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Boeing 737 Max 8 crash
dramaqueen
post Thu, 4 Apr 2019 - 21:11
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We’ve seen on the news that the cause of the Ethiopian Boeing 737 Max crash was as suspected: a malfunctioning angle-of-attack indicator causing the anti-stall device (MCAS) to put the aircraft into a dive.

But that’s not quite the whole story. Boeing were aware, before the crash, that if the pilots manage to over-ride the MCAS and regain control, the whole sequence can happen again unless they turn off the STAB TRIM switches (situated at the back of the throttle quadrant) within 5 seconds. If they are too late, they can try to haul the aeroplane out of the ensuing dive by pulling back on the control column in the usual way and applying the low-geared manual trim, but eventually the aircraft may be so heavily trimmed nose-down they wouldn’t have the physical strength to do so.

After the Lion Air crash, which appears to have happened for the same reason, the FAA issued an emergency Airworthiness Directive (AD) on November 7th: https://theaircurrent.com/wp-content/upload...MAX-AD-1107.pdf
It makes chilling reading: paragraph (e ), headed Unsafe Condition, reads:

“This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack indicator (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer. We are issuing this AD to address this potential nose-down trim, which could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain”.

So both Boeing and the FAA knew about a potentially disastrous defect in the MCAS system before the Ethiopian crash. It was suspected to have caused one fatal accident already. To save the aircraft would have required immediate problem recognition and action by the pilots - memory actions, without time to consult a check list. Even then, if it happens close to the ground there may not be time.

And what did the AD direct that airlines should do? Just add a couple of paragraphs to the Flight Manual. That’s all. There’s not even a requirement for specific pilot training, which is essential for emergency memory actions. Nor even a mention of the five-second time limit before the sequence happens again.

This post has been edited by dramaqueen: Thu, 4 Apr 2019 - 21:14
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post Thu, 4 Apr 2019 - 21:11
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The Rookie
post Fri, 5 Apr 2019 - 04:11
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As an engineer (automotive, 30+ years experience) I'm shocked and appalled by this, we routinely conduct something known as a DFMEA (Design Failure Mode Effect Analysis) so here are my takes outs that the DFMEA should/could have highlighted
1/ There are two Angle of Attack sensors, what happens if one goes wrong, which is it? Can three be fitted (I don't know the answer to that for the 737 Max) so if one reads different to the others you logically conclude which one is wrong. I don't know the type of AoA sensor used on the 737max but most are just a windvane that measures the direction of airflow relative to the aircraft, so fitting 3 would be simple.
2/ If the sensors disagree would it be more sensible to use the pessimistic one and initiate a nose drop to prevent a stall (thus risking deliberately flying the plane into the ground if you picked the wrong one) as Boeing have used or to use the optimistic one and take no action (noting this is meant to help prevent a pilot from stalling the plane, but its still the pilots primary responsibility).
3/ Angle of attack is the angle the wings make to the airflow, it's not the same as nose up or down compared to the horizontal, however such other information (airspeed, nose up or down angle compared to the horizontal, rate of climb etc) is available that it seems entirely plausible to model from that other information the angle of attack accurately enough to compare to the two sensors, or at least create a relaible inference as to which to trust, such that if the sensors disagree you have a frame of reference to decide which is correct or what (calculated value form all inputs) value to use for MCAS initiation.
4/ Given that they hadn't done any of 1-3, should the disagreement between sensors warning have always been a standard fit and not a $80,000 option (as Boeing have now decided on as part of the fix) with then a simple method of over riding it. Noting that a pilot trying to over ride it is using a checklist he hasn't had to be trained specifically on or taken simulator training on (and the MCAS isnt in the simulator training anyway). If the two AoA sensors are so far out should it have been a simple flick switch to then turn off the system as you already know the system has to be faulty.
Note, neither Lion or Ethiopian paid for the warning light, after all Boeing and the FAA had agreed it was optional so not 'required' for safe operation. Noting that $80,000 for a few lines of code, a controller i/o (that are both probably still there but redundant on the non option aircraft anyway) a few metres of wire and an LED is blatant profiteering anyway!

