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Concours77
23rd Nov 2018, 17:30
Post a schematic of the Horizontal Stabilizer TRIM architecture?

Bluntly, I am looking for function and response to Auto/Manual Trim commands.

An image of the AoA Vane installation would be helpful as well.

Possible MCAS operated as designed, that the signal offset from AoA sensor was Mx related.

If I’m a FCC, and I see two AoA values from the sensors, I am looking at two separate vane results. I don’t speak AoA, so I consider both values. The two are different, but only in absolute value. They are tracking in the airstream identically.

Was I programmed to average the two? If so, I might not think anything is wrong. What if humans consider malfunction of vanes only as aberrant behaviour? One smoothly tracking, the other jerky jerky! What if I don’t grok “Similar but different”?

What if one vane “thought” zero was zero, the other “thought” twenty degrees up was zero? Fine by me, I’m looking for herky jerky, not two lines with the same shape?

Not MY job to calibrate the silly things.

One other thing. If I was programmed to sum (average) the two vanes, and the average was less than Stall AoA, why would I trip the Stall Warning? I might still consider Nose Down Trim, in spite of Dave.

Hal

AI. Not long now.

gums
24th Nov 2018, 16:36
Salute!

I have an old FCOM, but no substitute for actual mechanical and electrical diagrams. And "code" of the black boxes wuld be even more valuable.

As you know, I am a fighter pilot that flew two very smart planes, then did system engineering for another 13 or 14 years in the weapon control and human interface aspects of military planes. I do not know anything about Councourse, as the public profile is scant, but from previous posts suspect technical savvy pilot or engineer of some sort.
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- I found a pic of the AoA sensor on a 737, and the pilot side is below the cockpit and just behind the radome. Another contributor has verified my suspicion that removing the radome could influence the AoA "plumbing". That was an early post on the main thread. From our very own Dave;Quote:Originally Posted by gums https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-post10297837.html#post10297837)The air data sensors appear to be just behind the radome and access panels for the radar. There's possibility of contamination or even a "loose" pnematic line depending upon the actual configuration of the air data sensors.Yes - specifically the LH ADM needs to have the pitot input disconnected to gain access to the radar.

GOL nearly lost a B738 seven years ago. Fresh from the factory, a radar failure required a software reload. When the technician reconnected the pitot hose, the connector was engaged, but not twisted in the bayonet fitting to lock it. The result was intermittent UAS, with vertical excursions even more extreme than the Lion Air.

That the crew and pax survived to tell the tale was, according to the report, largely due to a deadheading captain on the jumpseat who exhorted the PF (and the check captain in the RH seat) to "fly the d*mn airplane" (or words to that effect in Portuguese). In the Viper, we used AoA for a few things. a) when gear up, AoA versus gee limit function, whereby 25 deg +/- was max AoA at 1 gee and 15 deg at 9 gees b) leading edge flap scheduling, which was a funcion of mach and AoA and c) gear down, AoA bias applied to the basic gee command and a warning horn above 15 deg AoA.
Three sources: two cones and one hemispherical probe that was used for many functions. It used middle value, not an average. As with the gee and rate sensors in our system, it used the most benign value when down to two sensors. Not sure of all the failure criteria, but I think we had to go down to one sensor and then system went to standby gains. My tech data is unclear about the criteria. But the plane flew well with no AoA inputs if you were gentle, and the rate sensors helped a lot in that situation.

1) I see some overlap in the STS and MCAS and even the "feel" implementation, and will have to look again at the FCOM. So we have three systems that might be using AoA, and the MCAS for sure. Tink STS uses only CA, is a poor man's speed stability system.

2) I will look back at the FCOM I have re: 'feel" implementation, which appears to use independent AoA and other air data and does not physically move the stab or elevators.
2) I do not like either MCAS or STS moving the stab the way it is described. I would use the Airbus implementation where elevator command is the driver, and use the 737 "feel" system to help pilots realize they needed trim ( like all the old planes did when we were taught to trim out the pressures we were holding). I don't like aerodynamically influencing the plane just to provide speed stability or the sense of speed stability unless you go whole hog fly by wire with a mechanical backup.

Gotta go, but will check back later.

Gums sends...

Vessbot
24th Nov 2018, 17:33
and use the 737 "feel" system to help pilots realize they needed trim ( like all the old planes did when we were taught to trim out the pressures we were holding).

I was also wondering why they would use the trim instead of elevator feel to tailor the elevator forces, and FCeng made a post with a pretty good answer, in the STS thread. https://www.pprune.org/showthread.php?p=10303995

gums
24th Nov 2018, 19:31
Salute!

I don't see eye to eye with FC about this issue My comments in red:

First, if the unaugmented airplane were neutrally speed stable it would not require any steady elevator for a speed change and thus stiffening the column feel would not help as none would be needed to fly faster or slower. [dat's how Airbus and Viper work. 'bus is gee cmd like Viper, and speed stability is there due to aero design, so they use power for speed, best can tell. Viper was negative static stability until above 0.9M and had zero speed stability. Pitch was gee cmd only, unlike 'bus which corrected the gee cmd for pitch angle and bank angle] Further, if the unaugmented airplane were actually unstable with regard to speed it would require a push force to keep the nose from rising after slowing down and a pull force to keep the nose from falling after speeding up. [ very true. but the 737 is normal and reducing speed results in descending and increasing speed makes it climb. So STS moves the stab the "wrong" way to make the pilot trim the "right" way. GASP. That's what Mana saya over on the other tech thread, and he just trimmed the way he wanted/needed to, heh heh] These are clearly not the desired situation. Stiffening the column feel in that event would actually make the speed stability handling characteristics worse. [ I do not agree. Let the plane's normal aero speed stability work unless it is not aerodynamically stable. I expect the damn thing to nose over if I pull off power! I want it to keep my last trimmed speed/AoA]

Gums sends,,,,

gums
24th Nov 2018, 22:34
Salute!

