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.

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!

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 :-
https://www.pprune.org/tech-log/6157...l#post10430909

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.

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:).

I don't want to edit the above post in case it breaks the links in it so I add here:-

https://www.pprune.org/10430909-post1.html

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?
https://cimg8.ibsrv.net/gimg/pprune....5f36332e15.png

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!

Lastly - phew!
You can see what is actually in any post by quoting it and pressing "<> Source" in the Edit tool that opens.

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: https://www.pprune.org/tech-log/6193...izer-trim.html 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...

infrequentflyer789 Thanks for the link . very interesting , but that was on the 707 ( same age as my Cub :E ), 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.

This was the document I was referring to. Stunning comment about the removal of the rear column switches.

https://www.satcom.guru/2019/03/ethi...s-to-lion.html

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.

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.

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.

I do not think...
https://cimg4.ibsrv.net/gimg/pprune....fac41f93a3.jpg

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.

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).

But I thought that defective AoA was replaced before the second flight.

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 !!!!ty, than only FCC#1 do the stuff on the left side like stickshaker and on.
If the right AOA is !!!!ty, 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.

hans brinker
Yes, English is not my first language.
Try to read my posting in correlation to:
BTW: what on my posting is not correct?

Your post is correct in almost every respect. The difference hinges about the meaning of 'reset', which is not actually important in the context of this discussion. I did not go into details about the FCCs, since I was responding to a specific question by Arydberg .

As far as the pilots were concerned, they had no control over MCAS, and both they and the Lion Air maintenance technicians had no idea it existed.

It is my understanding that if the value improves MCAS will take out the trim input it put in (if MCAS is working correctly, not in the broken AOA scenario)


As far as your post being a reply to Gordon, what he said is correct. If the power is turned off and than turned on again MCAS will always use AOA 1. It is definitely correct English to call that a reset.

Probably best to define that as a Reset upon power up, or some such. The reset that has caused so much of the problem, MAY have been the fact that MCAS resets, and re-datumises following a function. The ratchet effect could be how the screw-jacks were at full travel.

MCAS has a long history in Boeing. Was first proposed on the 767 to fix issues, but vortex generators came up trumps. Both the KC-767 and KC-46 have MCAS, the USAF is currently reviewing whether the -46 MCAS has hidden gotchas.

The certification requirements for military aircraft are very different from passenger aircraft. MCAS may have been acceptable for the KC-46 tanker, but not for the closely related B767 passenger aircraft, which used aerodynamic fixes instead.

I found this from 2016, of some interest, and indeed a link to a discussion about the cut out switches on the centre pedestal. It mentions the MAX but shows the right switch with a mention of the Auto Pilot. Misinformation abounds.

Both on R&N but this one has some historic discussion with gums et al.

https://www.pprune.org/archive/index.php/t-576817.html

https://aviation.stackexchange.com/q...-on-the-boeing

Thanks for the pointer. This bit jumped out for me:

"Blip, relax, assess. Blip, relax, assess."

One wonders what the result of that trim technique would be with MCAS in play...:uhoh:

Greetings all!

New guy here, first post.

I'm still trying to get my head around why neither of the MAX accident crews simply applied sufficient nose up trim to neutralize the MCAS input. It could be an "airmanship" issue, but is there another possibility? Does anyone know if the 737 stab trim motor can actually stall in a high load environment? Is there an internal circuit breaker or thermal relief? It's a big electric motor, so there's bound to be some kind of protection if the stab was truly jammed.

Related question: The 737 has a single stab trim motor that operates at two speeds. The yoke trim switches actuate the trim motor in the high-speed mode. The MCAS (I believe) uses the low-speed mode. Is there a difference between the available torque in high-speed vs low-speed modes? For example, I have a two-speed electric drill that can stall out in the high-speed setting, but will continue to operate at low-speed. If the stab motor suffers from a similar phenomenon, I could see how the MCAS could function while the pilot-commanded trim would not.

You have this backwards. My understanding is that manually controlled electric trim operates at high speed with flaps down and at low speed with flaps up. MCAS operates the trim at high speed.

Your drill (if it's like my battery powered Makita and my corded Milwaukee) actually has a gear ratio change when you go from low to high range. I don't think the stab motor speed difference is achieved that way, and instead is done electrically in some manner.

Do you have a reference for that ?

On the NG (which of course doesn't have MCAS), low-speed yoke trim (i.e. with flaps up) is the same rate as high-speed A/P trim. High-speed yoke trim is 3x that rate and low-speed A/P trim is half the rate of high-speed A/P trim. I would have thought that MCAS would trim at the A/P rate, but I haven't seen that documented anywhere.

There's no gearbox in the stab trim motor.

Good description here: B737NG Flight Controls

Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?

Went back and checked, and the change in specs was the authority (from .6 to 2.5 degrees per activation). Article didn't mention speed. As noted elsewhere, the MCAS trim speed is faster than the flaps up main electric trim speed which is a problem. For all other trim functions, flaps extended trim speeds are faster than flaps up, and main electric trim speed is faster than automated inputs.

Authority (per increment / reset) was definitely increased from the original design of 0.6 degrees to 2.5, I don't know whether this was done by increasing speed or duration or both - the speed change wouldn't do it alone, but might have been required to "correct" the stick force fast enough.

Stab trim does go through a gearbox, between motor and jackscrew, and there are at least two clutches (nose up/down - not sure if ap/manual is also separate or is a speed change).

From NG AMM chapter 27:

What is also known is that MCAS trims at the flaps down autopilot speed - i.e. the "High speed". I don't know if this was a change at some stage due to finding low speed was insufficient for certification - it might have been. I also know how it was implemented (which is as I guessed it might be) - by blocking the flaps-up signal on MCAS engage.

References - elec trim functional diagrams below. One is from NG AMM, one is from an internet source but appears to be the equivalent diagram from MAX AMM. A whole lot looks to have changed with the wiring to shoehorn the MCAS function in...

Thanks for that info. I stand corrected.

To summarize:
Manual electrim trim speed when flaps are up and autopilot disengaged: 0,2 units per second
MCAS trim speed when flaps are up and autopilot disengaged: 0,27 units per second