Author Topic: Boeing 737 MAX  (Read 1780 times)

Offline Echnaton

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Re: Boeing 737 MAX
« Reply #15 on: April 22, 2019, 09:33:36 AM »
So if the Angle of Attack sensor is providing dodgy data, what can cause that? How does it obtain its data?



From what I have seen on Mentour, The AoA system consists of two independent vanes that point into the airflow around the nose of the plane. One on either side of the aircraft.  The fault can be in the physical vane itself, the way the vane's physical position is translated into data, or in the transmission of that data to the flight control computer.

Crucial to the problem is that MCAS uses the pilots AoA vane only.  The Lion Air plane that crashed had a fault in the AoA system that was noticed the day before, resulting in some maintenance work.  Its significance to MCAS was not understood and, IIFC, the flight crew on the fatal flight were not notified of the fault.

On the Ethiopian plane the faulty data caused the nearly continuous activation the captain's stick shaker. While the FOs column was not affected. This caused significant pilot confusion.  According to the Aviation Herald, Ethiopian Airlines had not yet update their flight manual with the  Emergency Airworthiness Directive Boeing issued in November, after the Lion Air crash. Information that if absorbed by the crew might have prevented the crash.

So like most accidents, both were a combination of a many things related to a new design. I interpret the preliminary information to be that the MCAS was not robust enough for its implementation at a crucial time in the flight without significantly more training than was suggested by Boeing or offered by the airlines to the pilots and maintenance crews. MCAS may not have been robust enough period. The vane appears to be the weakest point. But further investigations will undoubtedly reveal more.

« Last Edit: April 22, 2019, 09:39:23 AM by Echnaton »
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Offline Obviousman

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Re: Boeing 737 MAX
« Reply #16 on: April 22, 2019, 05:11:32 PM »
It's funny, in some dark way:

There were always discussions regarding the 'automation' / AFCS approach differences between Airbus and Boeing. It was said that on a Boeing, the pilot outranks the autopilot.

And then we have the Air France 447 incident - which involved an Airbus A330 - whose contributing factors were erroneous flight condition inputs (iced pitot tube, IIRC) and the pilots forced a stall condition when they should have been lowering the nose.

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #17 on: April 22, 2019, 06:43:58 PM »
On the 737 there is a noticeable and obvious indication of the operation of the automatic trim system on all 737s.  It is the motion of wheels on either side of the center instrument console. The wheels are connected to the jack screw by wires and allow for manual operation of the trim. 

In my understanding, most or all airliners have a trim system that automatically adjusts the stabilizer, and have for decades. The MCAS is a new implementation law on the 737 for a specific circumstance beyond normal flight trim. It was developed for use on a military aircraft and has been used successfully there. One significant difference is the military version took inputs from both AoA vanes.  Faulty inputs to the flight computer from the AoA systems seems to be a significant contributor to the MCAS activation, thus the Maxs misbehavior that lead to both crashes. The faulty operation forces an extremely high workload on the pilots that they are not properly trained for.  Resulting in some guessing at at time when there is little margin for recovery from the wrong guess.

The Mentour Pilot YouTube channel has a number of videos on this topic.  The channel  owner is a 737NG captain and has discussed the 737 trim operations in detail.

https://www.youtube.com/channel/UCwpHKudUkP5tNgmMdexB3ow/videos 


There is a very good one where he does a simulation of runaway trim, and it is clearly evident that the pilots' physical workloads are quite high.

The trim wheel has to be operated by pulling out a small handle and manually cranking the wheel backwards. That wheel is in an awkward position and is difficult to operate for one pilot. AIUI, this is because they are manually cranking the trim jackscrew; a difficult enough thing to do on the ground, made much harder by the airflow over the horizontal stabilizer. The other pilot's handle is 180° out on the wheel, and its only when both pilots are cranking it that it becomes easy to do. 

If the trim stab switches have been turned off after they have run into trouble, the trim jackscrew stops in the position it is in at that moment. If that is nose down at low altitude then we have a situation were both pilots have one hand in use to crank the trim, one pilot with his other hand on the yoke, leaving one pilot's hand to operating anything else that needs doing. All this is happening with the aircraft nose down falling out of the sky at only a couple of thousand feet above the ground... not good.