@ concourse

Attached or embedded is the 737 pitch block diagram, but no way to tell what is pure electric or hydraulic or cables.

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/391x523/737_pitch_f79cdc81cd65943de9af29a70ff44ea5956dbd40.jpg

Gums sends...

Hi_Tech
25th Nov 2018, 06:20
Gums: If you look at the dotted lines, they are all different. I hope the following edited fig makes sense.

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1152x648/fc_4a44aaeaf6ceb9619be67f228e9195b1bd7b0759.png

neville_nobody
25th Nov 2018, 06:56
AI. Not long now.
If this accident turns out to be as people suspect right now will probably do more damage to the AI argument then good. If designers can't fix the GIGO problem computers have pilots will be here forever.

CONSO
25th Nov 2018, 07:07
Gums: If you look at the dotted lines, they are all different. I hope the following edited fig makes sense.

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1152x648/fc_4a44aaeaf6ceb9619be67f228e9195b1bd7b0759.png

UHHH Most 737 stab trim motors are electric

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectionMethod=LatestReleased&Rendition=Primary&&dDocName=PCT_201019 (http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectio nMethod=LatestReleased&Rendition=Primary&&dDocName=PCT_20101 9)

Which gives a pdf file DS400-1A_B737 Stab Trim Upgrade.pdf
Eaton’s new Model 6355C Stabilizer Trim Motor features:
• Brushless three phase mo-tor design
• Low loss power bridge with IGBT switches
• Processor based motor commutation and velocity control
• Dual current limit (torque) control circuits
• Power up built-in test
• Continuous fault monitoring
• Fault storage (non-volatile memory)
• RS-232 test/maintenance interface
• Investment cast housing
• Two stage spur gear train
• Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train

Eaton’s Stabilizer Trim Motors
(STM) Model 6355B0001-02 and -03 have been used on Boeing’s 737-600/700/800/900 aircraft, starting with Line #1423, delivered in December 2003. The “B” model has been
superseded by Model 6355C, and replacement parts for the “B” model will be available for a limited time only — estimated at 3 years, based on usage. Parts scheduled for future obsolescence include:
• EMI FIlter Assembly
• Brushless DC Motor Assembly
• Voltage Reference
• Control Board Assemblies
• Isolated Drive Transmitter
• Isolated FET Driver

DaveReidUK
25th Nov 2018, 07:50
UHHH Most 737 stab trim motors are electric

All 737 models have electric stab trim motors.

wiedehopf
25th Nov 2018, 12:54
@CONSO that's hydraulic for the elevator not for the trim.

Anyway without the text that normally accompanies that diagram it's not too good to understand the system.

Concours77
25th Nov 2018, 13:12
If this accident turns out to be as people suspect right now will probably do more damage to the AI argument then good. If designers can't fix the GIGO problem computers have pilots will be here forever.

Maybe, but take note of the argument here: “sole source data”. Trying to simplify and economize reduced data. More data would “fix” the problem. The reliance on computed data is not harmed by this accident. The human element, Boeing v. Pilots is at center stage. There will always be deflected responsibility. The march continues to eliminate the “fragile human element....” Human decisions created human failing. AI avoids that problem. For good or evil, no?

seagull967
25th Nov 2018, 15:15
The actual electrical wiring to include MCAS switching would be very interesting. A screen shot of just the MCAS and electric trim wiring from a Mx manual. Anyone have that?

gums
25th Nov 2018, 17:57
Salute all!

Wow. Looks like a quorum of folks that wanna discuss the tragedy and "contribute". You never know when a regulator type or investigator or airline type digests our theories and such, but it might help to prevent another tragedy.
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You all know my background from the public profile, but too many "pruners" are "cute" with location and profile, including some here, so I always question some comments and assertions and opinions and..... The AF447 tech discussion was extremely technical and most contributors had extensive aviation and even "a/c type" background.
I was the lowly light puke that had flown the first fully FBW plane in service that had zero mechanical back up, had negative static stablity, and extreme body rates, AoA and gee values seen on almost every flight. But the heavy folks accepted me as well as Machinbird, and we got along fine.
I also worked with the human interfaces of the U.S. military platforms as a system engineer for about 14 years, so many of my opinions are based on "what would I do?", "How can I go back to a known condition?" , etc. And 99% of my control and display work was to prepare, target and launch a weapon. So I had to take into account many aspects of the process to ensure safety and reliability at the same time. With two no **** combat tours and only getting "shot down" once ( semi-crash landing with no power), and approx 4,000 hours in 5 warplanes plus some trainers, I was blessed to see many aero principles in action and all the "gotchya's". So with my location and bio out there on the Pprune profile and this missal, you all know where I come from.
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Present views/opinons on next post.

Gums presents...

Lonewolf_50
25th Nov 2018, 18:24
If this accident turns out to be as people suspect right now will probably do more damage to the AI argument then good. If designers can't fix the GIGO problem computers have pilots will be here forever. They all need someone to blame when the GIGO offers up a lot of G.

@gums: salute! :)
I'll be sparse on comments as my core competency in pilot training, and in training systems for new models and new mods, is hampered by not knowing this system, and thus grasping the differences, well enough. I have discovered from those in the industry that training is viewed somewhat differently within the industry, and within each company, and within each corporate culture. Things that I had assumed are 'true' generically for pilot training, and crew training, may not be.

