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737MAX Stab Trim architecture

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737MAX Stab Trim architecture

Old 27th Mar 2019, 01:52
  #161 (permalink)  
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There was a post on the 'Ethiopian' thread - which I'm sure you'll know, is covering both crashes - which showed a jpg ?? of a document spelling out the changes in the switches. As I understood it, it indeed said the switches in the MAX are now simply in series, a protection against switch welding on, and so forth. There was now not the option of Auto Pilot once even one of these switches were activated. Or, the AP no longer is routed through the switches.

The main issue now is to be sure either one of them cuts the motor/gearbox supply to the H Stabilizer, and indeed if that certainly stops the use of the thumb trim control of same.

IIRC, on the same sheet, they mentioned the REMOVAL of the rear (hidden) switch, in each of the columns) Making the point that since the MAX, reacting with a rearward nudge no longer had certain functionality. It must be said that there have been some counters to this information during subsequent posts, despite it explaining a lot if true.

Due to moderating making the page numbers change, I gave up any attempt at note-making - and even that page may have gone. I'll try and find it - but it will be on the morrow.
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Old 27th Mar 2019, 02:02
  #162 (permalink)  
 
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With PPRuNe page and post numbers in flux due to moderator actions within these busy / large threads I recommend referring to previous posts by entry date and time stamp. That is one thing that seems to remain constant. Of course that is subject to the users time zone. If you pull up PPRuNe and don't sign in I believe it shows universal time. I can find it a challenge to do the math for my time zone and then daylight savings comes along and shifts it all by an hour!
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Old 27th Mar 2019, 05:53
  #163 (permalink)  
 
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Originally Posted by FCeng84 View Post
With PPRuNe page and post numbers in flux due to moderator actions within these busy / large threads I recommend referring to previous posts by entry date and time stamp.
I think that the "permalink" works always (although I may have had a malfunction with it recently). Right click on "(permalink)" choose "Copy link address". I sometimes keep notes in a text file and paste in the permalinks. This is I think the best way to refer to posts or perhaps use the quote structure as described below.

Your most recent post permalink is :-
737MAX Stab Trim architecture

I have substituted "pprune " for the full PPRuNe domain name below.

PPRuNe /tech-log/615709-737max-stab-trim-architecture-9.html#post10430909

This sometimes gets automagically re-cast as
PPRuNe /showthread.php?p=10430909

I strongly suspect that 10430909 is a permanent unique post identifier. So the rest can be thrown away if you can be bothered to re-add PPRuNe /showthread.php?p=

Sadly google can't find by this post no.

Quoting a post using the Quote facility also creates a permanent link to it via the blue arrow thing. This link in quoted post text looks quite handy and I had not noticed it previously.

Originally Posted by FCeng84 View Post
Padding
This looks like this in the edit view (all spaces added by me to allow you to view details here.).
" [ Q U O T E =FCeng84;10430909]Padding[ / Q U O T E ] "

Without padding quote does not appear, a single full stop seems to be enough.

Last edited by jimjim1; 27th Mar 2019 at 06:41.
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Old 27th Mar 2019, 06:13
  #164 (permalink)  
 
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I don't want to edit the above post in case it breaks the links in it so I add here:-

737MAX Stab Trim architecture

PPRuNe /10430909-post1.html

Also works, it shows a SINGLE POST displaying the arbitrary post number of 1 (or any other number you choose). This also creates a valid permalink link to the post in normal thread view so that is only a click away.

Some of these may not be future proof if the forum software changes.

Also, I save stuff in notepad and quite often links get broken by an automatic line break ending up inserting a space in the link. Appears as the hexadecimal of the code for the space character. "%20".

If saved links don't work I always check for this first but BEWARE hex characters in links are very common so you need to find the right thing to change.

PPRuNe's longest ever thread and fluxiest ever post number?

Last edited by jimjim1; 27th Mar 2019 at 06:34.
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Old 27th Mar 2019, 13:28
  #165 (permalink)  
 
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You can point to posts like this too.
I have discovered how to put literals in posts with the n o p a r s e BBcode.
https://www.pprune.org/misc.php?do=bbcode

You type:-
[post]10430909[/post]
or
[post=10430909]Your desired label![/post]

Looks like this:-
https://www.pprune.org/showthread.php?p=10430909
or
Your desired label!
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Old 27th Mar 2019, 13:40
  #166 (permalink)  
 
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Lastly - phew!
You can see what is actually in any post by quoting it and pressing "<> Source" in the Edit tool that opens.
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Old 27th Mar 2019, 15:11
  #167 (permalink)  
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There are lots of Rumors going round in Africa ( the mother of rumors ..) about the Capt having done all the works as trained, incl STAB TRIM cut out and all, but could not overcome the final dive manually .
I do not believe in rumors, especially not in Africa, but a question for those here with experience on the 737.

