Guess the thread has gone "thread bare".
PUNT!
Gums....

You're kidding aren't you!

The only remedial action (in the short term at least) will be a concerted ass covering by the "Genius's" who decided to implement such a deadly system.

Salute PEI !
I agree with your view.
I have very strong feelings about modifications and even basic design charactreristics that can affect safety, Especially on the ubiquitous 737
The discussion on that famous mega-thread about "deep stall" versus "deeply stalled" surely had to have helped prevent another tragedy plus, maybe, refine some of the prodecures, control laws and warning indications, huh?. And I will bet that 737 folks who have seen our discussions and then the flight data now realize they could face uncommanded trim in a part of the envelope they did not expect three months ago. A really great part of this thread and the others is that folks know an existing procedure will help you get the plane back on the ground
The mods deleted the post from PEI that I responded to. We need better ROE here, and complaining about Mod editing is par for the course.

Merry Christmas and Happy New Year!

Gums...

737 Max AoA input to FDR
 

Does anyone know if there is a cockpit indication of the position of the elevator lead-screw, or is this just left to guesswork by the crew..?

There have been several comments about mod interference in this thread. I haven't made any changes, nor is there any evidence of changes by other mods that I can see. A couple of the comments are from gums and PEI, both of whom I hold in high regard, knowing who they are, so that concerns me, potentially. If you are concerned about edits in this forum (and there are some from time to time) please, by all means, send me a PM to query. I will follow the query up to see what the story might be. As most would be aware, I make very, very few mod changes to existing posts, unless they be totally over the top regarding normal forum mores.

Noted. However, one needs to be quite careful that one understands the poster's intent prior to running an edit on the post. Unless a post is rather silly or pointless, I prefer to run a query past the poster to ensure that we both are on the same page regarding the post in question's intent ....

Anyone care to discuss the possibility of elevator PCU failure or jam, elevator PCU mounting failure due overload or elevator structural failure due airload. I'm guessing the stab/elevator structure was not designed for 300kts IAS with full APND and full up elevator and associated G loading.

One eyewitness reported seeing paper like debris falling with the aircraft which imho may have been pieces of the elevator/stab structure having failed and separated.

KRUSTY, I see you are living up to your moniker, however the time for legal ass covering is basically past. What is even more important now is brand reputation.
That has to be handled well, or the costs will be astronomical.

For JT
With the scattering of topics/threads on the recent 737 Max problems, I think people are having difficulty remembering where they are on the Pprune topology. The Rumors and News forum has a different moderation philosophy than Tech Log.
I for one greatly appreciate your moderation approach. Best I can suggest is to understand why you would be receiving inappropriate criticisms and not let it get to you. If really concerned, you also can PM those who have commented adversely.

M,

Nothing much gets to me .. I have an inch thick hide in the nature of rhinos.

My concern is more with knowing if edits are being made. I have no problem with that but, sometimes, the PPRuNe structure does some strange things .. Main thing is to maintain the to and fro communications as appropriate. My philosophy has always been, and will remain, minimal moderator interference, consistent with good humour.

R&N, of course, has a very different clientele to that seen in this forum ... so the moderation probably needs to be a tad different.

Not an MCAS Fault
 

Again if you look at the trace of the left AoA Sensor, it all starts going 'pear-shaped' 3 minutes before the take-off roll.
You can notice the right AoA stays at a constant value, but the Left AoA starts to Drift, during the taxi run. Electrically such a drift can be caused by loose signal wires, or faulty digital logic chips.
Blaming the accident on MCAS is as much incorrect, as blaming it upon the elevator or stabiliser.. All of which operated normally.

Uhhh- " HAL" ( MCAS) kept on repeating itself since the AOA number never changed. HAL was designed to correct- override the pilot who had been trained that to disconnect hal , first method was to push/pull on yoke and he/she gets full control. But newborn HAL ignored such a disconnect- and simply paused for a few seconds.
Or do you really believe that any pilot would continue to trim to an extreme nose down position as shown by recovered jackscrews ?

Hi Conso, I think you are looking at the wrong end of the incident.. The error occurred 2 1/2 to 3 minutes before the take-off roll.
The left AoA Sensor starts to give erroneous data during the taxi run, maybe caused by strong tail-winds bashing the AoA Vane in the wrong direction.
From that point on, until 8 1/2 minutes later the flight was not viable.

from sunday seattle times article-

Substitute HAL for MCAS - ' I'm sorry dave- I cannot allow . . .'

I posted this in the Ethopian thread but perhaps it is more suited to this thread:

Can I ask a question about MCAS?

