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Has any one in this forum have access to B737 MAX AMM (Pages from AMM Chap 34-20-00) and if you can post the same system info for B737 MAX redundancy management of AOA signals.
I just cannot believe that one faulty AOA sensor can make the aircraft trim nose down. There has to be more protection in the system design for this not to happen. I think the FAA AD more or less confirms a poor design in B737 MAX. In the B777 which I am familiar with, each of the two ADIRUs (Air Data Inertial reference unit) receive both AOA inputs (There are two AOA sensors on most aircraft, same config on B737 also). This is compared with 'Calculated AOA' and a mid value is used. This is the redundancy built in the system on B777. Also each of the AOA sensor has two outputs, feed into two different computational channels. See the redundancy. There are actually 4 signals from two AOA sensors. The full text from the B777 AMM is as below. AOA Redundancy Management The AOA redundancy management logic uses a modified midvalue selection. The modified mid-value selection chooses the mid-value of these three AOA values: * Left corrected AOA * Right corrected AOA * Calculated AOA. The AOA redundancy management logic receives inputs from the inertial and air data systems to calculate the calculated AOA. So I am a bit curious about the B737 MAX AOA signal Logic. |
Originally Posted by ManaAdaSystem
(Post 10300929)
Take off today. 76 tons. So not low gross weight. I let the STS do it’s job (it was not supposed to do anything due to high gross weight?). It was trimming way longer than the claimed 10 seconds band it should trim after take off. I had to really push forward on the controls after it had finished trimming aft. So much that if I had let go, the nose would have pitched up really fast. I had to trim 5-6 turns forward before the aircraft was stable. That’s a lot! So, we have a system that brings the aircraft out of trim, and trims when it is not supposed to trim. Not super easy. Dangerous. I don’t think about this when I fly manually since the first thing I do is to trim forward in order to cancel out the STS. This system is plain stupid. Try it, and see for yourself. |
I thank you all for your contributions to this discussion. I think I get what you are trying to explain, but I am a simple soul and by my logic, any system that works agains my control inputs when I fly manually is making my work harder. And the STS works when the aircraft is heavy or light, GC fwd or aft, derated or full thrust. That is not how the STS is explained. I have flown the NG since they made it. I have never bothered with the STS before, I simply cancel it with manual trim. I was just when I saw the system discussed that I started to check what it did. So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens. For those of you saying «just let the STS do it’s job». No. You can’t because it will bring the aircraft well out of trim. It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall! |
It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall! |
Originally Posted by RetiredBA/BY
(Post 10304246)
Just an interested bystander as it’s a long time since I flew the 737, 200 and the very first 300s. I can’t remember the 73s or the 75 and 76 having a speed trim system as described here. Found them all delightfull aircraft to fly manually throughout the speed range. So, may I ask if it so vital, to meet the arbitrary speed stability requirements of the FAA that another system , with potential for malfunction with potentially serious consequences is really needed. Do we really NEED such a system and since it IS installed is there an instant cutout switch. When were the values of 3 pounds per 10 knots decided on and why are these exact values so important. Are current models of the 73 significantly more speed unstable than the earlier versions ? I still remember so clearly the problems caused by runaway tailplanes on my first jets, the Canberra and Valiant. Of course I know the flight control electronics, electrics, were nowhere near so advanced or capable as on present jets. Is the FAA using, or requiring Boeing to use a sledgehammer to crack a nut? Just curious! https://leehamnews.com/2018/11/14/bo...to-the-pilots/ |
Originally Posted by Capn Bloggs
(Post 10312432)
No it won't, as the speed reduces, the nose will drop, helped by the STS. The STS ensures the aircraft is speed-stable.