If you the pilot don't know what it(the bird) can and will do, what do you as a pilot do when it's doing that thing you don't know about?
(The test pilots I have worked with usually answer that with "get it back on the ground and don't go outside of known parameters on the way back" as often as not. )
Addressing that hole in the cheese is what draws me to the technical discussion.

gums
25th Nov 2018, 19:13
Salute!

Concourse has raised a very important legal point for this discussion on the main thread, and I agree 100%, but my personal preference here is to keep the legal aspects "legal" and concentrate on understanding what happened and identifying whatever it takes to keep this from happening again or another incident that could have been prevented.

Some here may even be flying the type and could use some "education".
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Being a pilot on my accident boards, I mostly look for causal and contributing factors that were out of human control at the time or prevented the human from overcoming the problem. So my interest in sfwe and hdwe is a large part of my personal investigation. I have seen out-and-out pilot judgement/skill crashes, basic smoking holes, but also one serious loss that involved "little understood" aspects of our autopilot implementation. So this MCAS brings back some sad memories.
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The trim gearbox is of more interest to me now than my poor opinion of the decision to move large control surfaces to "help" the pilot and not tell the pilot That gearbox is the last mechanical connection in the chain, right? And the control surface it moves has more pitch authority than the elevators. So I start there and work back to the first input to the doofer, whether from a human input or Hal input or combination.

How did the STS logic fail to provide up trim cmds as speed increased when I only see down cmds to the gearbox. Maybe the AoA prevented that, but I did not see a direct input to STS from AoA. Seems like STS uses speed data from the ADIRU and not raw "q" from the probes. So if the ADIRU logic asserts airspeed is FUBAR due to AoA problems, then what does the STS do? Inquiring minds want to know before my next flight.

A poster not so far back thot we may have seen the result of a cascade of events that had deliterious effects not allowed for or imagined. i.e. a single point failure that by itself should have only been an irritant, but other systems that used the output of that "module" reacted as designed and the chain of events created control surface movement that was not required and, indeed, was unsafe and could not be mitigated by the crew. In other words, the crew did not recognize what the problem was and kept trimming because every time they trimmed, the plane seemed to return to "normal".

The crew actions, even without knowing about MCAS, are an important part of this tragedy, but if things didn't break, they would not have had the opportunity to diagnose a new malfunction. So second to last flight did not identify the real problem because the "runaway trim" procedure seemed to have allowed a successful flight and a writeup. The accident flight would have been the same, and maybe next hop would be the tragedy.

Gotta go, and forgive my sermons.

Gums....

Lonewolf_50
25th Nov 2018, 20:44
In other words, the crew did not recognize what the problem was and kept trimming because every time they trimmed, the plane seemed to return to "normal". A "malfunction that fixes itself." Red Flag.
The crew actions, even without knowing about MCAS, are an important part of this tragedy, but if things didn't break, they would not have had the opportunity to diagnose a new malfunction. What besides (maybe) an AoA probe 'broke' in this case? Still unclear to me.
Diagnosing malfunctions is, when a problem has persisted for flight after flight, something for a post maintenance check flight. (IMO) not a revenue flight. (Hence my question about MEL?)
So second to last flight did not identify the real problem because the "runaway trim" procedure seemed to have allowed a successful flight and a writeup. But not an actual repair, it seems. So what is going on here: throwing parts in a problem, and hoping that fixes it? I'd hope the trouble shooting trees in the maintenance manuals would be a bit more expansive than that.
The accident flight would have been the same, and maybe next hop would be the tragedy. Maybe. depends on what actually broke.

gums
25th Nov 2018, 22:12
Salute Wolf!

Here's my rationale for my opinion/theory/whatever regarding the AoA sensor and its influence on various aircraft systems.

The cascading failures or unexpected operations of several systems seems evident once you start with the AoA that appears up to 20 degrees different than the other AoA sensor. So let us see the flow chart/system diagram.[AoA data to the air data black boxes and the crew sees unreliable airspeed. Bad AoA flag might not help here if crew does not understand the MCAS, but relation with stick shaker would prolly be recognized by 99% of the crews ]

- At start of the data, one AoA shows about 10 - 12 degrees higher than the other. After liftoff the delta increases to maybe 20 degrees and tracks the other AoA sensor perfectly.
- the overall system fault monitoring system does not flag the AoA disagreement as such [after takeoff. It shows the crew unreliable airspeed once pitot speed is above "x" and AoA data is bad, like 20 degrees delta. ]
- At liftoff we see several trim inputs from the "system", which seems from STS, and they look like STS is doing its job. i.e. trim up to maintain the trimmed speed. But stick shaker is now active.
- flaps are now fully retracted, stick shaker continues because AoA from one vane is very high and the circus starts.
- Hal trims down, gums trims up a bit because we are climbing like normal, increasing speed and that crazy STS is wired backwards.
- Nose down trim geting really strong, but trim switch on yoke seems to be working, But after I let off for 5 seconds I see nose down trim again. This ain't STS, I think.

Somewhere in the scenario the "feel" system is acting on the control yoke, and I do not have the personal flight experience to comment on how it feels and not sure about how it is supposed to work in concert with the other sub-systems and their logic/specs when they go south.

Meanwhile, pilot and MCAS are commanding up trim and down trim for 5 or 10 second intervals until something big happens and we see lotta power commanded and a divergent control pressure for each yoke.

Gums...

infrequentflyer789
25th Nov 2018, 22:14
A "malfunction that fixes itself." Red Flag.

I'm not sure. In effect the aircraft was repeatedly trimming against them, now I've read some of the archived discussions on Speed Trim/ STS both here and elsewhere and there are plenty of comments saying (paraphrased) "it keeps trimming against me", and plenty of replies saying "that's what it is designed to do". So this could easily be misidentified as STS simply being more enthusiastic than usual.