Is there a speed above which the air pressure on the control surface of the horizontal stab would start to prevent a standard human to activate normally the manual trim wheel ? and btw is this force accurately duplicated in a sim as speed builds up ?

I have flown some ( old) GA aircraft where above a certain speed the cables would just spin around the pulley not moving the stab ( e.g the Super Cub in a steep dive, where lever will turns with difficulty , but does not move the Stabilizer , and before you ask, this happens well below VNe )
I do not believe this would be possible in a modern airliner, but ask the question just in case. .
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Old 27th Mar 2019, 17:33
  #168 (permalink)  
 
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Originally Posted by ATC Watcher View Post
There are lots of Rumors going round in Africa ( the mother of rumors ..) about the Capt having done all the works as trained, incl STAB TRIM cut out and all, but could not overcome the final dive manually .
I do not believe in rumors, especially not in Africa, but a question for those here with experience on the 737.

Is there a speed above which the air pressure on the control surface of the horizontal stab would start to prevent a standard human to activate normally the manual trim wheel ? and btw is this force accurately duplicated in a sim as speed builds up ?

I have flown some ( old) GA aircraft where above a certain speed the cables would just spin around the pulley not moving the stab ( e.g the Super Cub in a steep dive, where lever will turns with difficulty , but does not move the Stabilizer , and before you ask, this happens well below VNe )
I do not believe this would be possible in a modern airliner, but ask the question just in case. .
The thread: Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim covers more-or-less the same thing I think. It's not that the cables spin without moving the stab (as with your Super Cub), it's that you won't be able to move them until aero loads are reduced.

I don't think it's clear (even having read that thread) whether or not it can still happen on newer 737s, but it may not have mattered for the ET crew. If your rumour is true it wouldn't be a great surprise, they were low (AGL) and fast, time to correct any mis-trim would be small. They had the unenviable choice between fighting the machine with the pickle switches (machine will win eventually, it's faster, it doesn't get tired, or distracted etc.) or hitting the cutouts and winding the handle, which is slow. The ground is rising...
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Old 28th Mar 2019, 07:32
  #169 (permalink)  
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infrequentflyer789 Thanks for the link . very interesting , but that was on the 707 ( same age as my Cub ), so ,as you said, not sure this apply in the same manner to the 737 NG or Max..

And does anyone knows if this forces accurately replicated in a SIM ?

As to the ET, well the rumors (again) are the FDR has been decoded successfully ( seems to have been some problems with the CVR ) so we should know pretty soon the sequence of events at least.
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Old 28th Mar 2019, 23:25
  #170 (permalink)  
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This was the document I was referring to. Stunning comment about the removal of the rear column switches.

The decision to disable the aft column cutout switch may not have been assessed as a hazard at all. Yet, this was the most threatening change made by Boeing, and very likely took away the last thread for survival on JT610 and I fear ET302.

How MCAS malfunctioned is irrelevant, the possibility exists that MCAS could malfunction. The aft column cutout switch has been a long-standing safety feature. Human factors must be taken into account. In the scenario where the stabilizer is running away nose down, the pilot may only fixate on pulling the column back in response. They may not be mentally capable to trim back or cutout the trim - instead they just keep pulling. That is where the aft column cutout switch saves the day. It very well could have been the last straw to save JT610."
https://www.satcom.guru/2019/03/ethi...s-to-lion.html
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Old 29th Mar 2019, 01:32
  #171 (permalink)  
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Well, this is the vital point. The author seems highly qualified and the dichotomy in what is fact or fiction so extreme during the R&N thread that it has become almost perfectly circular.

Has the darn switch been removed on the MAX, or not? He asked, Rhetorically.
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Old 29th Mar 2019, 03:45
  #172 (permalink)  
 
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I have a question, Apparently the AoA sensor is switched from right to left and left to right on successive flights of the Max 8. But in the case of the Lion crash the aircraft experienced problems with the working AoA on the preceding flight and after landing this AoA was replaced. But on the next flight i would expect the opposite AoA to be used. Thus it appears two AoA sensors were malfunctioning. This seems deeper than simply a single AoA. What am i missing.
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Old 29th Mar 2019, 05:55
  #173 (permalink)  
 
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Originally Posted by Takwis View Post
...No mention of the control column switches being removed, or having their function negated by another system, except the stab trim override switch.
Engaged MCAS overrides the column cutout switches.
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Old 29th Mar 2019, 06:50
  #174 (permalink)  
 
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Originally Posted by IFixPlanes View Post
Engaged MCAS overrides the column cutout switches.
I think it is clearer to say that the MCAS control signals run from the flight control computers to the stabiliser trim through a different circuit. This follows so that MCAS trim is not overridden, unlike speed trim which is inhibited by the column cutout switches.

