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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 17th Nov 2018, 19:24
  #1361 (permalink)  
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I think the problem with that, Mac, is that in rare cases a times-5 scenario would be real, and vitally important it be acted upon - even if only to warn.

The extremes experienced, especially in the last pitch up and dive, may well have been preceded by unprecedented in-flight streams of data, at least in civil flying. And yes, I'm taking into account the 447.


A by the way. Paniced. 'A combination of pissing ones pants whilst being in a panicked state.' I had to look it up, albeit in the Urban dictionary.

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Last edited by Loose rivets; 17th Nov 2018 at 19:36.
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Old 17th Nov 2018, 19:45
  #1362 (permalink)  
 
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Originally Posted by Mac the Knife View Post
"You could even apply a (fourth redundancy!) reasonableness test on the alpha vane's sensor values by writing more software that applies plain old logical common sense: --> Can that high alpha value, seen all of a sudden, and when the airspeed is good, and when g's aren't being pulled, be a valid value of alpha?"

Well, I'm not a pilot but I have written a lot of laboratory/experimental software. Before a chunk of data is allowed into the record, it is compared with the expected value and trend over the last 5 minutes - if it is >(say)5x the expected value it gets discarded as anomalous and an interpolated value substituted. Inputs must be sanity checked before being accepted.

Having said that, writing more and more code can easily introduce race conditions and more corner-case results, so it is far from easy.

Mac
That is basically the idea. Except maybe the "trend" you refer to would be governed by basic kinematic relationships & differential equations used in the short term in the context of an aircraft.
In a real-time dynamic environment of an aircraft, we can apply estimation algorithms to do a better job at detecting which AOA vane is bad. Sanity checks, reasonableness tests, whatever you want to call it, that Artificial Intelligence is only used to find a bad sensor. That way, it would take multiple failures of more than one subsystem instead of just taking one lone AOA vane value to cause an accident.
Certainly to use any algorithm would require healthy air data & inertial boxes, and the sanity check algorithm would need to be disabled if those redundant systems were determined sick. Those all go into the FTA/FMEA mentioned.

In the past, I've seen sanity check algorithms experience resistance from old management. They think "we've never had to do that before" and "we don't know how the FAA would certify the sanity check" and "Don't cost the company extra expense or you're fired." types of arguments. They are very conservative. 'Split Cockpit' design philosophy here extended to nose-down trim authority, which I think crossed a safety line.
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Old 17th Nov 2018, 19:51
  #1363 (permalink)  
 
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Originally Posted by Organfreak View Post
Another of hundreds of speculative posts: It appears that their (Boeing and FAA) idea of what most pilots would do in such a situation was over-optimistic, a product of wishful thinking.
What does that leave us with?

Should the training be mandated by the FAA against assumptions and is it affordable in time/cost?

It would seem that when an operator chooses to operate an airplane they are the one that assumes that their specified training is good enough. If you expect Airbus and Boeing etal. to accept risk then you must sign this off somehow.
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Old 17th Nov 2018, 20:02
  #1364 (permalink)  
 
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Only a software engineer but

.....from reading all the excellent info here, is it too simplistic to say that the airframe’s aerodynamic characteristics were a poor fit for the much-more powerful engines?

Thus extra software logic trees having to save the day, and said patches ultimately finding themselves cornered by certain exception scenarios.

I mean, it just sounds like you couldn’t safely put these engines on this airframe. Am I too simplistic?

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Old 17th Nov 2018, 20:16
  #1365 (permalink)  
 
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Originally Posted by lomapaseo View Post
What does that leave us with?
Should the training be mandated by the FAA against assumptions and is it affordable in time/cost?
It would seem that when an operator chooses to operate an airplane they are the one that assumes that their specified training is good enough. If you expect Airbus and Boeing etal. to accept risk then you must sign this off somehow.
This comes down to philosophy of regulation, doesn't it? I'd suggest, at this juncture, the fox must no longer be allowed to guard the chickens. The bottom line apparently trumps (if you'll pardon the expression), everything.

