Author Topic: Recent Ethiopian Boeing 737 Max 8 crash - speculation  (Read 943 times)

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Offline Fast Eddie B

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Recent Ethiopian Boeing 737 Max 8 crash - speculation
« on: March 19, 2019, 08:55:12 AM »
I searched, but was unable to find a thread on this topic.

I came across this speculation by a friend and fellow pilot, that I found well-informed and I thought I’d share.

***Warning contains my own uninformed speculation... I am posting it ONLY to see if people agree or disagree... ***

It seems clear to me that a chain of well known information could give us a possible explanation and the reasons for the recent 737 max 8 crashes.

Overall its the problem of adding envelope protection and automation to an older Boeing aircraft design that, unlike Airbus, was never designed for fly by wire automation, and an addition that is against the normal design philosophy of the manufacturer - - - Boeing.

Airbus has had fly by wire and computer based flight controls designed in from the begining, Boeing however have never before really embraced this idea. 

While the lunar module and many agile fighter jets would be unstable without fly by wire, Boeing have normally designed aircraft that fly 'naturally'..

This can be seen clearly in the design of the pilot interface to the aircraft, a big old fashioned yoke in the Boeing and a small side stick like a games controller in the Airbus and the Apollo era lunar module.... .

But here's the problem, when Boeing added bigger engines and other changes like fuselage length to the 737-max series they upset the natural flight characteristics, engines needed to be more forward in a different position, and had more thrust. I heard Boeing found this affected control, badly, at some edges of the flight envelope including the full power stall characteristics of the aircraft.

Pilots who practice full power stalls know they are scary at the best of times.  This aircraft was apparantly even more scary, so much so that it could probably not have been certified.

So automation shy Boeing were forced to depend on Airbus style software protection to prevent these edges of the flight envelope ever being reached.

Hense the MCAS system was born,  an override that would automatically forward trim the aircraft at high angles of attack approaching the stall in this configuration, flaps up full power, ( you know like the climb phase close to the ground just after take off) to eliminate the possibility of the stall...

Trouble is, the airframe was designed years ago. The trim on the 737 trims the angle of the WHOLE tail stabiliser.... not just the elevators. Yes that 'wing at the back' actually changes its position relative to the fusalage... its whole angle.... this is actually a very powerful, coarse control input in the wrong hands.... .. the pilot controlled elevator for pitch control at the back is only part of this whole flying surface... so I imagine that trim can, at full defection, be a stronger force than elevator input.

So a faulty angle of attack sensor could trigger this forward stabiliser trim if it detects a phantom stall... .  Boeing say this is what happened in the first lion Air crash... a faulty sensor.. but they blamed pilot training... really?

So what actually happens?  Well every 10 seconds or so, unless the whole trim system is disabled, the MCAS forces the nose down using this very powerful stabiliser trim, to avoid the phantom stall.

You can imagine a pilot fighting this in two ways, pulling back on that big yoke, and trimming backwards to reverse the false input, but every time he does the system kicks in again and undoes his fixes... there are two additional problems the poor pilot faces... .

Firstly because the aircraft thinks its stalling it starts shaking this yoke. imagine pulling to save your life on a yoke that is shaking, it's like the aircraft is disobeying wanting to nose down and still want you to fight it. But hang on...  secondarily the Yoke has an artifical feedback system designed to give flying the plane a more natural 'feel' for pilots.  This system can make the controls feel stiffer at high speed for example and looser at low speed, even though, like the steering wheel of a car the controls are power assisted.... in a stall guess what? this artificial feedback system kicks in to make it very hard to pull back on the yoke... its stiffens as if to say 'don't do that'...

So we have runaway trim, moving the whole rear stabiliser into a dangerous nose down flight position... possibly faster that the pilot can fix it, a shaking stick, and a yoke that actively resists pilot input to pull back! Even if the pilots disconnected the trim system (as per Boeing 'fix'...) its likely to be left in a nose heavy trim position... which takes time to fix...

Then the poor pilot has to try and pull up, using a small (relatively speaking) elevator, against a stiff, artificially resistant pressure on the yoke, possibly with a shaking stick.... all close to the ground? 

I can imagine easily how a runaway nose down trim, resistant and shaking controls and nose down path of the aircraft could quick result in loss of control, and ground impact, whatever the pilot did. Too little time to fight the disobedient aircraft..

To avoid it I guess they would need to very quickly identify what is happening (how can they when the problem has never been practiced?) disconnect the auto trim, manually fix the trim position quick enough to regain elevator authority, all while the stick is shaking and refusing or resisting to be pulled back?

