B737 Max MCAS notes from before the Ethiopian Accident
 

Attached are some notes I made in mid-Jan 2019 regarding the MCAS architecture and Lion Air crash

Salute!

Attention flyingfalcon!

We give up. We do not know what you want to hear, but for now I feel most here will let you continue to believe that trim affects speed and attitude. We have tried to erxplain AoA and attitude and reference systems and so forth. As planes became less and less like the WWI and early WW2 planes, the AoA versus Coeff of lift plot became less steep. The tendency to return to the "trimmed" AoA decreased. But given time, the plane would return to the trimmed AoA that is normally associated with one gee level flight speed.

The TRIM ON ALL BUT FBW SYSTEMS LIKE THE F-16 OR AIRBUS ATTEMPTS TO RETURN THE AIRCRAFT TO THE ANGLE OF ATTACK THAT IT IS TRIMMED FOR BEFORE AN UPSET OR CHANGE IN POWER. If that changes the pitch attitude with respect to the horizon, then BFD. So we give up and you win.

So let's move on, huh?

Gums sends...

Hi Alchad, that bit error could be the problem. However the data is Serial 32bit, so how does one bit get corrupted and not all the other bits. Bit 27 is also the third MSB (Most Significant Bit.)

https://en.wikipedia.org/wiki/ARINC_429

Looking at the Lion Air FDR, did reveal that the AoA error started to build up during the taxi run, 2 minutes before take-off.

Thanks - very nice
 

I am converted - we need a Like button.

I think that is likely the case but I'm new here. I would just share that some posts which are not spam have not been posted. So perhaps different mods moderate a little differently.

Thanks for your further explanation. And yes let's move on.

I have a separate question: Does anyone familiar with test piloting these crafts know that, before there is testing in the sims with airliner pilots. Is there actually a phase of testing that involves a pilot going up in the Max with the exact knowledge and training the airliner pilots will have (such that they wouldn't get any briefing on the MCAS)? Possibly with simulated load factors / weight of real cargo and passengers, ideally with even simulated changes in CoG from moving passengers throughout flight?

I gotta tell ya, John, this is the most exasperating forum I have ever tried to participate on.

For those who have concerns with the moderating, the following is the basic story -

(a) new posters are subject to draft post review for a number of posts before they get a direct post ability - that is site wide policy as far as I am aware

(b) while I am the main mod for this forum, there are others who have mod rights. Sometimes it is a case that mods are just not able to keep all the people happy all the time and, unfortunately, you are just going to have to accept that.

(c) my approach (which doesn't have any necessary influence on that adopted by my colleagues) is minimal editing/deleting and, where I delete (other than for obvious spam) I will let the poster know what's going on.

(d) if you have a post which you consider to be very important and it appears to have been deleted, by all means send me a copy by PM and, if I concur with your thoughts, I will endeavour to track down the reason for which it was deleted. Be aware, though, there will need to be a good reason for me to argue the case with my brother mods.

Would you happen to know the max nose down trim angle on the 737 NG and MAX?

I'd second that!.

Also, don't know if it is possible, but for threads like this and others which can get quite long, it would be very useful if posts like Le Flanneur's could be "stickied" at the top of he thread. Quite often some very useful pieces of information - Flight Data Recordings, instrument block diagrams etc etc are posted but then get lost. It would also be a reference for new posters to look before asking questions posted several times over.

As I said, don't know if it's possible, but would be nice to have....

Regards

Discussed extensively in the Lion Air thread. I don't recall the numbers, but I do remember the limits differing depending on whether it's the A/P, trim switches or trimwheel doing the trimming.

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Perhaps the only way to get these airplanes certified is to ditch the 'Hot-rod' engines and revert to the original engine type.
Vintage Model T Fords with Supercharged Chevy engines might be Ok for the Drag-Strip, but not for commercial transportation.
..

Stab Trim Cut-out switches relabelled on Max - why?
 

"Slacktide" posted a question (post 1337) on the Indonesian 610 thread asking why stabiliser cut out switches had been relabelled. As far as I could see, nobody had any views, just though I'd ask again as I'm also curious!

