B737 controlability-questions & surprises.

[Machinbird]

We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.

All were significantly out of trim nose down,
All experienced zero g or negative g during the pitch over,
All did not significantly reduce their angle of descent once they started down.

As pilots, we have to ask, how did this happen? How did these crews get themselves so badly out of trim and why couldn’t they at least try to pull out of their dives? The answer is beginning to emerge that they could no longer control their aircraft in pitch! I suppose it is not surprising that an aircraft that is trimmed heavily nose down might be hard to keep in level flight, but there are several effects that do not appear to be widely understood that are making this situation much worse than it initially appears! My education began with a PM question to FCeng84 on control jam prevention measures implemented in the 737. FCeng84 has agreed to the sharing of his replies.; This is not his or my complete response, however. There is more detail available.

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?
One of the characteristics of the 737 elevator control system that must be taken into consideration is the compliance (stretch) of the mechanical cables that run the length of the airplane from the control columns in the flight deck to the elevator input torque tube at the back end. When considering the behavior of the overall system, these cables are modeled as springs. That design came together well before my time so I don’t know if those cables were specified to have the spring constant (i.e., stiffness) that they do or if that was just a fallout of the materials used. Either way, the stretch of those cables contributes to both the handling qualities of the nominal system and the ability to cope with the consequences of a column jam.
For nominal operation (no failures) when the flight crew applies force to the column that force is transmitted through the mechanical cables to the elevator input torque tube. The motion of the torque tube (and thus the elevators) is less than it would be if these cables were infinitely stiff. Some of the column motion is essentially lost to the compliance of the mechanical cables. The amount of force it takes to move the columns (and similarly the amount of “lost motion”) is related to the stiffness of the feel and centering unit located at the back end of the airplane. At low speeds when the centering unit is softest (low forces) most of the motion of the column is transmitted to the elevator input torque tube. As speed increases and the centering unit stiffens, more and more of the column motion is lost to mechanical cable stretch and not reflected in elevator motion. The net result is that increasing airspeed causes the column stiffness to increase and the column to elevator gearing ratio to decrease. These two characteristics combine to result in a significant variation in elevator per pound of column force. This elevator per force gain variation is far greater than it would be if the mechanical cables were infinitely stiff.
An interesting side note is that when Boeing designed the 777 it was decided that the elevator control system mechanical gain variation described above should be preserved even though the 777 has no mechanical elevator control cables running the length of the fuselage. The 777 column linkages, variable feel mechanism, and the column position sensor pickoff point were designed to provide via mechanical means the same variation in column force to elevator gearing that was historically provided by the arrangement on the 737 (and subsequently 747, 757, and 767). There are specific elements in this design specified to have the proper amount of compliance to emulate the cable runs of the earlier non-FBW models.

Now back to the 737NG and the question of column jam mitigation. With one column jammed, the other column can be moved separately if sufficient force is applied to cause the cross cockpit column-to-column linkage to break free. In that event, motion of the column that is not jammed causes the mechanical cables between that column and the elevator input torque tube and the parallel cables running from the torque tube back up to the jammed column to stretch. The jammed column does not move, but the elevator input torque tube will move as it sits between the two sets of cables that are both being stretched. The forces will be significantly higher than nominal, but analysis and testing has shown that sufficient elevator motion can be commanded in this manner to provide for continued safe flight and landing with one column jammed.

Note that this system does not provide mitigation for a jam of the elevator input torque tube itself. Design and analysis has shown the probability of that event to be sufficiently remote that provisions for continued operation with that failure were not required.

To the question of elevator authority, a characteristic that has not had much (if any) play within PPRuNe recently is that of elevator blow down. The elevator actuators do not have the ability to generate enough hinge moment to push the elevators to their mechanical limits when operating at very high speeds. When this happens, max hydraulic pressure will extend the elevators as far as it can, but will be force limited. It is possible that toward the very end of the Lion Air 610 flight speed increase resulted in elevator blow down that further compromised the crew's ability to counter the stabilizer pitching moment via the columns/elevators.

After some back and forth discussion: FCeng84 had the following observation:
Either way, it seems clear that the flight crew were not able to generate enough nose up pitching moment using the elevators via the column to counter the mis trimmed horizontal stabilizer. Any time that a pilot flying such a plane with such a configuration (elevator and all moving horizontal stabilizer) senses that they are at risk of running out of pitch control authority they should get on the pitch trim in that direction first and ask questions later.

[flyzed]

We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.