We may know now that this was MCAS, but that is knowledge the pilots didn't have. What they did have (on previous flight) was UAS, which might reasonably be expected to cause odd behaviour from the speed trim system, plus stick shaker. So UAS is what got written up and "fixed".

If the pilots had the AOA disagree (an optional item they apparently didn't have) then maybe that would get written up, if they had known about MCAS maybe they would have suggested that as a cause and AOA would have been identified from that.


What besides (maybe) an AoA probe 'broke' in this case? Still unclear to me.


We don't know if the probe broke or was mis-fitted or why the previous probe was replaced. All we know is the the recorded value was 20deg out. I think these are analog signals so could even be a wiring fault?


Diagnosing malfunctions is, when a problem has persisted for flight after flight, something for a post maintenance check flight. (IMO) not a revenue flight. (Hence my question about MEL?)
But not an actual repair, it seems. So what is going on here: throwing parts in a problem, and hoping that fixes it? I'd hope the trouble shooting trees in the maintenance manuals would be a bit more expansive than that.
Maybe. depends on what actually broke.

Agree on check flight - but easy with benefit of hindsight. MELs and AMMs possibly unlikely to be helpful when the system showing the symptoms is undocumented and the sensor that is (maybe) broken gives no disagree error because the values are never shown to the pilots.

infrequentflyer789
25th Nov 2018, 22:32
The actual electrical wiring to include MCAS switching would be very interesting. A screen shot of just the MCAS and electric trim wiring from a Mx manual. Anyone have that?

I have acquired some chapters from NG AMM which I think cover what you are looking for, but it seems to have changed for the MAX, see this post (https://www.pprune.org/showthread.php?p=10313316) in the R&N thread.

And of course the NG doesn't have MCAS. It does have (very lightly documented) a similar sounding function, which is not named, which gives ND trim on the STS trim output (which is also the autopilot trim output) from the FCC. So in the NG case, no wiring changes, only software changes in FCC. Max is probably the same except at the column switch module (column position cutouts) and the cutouts on the console. And the software.

Unfortunately MAX AMMs are not likely to be out in the wild yet as the aircraft is in service in only a few places. Would be very interesting to compare one though (chapter 22)...

PEI_3721
25th Nov 2018, 22:34
gums, Lonewolf, et al, may I direct you to the questions at https://www.pprune.org/showthread.php?p=10319901
How much is actually known about MCAS or the integrity of AoA ?

Also see https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta-84.html#post10320312 #1675

DaveReidUK
25th Nov 2018, 23:36
So what is going on here: throwing parts in a problem, and hoping that fixes it?

Anyone who has ever worked in airline maintenance will have encountered that.

Before we hang the maintenance organisation out to dry, we really ought to wait until we know what was recorded in the tech log for those previous flights.

Machinbird
26th Nov 2018, 06:43
Can someone who works on current 737 models provide some information on the actual AOA sensors used? Some research on the subject shows that Boeing is using UTC made equipment on the 737.
I believe that they are based on a resolver principle https://en.wikipedia.org/wiki/Resolver_(electrical) , in which case there will be two AC signals coming out of each AOA sensor forming a sine and cosine signal pair that must be put together to readout the angle.
Since both right and left sensors appear to have been following the AOA, but the left sensor had a 20 degree offset, the point in the aircraft where the sine-cosine signal pair are combined before digitizing would be where to look for the source of the AOA error. Loss of either of the AC signals would create a stationary AOA reading.
I know that this is a bit technical for most people, but it might bear fruit as to the likely source of the AOA malfunction. .

Lonewolf_50
26th Nov 2018, 17:35
Anyone who has ever worked in airline maintenance will have encountered that. Yeah, which is why I asked. I've been involved in aircraft maintenance (though some years ago). I am not out to hang the maintenance organization, David. With this being a comparatively new bird, I'd love to see what back and forth the maintenance team had with their Boeing rep over this fault/malfunction ... but that's likely never going to come out in the public domain. I've seen a wide variety of OEM "expertise" in the field: some great, some not so great.
@PEI: Thanks, except the second like gives me "page not found" (I read your #1650 and #1674 by scanning the page, I think that is what you were pointing to).
@Infrequent: Thanks.
@gums: copy all, nothing further, out.

infrequentflyer789
27th Nov 2018, 15:17
Can someone who works on current 737 models provide some information on the actual AOA sensors used?


I can't meet that first requirement but in the absence of any more authoritative answer, I think you are likely correct based on 737NG information published (which matches some NG AMM chapters I might have acquired) here (https://www.satcom.guru/2018/11/stabilizer-trim.html). Search on that page for "SMYD Analog Int" and you'll find the SMYD diagram where in the top left we see "sin / cos / com" analog inputs from AOA sensor - bingo. Assuming it hasn't changed between NG and MAX.

However, although SMYD implements stick shaker, it doesn't do speed trim, that is the FCC (and they got both). The text beneath that diagram shows that SMYD outputs AOA to data acquisition, but not FCC/ADIRU. FCC gets AOA from ADIRU (where it is used in speed calculation), and as far as I can see ADIRU also gets it direct from the sensor.

Pretty sure (very rusty...) that decoding resolver output for position (not angular velocity) is just a matter of relative voltage magnitude of the sin/cos signals, so wouldn't be surprising to see that duplicated. That means you'd need to have two simultaneous conversion failures. I think, and if that bit is all still same as NG.


