737MAX Stab Trim architecture

[gums]

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

[KRUSTY 34]

Originally Posted by FCeng84

PEI - As you state, one of the very important starting points for any design effort is to clearly list the going in assumptions. Those include everything from failure rates and modes for input signals your system may be using to the failure characteristics of the equipment within your system to the expected human inputs if your system includes an operator interface.

In this instance with 737 Max MCAS I am sure that Boeing is taking this extremely seriously and carefully examining all of the assumptions that went into the original design. I am confident that the right experts within Boeing are working this issue in conjunction with certification authorities to determine if changes to the MCAS design, documentation, and/or training are needed and what those should be. The best path forward will be defined by those who know the system in the most detail so I will not speculate as to what particular changes in any of these three areas should be made.

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.

[gums]

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...

[PEI_3721]

737 Max AoA input to FDR

There are many discussion involving the AoA involvement in recent accidents, all based on the FDR, but where in the AoA system is the FDR recording made ?
Is this an analogue or digital value ?
Is the vane output analogue or digital; if analogue output and digital FDR input, where is the A 2 D made ?
Do the aircraft systems used different types on input. Assuming that fight guidance - STS and MCAS are digital, would stick shake be a separate analogue value, or as depicted in some diagrams a digital output of the FGC ?

[scifi]

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..?

[john_tullamarine]

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.

[PEI_3721]

John, I have no gripes with editorial action; apart from the difficulties arising from the accumulation of threads around the subject and the apparent inability of many contributors to read (to use Pprune search or a web search engine), then ‘think’ before posting.
These points alone - the apparent changing human condition, could be the main lesson for aviation regulators to learn from recent accidents.
Se la vie.

The following text from ‘System Failure; learn to think differently’ is a suitable backdrop for presenting ones position:-
“A difficulty is characterised by broad agreement on the nature of the problem and by some understanding of what a solution would look like, and it is bounded in terms of the time and resources required for its resolution.
In contrast, messes are characterised by no clear agreement about exactly what the problem is and by uncertainty and ambiguity about how improvements might be made, and they are unbounded in terms of the time and resources they could absorb, the scope of enquiry needed to understand and resolve them and the number of people who may need to be involved”.
and … , (I seek to ensure that the following does not apply to me):
“… when the problem is a difficulty an individual claiming to have the solution is an asset, but when the problem is a mess that individual is usually a large part of the problem!”
Jake Chapman, ‘Systems Failure’, https://www.demos.co.uk/files/systemfailure2.pdf


And being in the mood for quotes :-

“One of the main problems … in sharing their picture of the world with a wider audience is the knowledge gap.
One doesn’t need to be a writer to read and understand a novel, or know how to paint before being able to appreciate a picture, because both the painting and novel reflect our common experience. Some knowledge of what science is about, thought, is a prerequisite for both understanding and appreciation, because science is largely based on concepts whose detail is unfamiliar to most people.”
Len Fisher “How to dunk a doughnut.”

For ‘science’ substitute aviation, but the greater concern is if the knowledge gap, the ‘wider audience’, is within aviation.

[john_tullamarine]

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 ....

[hec7or]

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.

[Machinbird]

Originally Posted by KRUSTY 34

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.

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.

[john_tullamarine]

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.

[scifi]

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.

[CONSO]

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 ?

[scifi]

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.

[CONSO]

Originally Posted by scifi

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.

However, pilots and aviation experts say that what happened on the Lion Air flight doesn’t look like a standard stabilizer runaway, because that is defined as continuous uncommanded movement of the tail.On the accident flight, the tail movement wasn’t continuous; the pilots were able to counter the nose-down movement multiple times.In addition, the MCAS altered the control column response to the stabilizer movement. Pulling back on the column normally interrupts any stabilizer nose-down movement, but with MCAS operating that control column function was disabled. These differences certainly could have confused the Lion Air pilots as to what was going on.Since MCAS was supposed to activate only in extreme circumstances far outside the normal flight envelope, Boeing decided that 737 pilots needed no extra training on the system — and indeed that they didn’t even need to know about it. It was not mentioned in their flight manuals.

from sunday seattle times article-

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

[flyingfalcon16]

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.

[CONSO]

Originally Posted by flyingfalcon16

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

If the final safety analysis document was updated in parts, it certainly still contained the 0.6 limit in some places and the update was not widely communicated within the FAA technical evaluation team.“None of the engineers were aware of a higher limit,” said a second current FAA engineer.The discrepancy over this number is magnified by another element in the System Safety Analysis: The limit of the system’s authority to move the tail applies each time MCAS is triggered. And it can be triggered multiple times, as it was on the Lion Air flight.One current FAA safety engineer said that every time the pilots on the Lion Air flight reset the switches on their control columns to pull the nose back up, MCAS would have kicked in again and “allowed new increments of 2.5 degrees.” “So once they pushed a couple of times, they were at full stop,” meaning at the full extent of the tail swivel, he said.Peter Lemme, a former Boeing flight controls engineer who is now an avionics and satellite-communications consultant, said that because MCAS reset each time it was used, “it effectively has unlimited authority.”

[flyingfalcon16]

Originally Posted by CONSO

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.

[FGD135]

flyingfalcon16,

Here are your answers:

... applying nose down trim, will produce the effect of adding negative pitch attitude.

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.

So MCAS when activated is literally pushing the nose down. MCAS is sending nose down trim to bring the nose downward.

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

Does trim effect pitch attitude directly or is it providing column force so the pilot changes pitch?

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.

Would MCAS ever not be trying to bring the nose down?

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

Is it inaccurate to say MCAS uses nose down trim commands to apply negative pitch attitude to the plane?

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.

... does adding nose down trim add negative pitch attitude?

Not necessarily. See above.

​​​​​​​Would MCAS ever be active adding trim in a scenario of high AoA and the plane has negative pitch attitude?

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

​​​​​​​Is MCAS literally part of the trim system?

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.

[wiedehopf]

@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.