Last edited by Organfreak; 17th Nov 2018 at 20:17. Reason: typo
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Old 17th Nov 2018, 20:16
  #1366 (permalink)  
 
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In the past, I've seen sanity check algorithms experience resistance from old management. They think "we've never had to do that before" and "we don't know how the FAA would certify the sanity check" and "Don't cost the company extra expense or you're fired." types of arguments. They are very conservative. 'Split Cockpit' design philosophy here extended to nose-down trim authority, which I think crossed a safety line.
Quagmire, when Boeing became a delegated authority, they implemented robust protections against "Undo Pressure" - arguably much better than before delegation. When I was a DER, if I felt I was being pressured to approve something, my only real recourse was to go to the FAA - a very drastic step - and the FAA didn't really have a defined process to handle it. When Boeing initially moved towards becoming delegated, one of the first things they did was set up a process for 'Undo Pressure" - with training (both for the newly defined 'Authorized Representative" - AR - what the DER job morphed into - and for management. It worked - on the rare occasion when I thought management was getting out of line, all I had to do was say "This is starting to border on undo pressure" - and they'd immediately back off. Heck, one time I was catching flak from Rolls because I was refusing to approve some new FADEC software, a Boeing manager actually told Rolls to back off - I was just doing my job.
I don't know what happened during the certification of MCAS, but I seriously doubt it was management interference. Educated guess is that it simply wasn't identified as a flight critical system and didn't get the level of scrutiny appropriate for a system that can have catastrophic consequences if it malfunctions.
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Old 17th Nov 2018, 20:32
  #1367 (permalink)  
 
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And so B. "grandfathers on" Yep, it still has one fuse and two underslung engines...makes sense
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Old 17th Nov 2018, 21:38
  #1368 (permalink)  
 
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Last week FAA spokesman Greg Martin said

“the angle of attack values used by several systems, including the air data, the fight controls, the stall warning, etcetera, the safety analysis for each of these systems are currently being reviewed.”
On the face of it, a fairly boilerplate statement under the circumstances. However, I've been wondering if the MCAS would have been the only stall protection triggered by erroneous AoA data. The -800 FCOM states:

Stall Identification

Stall identification and control is enhanced by the yaw damper, the Elevator Feel Shift (EFS) module and the speed trim system. These three systems work together to help the pilot identify and prevent further movement into a stall condition.

During high AOA operations, the Stall Management/Yaw Damper (SMYD) reduces yaw damper commanded rudder movement.

The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.

As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables speed trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced.
Can anyone familiar with the systems comment on whether the EFS and/or speed trim protections would likely have been active? Looking at the Runaway Stabilizer checklist, I'm struck by the fact that in the circumstances either or both the EFS and/or speed trim protections may have posed a serious impediment to recovering from an MCAS induced stab trim pitch down.

Control airplane pitch attitude manually with control column ...
may be somewhat problematic if the EFS is increasing forward control column force to approximately four times normal feel pressure, and

​​​​​​... and main electric trim as needed.
If the trim system senses that the AoA is already at or aboveabove stickshaker AoA will it recognise nose up trim commands using the main electric trim? (ie will it allow the aircraft to be trimmed further into the 'perceived' stall?)

Last edited by MickG0105; 17th Nov 2018 at 23:17. Reason: Word choice correction
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Old 17th Nov 2018, 22:21
  #1369 (permalink)  
 
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In the GA world, there is a probeless AOA , description here, based on Sperry Patent #3,948,096.
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Old 17th Nov 2018, 22:51
  #1370 (permalink)  
 
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Originally Posted by tdracer View Post
Quagmire, when Boeing became a delegated authority, they implemented robust protections against "Undo Pressure" - arguably much better than before delegation. ............ Educated guess is that it simply wasn't identified as a flight critical system and didn't get the level of scrutiny appropriate for a system that can have catastrophic consequences if it malfunctions.
Nice to hear DER pressure has been alleviated somewhat. ... Agreed, pitch trim in a split cockpit from 1 bad AOA vane was being thrown to the pilots to sort out, because it could be handled. Pilot backup in this case. This philosophy should be questioned.

Originally Posted by MarcK View Post
In the GA world, there is a probeless AOA , description here, based on Sperry Patent #3,948,096.
Yes, the patent describes a way of geting AOA from other non-vane sensors (air data & inertial) via complementary filtering & kinematic reconstruction, an early stab at analytical redundancy which can be very useful.. In fact all sorts of variations have been tried out over the years...Sperry was the first company I worked for, and the patent was by Harry Miller. (The B-2 does some amazing software tricks.)