It has never been about flying at 150 kts in a «speed stable» condition. I admit I don’t know how far towards a stall the STS will bring the aircraft, but with the amount of back pressure it creates, I think it will get pretty close before anything happens. If it tries to trim the aircraft back to the «stable speed» of 150 kts in clean config, things will get interesting. By the way, which aircraft type do you fly, Bloggs? |
Forgive me, I have only been flying for 30 years and nearly 20 on the NG So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens. For those of you saying «just let the STS do it’s job». No. You can’t because it will bring the aircraft well out of trim. So either be a pilot or go fly an Airbus. Believe me, you’ll have a lot more “why is it doing this” questions on an Airbus. |
Originally Posted by Derfred
(Post 10312859)
So, after 20 years of 737NG experience, you suddenly have a problem with STS because you read something on PPRuNe. At the risk of repeating myself, who told you to fly the aircraft like that? That is not how it is intended to be flown. It is designed with speed stability, which means that an intentional speed change requires trim, like any non-FBW aircraft. So TRIM FFS. Like you’ve been doing for 20 years. No-one is saying that. They’re just saying don’t expect it to trim to what you want. That’s not what it’s there for. Trimming is your job. Boeing designs aircraft for pilots. So either be a pilot or go fly an Airbus. Believe me, you’ll have a lot more “why is it doing this” questions on an Airbus. I have just not bothered to see what it really does before now. What is wrong with that? Keep defending a system that works continously against the pilot. Never had any other aircraft do that before. I have never flown an Airbus. Why bring it into this discussion? And why are you implying that pilots who fly Airbus aircraft are not pilots? Do you work for Boeing? |
Originally Posted by ManaAdaSystem
(Post 10312883)
Keep defending a system that works continously against the pilot. Never had any other aircraft do that before. |
Originally Posted by Vessbot
(Post 10312888)
By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.
But I still don’t like it. |
Originally Posted by ManaAdaSystem
(Post 10312978)
And in three sentences, you have explained the STS perfectly. Thanks! But I still don’t like it. |
Originally Posted by Vessbot
(Post 10313081)
Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.
I’ve been fly the MAX for 16 months now and it’s a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt.... The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000’ to go) is back up at 1500fpm Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season. |
Originally Posted by Vessbot
(Post 10313081)
Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.
So, I will just continue to trim against it, just as as before. Have a nice weekend! |
Originally Posted by EIFFS
(Post 10313189)
I’ve been fly the MAX for 16 months now and it’s a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt.... The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000’ to go) is back up at 1500fpm Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season. |
According to boeing how is the Lior air accident prevented? |
Originally Posted by ManaAdaSystem
(Post 10313244)
NG -800 has a bad rep when it comes to landing mishaps. I really hope your predicitons about the Max does not come through. There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another “sub system” new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing. The Landing Attitude Modifier (LAM) system performs two functions. The first LAM function applies when the flaps are in the 30 or 40 position. To maintain acceptable nose landing gear contact margin, LAM symmetrically deploys flight spoilers on approach to reduce lift and force the airplane to use a higher angle of attack. The amount of spoiler deflection depends on the approach speed. Deflection begins at approximately 10 knots above VREF. The second LAM function applies when flaps are positions 15 through 30 and the thrust levers are near idle. This function also symmetrically deploys flight spoilers, in order to generate additional drag The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets) Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many don’t ) and you’ve correctly completed a landing distance calculation ( again many don’t) the NG SFP & 737-800 (MAX) model have excellent stopping capability. As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO |
Originally Posted by EIFFS
(Post 10313472)
i There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another “sub system” new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing. The Landing Attitude Modifier (LAM) system performs two functions. The first LAM function applies when the flaps are in the 30 or 40 position. To maintain acceptable nose landing gear contact margin, LAM symmetrically deploys flight spoilers on approach to reduce lift and force the airplane to use a higher angle of attack. The amount of spoiler deflection depends on the approach speed. Deflection begins at approximately 10 knots above VREF. The second LAM function applies when flaps are positions 15 through 30 and the thrust levers are near idle. This function also symmetrically deploys flight spoilers, in order to generate additional drag The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets) Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many don’t ) and you’ve correctly completed a landing distance calculation ( again many don’t) the NG SFP & 737-800 (MAX) model have excellent stopping capability. As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO -800 and FL 40 = pitch nearly 0 degrees. I see this as one of the reasons why pilots have a tendency to go above the glide when they get close to the runway. |
Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.
I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two. Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing. FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention. However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail. The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability. |
Vessbot's analysis is excellent. The only other thing he should added is that it works fine. I've only ever had one significant failure in several thousand hours on the type. Dont over think it. Runaway Stab. Checklist works fine if you have a problem and all else fails put the little feet of the PFD aircraft symbol on the horizon and set 80% N1 then work the problem. Fly the f@#king aircraft. Not that hard.
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Originally Posted by LEOCh
(Post 10318780)
Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.
I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two. Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing. FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention. However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail. The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability.