Regards


Post below:

It appears that the two trim systems which were labeled "Main Elect" and "Autopilot" on the NG are now labeled "Pri" and "B/U" on the Max. One would ASSume that this means Primary and Backup. It would be useful to know if there are any operational changes to the system besides the labels. It is unlikely that they would have made have made a change to the labels without a reason.

737 NG

https://cimg5.ibsrv.net/gimg/pprune....b7978fc4da.jpg

737 MAX


https://cimg7.ibsrv.net/gimg/pprune....4ae6cdebb5.jpg

I think I asked about this too at one point on one thread, I vaguely recall that there was a sort-of answer that they are now "primary" and "backup" cutouts and both of them cut both (auto and main-elect) circuits. This implies there is no way to cutout STS/MCAS and still have manual electric trim, I think. However, I am not sure we got an answer as to why they have changed or whether this change has been properly communicated to pilots.

Take this quote from a MAX ASRS report:
Then add in the fact that these re-labelled and (possibly) re-purposed switches are the very ones the accident crews were expected to use to control MCAS...

I should stop there (or earlier) since I'm only an engineer flying an armchair, but this smells too much like the times in my career when development/sales/delivery/training have all blamed the end user for a ****up, yet when I have been the one to sit down with the end users I have found that between development/sales/delivery/training we have produced a system that an average user was inevitably going to ****up at some point. I'm lucky, those times have not been in aviation, the worst consequence was data loss (and maybe contract loss, bonus loss...), but nobody died - but it shouldn't happen in aviation, it should be caught before it gets in front of an "average end user" with a plane load of pax behind them.

Alchad

I am quoting from memory, but the explanation somewhere on this forum is:
1. On the NG there are only two inputs for electrical trim, the autopilot flight control computers and the yokes. Each has their own switch, wired in parallel.
2. On the MAX there are three inputs, if you add MCAS. Since adding a third switch is complicated, they wired the switches in series. This provides an extra level of inhibition of runaway trim, since one of the two could theoretically get stuck in the on position (due to short circuits).
I hope this is correct and helpful. The implications of these changes are beyond my scope.

Edit: I posted simultaneously with infrequentflyer789 whose comment overlaps with and supports my recollection.

Think this was it post 2439 on the Ethiopian thread

@RUTUS, Since there are no other electrical connections in that diagram, the logical conclusion is that indeed on the NG any kind of automatic trim changes can be disabled by the AP cutout switch, and also by the column cutout switch connected in series with it.

I can't find a similar diagram for the MAX, but I remember reading that the cutout system has been redesigned. For example the two cutout switches have been renamed, from MAIN ELECT and AUTO PILOT, to PRI and B/U (primary and backup).

And, if I remember correctly, those two switches don't longer have independent functionality on the MAX, because they are connected together in series. If one of them gets stuck or fails shorted, the other can act a backup for it, so on the MAX both manual cutout switches would disable any kind of electric trim, manual or automatic.

I don't have further details about that, and I wouldn't want to speculate about exactly how it works on the MAX in combination with the column cutout switches, but this has been discussed previously in the Lion Air thread, you may try to look there for more details.

Alchad

Thanks for copying the original post. I hope my comment helped track that down, and was not too creative an interpretation. It seems nobody is 100% sure? In any case the action for runaway trim is always both off, regardless of cause or model type.

Edit: Amended my earlier comment.

Where does STS come from?

My quote was definitely faulty. Speed trim comes from the flight control computer. See: https://www.pprune.org/showpost.php?...4&postcount=37

Edit: Reading that detailed reference suggests two points. 1. STS and MCAS run on a single FCC at a time, which alternates between flights. 2. AFAIK the autopilot runs on both FCCs, but either is crew selectable while inflight.

The autopilot can be disabled, but STS and MCAS are used in manual flight, and cannot be disabled. How this all fits together raises many more questions (completely outside my scope).

One thought regarding the pedestal cut out switches.
I find it lacking in functionality that you can not disconnect all automatic trim like STS, Mach trim and MCAS and still continue to have manual electric trim available. The two switches should have been configured so that one cut only the automatic trim functions and the other cut all electric trim.

That would in my opinion have given more alternatives for the crew as I am sure any crew would very much like to have manual electric trim available after any run away automatic trim had been cut out.

If I was FAA this would be (the) one physical item/functionality to change before lifting the grounding of the Max.