I think this must qualify as a design flaw, that results on trying to add automation and envelope protection onto an aircraft concept and pilot interface where so many other design decisions have assumed automation is not present.

Thank goodness they have grounded the aircraft. Finally.  But what can Boeing do to fix it?  I suggest replacing MCAS with a good old fashioned Stall Warning horn, and let the pilots fly.. put a placard on the console saying 'Full power stalls prohibited'.. what else can they do?

Discuss?


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Offline brilligtove

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #1 on: March 19, 2019, 09:44:15 AM »
I'm very interested in hearing what the pilots in here have to say.
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Offline superdave

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #2 on: March 19, 2019, 09:58:12 AM »
well i have no idea how true that is but it sounds utterly horrific.
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Offline Fast Eddie B

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #3 on: March 19, 2019, 10:11:49 AM »
I'm very interested in hearing what the pilots in here have to say.

Quickly, I have a Commercial Multiengine license and am a CFI with about 6,700 hours, the majority instructing. Zero time in jets, though.

NOTHING is more instantly terrifying than not having the controls respond as you expect them to. I've hit moderate wingtip vortices that began an uncommanded roll, and it puts your heart in your mouth instantly. Far worse than an engine failure, let's say, where one may have the benefit of time to troubleshoot and plan.

There's another case where a single-point failure brought down a commercial transport jet - in short, the airspeed indicator was not indicating properly and incorrectly showed airspeed was rapidly increasing. The pilots kept pulling back on the yoke to slow it down. When the stick-shaker activated to warn of a stall, they looked at the airspeed and mistook it for a Mach buffet. As I said, terrifying. I'll see if I can find a link to that accident and I'll post it.

Offline Fast Eddie B

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Offline DevoutCatalyst

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #5 on: March 19, 2019, 10:42:36 AM »
The YouTube pilots are commenting. Not enough data yet. This brief video shows systems involved,


Offline Belgarath

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #6 on: March 19, 2019, 07:59:58 PM »
Not enough data yet but that speculation is quite wild and very likely a bunch of crap. 

I really hate to speculate in these matters because such speculation is better left to SCARY MARY on CNN but most of what I’ve seen is bunk

So my speculation which is most likely wrong is quite simple.  There are sensors that detect a nose high/high angle of attack issue.  These sensors sometimes fail.

This is not a run away trim, it’s most likely the stick pusher that lowers the nose when the system detects an angle of attack that is too high. 

It is also likely that the stick push is then overpowering the pilot.  There are reports that the crew tried to overcome the stick push 29 different times. You don’t have 29 distinct events with a runaway trim.

A runaway trim, on the other hand, would be just a continuous nose down push that got worse and worse. And a runaway trip is not new.  There are tons of well proven safeguards to cut the trim completely out of the system if it does run away. 


Either way we should avoid speculation. 

ETA:  I do not have any time in a Boeing.  I’m familiar with their philosophy and how their flight control systems are designed.  I have roughly 12000 hours with about 5000 in transport category aircraft.  2000 in the Airbus 320 series. 



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Offline brilligtove

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #7 on: March 19, 2019, 09:58:24 PM »
I appreciate your insight.
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Offline PatrickG

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Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #8 on: March 20, 2019, 12:30:39 AM »
This article gives more detail. I also had a discussion with a B747 and A380 Pilot about this.

In short: it does look like a runaway trim.  The MCAS system does not have redundancy, so with erroneous Angle-of-Attack sensor input it can command a nose down trim. This was limited to 2.5 degrees per event but apparently each event could send the elevator trim another 2.5 degrees until it hits the full down location.  That cannot be overcome by pulling the yoke.

The MCAS system does not apply the stick pusher, it only acts on the elevator trim. 

The application of the trim turns the trim wheels and is quiet audible in the B737. But a check pilot assured me that it happens that pilots miss this cue for out-of-trim.  The potential runaway trim seems to me a serious design flaw.

A panicking crew might not fix the trim issue but instead pull the yoke.

The real problem is that the B737 is not fly-by-wire. Modern airplanes perform the ‘envelope protection’ automatically via the computer between the stick and the control surfaces.  The new planes such that the A320 and A350 B787 have the same problem of low hanging powerful engines pulling the nose up at full power.  This is elegantly solved by the flight control computers.
The B737 has old fashioned cables and hydrolics instead. The B737’s MCAS system is a hack that attempts to do the envelope protection via the trim wheel. This was chosen because Certifying a FBW system and also training pilots for it was considered too much of a change and too time consuming. 

Boeing will have to bite the bullet and design a successor to the 737 that is nicely fly-by-wire and uses that latest materials.