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Mana - When Boeing developed the C*U control law that is the augmented pitch axis of the 777 a very deliberate design decision was made to provide positive speed stability that would require pilot action to "trim" column forces that build up when airspeed changes. C*U speed stability is implemented by managing within the control law a reference speed and requiring steady column force to fly at speeds away from that reference. Pitch trim on the 777 works to slew the reference speed up and down. The 777 does not drive the stabilizer directly to create speed stability, but it does augment the response characteristics in such a way that column force builds up that must be trimmed off. Without trim, the 777 seeks to return to the reference speed. 777 pilots must trim against the speed stability provided by C*U whenever they change speed regardless of the unaugmented speed stability characteristics of the bare airplane itself. In this way, the pilot task with regard to speed changes is not that different between 777 and 737.
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Originally Posted by FCeng84
(Post 10322332)
Mana - When Boeing developed the C*U control law that is the augmented pitch axis of the 777 a very deliberate design decision was made to provide positive speed stability that would require pilot action to "trim" column forces that build up when airspeed changes. C*U speed stability is implemented by managing within the control law a reference speed and requiring steady column force to fly at speeds away from that reference. Pitch trim on the 777 works to slew the reference speed up and down. The 777 does not drive the stabilizer directly to create speed stability, but it does augment the response characteristics in such a way that column force builds up that must be trimmed off. Without trim, the 777 seeks to return to the reference speed. 777 pilots must trim against the speed stability provided by C*U whenever they change speed regardless of the unaugmented speed stability characteristics of the bare airplane itself. In this way, the pilot task with regard to speed changes is not that different between 777 and 737.
If I understand the 777 system correctly, you do not need at lot of trim if a short trim activation resets the speed reference? I know what I would prefer my aircraft to do. |
I don’t think that it is a major difference between European and US certification rules as such, just a difference in control design. After all there are no changing stick forces in an airbus, the only force is a spring setup in the stick itself to return it to neutral, completely independent of control law and speed. Trim is always automatic except in direct law, and most people forget at first to trim manually if they happen to find themselves in direct law, which is the last fallback level available. That said, the airbus FBW design is older than the Boeing one, and I think that Boeing learned from what happened with the Bus. By the way, it is absolutely astonishing how fast one de-learns trimming on switching from Boeing to Airbus. Just my personal observation. Simply because it is never needed in normal line operation. |
The 777 flight control system is excellent. It flies like a large, more stable 737. Trimming is natural and intuitive and exactly like flying a non-few aircraft. It also keeps you in practice for the two reversionary modes, where manual trim is required. It also means the handling characteristics in reversionary modes are very similar to normal mode and require less capacity in a non-normal situation. C* U is excellent - Boeing certainly as Denti says looked hard and learned lessons from the Airbus approach. |
Salute Denti!
A great point about how fast one de-learns trimming on switching As denti says, the move from a "conventional" plane to the new, improved one is not always a big deal. We old curmudgeons that resisted the Viper limiters abandoned the prejudice real fast. We also had no problem NOT TRIMMING for speed/AoA. It was more trimming for attitude, and the 'bus does that without a "coolie" hat button or whatever. If you are holding stick fore or aft, the the FLCS tries to reduce that pressure/displacement/force. AF447 showed one problem with that philosophy, in that we had a fully deflected stab after "x" seconds since the pilot held back stick forever and the FLCS tried to reduce the back stick displacement. Gums... |
Intermim Report's Out.
http://knkt.dephub.go.id/knkt/ntsc_a...y%20Report.pdf As mentioned elsewhere, the MCAS system didn't get a mention. As also mentioned elsewhere, multiple previous sectors were flown using manual trim wheel, apparently, to handle the defect that never got quite fixed. The CVR still not recovered. I encourage your attention to pages 14 and 16 of the report: the difference in the parameters on the accident flight and the previous flight. |
MCAS is mentioned in the BOEING letter.
Chapter 5.12, p62 |
Originally Posted by Lonewolf_50
(Post 10323788)
As also mentioned elsewhere, multiple previous sectors were flown using manual trim wheel, apparently, to handle the defect that never got quite fixed.