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Offline smartcooky

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Re: Boeing 737 MAX
« Reply #18 on: April 22, 2019, 08:16:45 PM »
It's funny, in some dark way:

There were always discussions regarding the 'automation' / AFCS approach differences between Airbus and Boeing. It was said that on a Boeing, the pilot outranks the autopilot.

And then we have the Air France 447 incident - which involved an Airbus A330 - whose contributing factors were erroneous flight condition inputs (iced pitot tube, IIRC) and the pilots forced a stall condition when they should have been lowering the nose.


One pilot was forcing a stall condition, but because the control system was a side-stick, in a dark cockpit, with no force feedback to the other side-stick, the other pilot didn't realize the first pilot was pulling back.

Sully reckons that accident would never have happened on a Boeing because both yokes move when either pilot operates his. The yoke on your side pinning you in your seat is a pretty big clue tat the other pilot is pulling back as hard as he can.
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Offline Bop

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Re: Boeing 737 MAX
« Reply #19 on: April 26, 2019, 12:43:41 PM »
So if the Angle of Attack sensor is providing dodgy data, what can cause that? How does it obtain its data?



From what I have seen on Mentour, The AoA system consists of two independent vanes that point into the airflow around the nose of the plane. One on either side of the aircraft.  The fault can be in the physical vane itself, the way the vane's physical position is translated into data, or in the transmission of that data to the flight control computer.

Crucial to the problem is that MCAS uses the pilots AoA vane only.  The Lion Air plane that crashed had a fault in the AoA system that was noticed the day before, resulting in some maintenance work.  Its significance to MCAS was not understood and, IIFC, the flight crew on the fatal flight were not notified of the fault.

On the Ethiopian plane the faulty data caused the nearly continuous activation the captain's stick shaker. While the FOs column was not affected. This caused significant pilot confusion.  According to the Aviation Herald, Ethiopian Airlines had not yet update their flight manual with the  Emergency Airworthiness Directive Boeing issued in November, after the Lion Air crash. Information that if absorbed by the crew might have prevented the crash.

So like most accidents, both were a combination of a many things related to a new design. I interpret the preliminary information to be that the MCAS was not robust enough for its implementation at a crucial time in the flight without significantly more training than was suggested by Boeing or offered by the airlines to the pilots and maintenance crews. MCAS may not have been robust enough period. The vane appears to be the weakest point. But further investigations will undoubtedly reveal more.
From the data recovered it looks like ET302 may have had a vane detachment (they have an internal counterweight that would swing it to maximum deflection, and there was a brief period where it 'almost' returned to normal readings, this was at a period of extreme negative G forces, suggesting that the reading was from the counterweight 'floating'- of note was that at that same instant the sensor reading failed to full deflection,the heater sensor failed as well.....

Another thing that may have led to the crews confusion is that although the captains shaker only was going off, apparently the yokes are connected, so the FO yoke may have had some stickshaker bleeding through

A third point of confusion was the airlines requirements for 'no additional training' required for the MAX, but one major difference was the trim yoke switches, in the earlier 737 variants, you could disable all AP functions with one, the other disabled AP control over the stabilizers- BUT allowed electrical trim to continue working. The MAX changed this to disable all AP with both switches- this may explain why the MCAS  was turned back on again a couple of times late in the flight- at the speed they were going by that stage, it is literally impossible to trim out the MCAS input to the stabilizers by hand with the trim wheels, and they tried to activate the electrical trim motors as per previous models, only for this to not work as expected (a change which was only noted in the MAX manuals with a 'throwaway line' that the function of the yoke switches had been renamed- nothing said about the fact they had been rewired in a different configuration- no training need then!!!)

Worse when they finally reactivated what they thought was the elec trim circuits (or possibly they deliberately reactivated the entire system- they didn't much to lose by that stage) , it also reactivated the faulty MCAS- but this wouldn't have apparent at first, as it was still held off until they stopped trying to retrim, it waits 5 seconds and then starts winding even MORE trim on...

Apparently the 'official' way to get out of the overloaded trim is to decrease thrust and nose down, while winding off (anything up to a hundred turns!!!!)- not really an option when you are only 1000ft off the ground... (and some media reports have been misinterpreting their altitude to boot which doesn't help) yes they did get to nearly 9000ft before it all went pear shaped, giving the impression they had climbed a bit, what many don't seem to know is that Bole is 7625ft above sea level and where they crashed is higher again- so they were never far off the ground at all...