It's my understanding applying nose down trim, will produce the effect of adding negative pitch attitude. So MCAS when activated is literally pushing the nose down. MCAS is sending nose down trim to bring the nose downward. Is this a completely correct statement from an engineering perspective? Is there any reason to think of MCAS another way? Does trim effect pitch attitude directly or is it providing column force so the pilot changes pitch? Would MCAS ever not be trying to bring the nose down? Is it inaccurate to say MCAS uses nose down trim commands to apply negative pitch attitude to the plane? In other words, does adding nose down trim add negative pitch attitude? Would MCAS ever be active adding trim in a scenario of high AoA and the plane has negative pitch attitude? Is MCAS literally part of the trim system?

Sorry about all the questions. I've been trying to understand this system and it seems to be described a few different ways.

from seattle times

What a gather from that is that by sending commands to the stab, MCAS actually can directly effects pitch attitude and not just colum forces. In fact it effects pitch even more so than the elevators. That is, it has more pitch authority. So in the Lion Air incident, the FDR recorded the pilots pulling back with 100lbs force before crash. It seems they could not over power the negative pitch attitude created by a full nose down stab trim.

flyingfalcon16,

Here are your answers:

Not necessarily. The nose down trim will create the tendency for the nose to pitch down. This "tendency" is better known as a "moment". There are many pitching moments acting at any given time. Which way the aircraft pitches, if at all, depends on what the sum of those moments is. If the sum was zero, for example, and nose down trim was applied, then yes, you would expect a pitch down result, but the application of nose-down trim does not guarantee a nose-down, or negative, pitch attitude.

Note that a pitch-down does not necessarily result in a "negative pitch attitude". In aerodynamics, the latter is considered to be a pitch attitude whose angle is below the horizon. An aircraft with a pitch attitude of positive 45 degrees, for example, could experience a pitch-down such that it's pitch attitude was now positive 30 degrees. Both these attitudes are considered positive pitch attitudes.

MCAS creates a nose-down pitching moment. As stated above, that moment may or may not result in an actual pitch-down.

Trim changes create changes to the pitching moment. This is true for everything on the aircraft. Extending undercarriage and flaps, increasing or decreasing thrust, etc - all create pitching moments. At any time in flight, there are many pitching moments. Whether the aircraft pitches up or down, and at what rate, depends on the sum of all those moments. If the sum is zero, for example, then there will be no pitching.

MCAS is there to create pitch-down moments only, and only when the aircraft is at high angles of attack (AoA) - supposedly.

Yes, this is inaccurate. The accurate statement would be, "MCAS uses nose down trim commands to effect pitch-down moments". This is the pure intent of the MCAS - the pitch-down moment - nothing to do with the attitude of the aircraft.

Not necessarily. See above.

As I understand it, yes, the MCAS could apply trim in that scenario.

No. It is a completely separate system, whose only means to effect a pitch-down moment is via the stabiliser trim. Boeing would have dearly loved to have had other ways available to do this, but did not.

@flyingfalcon

I get the impression you are mixing AoA and pitch attitude.

If you are flying level (not descending or climbing) and there are no significant up/downdrafts then pitch attitude and AoA are linked.

But when you are descending fast you can have a high AoA while having a negative pitch attitude.
So in that situation MCAS would still activate and if the AoA value is correct it would indeed be correct for it to activate.

Assuming the pilot does not pull further on the stick compensating the nose down trim, MCAS will make the plane pitch down.

To clarify, I didn't mean to suggest MCAS will result in negative pitch attitude, but that it adds negative pitch attitude. Your wording is better. MCAS creates a pitch down moment. Thx for the example too.

So isn't it accurate to say, since MCAS creates a pitching down moment by adjusting trim, it is effecting the final attitude of the aircraft, which is determined by all pitch moments acting on the plane? Or put very shortly: MCAS effects the pitch attitude of the plane. Since by changing the aerodynamics of the plane, trim effects pitch.

I see, thank you for those examples.

We typically see it described as preventing a stall, I guess that's why it seems odd to see it working when the plane is in a dive. In this case I suppose it would still be keeping the plane from pitching upward to quickly.

I wonder if the erratic oscillating pitching up and down movement we see in the recent crash scenarios could be MCAS over activating or not resetting properly to flight scenarios of both negative and positive pitch attitude and high AoA?

In regards to MCAS activation, in the Boeing 737 training manual it describes the system like this: "The MCAS only operates at extreme high speed pitch up conditions that are outside the normal operating envelope."

So while high AoA can happen while having negative pitch attitude, isn't it saying in the manual MCAS is only activated at pitch up conditions only? Or does this just mean the pilot is pulling up causing a pitch up condition which could occur during negative pitch attitude of the craft?

Not accurate to say that. Yes, it is creating a pitch-down moment, but that moment doesn't necessarily result in any change to the final attitude of the aircraft.

