Author Topic: Boeing 737 MAX  (Read 2327 times)

Offline Glom

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Re: Boeing 737 MAX
« Reply #30 on: May 08, 2019, 03:50:25 AM »
It seems that it was the retraction of the flaps that caused MCAS to engage, so those who wish to blame the pilots are saying that had they followed the procedure and not retracted the flaps, they wouldn't have died.

The problem is that Boeing still designed a plane that actively tries to kill you if you retract the flaps.

You shouldn't retract the flaps with a stick shaker because a stick shaker is supposed to mean you're near the stall.  Clearly that was incorrect in this case. It was obvious they were nowhere near the stall so blindly following the procedure because duh stick shaker would not problematic too.

Offline Obviousman

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Re: Boeing 737 MAX
« Reply #31 on: May 08, 2019, 04:59:33 AM »
Always have to remember the 'flappy' mantra:

- Gear up
- Flaps up
- Shut up

Offline bknight

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Re: Boeing 737 MAX
« Reply #32 on: May 08, 2019, 09:11:03 AM »
Always have to remember the 'flappy' mantra:

- Gear up
- Flaps up
- Shut up

Nothing about the throttle?
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Offline smartcooky

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Re: Boeing 737 MAX
« Reply #33 on: May 08, 2019, 08:00:38 PM »
It seems that it was the retraction of the flaps that caused MCAS to engage, so those who wish to blame the pilots are saying that had they followed the procedure and not retracted the flaps, they wouldn't have died.

The problem with that theory is that the pilots would have to already know that retracting the flaps would cause MCAS to engage; and since retracting flaps is a standard part of climbout, they would not have realised that it was the cause of their pitch down problems. Its going to be difficult for the pilots to know that when they didn't even know MCAS was even a thing - it wasn't in the Flight Manual, the QRH or any of the 737 Max documentation such as the training manual. Nor was there any mention of it in the 2½ hour iPad course about the 737 Max. The only people who even knew MCAS existed were the engineers who designed it, and even they had no idea that it could cause this problem.

To be clear, MCAS was implemented to make the flight handing characteristics of the 737 Max as near as possible the same as the 737 NG.  Its was supposed to be something that worked in the background.

The problem is that Boeing still designed a plane that actively tries to kill you if you retract the flaps.

You shouldn't retract the flaps with a stick shaker because a stick shaker is supposed to mean you're near the stall.  Clearly that was incorrect in this case. It was obvious they were nowhere near the stall so blindly following the procedure because duh stick shaker would not problematic too.

Tragically, had the pilots been briefed as to what MCAS was and how it works, they MIGHT have been able to figure out that extending the flaps to the minimum setting, Flaps 1, would have disengaged the MCAS, full control would have been restored, and as long as they kept their speed below the overspeed limit for that flap setting (about 200 kias IIRC)  they could have circled back to the airport for an emergency landing.

As it was, they had no idea what they were fighting. The aircraft appeared to have a mind of its own, and at low speed and low altitude, they just didn't have enough time work it out.
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Offline Glom

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Re: Boeing 737 MAX
« Reply #34 on: May 09, 2019, 01:12:54 PM »
And of course these are the types of scenarios you train for. But no training was a design objective.

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #35 on: May 12, 2019, 07:43:19 AM »
Oh dear, it looks like Boeing committed another unforgivable blunder; they changed the functionality of the stab trim switches on the 737 Max without telling anyone.

Those following the story will recall these two switches on the back right of the centre console...



On previous versions of the 737 such as the 737 NG the left switch only deactivates the yoke trim switches that pilots use to control the horizontal stabilizer, and the right switch only deactivates autopilot control of the horizontal stabiliser.

However, on the 737 MAX , this was changed so that both switches performed the same function - setting either one to cut-out would disable the yoke trim switches and the autopilot functions, such as MCAS.

This means that, in the case of a runaway trim problem, a pilot flying the 737 NG could set the right switch to cutout, to turn off autopilot trim inputs, and still have manual electrical control of the horizontal stabiliser with the trim switches on the yoke. However, if they were flying the 737 MAX, this would not work - setting either switch to cut-out turned the whole stab trim system off!

Boeing made this change without informing pilots, and without including it in the iPad "Differences Training" course or any of the training documentation.
 
https://www.seattletimes.com/business/boeing-aerospace/boeing-altered-key-switches-in-737-max-cockpit-limiting-ability-to-shut-off-mcas/
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Offline Bop

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Re: Boeing 737 MAX
« Reply #36 on: May 12, 2019, 12:13:00 PM »
I mentioned that  previously, and I suspect it was why the MCAS was re-enabled towards the end, they tried to get the electric trim working 'as per spec' as manual trim was by this stage out of the question, and in the process re-enabled the MCAS, dooming them...
Problem was that Boeing 'changed the spec' without telling anyone...
I have a diagram of the wiring system floating around somewhere at home, when I get back, I'll see if I can dig it up, it clearly showed the MAX now has both switches now wired together in series, where previous versions had them separated, with one switching off all electrical assist (what the MAX now has with BOTH switches) and the other switching off AP functions, but leaving the pilots trim switches on the yoke able to drive the electric motor (now nonexistent on the MAX!!!!)