Of course none of this is guaranteed, but there are some hints from the released data so far that points to this possibly being the issue (I am shamelessly passing on some pretty smart peoples ideas here, I only fly private GA, but a mate flies 737NG's and there is a lot of discussion of the MAX issue obviously)

 Be interesting to see what comes out officially in the end...


Offline Bop

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Re: Boeing 737 MAX
« Reply #20 on: April 27, 2019, 06:40:20 AM »


This was taken from a video on a sim, showing the difficulty they would have had at that speed retrimming. remembering that they would have had to wind off tens of turns at the minimum...

This is a transcript of the now deleted video

Quote
The original sim session transcript [C=CAPT; F=FO]:
C: We have an IAS disagree.
C: So, IAS disagree memory items.
F: Autopilot if engaged, disengage.
C: Disengaged!
F: Autothrottle if engaged, disengage.
C: Disengaged!
F: Flight directors - Both up
F: With flaps up established a flight path 4 degrees and 75% N1.
C: So, 75% N1.
F: We have 77, 76,...
C: A little bit less...
F: And, there you go.
C: 4 degrees.
F: 4 degrees.

C: So I am trying to establish this now.
F: Check!
F: We are descending...?
F: We probably... Are you feeling troubled with...
F :Any trouble with the flight control?
C: Yeah, I'm trying to trim it but...
C: It continues to trim against me when I'm trimming
C: So state the malfunction, please.
F: Yeah, this doesn't look right. Looks like uh...
F: How do you feel the stabilizer, the trim system?
F: Can you control it?
C: I'm trimming it. It is responding but...
F: It's a runaway stabilizer, if you agree?
C: For every time that I trim backward, it keeps trimming forward.
F: It's trimming forward. Yeah, it's runaway stabilizer.
C: So, runaway stabilizer memory items...
C: And i'm trying to keep this thing at 4 degrees.
F: Control column, hold firmly.
C: I am... [CAPT is holding the yoke firmly with both hands]
F: Autopilot - if engaged, disengage.
C: It's disengaged.
F: Autothrottle - if engaged, disengage.
C: It's..., if you can disengage it for me, make sure that it's disengaged.
F: It's disengaged.
F: And, do you feel that the failure stop?
F: Negative?
C: No, it's still moving.
F: Stab trim cutoff switches to cutoff.
F: OK. It stops. It looks like it stops.
C: You can see now I'm using almost full back pressure here.
F: Exactly.
C: How many degrees nose down?
F: We have 4 units nose down now
C: 4 units nose down?
F: Yup.
C: OK, I'm struggling.
C: I'm actually using almost my full force to keep the aircraft level here.
F: Do you want me to help you?
C: What I would like to do.
C: Just for the sake of exercise, can you trim this forward? [to simulate MCAS trim AND]
C: See if we can reach even zero nose down.
C: And see if I can even hold it.

[FO is trying to crank the trim wheel to reach zero nose down, simulating MCAS AND]

C: So, now we are doing this just as an exercise!
C: Do not try this at home.
C: This...
C: We are at 300 knots now.
F: I'm fighting.
C: I'm sttrugling to to keep this aircraft flying.
F: My god! [FO surprised at how hard it is to trim further nose down]
C: Yeah, the thing is with higher speed the force on the stabilizer will be higher and higher as well.
C: So it becomes almost impossible to move it.
C: So we are now at about 3 degrees.
F: Yup. [FO still tries to continue trimming nose down, the wheels is so difficult to spin]
C: We're still about 3 degrees away from full nose down trim.
C: And I am using everything that I have. [CAPT still holding on to his yoke with both hands]
F: My God ! [the trim wheel barely move for the down trim]
C: This is realistic guys.
C: This is how much of effort it would take to trim the stabilizer at this kind of speed.
C: Umph... [Capt is still trying to hold on to his yoke with his hands]
C: I'm just in control of it, though. But it's getting harder and harder.
C: And remember we're still 2.5 degrees away...
F: My God! [FO still struggles to spin the refused-to-be-spun trim wheel]
C: It's not possible, is it?
C: All right, we stop at that.