Consider the aircraft being hand-flown in straight and level flight with the thrust at idle. In this situation, the speed is decaying, but to maintain level flight, the pilot is steadily increasing the rearward pressure on the control column. The aircraft is maintaining altitude but approaching the stall. Note that the pitch attitude is increasing (becoming more positive, or "nose high").

It is this situation (and similar) that Boeing had in mind when they conceived of the MCAS. Certification standards require the nose-down moment (or control column forces?) to linearly increase in this scenario, but Boeing found this wasn't happening satisfactorily and needed some way to impose more down moment - hence the MCAS.

In this same scenario, but with the MCAS working as designed, the pilot must exert a steadily increasing back pressure on the column in order to maintain level flight. The aircraft is now more stall resistant. This is what Boeing had in mind.

This example shows that there is no hard and fast link between MCAS operation and downward changes of pitch attitude. In fact, the pitch attitude has increased throughout (become more positive, or "nose up"), with the effect of MCAS being to increase the control column forces being experienced by the pilot.

No. MCAS may affect the pitch attitude of the plane. Whether it does or doesn't depends on what else is going on. It was Boeing's belief that MCAS would never affect the pitch attitude of the plane.

Right, but it's effecting pitch attitude. So maybe the actual pitch attitude doesn't change, though I think considering it's design it would be hoping the pitch did decrease as it's primarily meant to become active in a stall scenario. From MCAS' point of view, it is effecting the pitch attitude by adding nose down trim. It is "trying to pull the nose down" whether that results in a net effect of the pitch changing or not. Put another way, I would expect that when it's active, the nose would pitching down more than if it was not.

Instead of giving MCAS authority to control the aircraft, I propose a solution... When MCAS activates, just allow it to illuminate an annunciation in big red flashing letters...'AoA High, Stall Warning'
and let the pilots perform the correction..

That's not a solution at all. The B737 already has a stick shaker and stall warning. The problem is that the elevator does not have sufficient authority, and the stabiliser needs to be rapidly trimmed down, just at the time when the pilot already has a high workload.

MCAS is not there to address an elevator authority issue. Elevator has plenty of authority. The requirement for MCAS comes from FARs calling for steady increase in stick force as AOA increases regardless of what other mechanisms (shakers, aural warnings, column feel stiffening, etc.) are activated to alert crew to abnormally high AOA.

MCAS is qualified in many sources as fundamentally a stall prevention system.

The original Boeing document provided to the FAA included a description specifying a limit to how much the system could move the horizontal tail — a limit of 0.6 degrees (turned out to be actually 2.5 deg), out of a physical maximum of just less than 5 degrees of nose-down movement (20+deg with the new 2.5 deg rotation (don't forget to add it can reset indefinitely)). From this statement, it's pretty clear that the intention was to pull the nose down hence the wording, "5 degrees of nose-down movement".

Since MCAS changes the position of the horizontal stab it is absolutely effecting the pitch attitude of the plane. Or if you rather, it is effecting the pitch down forces on the plane.

All of those sources would be media, seeking to explain the system to the layman as simply as possible. For the media, trying to explain in terms of pitch-down moments would be way too technical. You won't hear Boeing, the FAA, or anybody on the inside describing MCAS as being for stall prevention.

No. A system that pulls the nose down would be for stall prevention/recovery. If certification standards required that on the MAX, then Boeing would have simply added a stick pusher. The stick pusher is a vastly more simple and reliable system and has been in use on airliners for many decades now. Why would Boeing have adopted the new and unproven MCAS when they could have simply used a stick pusher?

Check how MCAS is described by insider sources. Here is one such source:

737 MAX - MCAS

When you say "effecting", I think you mean "affecting". That first sentence is false. The second is true. For the first, if you had said, "influencing the pitch attitude", then it would have been true.

2 other flights
 

Here are 2 more accidents that may bear looking into. Both happened with 737 - 800 and 737 - 8AS aircraft and both soon after takeoff. In both cases the cause of the crash was ruled pilot error. You can look them up on Wikipedia as i cannot post URLs
5 May 2007 Kenya Airways Flight 507
25 Jan 2010 Ethiopian Airlines Flight 409

"5 degrees of nose-down movement" is an official statement by Boeing. It wasn't from the media. I think that clearly implies the system is designed to create nose-down movement.

Right that's better wording. MCAS affects pitch attitude which can, in the scenario of avoiding a stall, effect the pitch attitude. Since it's changing the stab position it has incredible amount of pitch authority, so I think it isn't really that inaccurate to say "effects". Or MCAS has an effect on pitch attitude. It may not if the pilot uses the elevators to counteract the pitch down force. But that's technically adding another force outside the MCAS system itself. All other things being equal, if MCAS activates, the pitch attitude will change / the nose will drop (more so than if it wasn't active).

