevator balance and tab design alone would not be sufficient to mitigate the undesirable open-loop 737 pitch characteristics we have been discussing.

Regards,

FCeng84
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Old 5th Feb 2019, 08:24
  #38 (permalink)  
 
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FCeng84,

Spot on, thank you for such a clear explanation. STS and MCAS are there to correct speed instability in certain parts of the flight envelope. This was a point I was trying to make several months ago (perhaps inadequately) when I was arguing that MCAS was not an anti-stall device but a stability augmentation device. An exercise in semantics, perhaps, but there is a difference.
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Old 5th Feb 2019, 10:24
  #39 (permalink)  
 
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I have studied and flown actual unusual attitude recoveries in PA28 and Zlin aircraft and A320/321/330 and B737 in the SIM, but at no time has anyone mentioned, demonstrated or allowed me to practice rolling from a nose high attitude to allow a nose to drop. This is my point.

If you find it extraordinary, have a go at my instructors and flight training school.
I am not the slightest surprised if no one has demonstrated the nose high recovery in the 737 simulator where rolling to the nearest horizon may be the only way to lower the nose. Boeing wouldn't publish that advice in their FCTM unless it was a proven method to prevent a nose high stall. Unless military trained, not many instructors are even aware of this escape method which is precisely why no one has shown you in the simulator.

Many years ago an airline captain colleague of mine who flew fighters in WW2 was flying a Tiger Moth. With no warning,an elevator cable defect caused the aircraft to pitch up sharply at lift off. The elevator was ineffective in lowering the nose. He managed to roll the aircraft to 80 degrees angle of bank which dropped the nose to the horizon as he planned. The same sequence occurred again - that is a severe uncontrollable pitch up. Again he rolled hard to get the nose to drop to the horizon.
.
He completed three gyrations like this around the airport until he managed to get the wings level at 20 feet and touched down on three points just as the next pitch up was occurring. In his report he attributed his survival to an instructor who taught him this manoeuvre even before his first solo - which was in a Tiger Moth as it turned out.
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Old 5th Feb 2019, 10:34
  #40 (permalink)  
 
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FCeng84, re #36.
Post #34 seeks to explain the question about the apparent inability to control pitch (#1), but without involving additional system failures; Occam’s Razor, etc.
In addition, the view based on control displacement was in part to separate the discussion from thread drifts into ‘force’ control input, and also better relate to the piloting problems of flying degraded aircraft.
As such, repeating your quote on jammed controls was inappropriate and misplaced; sorry, #34 edited.

However, are the comments in #34 sufficiently correct from an engineering / design viewpoint, and would these apply to the 737 ?

Re STS, MCAS, thanks again; repetition is a necessary burden in open forums.
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