All were significantly out of trim nose down,
All experienced zero g or negative g during the pitch over,
All did not significantly reduce their angle of descent once they started down.

As pilots, we have to ask, how did this happen? How did these crews get themselves so badly out of trim and why couldn’t they at least try to pull out of their dives? The answer is beginning to emerge that they could no longer control their aircraft in pitch! I suppose it is not surprising that an aircraft that is trimmed heavily nose down might be hard to keep in level flight, but there are several effects that do not appear to be widely understood that are making this situation much worse than it initially appears! My education began with a PM question to FCeng84 on control jam prevention measures implemented in the 737. FCeng84 has agreed to the sharing of his replies.; This is not his or my complete response, however. There is more detail available.


After some back and forth discussion: FCeng84 had the following observation:

Thank You Machinbird, very interesting analysis, you and FCeng84 have done an exceptional study.

[Fursty Ferret]

An interesting side note is that when Boeing designed the 777 it was decided that the elevator control system mechanical gain variation described above should be preserved even though the 777 has no mechanical elevator control cables running the length of the fuselage.

Suddenly the sloppy handling characteristics of the 787 make sense. Wonder if this explains the column snatch and out of trim condition when the autopilot is disconnected?

[172_driver]

Thank you for an interesting technical description!

From a pilot's perspective.

As pilots, we have to ask, how did this happen? How did these crews get themselves so badly out of trim and why couldn’t they at least try to pull out of their dives? The answer is beginning to emerge that they could no longer control their aircraft in pitch!

Any time that a pilot flying such a plane with such a configuration (elevator and all moving horizontal stabilizer) senses that they are at risk of running out of pitch control authority they should get on the pitch trim in that direction first and ask questions later.

You came to the conclusion yourself. While you shouldn't control the aircraft with pitch trim, elevator input and trim wheel motion must come close together or control column forces will quickly become uncomfortably high. The QRH Maneuver section mentions pitch trim may be necessary for both Nose High and Nose Low recoveries.

[Capt Quentin McHale]

FCeng84,

With respect, but sorry, I can"t get my head around "cables modelled as springs" and "cables stretching" with the "ability to cope with the consequences of a column jam". I can't see too many pilots "stretching" 3/16th dia carbon steel cables via the control columns!!! So what does the elevator breakout mechanism under the cockpit floor for a left or right cable jam do, and, the pogo"s at the elevator pcu's do then???

What exactly are your qualifications? Are you a engineer/mechanic/LAE/LAME???

Rgds McHale.

[Judd]

Any time that a pilot flying such a plane with such a configuration (elevator and all moving horizontal stabilizer) senses that they are at risk of running out of pitch control authority they should get on the pitch trim in that direction first and ask questions later.

Keeping in mind that if the pitch attitude is rapidly increasing, it may be better to rapidly roll the aircraft to the nearest horizon which allows the nose to drop while simultaneously using appropriate stab trim. This advice is clearly explained in the 737 FCTM. What has always concerned me is the majority of 737 pilots I have flown with during simulator training, never had this manoeuvre demonstrated during their type rating or recurrent simulator training. They are told by their instructors to "read your FCTM" as if that is sufficient advice to ensure hands-on competence. Well, it isn't.

[Vessbot]

I can't see too many pilots "stretching" 3/16th dia carbon steel cables via the control columns!!!

Every material is like Jell-O, even diamond. It's just a matter of how much or how little.

And over the length from the nose to the tail, a little adds up to a lot.

[Machinbird]

Capt Quentin McHale, I think the answer to your question lies in visualizing the cable run to the back end of the aircraft. The cable is likely supported at regular intervals along its length, and forms catenary arcs in between the points of support.
(In physics and geometry, a catenary is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends.) The droop in each catenary segment is dependent upon cable tension.
As you increase tension in the cables, they approach a straight line segmenst between the points of support and act like tension springs.
Another factor to consider is that with large aircraft, there is a significant difference in the rates of thermal expansion between the steel control cables and the aluminum structure. Larger aircraft have tension compensating devices incorporated at the points of cable termination to allow for these differences. These tension compensating devices are normally springs.

[gums]

Salute!

TNX, 'bird.

As to "stretch" in the cables, there must be some kinda "tensioning" component to the cable runs, as well as the circuitous route the cables must take, as 'bird mentioned. So using a spring for the models seems O.K.