Since both right and left sensors appear to have been following the AOA, but the left sensor had a 20 degree offset, the point in the aircraft where the sine-cosine signal pair are combined before digitizing would be where to look for the source of the AOA error. Loss of either of the AC signals would create a stationary AOA reading.

Partial loss, or rather additional impedance in one signal line might do what? I think it could lead to offset reading, not sure it would be a consistent offset though, been doing software-only for too long.

Broken sensor or misfitted sensor is probably the place to look first - not least because identifying a wiring issue now is going to be tricky...

Concours77
27th Nov 2018, 17:12
SEE Pitch TRIM FDR GRAPH.

Each cycle in the relatively consistent graph of STAB TRIM in the following graph shows a steady TRIM at the top as a “plateau”. The opposite “bottom” transition to Nose UP has no flat portion, it is sharp, and involves arresting the Tail Plane from movement towards Nose Down, to movement towards Nose Up.

I am postulating that the flat portion of the graph at the top of each couplet demonstrates MCAS OFF.

So five seconds on a graph that is absent the time, in the x axis.

As MCAS is causing the Tailplane to raise its leading edge, there is up Elevator. This deflection forces the trailing edge of the tailplane Down, which imparts a force upwards on the leading edge, In the direction of travel caused by ND Trim.

Note How sharp the bottom line is in reversal of TRIM force from ND to NU, as pilot switches on manual NU TRIM.

From an elevator boosted movement up at the leading edge, the assembly is halted, and direction is reversed.

There are twenty two of these robust reversals of Tailplane AoA. From the graph, there is a sudden loss of NU TRIM, followed by uniquely ND travel of the TRIM? This is just prior to LOC and impact.

What could cause the sudden loss of TRIM input NOSE UP? Broken rocker switch at the yoke? Loss of energy at the jackscrew?

Broken, jammed (my best guess) actuator or seizure of ball nut? NOSE DOWN.

Whatever the immediate cause of runaway (sic) Trim, There appears to me to be a record of mechanical failure which caused the final dive.

With elevator deflected full NU, the Tailplane at Nose Down, and the jackscrew “pushing” the entire assembly Nose Down, the signal changes via manual trim to Nose UP, which involves the jackscrew stopping, then “pulling” the leading edge of the TailPlane down in transit to NU.

The jackscrew has a limit load of 25,000 pounds, (hat tip to CONSO).

Each reversal from ND to NU involves a shock load of “x”, There is not a concomitant shock load at the top of the couple, because MCAS is “resting”. This pause allows the momentum of the Tailplane to stop, eliminating shock load.

Why did PF continue this cat/mouse action of reversal of TRIM for twenty two iterations? Was he trying to allow a mechanic the time to trouble shoot, whilst he actioned the MCAS TRIM ND to Manual TRIM NU?

Something broke/seized, jammed or failed in some other way?

imo

SteinarN
27th Nov 2018, 18:05
One idea, does the tail plane trim motor have any thermal protection that could have tripped in any way rendering the motor useless at the end?

Concours77
27th Nov 2018, 18:26
One idea, does the tail plane trim motor have any thermal protection that could have tripped in any way rendering the motor useless at the end?

Howdy,

Relative to my post above, the (electric) motor actuator had to have endured highly stressed loading. Heat could easily have compromised the action... I don’t know about “useless”, but switching could have been highly degraded. At the final one way increase in Nose Down Trim, the HS could have been frozen, but the action shows a steady increase in leading edge movement UP, not consistent with duff motor?

birdspeed
27th Nov 2018, 18:47
Concours77, as the stab seems to trim to full ND at a normal rate, I don’t think there is a mechanical failure of the stab jackscrew.
What perhaps happens is a breakout of the control yokes just before the dive. FDR shows a difference in the applied control force on the yokes. I believe when elevator breakout occurs there is sudden loss of elevator effectiveness. This would cause a sudden bunt over, with a wtf moment and the pilots stopped any sustained opposite trim.

what do you think?

Concours77
27th Nov 2018, 19:00
Concours77, as the stab seems to trim to full ND at a normal rate, I don’t think there is a mechanical failure of the stab jackscrew.
What perhaps happens is a breakout of the control yokes just before the dive. FDR shows a difference in the applied control force on the yokes. I believe when elevator breakout occurs there is sudden loss of elevator effectiveness. This would cause a sudden bunt over, with a wtf moment and the pilots stopped any sustained opposite trim.

what do you think?




“Switching”..... NOSE UP Trim was Lost, inhibited. There are two small spikes in the downward line, but obviously not significantly effective. Weak, transient or failed switch at the yoke? I noted the steady increase of ND Trim, so yes, the jackscrew was (remained) effective....

NU had lost its effect. Signal loss in that channel to the motor? Heat?

Loss (literally) of elevator? That is consistent with loss of all downward force on the Tailplane. Nothing left to slow TRIM ND? The problem with that is that mightn’t a “release” of Elevator reduce drag on ND (MCAS) TRIM? Showing an increase of rate ND?

(That’s backwards, the loss of Nose Up Elevator would reduce force on Tailplane trailing edge down, slowing the rate of ND TRIM).....

imo

gums
27th Nov 2018, 20:51
Salute!

- I still look for evidence of a mechanical failure, but looking at the traces..... I see no clear evidence of mechanical failure, although the overheat possibility put forward may be relevant. And then there's previous flight and maybe not enuf lube on the screw as we saw in Alaska crash. I am leaning more and more toward an electronic cause by Hal and his minions. They seem to be able to move the stab without pilot consent.
- Stab is way more powerful than the elevators. Even with full deflection of the elevator, once above a certain "q", the elevator will not be able to overcome full stab deflection.
- No big rate changes on the stab position. Same slope of movement as all previous down cmds.
- At "the event", engine data shows significant power demand for a few seconds. and then one large down command from Hal, and stab moves. Another large down cmd then comes from Hal.
- By now, we see large control forces by both folks and eventually 100 pounds by the pilot. Damn, but this is scary.