C: The reason that we have to try...
C: The reason we have to trim this manually is because the normal trim system wouldn't do this, OK.
C: It would require manual trim to get it away from this.
C: That's fine.
C: Trim it backward. [This time to illustrate the effort to trim the nose back up after "MCAS" brought the AC further nose down]
C: Trim it backward as you can.
F: Oh my God! I couldn't... [FO can't spin the wheel to trim up]
C: OK.
C: Eh...
C: Juan, press the red button! [CAPT called the sim operator...]
C: Press the red button now. [to stop the sim session]
C: This is at 340 knots.
C: And the trim is at...It's still at almost 2.5 degrees.
F: Yeah, 2.5 degrees.

Offline Echnaton

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Re: Boeing 737 MAX
« Reply #21 on: April 27, 2019, 04:52:15 PM »
A bit of news from Aviation Herald

Quote
On Apr 27th 2019 it became known, that four independent whistleblowers, current and former Boeing employees, had called the FAA hotline for whistleblowers regarding aviation safety concerns on Apr 5th 2019. The concerns reported were wiring damage to the AoA related wiring as result of foreign object damage as well as concerns with the TRIM CUTOUT switches. The FAA believes these reports may open completely new investigative angles into the causes of the two crashes in Indonesia and Ethiopia.

https://avherald.com/h?article=4c534c4a/0045&opt=0

Faults in the AoA vane continue to be prominent in the information coming out. 
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Offline Bop

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Re: Boeing 737 MAX
« Reply #22 on: April 28, 2019, 04:00:43 AM »
Ouchies, that's exactly whats been floating around the pilotsphere for probably a week or more...

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #23 on: April 28, 2019, 03:52:40 PM »
Bop

I've nicked your post to repost at ISF in the thread there. I hope you're OK with that.
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Offline Bop

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Re: Boeing 737 MAX
« Reply #24 on: April 29, 2019, 06:26:02 AM »
Thats ok, I pinched bits myself
(waves hi over there- waves vaguely)

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #25 on: April 29, 2019, 07:38:17 AM »
Thats ok, I pinched bits myself
(waves hi over there- waves vaguely)

Aha.. the penny drops... Bop = Dabop
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Offline Bop

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Re: Boeing 737 MAX
« Reply #26 on: May 05, 2019, 11:25:09 PM »
There's been a lot of criticism of both the FAA and Boeing about the way safety testing was done, there was an article in the Seattle Times that not  only suggests that major corners were indeed being cut, but there is a noxious attitude in Boeing that led to the MAX incidents being almost inevitable....

The chances of other Aviation regulators accepting any re-certification from the FAA as being above board just went through the floor....

https://www.seattletimes.com/business/boeing-aerospace/engineers-say-boeing-pushed-to-limit-safety-testing-in-race-to-certify-planes-including-737-max/

Quote
In 2016, as Boeing raced to get the 737 MAX certified by the Federal Aviation Administration (FAA), a senior company engineer whose job was to act on behalf of the FAA balked at Boeing management demands for less stringent testing of the fire-suppression system around the jet’s new LEAP engines.

That June he convened a meeting of all the certification engineers in his unit, who collectively agreed with his assessment. Management initially rejected their position, and only after another senior engineer from outside the MAX program intervened did managers finally agree to beef up the testing to a level the engineer could accept, according to two people familiar with the matter.

But his insistence on a higher level of safety scrutiny cost Boeing time and money.

Less than a month after his peers had backed him, Boeing abruptly removed him from the program even before conducting the testing he’d advocated.

The episode underscores what The Seattle Times found after a review of documents and interviews with more than a dozen current and former Boeing engineers who have been involved in airplane certification in recent years, including on the 737 MAX: Many engineers, employed by Boeing while officially designated to be the FAA’s eyes and ears, faced heavy pressure from Boeing managers to limit safety analysis and testing so the company could meet its schedule and keep down costs.

That pressure increased when the FAA stopped dealing directly with those designated employees — called “Authorized Representatives” or ARs — and let Boeing managers determine what was presented to the regulatory agency.

“The ARs have nobody supporting them. Nobody has their backs,” said one former Authorized Representative who worked on the 737 MAX and who provided details of the engineer’s removal from the program. “The system is absolutely broken.”

FAA-designated oversight engineers are supposed to enjoy protection from management pressure. Removing one who proves a stickler for safety regulations will inevitably produce a chilling effect on others who see the consequences of being too rigid about safety concerns, said John Goglia, former member of the National Transportation Safety Board (NTSB).