Both 'bird and I and some others here flew the military lites, and we had zero cables or even pushrods to the control surfaces. They were moved by irreversible hydraulic actuators according to the hyraulic pressure exerted from us moving the stick or yoke. In my last jet, we had constant pressure to the servo-actuators conmtrolled by electrical signals Hal sent there according to our commands (and his), hence "the electric jet". So modeling the "cables" does not apply.

As far as rolling the plane for runaway trim, that's a player for nose up. Might be uncomfortable for the SLF, but gives you a little time to use trim wheel or button, or turn off the system. Nose down is a bear, 'cause you would have to roll inverted.

The 737 wing shape ( camber/washout/sweep) and the relatively small elevator are players here. The plane apparently has a low critical mach before having "nose tuck" problems. And then there's the elevator effectiveness once at or above that mach number. So the advice to use trim when having pitch problems is good advice. Yeager used his mechanical pitch trim for the stabilizer before losing control because his elevator didn't work right once the shock waves came into play. He told me that this problem was not understood at the time, but an engineer told him about the possibility. When the opportunity arose, he trimmed his way back under control ( "luck is when preparation meets opportunity") As a result, our fighters went to all-moving stabilzers for pitch. This was facilitated by the irreversible hydraulics to move the stab as I mentioned earlier.

Gums comments....



.

[Jwscud]

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.

[Machinbird]

Jwscud, from the engine fuel flow traces from Lionair JT610, there was a substantial increase in fuel flow as they began to lose control. They were trying to get the nose back up, but it didn't help it seems.
I suspect that they encountered negative g as they pitched over. Hand placement on the yokes, and fear caused them to yank at the controls and forget about the trim.
We have to remember however that it only takes a few pounds of force at most to actuate the control valves on the hydraulic cylinders. The rest of the force you are expending is wrestling with the feel system. That feel system is just a mechanical/hydraullic gorilla at the back end of the aircraft that you are fighting with. You are not big enough to win that battle. The aerodynamic forces are just simulated by the feel system.

The aircraft was designed to be flown in trim. You have to always remember that fact when in the heat of battle.
In fact, there is a mechanical link between the stabilizer position and the feel computer. Its function is to compensate for cg location. When you are well out of trim, you are in effect giving the feel computer bad information.

[gums]

Salute!

Good points, 'bird. And I don't like all the mechanical stuff for feel that is a function of the FCC boxes and such. Seems very complicated. And I still hang on to some kinda mechanical failure right at the end game, as up until then the crew was able to trim with the switches and then have the 5 second delay before Hal trimmed nose down. Hell, it could have even been a thermal problem with that electric motor, as how many cycles in a 7 or 8 minute period did they test?

Any 'bus drivers here? Explain to us your "feel". From what we saw in the books few years back, it looks about like the Viper. In other words, none except for a spring!!! The pilot "feels" what the plane is doing for the most part. Of course, the 'bus pilot commands an attitude-corrected gee and a modified roll rate. But I can not find "force feedback" on the two sticks.

Gums sends...

[megan]

We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.

All were significantly out of trim nose down,
All experienced zero g or negative g during the pitch over

We are not seeing a repeat of earlier accidents where the trim motors couldn't generate sufficient torque to overcome the stabiliser loads?

Alaska Air Flight 261 crash: What is a horizontal stabilizer? Are there previous crashes caused by stabilizer malfunctions? (http://www.airlinesafety.com/faq/faq10.htm)

[Machinbird]

We are not seeing a repeat of earlier accidents where the trim motors couldn't generate sufficient torque to overcome the stabiliser loads?
Good question megan. When looking at the trim inputs on JT610 as they lost control, there appear to be some very short stabs at nose up trim that would not have been enough to affect the outcome.
Why they did not continue to hold the trim buttons down is not known at this time. Maybe when the CVR data is disseminated we will know.
Could be that they felt it wasn't fast enough to help their situation. Could be that the trim wheel wasn't turning.
Apply Occam's Razor and you shouldn't be too far from the truth.

[hans brinker]

Salute!

Good points, 'bird. And I don't like all the mechanical stuff for feel that is a function of the FCC boxes and such. Seems very complicated. And I still hang on to some kinda mechanical failure right at the end game, as up until then the crew was able to trim with the switches and then have the 5 second delay before Hal trimmed nose down. Hell, it could have even been a thermal problem with that electric motor, as how many cycles in a 7 or 8 minute period did they test?

Any 'bus drivers here? Explain to us your "feel". From what we saw in the books few years back, it looks about like the Viper. In other words, none except for a spring!!! The pilot "feels" what the plane is doing for the most part. Of course, the 'bus pilot commands an attitude-corrected gee and a modified roll rate. But I can not find "force feedback" on the two sticks.