Gums posits....

jimtx
27th Nov 2018, 22:01
Salute!

- I still look for evidence of a mechanical failure, but looking at the traces..... I see no clear evidence of mechanical failure, although the overheat possibility put forward may be relevant. And then there's previous flight and maybe not enuf lube on the screw as we saw in Alaska crash. I am leaning more and more toward an electronic cause by Hal and his minions. They seem to be able to move the stab without pilot consent.
- Stab is way more powerful than the elevators. Even with full deflection of the elevator, once above a certain "q", the elevator will not be able to overcome full stab deflection.
- No big rate changes on the stab position. Same slope of movement as all previous down cmds.
- At "the event", engine data shows significant power demand for a few seconds. and then one large down command from Hal, and stab moves. Another large down cmd then comes from Hal.
- By now, we see large control forces by both folks and eventually 100 pounds by the pilot. Damn, but this is scary.

Gums posits....

Wouldn't the increase in airspeed and high opposite elevator input fit the Boeing warnings for the 727 and older 737s that you may need to relax elevator input to allow trim in the same direction as your input. The elevator airload overpowers the jackscrew leverage. They might have been pulling and trimming but the stab wasn't moving. I saw this in a 727 simulator a long time ago. Is that warning still in the newer 737 FCOMs?

gums
27th Nov 2018, 23:09
Salute Jim!

Worth talking about the elevator authority and such.

High "q" will definitely make that big stab overpower the much smaller elevator.

Apparently, this bird ( 737) seems to act like most of us expected in the old days and even today until they let us fly a plane with negative or neutral static stability[see added below] . We trimmed to reduce control pressure/movement. That got you back t the original AoA/speed. So you could have a lot of elevator deflection but the trim operation would allow less and less control pressure until you reached the desired trim for speed/AoA. And then we had STS, which trimmed the stabilizer without pilot input. Huh???? This ain't no Airbus fly by wire FLCS. But rational was to tell the pilot ( I use pilot to assert whoever was in charge of moving controls), he needed to trim for the new speed/AoA. What airplane have they flown that did not need a new trim after changin speed/AoA?? Do they need to be "reminded"? SHeesh.

And then we get another tweak to the plane's flight control system.. MCAS. And its operation and such are not well known by the rank-and-file aircrew. How much interplay between MCAS and STS and the articicial pitch "feel"?

Gums sends...

ADDED: I wish to clarify my assertion about static stability or speed stability. Some folks think the FBW Airbus planes have neutral speed stability. So most of the time speed is controlled by power. The fact is that at least the AB330 that went into the ocean (AF447) showed very good stability and fairly benign stall characteristics. No violent wing rock or buffet. The stall "protection" was inhibited due to the low airspeed value feeding the flight control computers, so HAL did not keep the pilot from commanding back stick and the stabilizer from trimming to full nose up. From reading about the Concorde, which had a lotta fly by wire, even it had conventional speed stability.
The Boeing STS is a funny way of telling pilots they have to re-trim if speed changes. Duhhh? Oh well, I would rather have the plane try to get back to last trimmed speed and I can trim up or down to achieve the new desired climb angle or new airspeed//AoA. And remember that we' re really trimming to AoA. I do not unnerstan moving a large control surface to tell pilots they need to trim versus using the "feel" system that the plane already has and has done fine for decades.

Machinbird
3rd Dec 2018, 07:36
Concours77, as the stab seems to trim to full ND at a normal rate, I don’t think there is a mechanical failure of the stab jackscrew.
What perhaps happens is a breakout of the control yokes just before the dive. FDR shows a difference in the applied control force on the yokes. I believe when elevator breakout occurs there is sudden loss of elevator effectiveness. This would cause a sudden bunt over, with a wtf moment and the pilots stopped any sustained opposite trim.
Birdspeed, you may well have the final piece to the puzzle.
Here is the cockpit NG flight control portion diagram courtesy of Infrequentflyer:
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/640x420/screen_2bshot_2b2018_11_14_2bat_2b10_35_00_2bam_5cb5e5697196 e1906a6728081987d0c01473aabe.png

NG cockpit control system

See the little goodie named 'Breakout Mechanism'
That is there to restore controlability when something jams part of the flight control system. It's function is to separate the flight control system into two halves.With enough force differential between the two control columns, a cam system will disengage and allow the two sides of the control system to operate independently. From the FDR elevator force charts, the crew were applying force at the limits of their ability against the elevator feel system, and there is no sense in making a disconnect that requires more force than can be exerted. Just how far back in the aircraft would the system remain separated is the next question. It appears that the system would be separated back to the hydraulic power control unit input torque tube. The NG flight control system appears to have benefited from jammed flight control incidents involving earlier models. See this reference:NTSB Recommendation (https://www.ntsb.gov/safety/safety-recs/recletters/A-11-007-011.pdf)
My key takeaway is the following statement: "Further,the elevator system on 737-600 through -900 series
airplanes was improved by the addition of several mechanical override mechanisms. While these
override mechanisms do not mitigate all possible jam conditions, in general, in the event of a
system jam, the mechanisms allow both elevators to be controlled by the movement of the
unaffected control column."
However my reading on flight with jammed controls indicates that the crew should expect reduced control effectiveness. Whether or not flight with disconnected but not jammed pitch controls would experience decreased control effectiveness is dependent on the location and design of the additional override mechanisms required. What I suspect is that there are spring cartridges located both before and after the power control unit input torque tube.