Looking at FR24 data the typical altitude excursions associated with the MCAS trim and stickshaker are only present on the accident and previous flight DPS to Jakarta. Reading pages 7, 8 and 9 it appears to me the plane intermittently didn't show any speed or altitude on the captains side. If i understand correctly this could happen if the left AoA signal was missing as static pressure which is needed for speed and altitude gets corrected by AoA. So intermittently failing AoA sensor might have been the problem. Now in Denpasar the AoA sensor was replaced. This is the most likely point for the introduction of the 20 degree offset. A stuck sensor that just transmits one value might happen as a failure. But tracking a 20 degree offset is very unlikely to occur without some change being made. Possible reasons for the 20 degree offset i can imagine are: Wrong part number Faulty manufacturing Very creative attachment to the plane AoA vane bent after installation and test To quote the report: For troubleshooting due to repetitive problem perform replaced angle of attack sensor in accordance with Aircraft Maintenance Manual (AMM) Task 34-21-05-000-001 and task 34-21-05-400-801 carried out. Installation test and heater system test result good. Now either that test was not performed or they made a mistake here. |
Originally Posted by wiedehopf
(Post 10324020)
Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here. |
The difference between the C* pitch control approach taken by Airbus and the C*U approach taken by Boeing is not so much a difference in cert authority requirements as it is a difference in how these two groups chose to address speed stability requirements. Boeing with C*U chose to continue to meet the existing requirements for stick force per knot as speed changes. Airbus chose to seek a special condition to allow their design not to require any force for speed changes within the normal envelope. Airbus was able to convince cert authorities that because their system provides protections for both overspeed and underspeed conditions there was no need to provide pitch controller force cues for speed changes that do not exceed upper or lower limits. As a result, the Airbus C* system does not require the pilot input to manage pitch trim where the Boeing C*U system does. The Airbus system presents the pilots with less workload. The Boeing system is more conventional when compared with non FBW airplanes. As noted earlier, the Boeing system promotes the pilot maintaining pitch trim skills that come in very handy if failures result in dropping to a degraded mode. It may be easy for pilots to unlearn their pitch trim skills when transitioning to a system that does not require pitch trim. Hopefully those same crews are not caught out too far when a failure drops them to a mode that requires manual control of the stabilizer to keep the airplane in pitch trim.
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With that logic all FBW aircraft should operate in degraded mode at all times. That would keep the pilots ready always. |
Originally Posted by wiedehopf
(Post 10324020)
Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here. turning the vane and checking at what point stick shaker triggers. I don't have the relevant bits of NG or MAX AMM to see if it is the same, but it seems likely it will be similar. We know from the log that they recorded doing an "installation test", but we don't know exactly what. However, if as with the classic, the test is specified as the last step of the install procedure, do you actually need to note the test number given that you have recorded completing the install procedure? Not sure. There is further oddity in the traces too - the 20deg offset is actually not consistent. At the beginning of the previous flight, while on the ground, it looks to be about 10, falling and briefly going -ve, then rising to a constant 20 once the airspeed goes up. At the end of that flight, when airspeed drops, the offset appears to increase a little, and that higher offset is the same at start of next flight, until airspeed comes up and then it goes back to 20. So we may be looking at a sensor fault that is dependent on airspeed and therefore wouldn't have been found on a ground test anyway. On the other hand the vanes are not alive without airspeed and may settle at different points anyway. But that drop at the start of the previous flight may be significant, probably at the end of taxi, the right AOA bounces a bit too, but the left much more. Something came loose? Image to show what I am rambling on about: https://cimg9.ibsrv.net/gimg/pprune....aa36e8b203.png Note that if you are thinking sticky vane (I was), the test procedure on the classic contains the following: slowly rotate sensor vane between stops using light finger pressure. Check that vane rotates without binding or variations in torque |
Originally Posted by MickG0105
(Post 10324100)
Or it's not a raw data (sensor) problem rather it's a processed data (ADIRU) problem. The Left ADIRU seems to pop up quite a bit in the AFML Resolution Descriptions. I've got a fiver on a fault in the Left ADIRU featuring as a contributing factor.
Also, on the NG at least, the data acquisition point for AOA is downstream of SMYD, so we are seeing, in the traces, what SMYD saw/calculated, not ADIRU output. Unless the MAX air data architecture is significantly different, but I don't think it will be. |
Concerning MCAS on the accident flight.
In the tech log no report of SS on previous flight...!? Anyhow "the DFDR showed the stick shaker activated during the rotation and remained active throughout the flight." So the first activation of SS was after replacement of LH AoA sensor. Is it possible to install a RH AoA sensor in the LH position? Part number confusion...? Just asking. |
Originally Posted by ManaAdaSystem
(Post 10324419)
With that logic all FBW aircraft should operate in degraded mode at all times. That would keep the pilots ready always. |
Originally Posted by infrequentflyer789
(Post 10324434)
There is further oddity in the traces too - the 20deg offset is actually not consistent. [...]
https://cimg9.ibsrv.net/gimg/pprune....aa36e8b203.png In summary: he doesn't have a failure mode that fits all the trace data either. |
Infrequentflyer, I think we can easily explain the WTF point on the graph you presented.