Gums sends...


And I still hang on to some kinda mechanical failure right at the end game, as up until then the crew was able to trim with the switches and then have the 5 second delay before Hal trimmed nose down.
I remember reading somewhere that it looked like the PIC had been flying and correcting the autotrim inputs, but just before the end handed control to the FO, presumably to troubleshoot. It said the FO made small trim inputs counter to the autotrim, but each of those inputs allowed the autotrim to add the full 2.5(?) units of AND, and no one switched off the autotrim. I am afraid the final result will find no difference in problems from the previous leg, just a different outcome.

As a (somewhat reluctant) bus driver myself, the systems are too different to make a valid comparison, for me at least!

[eckhard]

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?

[gums]

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..

[underfire]

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/452x1034/lhailinputng_fe4aa88f1ef4a9d1ab68081a4b8e7daa40be0dc6.jpg


https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/570x568/elevfeelspring_343dc47e07940a6ef634a3a8fff2676ad52798f9.jpg

note all of the connections....

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/955x582/737aileronfcs_5616d248a967cabd7f21d2c4cfe3f0b1636e9100.png

and for the hell of it...

The thermal expansion coefficients were as follows:
the steel-wire rope cable was 1.92×10−5/°C ;
the steel strand cable was 1.38×10−5/°C ;
the steel tendon cable was 1.87×10−5/°C;
and the steel rod was 1.19×10−5/°C.

[RVF750]

For what it's worth, I'd say the main problem is most modern 737 drivers sit with very low hour cadets in the right seat, and Boeing's training is very much focused on automatic flight. When they reach the left seat, a lot of Captains came up the same way and have done very little hand flying. Neither pilot is very confident hand flying and very few practice regularly. Thus when it goes pear-shaped, they are not ideally trained to cope. Grumpy old git. That's me.

[Sidestick_n_Rudder]

Gums says
Any 'bus drivers here? Explain to us your "feel". From what we saw in the books few years back, it looks about like the Viper. In other words, none except for a spring!!! The pilot "feels" what the plane is doing for the most part. Of course, the 'bus pilot commands an attitude-corrected gee and a modified roll rate. But I can not find "force feedback" on the two sticks.

Bus sidesticks just have a simple spring, there’s no force-feedback computer or any mechanical device. Control sensitivity vs speed is taken care of by FBW and C* control law (or whatever has been programmed by Airbus, as they keep the details secret). The stick force is only dependent on stick displacement, not airspeed.

However, as airspeed increases, you need less elevator deflection for the same “gee”, which the FBW system commands.

It’s not the case in Direct Law, where you get elevator deflection as a function of sidestick deflection and there’s no force-feel, there’s nothing to stop you from overs-stressing the airframe at high speed with abrupt control movements. Because of that Airbus is paranoid about Direct Law and avoids it like plague in case of malfunctions. Also, there’s plenty of warnings in the FCOM about handling in Direct Law.

As as the last point, FWIW. While I have never flown a real Viper (would love to!), I had an opportunity to fly an F16 simulator at a friendly airbase and was surprised (almost disappointed) to find that the thing handled very similar to the A320, that I flew at the time! As an Airbus pilot, I felt right at home with the Viper side-stick, from the very first minute.

Also, had a friend or two, that moved from the Viper to the 320 and they confirmed that A320 behaves pretty much like a fully loaded F16 - even speeds/climb rates are similar :)

[Goldenrivett]

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-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas-jt610/

[gums]

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.

[FCeng84]

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.

[eckhard]

Ok, thanks for the information.

[Capt Quentin McHale]

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 moves 4 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.

[FCeng84]

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

[JenCluse]

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?

[JenCluse]

I'm with you, Mr/s Git.

[Centaurus]

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.

[gums]

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...

[megan]

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.

[Uplinker]

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 !

[Centaurus]

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.

[PEI_3721]

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.

[Uplinker]

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 :ok:)


.

[FCeng84]

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

[FCeng84]

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

[Bergerie1]

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.

[sheppey]

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.

[PEI_3721]

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.

[six string]

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.

[FCeng84]

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

[PEI_3721]

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.

[gums]

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....

[FCeng84]

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

[FCeng84]

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

[hans brinker]

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.

[Bergerie1]

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.

[ManaAdaSystem]

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.

[sheppey]

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.

[FCeng84]

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.

[Bergerie1]

FCeng84,
Thank you for making that so clear.

[Bergerie1]

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.