Machinbird
5th Dec 2018, 18:15
Tucked into your control system pathways, you will also find a little gadget called a POGO. These give you the ability to maintain control although your flight control system is partially jammed. This item fits the category of "mechanical override systems" referred to in the previous posting. Rather than describe them myself, I figured that I would let a professional take that task. See Pogo Load Limiter (http://daerospace.com/mechanical-systems/load-limiter-pogo/)
Now take a look at this simplified diagram of the elevator control on the 737NG (courtesy of Peter Lemme's Blog) Do you see the POGO?
Well, I said this is simplified. There has to be another set of Pogo-like mechanisms connecting the left and right control cables to the elevator input torque tube or else there would be no point to having the breakout mechanism discussed in my previous post.The two columns would be locked together through the elevator input torque tube. Consider the implications of what happens when the crew hauls back hard against the elevator feel and centering unit resistance!!
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/640x384/screen_2bshot_2b2018_11_11_2bat_2b9_07_08_2bam_c27c6063d0a2b 21cd4aaf799135092a953e5fbec.png

737-300 Elevator Control

gums
5th Dec 2018, 18:49
Salute!

Good diagram, ‘bird.

looks like a lot more than a simple 1960’s design that only had hydraulic “assist” .

Gums sends.....

megan
9th Dec 2018, 02:48
Apparently, this bird ( 737) seems to act like most of us expected in the old days and even today until they let us fly a plane with negative or neutral static stability. We trimmed to reduce control pressure/movement. That got you back t the original AoA/speed. So you could have a lot of elevator deflection but the trim operation would allow less and less control pressure until you reached the desired trim for speed/AoA. And then we had STS, which trimmed the stabilizer without pilot input. Huh???? This ain't no Airbus fly by wire FLCS. But rational was to tell the pilot ( I use pilot to assert whoever was in charge of moving controls), he needed to trim for the new speed/AoA. What airplane have they flown that did not need a new trim after changin speed/AoA?? Do they need to be "reminded"? SHeesh.

And then we get another tweak to the plane's flight control system.. MCAS. And its operation and such are not well known by the rank-and-file aircrew. How much interplay between MCAS and STS and the articicial pitch "feel"?I wonder if the systems incorporated may prove to be installed to satisfy some certification clause, but not actually bringing any real benefit in the ability of the pilot to exercise control.

The helicopter industry had just this occur with the introduction of the pitch bias actuator (PBA) on the S-76 and Blackhawk.

The FAA requires that you must have Longitudinal Static Stability (LSS), where the stick position for any speed must be ahead of a lesser speed. Without the PBA the helos demonstrated negative stick gradient in certain areas of the envelope, which certification said was a no, no. The PBA provides longitudinal cyclic displacement proportional to airspeed. The DAFCS commands the PBA as a function of pitch attitude, pitch rate, and airspeed. Failure modes were,
1. Attitude failure--bias actuator centered
2. Pitch rate failure--faded out pitch rate component
3. Airspeed failure--actuator goes to 120-knot position and attitude and rate continues to function

In practice pilots flying were unable to tell if the PBA was active or not, so the authorities were ultimately convinced that the PBA was not needed, and the system was removed from both the S-76 and Blackhawk. Negative Stick Gradient was a problem only for certification standards and did not impose a handling problem for the Pilot.

gums
11th Dec 2018, 01:08
Salute Megan!
You can attribute my quotes, but I recognized it right away.
From megan
I wonder if the systems incorporated may prove to be installed to satisfy some certification clause, but not actually bringing any real benefit in the ability of the pilot to exercise control.


I feel that Megan has nailed it above, but it would be nice for us to see the pitch authority at various AoA and stick force gradient that seems to be a factor in certification. Many here are real aero folks and mostly pilots. We can handle it.

Being from the military "cert" community, I flew a few designs that would prolly have not been certified by the U.S. or other country aviation bureaus. I even flew one that failed several pitch and roll gradient specs, but USAF produce a few hundred and we flew the suckers because we had good checkouts and were briefed at length on the "waivers". I contrast this with the hapless crew on that Lion Air jet.

Added..... I flew one type that behaved just like all the aerodynamic classes said it should. Right up to about a degree or so of the critical AoA, and then whahooo!!! It was the VooDoo, so youse can look it up, but the real explantion is hard to find. The thing gave plenty warning when subsonic unless you yanked back real quick. But we had a "pusher" that used AoA and pitch rate and stick force to "help" us. However, the stick force per gee and rate and such was perfectly within the military certification requiremnts until that tiny degree of AoA before the pitch-up. And then any more back stick /AoA exhibited what we called the stick "getting light", and if you persisted, then you witnessed the wahoo, heh heh.

Glad we can discuss stuff here. Guess the casual folks on the main forum think they must be aero engineers of test pilots to be here.

Gums sends...

PEI_3721
11th Dec 2018, 07:34
megan, gums, many of these points are also in thread https://www.pprune.org/tech-log/614997-b-737-speed-trim-system-5.html, post 81 onwards.
And additionally some other, as yet, unanswered thoughts on ‘collateral’ effects, including those which ‘get into the trim system’ per se.

gums
11th Dec 2018, 14:58
Salute PEI ,

Yeppers, I saw the great posts and such and learned a lot, but the other thread did not get into the MCAS for about 60 + posts. It is a really great discussion about stability and how the STS works and why.

Since the MCAS seems to be of major concern for the Lion accident, and because my strong feeling is we pilots should know about a significant change to our flight controls, I gravitated to this thread..

One thing that mabe FCeng can explain is the interaction between STS and MCAS. Seems to this old pilot that at certain flight conditions they are opposing each other. In other words if speed/AoA is changing, then one system is trimming opposite the other. Or am I way off base?