MickG0105 reported above: Last week FAA spokesman Greg Martin said, Quote:“the angle of attack values used by several systems, including the air data, the fight controls, the stall warning, etcetera, the safety analysis for each of these systems are currently being reviewed.”
When writing pitch control laws, the rule is to avoid use of dual sensors in favor of triplex sensors only, where the elevator is involved especially. AOA failures shouldn't affect autopilot modes anyway. This nose down stall protection is similar.
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Old 17th Nov 2018, 23:44
  #1371 (permalink)  
 
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Originally Posted by MickG0105 View Post
If the trim system senses that the AoA is already at or aboveabove stickshaker AoA will it recognise nose up trim commands using the main electric trim? (ie will it allow the aircraft to be trimmed further into the 'perceived' stall?)
Good insights MickG0105, good addition to the knowledge base.

My understanding is that manual electric trim overrides the Speed Trim System (STS) for 5 seconds, ie if a manual electric trim input is added every 4.9 seconds, STS will not apply nose down trip at high AoA. However as you correctly note, the Elevator forces to hold this high AoA will increase dramatically due to the EFS operation. Pushing forward will feel easy, any back pressure will be significantly more difficult. This protection is another subtle way the system is saying "do you really want to keep pitching nose up", together with the other clues stick shaker and the "stall stall stall" callouts added after the Turkish accident IIRC.

The MCAS however, appears operate at high AoA unless the stab trim switches are in the cutout position. These switch are never touched in normal operation.

Here is a recent stall training video, I believe it may be the MAX. Unfortunately, the trim wheel is difficult to see, and it's not clear if the PF or STS/MCAS is trimming.

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Old 18th Nov 2018, 03:51
  #1372 (permalink)  
 
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How about this for an out of range response.
in case computer senses unreliable data, or in case of pilot disengaging Autopilot.
1 audible alarms are silenced
2 flight display dissapears and is replaced by a full screen attitude display from the selected gyro (the other two gyros displayed small to one side)
3 tastefully displayed at the perimiter of said attitude display could be some suggested thrust settings for common scenarios
4 any further audible prompts must only be driven from the selected gyro

I cannot see the logic of allowing alarms to continue if their data is suspect!
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Old 18th Nov 2018, 05:54
  #1373 (permalink)  
 
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Originally Posted by CurtainTwitcher View Post
Good insights MickG0105, good addition to the knowledge base.

My understanding is that manual electric trim overrides the Speed Trim System (STS) for 5 seconds, ie if a manual electric trim input is added every 4.9 seconds, STS will not apply nose down trip at high AoA. However as you correctly note, the Elevator forces to hold this high AoA will increase dramatically due to the EFS operation. Pushing forward will feel easy, any back pressure will be significantly more difficult. This protection is another subtle way the system is saying "do you really want to keep pitching nose up", together with the other clues stick shaker and the "stall stall stall" callouts added after the Turkish accident IIRC.

The MCAS however, appears operate at high AoA unless the stab trim switches are in the cutout position. These switch are never touched in normal operation.

Here is a recent stall training video, I believe it may be the MAX. Unfortunately, the trim wheel is difficult to see, and it's not clear if the PF or STS/MCAS is trimming.

Hey CT,

That's not a MAX. The simulator software referred to in the video is very new in airline operations. The old software was found to be unrealistic in terms of upset recovery, feed back and stall recovery. The new software gives the pilots G feedback in all axis, limits and parameters for unusual attitude training.

The positive and negative G limits on the 73 are very easy to exceed. Especially in yaw. Nobody really knew they were doing it, until this software was introduced.
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Old 18th Nov 2018, 06:50
  #1374 (permalink)  
 
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Originally Posted by QuagmireAirlines View Post
True there. Odd & tragic & stupid how just ONE AOA (alpha) sensor can cause nose down pitch trim. One can argue about the Human Factors involved here concerning stress, panic, time to decide what to do, etc., but I tend to come down on the side of "don't over-stress tired human brains", from the Flight Controls Engineer viewpoint I have.