In addition why was there no training material on this feature at all, not in simulator training, could it be it was part of Boeing's project remit for minimal pilot retraining so as to make it more marketable that was in fact at the expense of safety?

For info, an AoA vane, this is the usual way of measuring AoA but I can't find info on what is fitted to the Max.

An interesting read for the inquisitive of mind with other types included
https://utcaerospacesystems.com/wp-content/...AOA-Systems.pdf

What is a DFMEA
https://quality-one.com/dfmea/

This post has been edited by The Rookie: Fri, 5 Apr 2019 - 04:38


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Redivi
post Fri, 5 Apr 2019 - 08:04
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I had to occasionally perform DFMEA when I worked in the chemical industry
An individual component that has the potential for a catastrophic failure is an absolute no-no especially in industries such as aircraft manufacture

Unbelievable when it's a sensitive instrument waving in the breeze where its only protection against bird-strike or a dropped tool is a "Caution" sign

This has all the signs of a company taking a gamble with a short-cut to address time and cost pressures imposed by customers

By coincidence, I'm spending the next few days writing a report for a court case where this happened, fortunately with only financial consequences

This post has been edited by Redivi: Fri, 5 Apr 2019 - 08:07
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Fredd
post Fri, 5 Apr 2019 - 10:07
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QUOTE (The Rookie @ Fri, 5 Apr 2019 - 05:11) *
For info, an AoA vane, this is the usual way of measuring AoA but I can't find info on what is fitted to the Max.

That is indeed the (vane) type of AOA sensor fitted to the 737. There is a more robust cylindrical probe type which is typically used on high speed military aircraft, but at low speed the vane type is more responsive and so is common on airliners.

I can assure you that carrying out FMEA is very much standard practice in the aircraft industry, although quite how this dreadful kludge made it through if they were doing them properly is a mystery to me.

I suspect the root cause of this is that the MCAS was seen as simply a stability augmentation system to improve the aircraft's handling characteristics, making them feel more like the "classic" 737 and avoid any nasty scares for the pilots due to nose-up trim caused by the more powerful engines on the Max. Limited authority systems like this are not at all uncommon, and wouldn't be considered safety critical - hence the reliance on a single AOA sensor, for example. What seems to have been completely missed is the confusion to the crew caused by the frequent, intermittent nose-down trim, particularly when it was erroneous. Piggy-backing off the normal trim system makes it worse because it seems the only way to disable the thing is to disable the trim actuators, which if all this erroneous trimming has gone on long enough may be the only way for the pilots to have hauled the nose back up. Limiting MCAS authority and adding a comparison with a second AOA sensor and a "disagree" warning will both help, but it's still fundamentally a horrible design. I think the shock for most aerospace engineers is that a company with a reputation like Boeing's could have thought this was acceptable.


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dramaqueen
post Fri, 5 Apr 2019 - 11:28
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QUOTE (Fredd @ Fri, 5 Apr 2019 - 11:07) *
I suspect the root cause of this is that the MCAS was seen as simply a stability augmentation system to improve the aircraft's handling characteristics, making them feel more like the "classic" 737 and avoid any nasty scares for the pilots due to nose-up trim caused by the more powerful engines on the Max.


That’s the way they would like people to see it. In reality, it’s a full-on anti-stall device. A Boeing diagram actually states: “MCAS pushes the jet’s nose down to reduce the risk of stalling.” This sort of anti stall device - a modern variant of the pneumatic stick push on the BAC 1-11 - is very rare. It’s very rarely necessary. Pilots are taught from the cradle how to identify and deal with an incipient stall. Only if the aircaft’s stall characteristics are critically dangerous would such a system be necessary.