Article: https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/

Offline superdave

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #9 on: March 20, 2019, 09:47:47 AM »
I'm no engineer but...

Wait, I am. 

I know hindsight 20 20 yada yada but this seems like an epic fail that should have been caught.  Avoiding unwanted positive feedback loops is control systems 101.
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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #10 on: March 20, 2019, 10:31:47 AM »
Boeing identified a problem (the problem) soon after the first crash.

They developed a fix. The FAA regulators disagreed about certain aspects of the fix and Boeing had to work out adjustments with the regulators.

Then the government shutdown happened and the FAA regulators couldn’t work on it.

After the shutdown the fix was approved and was scheduled to roll out in early April.

Then the second crash occurred.

The fleet is grounded and the same fix will probably be implemented, unless investigation of the second crash reveals other issues.

It’s hard to say if the FAA had a director if they could have acted faster (possibly) or if the government hadn’t shut down if the fix could have been implemented soon enough to prevent they second crash.


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Offline swan

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #11 on: March 20, 2019, 11:29:50 AM »
Though PatrickG explained the current issue pretty well (at least as I have understood it) already, here are some videos from "Mentour Pilot" (a 737 passenger plane pilot in Europe) with some decent explanations on these issues (careful to avoid undue speculation), control systems, etc.







At the very least, if there's some odd thing you've noticed when flying then do a search on his channel and you'll likely find a decent explanation.

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #12 on: March 20, 2019, 05:07:04 PM »
According to the  Orange Menace, planes are too complex :(
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Offline xenu

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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #13 on: March 20, 2019, 08:18:55 PM »
I've been a mechanic on jets for a long time. I have about 33 years working for the airlines. I like what Belgarath said, most of what people say is off the cuff and they are just guessing, when it comes down to it Boeing and the FAA/NTSB are still looking into this. I want to say up front I have not worked on the MAX much. We do not get it in Chicago much. I did go through the 5 day school about a year ago. So take what I say with a grain of salt. My airline has about 30 of them. We fly a lot of 737NG.

The 737 Max is a different plane then a 737NG( Next Generation a normal 737). It borrows a lot of technology from the 787. The flight deck and the troubleshooting with the 787 are very close. The 787 is basically a flying computer and generator and uses a lot of fiber optics. The 737Max is still on the old 737 frame but like I said incorporates a lot of the 787 technology. The troubleshooting on it is really neat.

So the problem. The 737 has an angle of attack(AOA) sensor on the front of the airplane. This is a vane on the front of the aircraft that lets the airplane know the angle of the wind to the aircraft. The plane in Ethiopia from  what I have gathered had one angle of attack sensor but the airline I work for bought an option for a second one, so we have a back up sensor. When there is a fault a maintenance message will show up and will say "MCAS MAX CMD LIMIT" This fault can only occur in flight. It also means that the max CMD limit has been reached inflight. So when I looked in the IFIM( fault isolation manual) there are 2 ways that this is generated. One is through FCC-A and FCC-B. I am not sure if it is two separate computers or just two channels in one computer. I didn't look into it that deep. One thing that is interesting is that if you have a FCC-A or B the FIM says to check both AOA sensors. So it sounds like it is getting both signals and comparing the results. If they disagree then I would think that it would fault the system and then not activate MCAS. 

I talked to one of our 737 pilots and he didn't seem that concerned. He said that we have a redundant system and seemed happy with the training to handle the problem if it occurred. I can see why Boeing at first said it was a training issue because they seemed to have a procedure if this happened. On the other hand this should not have happened. The system I believe is fairly new and was needed because of the change in flight characteristics with the design of the new plane. What this really shows is that the FAA is really to small to over see the industry as it has become and has given the manufactures to much control over the certification process. The FAA does not have enough inspectors to over see properly over sea overhaul bases.

I hope I made sense.
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Re: Recent Ethiopian Boeing 737 Max 8 crash - speculation
« Reply #14 on: March 20, 2019, 08:51:44 PM »
If they disagree then I would think that it would fault the system and then not activate MCAS.

Thanks for your input, xenu.  As I understand the issue from the videos above, while it makes sense that the MCAS would have a passive fail-safe if there is a discrepancy between the two AOA sensors, that does not appear to be the case. If either of the AOA sensors indicates a stall condition, the MCAS activates and it is the pilot’s responsibility to identify the issue as a runaway stabilizer and perform the runaway stabilizer memory tasks, which include manually disabling the MCAS by turning off the motor. The pilots on the previous flight of this same plane had done precisely that.
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