Assuming the crash was caused by the AOA alpha vane ... continuing on:

"Analytical Redundancy" would have prevented this problem, as it would add a third alpha to the dual vane measurement to break the tie between 2 vane sensors. ... Note that alpha = theta - gamma, received through air data & inertial sensors apart from the alpha vanes. That caclulated third alpha makes the system triple redundant, along with using median value selection.

You could even apply a (fourth redundancy!) reasonableness test on the alpha vane's sensor values by writing more software that applies plain old logical common sense: --> Can that high alpha value, seen all of a sudden, and when the airspeed is good, and when g's aren't being pulled, be a valid value of alpha? Remember we have pitch rate sensors, accelerometers, pitot-static tubes, etc. to help the alpha fault detection out. Remember alpha-dot = pitch rate - (accelerometer / speed); we can use all the kinematic laws to do this.

All the above is what a very fast, sharp, smart pilot or flight engineer would do in a millisecond if they saw an alpha vane sensor measurement hard-over (high) all of a sudden. It would be obvious to an observer actually seeing the sensor values along with the trimmed aircraft state to decide it's a bad sensor. That is if a human could see & monitor the raw sensor values in real-time to make that clear judgement. Computers can.

Algorithms such as the above, along with complementary or Kalman filtering to filter out spectral noise and emphasize the short term estimation of valid alpha would also help. I won't go into the signal processing or kinematic blending algorithms here, but I will say you wouldn't need much.

And, finally, where was the FTA & FMEA in all this? I guess they thought the nose-down trim would be easily countered by pilots & was not deemed catastrophic. Again, Human Factors, and I'm on the side of "don't put the plane in peril and expected tired or paniced pilots figure it out".
In answer to the question raised in post#1363 above and several other posts after this one, I had made a query to get an answer anyone trained on MAX and has access to MAX AMM (Maintenance Manual). The question of omission or rejection of wrong data is nicely dealt in B777 ADIRU, which also has only two AOA sensors (This acft is an early 1990 design). I am sure B737 could not be certified without similar protection. If they have not done so, it is a serious shortcomin in design. Sensors do fail or damaged by FOD. If in 1990 design this was foreseen, why overlook it in 2015 design?
B737 also has two ADIRUs like B777. So I wanted to find out from any one the correct config on B737 MAX. But no one has replied so far.

My Post #841
There has to be more protection in the system design for this not to happen. (Single source failure)
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. Also there are two computed AoA values. So in total 6 signals.

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.


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 am just curious, and hope it does not bore other users. END

However no one has replied. So we have wait for the interim report to come out to understand what happened to LION Air. For other concerned B737 MAX pilots, they have to follow the new procedure, and we have to keep speculating endlessly.
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Old 18th Nov 2018, 07:46
  #1375 (permalink)  
 
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Originally Posted by Hi_Tech View Post
There are two AOA sensors on most aircraft, same config on B737 also
While that's true of Boeings, Airbus aircraft - both narrow- and wide-body - typically have 3 AoA sensors, one on the port side nose and two on the starboard.
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Old 18th Nov 2018, 09:03
  #1376 (permalink)  
 
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Originally Posted by Hi_Tech View Post
There has to be more protection in the system design for this not to happen. (Single source failure)
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. Also there are two computed AoA values. So in total 6 signals.
I don't have access to the MAX AMM, but on the NG, there are two AOA sensors - each with two resolvers. The left AOA signals go to the left ADIRU and the SMYD 1 (Stall Management Yaw Damper) box and the right AOA sensor signals go to the right ADIRU and SMYD 2. The two ADIRUs and the two SMYDs cross check.

The resolvers are all analogue outputs.

I can only presume that the MAX and NG have very similar ADIRU and SMYD set-ups - with the MAX having additional MCAS software/logic in the SMYD stall warning functions.
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Old 18th Nov 2018, 09:10
  #1377 (permalink)  
 
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Originally Posted by QuagmireAirlines View Post
True there. Odd & tragic & stupid how just ONE AOA (alpha) sensor can cause nose down pitch trim. One can argue about the Human Factors involved here concerning stress, panic, time to decide what to do, etc., but I tend to come down on the side of "don't over-stress tired human brains", from the Flight Controls Engineer viewpoint I have.