Siince the aircraft is not yet airborne, that is probably brief jet blast impingement on the nose of the aircraft. Additionally, Mr. Lemme has called attention to the difference in signal between left and right AOA while taxiing. The left AOA was noisy and the right AOA was rather smooth in that segment of the FDR data. IMO that would be typical of a sneak circuit that is being excited by vibration transmitted by the landing gear into the fuselage. Sneak circuits commonly originate at bent connector pins, conductors that are chafed by adjacent structure, or wires that become pinched when other components are installed, but there are also many other possibilities. |
Originally Posted by Machinbird
(Post 10327307)
Infrequentflyer, I think we can easily explain the WTF point on the graph you presented.
Siince the aircraft is not yet airborne, that is probably brief jet blast impingement on the nose of the aircraft. Additionally, Mr. Lemme has called attention to the difference in signal between left and right AOA while taxiing. The left AOA was noisy and the right AOA was rather smooth in that segment of the FDR data. IMO that would be typical of a sneak circuit that is being excited by vibration transmitted by the landing gear into the fuselage. An intermittent wiring or connection fault, at or near the sensor, could account for the issues that triggered the sensor replacement and the worsening of the problem after it was replaced. If it is vibration triggered then ground testing might fail to find it (and the AOA sensor might test out fine). Be interesting to know if that AOA sensor had been replaced before or was factory fitted. |
From the descriptions in this thread and external inks, #97, and particularly https://www.satcom.guru/2018/11/stabilizer-trim.html, the STS is a ‘crutch’ on late series 737 to meet low speed stability requirements. MCAS appears to be a similar ‘crutch’ but addressing more specific nose up issues when approaching or at stall (25.203), and when turning where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines -737 MAX. However, it is difficult to understand the effects of input failures (e.g. AoA) amongst the complex computations and interactions in these trim systems. (Forgive the ‘Non PC’ quip, but a failure in heavily ‘crutched’ systems, literally leaves you with no leg to stand on.) - ‘close coupled’ systems. The association of the Elevator Feel and Centering Unit with trim has been described, and the reasons for separating the (independent?) pneumatic ‘muscle’ from the electronic logic in the trim system - mechanical, elevator feedback. Is it possible that a false AoA will trigger a change in the trimmed - elevator neutral shift unit due to a false change in the feel / centering unit ? If so, then a consequence might be an erroneous trim datum, which may also be misplaced by MCAS and / or electric pitch trim, so that no stable trim condition can be achieved. A pilot will continuously ‘chase’ a stable pitch condition - there is no trimmed position for speed (trying to fly in-trim with the elevator offset). Furthermore with similar mechanism, would trim / AoA offset cause the feel unit to position at it’s maximum extent, and so doing limit the available amount of stabilizer / elevator for control ? (and / or stick force limit). Aerodynamic slat operation also appears as a connection within the various ‘trim’ computations, relating to AoA input. Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ? Possibly with significant adverse pitching moments competing with pitch control and trim. Or even with the ‘handed’, separate-side AoA inputs and computations, could there be asymmetric slat deployment. (The slat thoughts above come from the apparent roll control issues in the Lion accident.) |
Originally Posted by safetypee
(Post 10327740)
From the descriptions in this thread and external inks, #97, and particularly https://www.satcom.guru/2018/11/stabilizer-trim.html, the STS is a ‘crutch’ on late series 737 to meet low speed stability requirements. MCAS appears to be a similar ‘crutch’ but addressing more specific nose up issues when approaching or at stall (25.203), and when turning where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines -737 MAX. There was/is a similar mechanism added to the NG as part of speed trim function, my guess is MCAS evolved from it and replaced it. The NG AMM says: Near stall, the speed trim function trims the stabilizer to a nose down condition to allow for trim above the stickshaker AOA and idle thrust. The trim continues until the stabilizer gets to its limits or the aft column cutout position is exceeded
Originally Posted by safetypee
(Post 10327740)
However, it is difficult to understand the effects of input failures (e.g. AoA) amongst the complex computations and interactions in these trim systems.
(Forgive the ‘Non PC’ quip, but a failure in heavily ‘crutched’ systems, literally leaves you with no leg to stand on.) - ‘close coupled’ systems. Essentially the automation paradox again. Is it possible that a false AoA will trigger a change in the trimmed - elevator neutral shift unit due to a false change in the feel / centering unit ? [...] Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ? |
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