Boeing apparently didn't consider this to be a major enough change to mention in their ipad slideshow, past 'we renamed the switches'...

:-O

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #37 on: May 12, 2019, 09:53:32 PM »
I have a diagram of the wiring system floating around somewhere at home, when I get back, I'll see if I can dig it up, it clearly showed the MAX now has both switches now wired together in series, where previous versions had them separated, with one switching off all electrical assist (what the MAX now has with BOTH switches) and the other switching off AP functions, but leaving the pilots trim switches on the yoke able to drive the electric motor (now nonexistent on the MAX!!!!)

Well that is appalling. If one of my tech bosses asked me to do it that way, I would refuse, and I certainly would not sign off on it.

The correct way of doing this would be to remove the second switch, and make a new switch panel with only the single remaining switch on it. The only reason I can see for doing it the way they did is to save a few dollars on new panelling. Its another clear attempt the cut corners and costs.
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Offline Glom

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Re: Boeing 737 MAX
« Reply #38 on: May 13, 2019, 04:07:31 AM »
Is that verified?

That sounds too ridiculous to be true.

Offline smartcooky

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Re: Boeing 737 MAX
« Reply #39 on: May 13, 2019, 06:55:46 AM »
Is that verified?

That sounds too ridiculous to be true.


Stab Trim block diagram for 737NG


Stab Trim wiring diagram for 737MAX

Please note that the top diagram is a block diagram rather than a wiring diagram but it will get the point across (I can't find an actual wiring diagram for the 737NG Trim system, but I know enough about these sorts of documents to have a fair idea what it will look like.

You can see that in 737NG (top diagram) the two switches are separate. The left ("Main Elect") switch kills electrical power to the Stab Trim motor (so stops the whole stab trim system from working) while the right ("Autopilot") switch only turns off the autopilot inputs to the Stab Trim motor, leaving the pilots able to electrically operate the trim system.

However on the 737MAX (bottom diagram) the two switches are wired in series. Also note that A on the left switch connects to A on the Electric Stab Trim Motor Block (on the right side), and B on the right switch connects to B on the FCC (Autopilot) block (bottom centre). As I understand this diagram, switching off either the left or right switch will turn off power to the Stab Trim motor, and this means the pilots will have no manual electrical control of the trim system from their yoke trim switches. Their only option then is to use the trim wheels and we have already seen what a difficult task that is.
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Offline Bop

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Re: Boeing 737 MAX
« Reply #40 on: May 13, 2019, 10:04:24 AM »
Well that is appalling. If one of my tech bosses asked me to do it that way, I would refuse, and I certainly would not sign off on it.

The correct way of doing this would be to remove the second switch, and make a new switch panel with only the single remaining switch on it. The only reason I can see for doing it the way they did is to save a few dollars on new panelling. Its another clear attempt the cut corners and costs.
From the various pilots, it was more a training issue, one of the contract requirements was that the planes had to be a 'standard 737' ie no retraining/re-certifications for the pilots necessary, and there was a major penalty if this was not achieved (from memory just over 100 million dollars- for one airline alone!)
This obviously led to management pushing for the software to correct the trim issues 'invisibly' so the pilots simply didn't have to be told...
In fact even after the first crash, where I suspect Boeing management had a pretty good idea what had gone wrong very quickly- they simply did a quick 'here's what to do' briefing on an Ipad and called it good enough and still didn't release much info on the new system

Is that verified?

That sounds too ridiculous to be true.
Unfortunately yes
(I see Smartcooky has already put up some diagrams, I believe I still have the NG circuit diagrams as well at home when I get back there ...)

Offline Glom

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Re: Boeing 737 MAX
« Reply #41 on: May 13, 2019, 10:22:46 AM »
Well then. Boeing are in for a right boening.

Offline Bop

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Re: Boeing 737 MAX
« Reply #42 on: May 13, 2019, 05:38:47 PM »
Yes indeed, and the FAA along with them (due to the whole 'selfcertification' thing- which is also a relatively new thing...

It's looking like many other countries regulators are going to be far less 'trusting' of FAA declarations, with the UK , EU and China to date saying that may require 'additional' certification checks before lifting the flying ban in their countries- a real slap in the face to the FAA!

 :o

It could lead to the 737MAX being allowed to fly in the USA only, and banned from international flights until Boeing gets re-certified in each individual country that demands it

Here in Australia, Virgin was supposed to be getting 50 new hulls  by mid/end of this year, they have pushed them out to 2021- which doesn't look good for any continuing orders either, they obviously aren't expecting to be able to use them any time soon...