That’s what happened here. The new engines and their new position on the wing caused a longitudinal instability which can result in an abnormal pitch-up at high angles of attack. Abnormal nose-up pitching is not allowable under 14 CFR 25.203(a ) https://www.law.cornell.edu/cfr/text/14/25.203“stall characteristics” -see https://www.law.cornell.edu/cfr/text/14/25.203
Boeing tried to rectify this with various aerodynamic adjustments but they were still not enough to pass the regulations. Hence the “stick push”.

The MCAS operation is extremely safety critical. Boeing actually states: “It is possible for the stabilizer to reach the nose-down limit unless the system inputs are counteracted completely by pilot trim inputs and both Stab Trim Cutout switches are moved to Cutout.”

From a pilot’s point of view I can’t think of a more horrifying scenario. The fact that Boeing knew of the potential danger makes it a great deal worse. Corporate manslaughter?
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Steve_999
post Fri, 5 Apr 2019 - 11:32
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And, irrespective of anything the manufacturer says about a resolution, how many people are going to knowingly fly in one of these in the future?
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Fredd
post Fri, 5 Apr 2019 - 11:50
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QUOTE (dramaqueen @ Fri, 5 Apr 2019 - 12:28) *
QUOTE (Fredd @ Fri, 5 Apr 2019 - 11:07) *
I suspect the root cause of this is that the MCAS was seen as simply a stability augmentation system to improve the aircraft's handling characteristics, making them feel more like the "classic" 737 and avoid any nasty scares for the pilots due to nose-up trim caused by the more powerful engines on the Max.


That’s the way they would like people to see it. In reality, it’s a full-on anti-stall device. A Boeing diagram actually states: “MCAS pushes the jet’s nose down to reduce the risk of stalling.” This sort of anti stall device - a modern variant of the pneumatic stick push on the BAC 1-11 - is very rare. It’s very rarely necessary. Pilots are taught from the cradle how to identify and deal with an incipient stall. Only if the aircaft’s stall characteristics are critically dangerous would such a system be necessary.

I think you're getting ahead of yourself there - do you know what the characteristics of the aircraft are without MCAS? All airliners with under-slung engines (which is to say, the vast majority) will have pitch-up with increasing engine power; it'll be more pronounced with the Max than the previous generations, but it's not necessarily dangerous in itself or beyond the ability of a typical pilot to handle. I do believe the reason for wanting an augmentation system (which is nothing like a full-authority stick-pusher, or shouldn't be anyway) was to be able to sell a common type approval, no training required story to existing 737 customers. Unfortunately they seem to have screwed it up. Their ham-fisted approach to the aftermath is doing them no favours, either.

QUOTE (Steve_999 @ Fri, 5 Apr 2019 - 12:32) *
And, irrespective of anything the manufacturer says about a resolution, how many people are going to knowingly fly in one of these in the future?

Didn't hurt the DC-10/MD-11 - peoples' memories tend to be short.


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notmeatloaf
post Fri, 5 Apr 2019 - 12:44
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Its pointless to say "we do this in automotive". There are plenty of catastrophic mechanical failures possible on cars and the methods to prevent them are more crude. I had a castle nut fail and most of the front offside wheel and suspension fell off on a Golf.

Commercial aircraft have incredibly complex software and its impossible for any engineer to say there are no unexpected behaviors. Software is always prone to human error. You see the same with self driving cars - any accident is news despite the fact they are incredibly safe versus a human driver. Everything is obvious in hindsight.

This post has been edited by notmeatloaf: Fri, 5 Apr 2019 - 12:46
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The Rookie
post Fri, 5 Apr 2019 - 14:56
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DFMEA don’t prevent failures, but they certainly help, there are so many obvious issues with MCAS without doing even a complex analysis it looks clear to me that someone dropped the ball.


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captain swoop
post Sat, 6 Apr 2019 - 00:54
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My issue with with MCAS is that it actively takes control away from the pilot, and keeps it away from him.

If MCAS can predict the onset of a potential stall, and take action, why not instead, just have a warning bell/horn and/or a 'Bitchin Betty' - "Warning: Stall Imminent!"

