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B737 controlability-questions & surprises.
We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.
Originally Posted by Machinbird
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?
Originally Posted by FCeng84
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. 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. |
Originally Posted by Machinbird
(Post 10365348)
We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.
After some back and forth discussion: FCeng84 had the following observation: |
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. |
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. |
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. |
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. |
Originally Posted by Capt Quentin McHale
(Post 10365528)
I can't see too many pilots "stretching" 3/16th dia carbon steel cables via the control columns!!!
And over the length from the nose to the tail, a little adds up to a lot. |
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. |
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.... . |
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. |
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. |
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... |
We have seen three B737 fly into the ground/water at steep angles in recent years, and there appears to be a common thread.
Alaska Air Flight 261 crash: What is a horizontal stabilizer? Are there previous crashes caused by stabilizer malfunctions? |
We are not seeing a repeat of earlier accidents where the trim motors couldn't generate sufficient torque to overcome the stabiliser loads? 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. |
Originally Posted by gums
(Post 10365914)
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! |
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? |
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.. |
https://cimg5.ibsrv.net/gimg/pprune....aa40be0dc6.jpg
https://cimg9.ibsrv.net/gimg/pprune....6ad52798f9.jpg note all of the connections.... https://cimg6.ibsrv.net/gimg/pprune....b1636e9100.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. |
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.
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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. 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 :) |
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