Assuming the crash was caused by the AOA alpha vane ... continuing on:

"Analytical Redundancy" would have prevented this problem, as it would add a third alpha to the dual vane measurement to break the tie between 2 vane sensors. ... Note that alpha = theta - gamma, received through air data & inertial sensors apart from the alpha vanes. That caclulated third alpha makes the system triple redundant, along with using median value selection.

You could even apply a (fourth redundancy!) reasonableness test on the alpha vane's sensor values by writing more software that applies plain old logical common sense: --> Can that high alpha value, seen all of a sudden, and when the airspeed is good, and when g's aren't being pulled, be a valid value of alpha? Remember we have pitch rate sensors, accelerometers, pitot-static tubes, etc. to help the alpha fault detection out. Remember alpha-dot = pitch rate - (accelerometer / speed); we can use all the kinematic laws to do this.
This certainly makes sense, but I'm not sure that two AoA vanes really constitute two sensors. Instead they are a special case as they are only semi-independant, (i.e expected to disagree somewhat when the plane is rolling and yawing). Analysing the difference between the two to try and detect a fault is possible based on other sensors, but adds the possibility of additional complex failures.

Originally Posted by Hi_Tech View Post
My Post #841
There has to be more protection in the system design for this not to happen. (Single source failure)
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. Also there are two computed AoA values. So in total 6 signals.

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.
Thanks for posting, and this 777 system is doing essentially what Quagmire was discussing, using additional sensor data to combine with the two AoA vanes in order to produce a pretty robust AoA. But if the same system is present on the MAX and the mid-value AoA is used for input to MCAS (and stick shaker, stall elevator feel changes), the single AoA sensor failure should not have caused MCAS downtrim in this accident. Some additional failure that also corrupted the computed AoA would be necessary to combine with the bad external sensor AoA data.
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Old 18th Nov 2018, 10:11
  #1378 (permalink)  
 
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is there someone with authority and detailed 737 knowledge who can explain to interested but uninformed nitwits like me whether any consensus as to cause of this tragedy is emerging?
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Old 18th Nov 2018, 10:53
  #1379 (permalink)  
 
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This thread contains a lot of interesting and intelligent proposals to improve pilot training, aircraft system design and certification procedures. But even with all these things still further improved (which is ok of course), modern jets like all other complex systems will go on to produce odd quirks, bugs and failures. Troubleshooting by pilots and maintenance does not get easier with added complexity and some basic failures like a malfunctioning sensor get camouflaged by quadruple redundant systems, voting algorithms, Kalman filters, Fuzzy Logic, Artificial Intelligence and you name it. We all know this.

But what I cannot understand is why an airline organisation does not prevent loading poor SLF and non-pilot crew into an aircraft that had serious problems documented on the three previous flights. This without a thorough assessment of the problem(s) on ground, corrective action and then some test flights by crews prepared for dealing with such problems. That is the real killer in this instance, and not missing pilot knowhow or design/certification flaws. Airworthiness is not only about stamps and signatures on some forms. As a lowly aircraft owner and pilot I will not take along passengers for a ride if my aircraft is plagued by unresolved troubles or untested repairs of such. And I hope the airline I buy my next ticket as a passenger will not do so either. Then I will embark with confidence, be it a Boeing, Airbus, Embraer et altera.
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Old 18th Nov 2018, 11:29
  #1380 (permalink)  
 
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Originally Posted by cooperplace View Post
is there someone with authority and detailed 737 knowledge who can explain to interested but uninformed nitwits like me whether any consensus as to cause of this tragedy is emerging?
There can't be consensus in speculation.
Read the thread for that speculation.

Also i have neither authority nor detailed 737 knowledge but i have read the thread.

It probably won't be a single cause, some more likely causes in arbritrary ordering are:
Bad maintenance and not going for a test flight after repeated serious air data problems
Pilots overloaded not fixing the trim
Boeing system design

Apart from that you will have to wait for the report.
Anyway even with full FDR data which this forum does not have they still want to wait for the CVR to be found because they are not sure what happened exactly.
(Maybe they will just publish some interim report when they deem finding the CVR unlikely)
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