I learned to fly in a Cherokee 140. It had a red stall warning light and a "whiny" horn that would sometimes go off intermittently about 50 to 300 feet AGL on climbout, particularly in blustery conditions. The response was usually to just ease the yoke forwards a little to slightly lower the RoC.

If it had a system that pitched the nose down autonomously at 50 feet, I don't think I would be here to post this!
Also, MCAS goes through a single sensor, a single point of failure. Apparently the remedy for the MCAS failure is to manually trim. They must have never tested this because a test was done by a qualified training pilot in a 737 simulator and he found he could not save the plane using the Boeing supplied procedure.

QUOTE
In the test, the two European pilots in the 737 simulator set up a situation reflecting what happens when the pre-software fix MCAS is activated: They moved the stabilizer to push the nose down. They set the indicators to show disagreement over the air speed and followed normal procedures to address that, which increases airspeed.
They then followed the instructions Boeing recommended and, as airspeed increases, the forces on the control column and on the stabilizer wheel become increasingly strong.
After just a few minutes, with the plane still nose down, the Swedish 737 training pilot is exerting all his might to hold the control column, locking his upper arms around it. Meanwhile, on his right, the first officer tries vainly to turn the stabilizer wheel, barely able to budge it by the end.
If this had been a real flight, these two very competent 737 pilots would have been lost.


https://www.seattletimes.com/business/boein...hiopian-flight/

This post has been edited by captain swoop: Sat, 6 Apr 2019 - 00:57
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Guest_Charlie1010_*
post Sat, 6 Apr 2019 - 06:43
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Having been involved in aviation all my life I would never get on a DC10 or 11 to fly.
The MD-11 has had problems with its flight control systems that have resulted in multiple accidents and incidents since the aircraft's introduction.
And the DC10?

https://en.m.wikipedia.org/wiki/Category:Ac...l_Douglas_DC-10

I will not fly on an aircraft (737max) with such a badly thought out process for improving economy.
Moving the larger engines because of FOD and ground clearance making it unstable then using software which isn’t fully explained to the pilots to automatically guide the plane down repeatedly.
It’s all here and scary.

https://theaircurrent.com/aviation-safety/w...tem-mcas-jt610/

My memory isn’t short.

The airlines had to do extra training because of the problems with the MD11.

From Wikipedia

In an effort to improve fuel efficiency, McDonnell Douglas designed the MD-11's center of gravity to be much further aft than other commercial aircraft. There was also a fuel-ballast tank in the MD-11's horizontal stabilizer since its tailplane was smaller than the DC-10's to improve fuel efficiency, but this was found to inhibit the MD-11's crosswind performance.[7] These design features significantly reduced the MD-11's margin for error during the takeoff and landing phases, making it more difficult to handle than the smaller DC-10.[8] A number of operators have introduced special training to assist crews in safely handling the MD-11's critical phases of flight.[56]
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bama
post Sat, 6 Apr 2019 - 16:42
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weren't they in a rush to get the 737 finalised due to a competitor having a model that would eat Boeing's lunch of they didn't finish by a certain date ?


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Which facts in any situation or problem are “essential” and what makes them “essential”? If the “essential” facts are said to depend on the principles involved, then the whole business, all too obviously, goes right around in a circle. In the light of one principle or set of principles, one bunch of facts will be the “essential” ones; in the light of another principle or set of principles, a different bunch of facts will be “essential.” In order to settle on the right facts you first have to pick your principles, although the whole point of finding the facts was to indicate which principles apply.

Note that I am not legally qualified and any and all statements made are "Reserved". Liability for application lies with the reader.
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Guest_Charlie1010_*
post Sun, 7 Apr 2019 - 07:15
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As the head of Southwest’s pilot union explained a few months ago, pilots were “kept in the dark” regarding this:

“We do not like the fact that a new system was put on the aircraft and wasn’t disclosed to anyone or put in the manuals.

Is there anything else on the MAX Boeing has not told the operators? If there is, we need to be informed.”
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