“It negates the whole system,” said Goglia. “The FAA should have come down on that really hard.”

Following two deadly 737 MAX crashes off the coast of Indonesia and in Ethiopia that killed 346 people, and the subsequent grounding of the airplane worldwide, the certification of the jet has come under intense scrutiny, including a slew of lawsuits, congressional hearings and a criminal investigation.

None of the people interviewed were involved in certifying the Maneuvering Characteristics Augmentation System, the flight-control software implicated in the two crashes. But one area of scrutiny is sure to be the delegated system under which Boeing employees, paid by the company but acting as FAA designees, did the detailed certification work. It may slow down plans by the FAA and Boeing for a future certification regimen that would further erode the FAA’s oversight.

Boeing, in a statement responding to Seattle Times questions, said that FAA procedures, including regular, FAA-mandated training, “ensure Boeing employees serving in this capacity act independently on behalf of the FAA.”

It added that “there are processes in place to carefully evaluate any concerns regarding the AR’s ability to act independently.” The company declined to comment on individual cases cited in this story.

Yet as the FAA has increasingly delegated certification tasks to Boeing itself, it’s also made changes to the reporting structure that leave its designees to fend for themselves inside the company.

While a few former employees involved in certifications said they handled the pressure as a regular part of the job, others described the work environment as hostile, focused on achieving FAA approval within schedule and cost targets. Some of those workers spoke on condition of anonymity to protect professional relationships or for fear of retribution.
This echoes the findings of a Seattle Times investigation in March of what happened on the FAA side of the MAX certification. Within the FAA, its safety engineers worked under constant pressure from their managers to delegate more and more work to Boeing itself, and to speedily approve the safety assessments the Boeing designees came up with.

On the Boeing side of that process, the removal of the senior engineer acting as an FAA Authorized Rep was an extreme example that highlights the broader negative impact of two changes: The FAA no longer appoints its own ARs, instead leaving that to Boeing. And these designees now rarely interact with the FAA directly, according to former Boeing ARs interviewed by The Times.

They said these changes have stripped them of protection and given managers more opportunity to push for shortcuts.

In a statement, the FAA said it oversees the Boeing certification system “to ensure procedures are followed.” The agency also said it has “received no whistleblower complaints or any other reports … alleging pressure to speed up 737 MAX certification.”

Boeing managers are supposed to undergo “undue pressure” training to ensure that they aren’t crossing boundaries with the FAA’s representatives. And some ARs said that, despite some tensions, their managers were respectful of the role.

Fred Stong, an AR who worked on electrical systems at Boeing, said his experience was that everyone works through differences to reach common ground. He said he was always assertive in his role and didn’t face any problems.

Ouchies
I can't see any possible issues with allowing Boeing to basically control the engineers appointed to keep safety up with economics being the secondary consideration, nope, that could NEVER end in tears....
/sarcasm...

Offline Glom

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Re: Boeing 737 MAX
« Reply #27 on: May 07, 2019, 04:10:56 AM »
I'm hearing talk about the flight crew retracting the flaps in response to potential overspeed and this is contrary to procedures because you should remove power first to arrest speed increase.

But even so, how did this contribute to the crash? They had plenty of energy, the aircraft would have still flown clean.

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #28 on: May 07, 2019, 08:41:55 AM »
I'm hearing talk about the flight crew retracting the flaps in response to potential overspeed and this is contrary to procedures because you should remove power first to arrest speed increase.

But even so, how did this contribute to the crash? They had plenty of energy, the aircraft would have still flown clean.

It looks like they had the power setting at 94% for take-off, and it remained there for the entire duration of the flight until it crashed. That would be very usual, it means means the aircraft was traveling much faster than it ought to have been... its no wonder they had the over-speed clacker going off. High speeds like that make it very difficult to manually trim using the trim-wheels because of the excessive airflow over the horizontal stabilizers.

According to the report I read, the flaps were extended 0.019° - an insignificant amount.
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Offline smartcooky

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Re: Boeing 737 MAX
« Reply #29 on: May 07, 2019, 08:33:14 PM »
https://www.theverge.com/2019/5/2/18518176/boeing-737-max-crash-problems-human-error-mcas-faa

Longish article - well worth the time to read for aviation tech enthusiasts
► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
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