Put another way, your disagreement would be like saying the aerodynamic forces on the tail have no effect on pitch attitude. They only have an affect. I'm not sure that's more accurate but if it is it seems pedantic.

From the link you pasted (which I've read at length before): "as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall."

I don't think it's inaccurate to think of MCAS as a system to avoid a possible stall. Effectively, pulling the nose down by changing stab position.

I would post a good video of an airliner pilot explaining it as essentially a stall prevention system but I can't because I don't have enough posts to paste links. His take is the engines change the stall characteristics of the plane such that under a stall condition the elevator forces may not be enough to bring the nose down, so an adjustment to the stab is required.

So nose down trim affects pitch attitude. Which can and certainly in the latest crashes effect the pitch. By describing it the way you do you seem to imply MCAS has no pitch authority. Changing the position of the entire "tail wing" has a profound effect on the lift forces generated by the tail and the final pitch attitude of the plane.

Salute!
@flyingfalcon

One thing that confuses many folks is that airplanes and the folks in them are operating in a different geometrical/spatial frame of reference than when sitting in a chair in front ot a screen or reading a book. They are also operating in a different dynamic frame of reference, as they are usually moving with the pointy end forward! Heh heh.
So you have to leave your chair behind in the computer room or living room, huh? You have to put yourself in the "aerospace" vehicle and your new frames of reference You have your body coordinates and the Earth's space coordinates. The body coordinates are related to your chair - up/down, left/right and front/back. And motion is along those axis plus rotation about them. The six degrees of freedom we hear about, huh?

Your plane moves your body coordinates within the space coordinates. It's up to you to deal with both if you wanna get someplace or put on a great acro routine. Your "attitude" is your coordinate system's relationship to the space coordinates. It's how people outside see your vehicle. And AoA is related to your velocity vector WRT the air in those space coordinates. So are all the functions of the other appendages of your vehicle .

With a neutral, fixed elevator , changing the AoA of the horizontal stabilizer definitely changes the AoA of the main wing and the whole vehicle. Hell, every fighter and even many civilian planes have no elevator! They use that big stab to control AoA. So maybe you are still in your living room chair reference frame and see the effects of that change in AoA as "attitude".
That's the impression we are getting from your posts. Watch a big plane land and it's obvious that the thing is not "pointing" along its flight path because of its AoA. Nose up attitude, downward flight path.

Gums sends...

Because elevator control alone is not enough to overcome the pitch up forces added by the new engines in a high AoA stall scenario.

I am not too sure how many correspondents in this thread have ever flown an aircraft, but the first lesson pilots learn, is that trimming the tailplane controls the speed of the aircraft. This trimming is either done at the front of the all moving tailplane with the jackscrew, or at the rear with the elevator.
Even in the simplest form of aircraft, a glider, you can bimble around for hours looking for thermals, with the trim set towards the rear, for 40 knots. But to give some increase in control for landing you actually need to speed up to 60 knots which equates to putting the trim just a bit further forward than mid-way. If you put the trim fully forward the speed will increase even further.
So The Lion Air pilots must have seen their trim going towards the very high speed end of the range, but unfortunately did nothing to disconnect the automatics, and fly manually. Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.

Salute!

from scifi:

If they thot that, then they were correct. Might be because using the wheel switches they could get nose up trim, ya think? The stab switches on the pedastal turn off electric power to the stab for all of the pseudo-manual functions. Then they have to crank that manual trim wheel ten turns just to get what HAL did in a half a second!!!

Gums sends...

I'm trying to respond to qualify my remarks but they won't approve many of my posts. This is very heavily curated forum. If you look on one of Nasa's sites you can find this statement: Trim controls speed and attitude.

I'm trying to respond to qualify my remarks but they won't approve many of my posts. This is very heavily curated forum

The site has a requirement that new posters with less than how ever many posts have each each new post invigilated and then released to view. Once you get to the magic number, your posts are treated as OK by default. This is to put a check on out of left field inappropriate posts by new folk and, more importantly, spammers. Unfortunately, like most sites, we get our share of the latter but you don't get bothered by them because of the mod invigilation requirement.

Unfortunately, that means that one of us has to have a look at the post and, given that we are part time mods, that takes a little while longer than instantly on most occasions. So, in respect of your last post, I have just done my run through the queue and released it a few minutes ago. Next time someone might coincidentally review a post near immediately following its drafting .. all depends on the luck of the draw. However, you should expect to see your new posts appear in a reasonable timeframe, if not instantaneously.

I assure you that this forum is very relaxed in its moderation activity. As a general rule, the folk who frequent TL, in the main, are "nice" people and we don't have much in the way of problems ....

I have been told more than once that this kind of statement contains several misperceptions. I leave it to others to provide the correct answer.