We know that the wiring and labels of the pedestal cutout switches have been modified in the MAX, so it makes sense that the changes for MCAS were done as part of that process. This was discussed earlier (can't find the reference).

It would not make sense to have wiring going from any of the switches to the FCC, which is my interpretation of your statement. AFAIK the B737 MAX is still fundamentally a manual aircraft, and all software functions are implemented upstream of those downstream cutoff switches.

These wiring differences have profound implications for pilots trained on NG, when trying to diagnose an MCAS fault on the MAX. They probably explain most of the Lion Air pilots actions and confusion.

Last edited by GordonR_Cape; 29th Mar 2019 at 11:58. Reason: Factual inaccuracy.
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Old 29th Mar 2019, 07:23
  #175 (permalink)  
 
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I do not think...



Last edited by IFixPlanes; 29th Mar 2019 at 07:58. Reason: Done
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Old 29th Mar 2019, 11:54
  #176 (permalink)  
 
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IFixPlanes

Point taken, and thanks for the diagram! I do not recall seeing that posted on this forum before.

I bet the pilots would have had fun studying that diagram, both during training, and while trying to control a misbehaving MCAS system.

Edit: I have amended my previous comment to remove the parts that were factually inaccurate. The remainder remains valid IMO.

Last edited by GordonR_Cape; 29th Mar 2019 at 12:00. Reason: Add context.
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Old 29th Mar 2019, 15:39
  #177 (permalink)  
 
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Originally Posted by Arydberg View Post
I have a question, Apparently the AoA sensor is switched from right to left and left to right on successive flights of the Max 8. But in the case of the Lion crash the aircraft experienced problems with the working AoA on the preceding flight and after landing this AoA was replaced. But on the next flight i would expect the opposite AoA to be used. Thus it appears two AoA sensors were malfunctioning. This seems deeper than simply a single AoA. What am i missing.
This was discussed previously. Apparently turning off power to the aircraft always resets MCAS to the captain's side AOA. The next flight carried out would then alternate the AOA side as you describe. Obviously during maintenance to replace the AOA sensor the technicians would have to turn off the power. This reset the AOA to the captain's side again.

BTW, since there was no way to change which AOA sensor used by MCAS while in flight (unlike the twin autopilots), there was zero use of redundancy (and only a minuscule escape route). IMO the twists and turns in this story are deeply bizarre (and sometimes lost in the multiple posts on this thread).
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Old 29th Mar 2019, 15:42
  #178 (permalink)  
 
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But I thought that defective AoA was replaced before the second flight.
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Old 29th Mar 2019, 16:19
  #179 (permalink)  
 
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MCAS is a funktion of the FCC.
There is no reset.
If either one of the FCCs get a given AOA value that represent Stall for him and all these things are given
- MCAS is enable for airplane model by program pin selection
- Autopilot is disengaged
- Flaps are up
- Pilots are not commanding stabilizer trim (Manual mode).

he activate the MCAS.

If the left AOA is shitty, than only FCC#1 do the stuff on the left side like stickshaker and on.
If the right AOA is shitty, than only FCC#2 do the stuff on the right side.
If the AOA of the aircraft is really that high to activate the MCAS than both FCCs do their stuff on their side (i.e. stickshaker on both sides)

If the value of an AOA improves, the FCCs stop their MCAS signal - yes you can call this a reset...
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Old 29th Mar 2019, 16:50
  #180 (permalink)  
 
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Originally Posted by IFixPlanes View Post
MCAS is a funktion of the FCC.
There is no reset.
If either one of the FCCs get a given AOA value that represent Stall for him and all these things are given
- MCAS is enable for airplane model by program pin selection
- Autopilot is disengaged
- Flaps are up
- Pilots are not commanding stabilizer trim (Manual mode).

he activate the MCAS.

If the left AOA is shitty, than only FCC#1 do the stuff on the left side like stickshaker and on.
If the right AOA is shitty, than only FCC#2 do the stuff on the right side.
If the AOA of the aircraft is really that high to activate the MCAS than both FCCs do their stuff on their side (i.e. stickshaker on both sides)

If the value of an AOA improves, the FCCs stop their MCAS signal - yes you can call this a reset...
Maybe English is not your first language (neither is it mine), but your choice of words doesn't help your argument.

Yes, there is a "reset", at least there is in the meaning of "MCAS will become active again, and add an input to the THS". One of these resets is 5 seconds after the pilots stop using the thumb trim switch. If the pilots do not use the thumb trim switch after an trim input by MCAS due to faulty AOA, there won't be another MCAS input. If the value of the AOA was correct, MCAS will trim the nose back up after the AOA is back to normal range.

If you mean there is no specific MCAS reset button on the flight deck you are right, but your post isn't very clear/correct/helpful/relevant.
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