Gums asks...

Vessbot
11th Dec 2018, 18:07
Salute PEI ,
One thing that mabe FCeng can explain is the interaction between STS and MCAS. Seems to this old pilot that at certain flight conditions they are opposing each other. In other words if speed/AoA is changing, then one system is trimming opposite the other. Or am I way off base?


There would have to be a predetermined hierarchy of precedence of simultaneous/opposing inputs. And surely MCAS (i.e., the high alpha protection) would be above STS and its general speed stability. And this is nothing special, my jet has electric trim only with probably half a dozen inputs, each in its order in this type of hierarchy.

Glad we can discuss stuff here. Guess the casual folks on the main forum think they must be aero engineers of test pilots to be here.

Me too. I'm on a few US-centric forums and the main topics there seem to be about industry complaints. But if you get too much into technical stuff (except maybe ATC procedures) you're too much of a nerd.

FCeng84
11th Dec 2018, 18:22
Gums - glad to share the level of system understanding that I can. STS and MCAS are both functions implemented within the autopilot and thus use the same path to the stabilizer that the autopilot uses for autotrim when it is engaged and controlling the elevator. In the logic MCAS has priority over STS which to me makes perfect sense as MCAS is there to address less than desirable Cm-alpha characteristics while STS is there to address less than desirable pitching moment vs. speed (would that be Cm-vcas or Cm-u?) characteristics. STS is more of a phugoid thing while MCAS is more related to short period. In my opinion, short period stability characteristics are more important than phugoid stability characteristics. Things can get out of sorts much quicker when related to short period than phugoid.

gums
11th Dec 2018, 18:55
Salute Vess!

Sounds like the L39 has only a coolie hat switch on top of the stick, heh ?
++++++++++++++++++++++++++++++++++++++=
I must admit that I only flew one jet that "trimmed" all by itself, and that counts 4 others that went the gauntlet from pure cables, pushrods and pulleys, thru hydrualic assisted control surfaces, irreversible hydraulic systems and then a full FBW system with zero mechanical connections to control surfaces. The auto trim was in the VooDoo, which had a most cosmic A/P. Only trim from HAL was when I selected A/P for attitude control, speed, heading, altitude etc. I also had a A/P coupler that flew the attack scenario and a ILS approach without throttle control. Least I could crash if I didn't pay attention to power, heh heh. But that would be my fault.
++++++++++++++++++++++++++++++++
First I saw about auto trim from Hal for the next generation lights was back in the 70's when the Viper FLCS was being discussed in our small community. The non-aero engineer pilots thot the thing had auto-trim. Nope. It trimmed to the commanded gee, and still does 40 years later. We did not have to trim if we slowed down or speeded up. Roll was slightly different due to potemtial rigging of the ailerons and speed could force roll in one direction or the other. This was negligible most of the time because the control law was zero roll rate or whatever you had set.

Even the 'bus is not a pure auto trim, but it's the closest I can find for most flight parameters if auto-throttle is engaged..

I would like to see an official flow chart from Boeing that shows the interaction of STS and MCAS. Inputs and outputs to the stab and cockpit displays when things were awry.

Gums sends...

gums
11th Dec 2018, 19:02
Salute FC !

Oh great, and that answers my basic question anout priority.

I also understand the phugoid at normal AoA and Aoa changes, but seems the STS implementation is rather harsh, heh heh. I have bad vibes about pilots that do not recognize a need to trim. But if they wanna hold a few pounds of pressure for 3 hours in one of the heavies, that is their call.

Gums sends...

Vessbot
11th Dec 2018, 19:17
Salute Vess!
Sounds like the L39 has only a coolie hat switch on top of the stick, heh ?


Salute! Yes except the coolie hat part of it on the one I flew was missing, so there was just a little 4-way metal nub that hurt your thumb! But that's a different jet than what I was talking about, (my profile's out of date) the CRJ. There are only electric inputs, and the hierarchy goes something like:

Captain's manual switches
FO's manual switches
Autopilot inputs
"Autotrim" (vague naming award of the year... this moves when the flaps are moving in the first few notches)
Mach trim

Are there any other secret cooks in the kitchen? Who knows!

(Come to think of it, I think the L-39 autotrimmed with flap movement too... but it's been a while)

megan
12th Dec 2018, 02:11
gums said - You can attribute my quotes, but I recognized it right awaySorry gums, a habit I should get out of.

jimtx
12th Dec 2018, 04:48
Gums - glad to share the level of system understanding that I can. STS and MCAS are both functions implemented within the autopilot and thus use the same path to the stabilizer that the autopilot uses for autotrim when it is engaged and controlling the elevator. In the logic MCAS has priority over STS which to me makes perfect sense as MCAS is there to address less than desirable Cm-alpha characteristics while STS is there to address less than desirable pitching moment vs. speed (would that be Cm-vcas or Cm-u?) characteristics. STS is more of a phugoid thing while MCAS is more related to short period. In my opinion, short period stability characteristics are more important than phugoid stability characteristics. Things can get out of sorts much quicker when related to short period than phugoid.

I'm confused. Humans have to be protected by MCAS and STS but when the autopilot is used it doesn't care what the stick forces are? It just flys attitude and airspeed and whatever other constraints are input and is quick enough to see an unwanted pitchup that was caused by the Max engines or an unwanted regime that the STS would protect a Human against, whatever that was. I suspect that Gums has already answered the question of whether he would need that help but of course history does say that some of his cohorts failed in that respect. You still haven't answered my question about why steep turns were referenced in the Boeing/FAA literature.

FCeng84
12th Dec 2018, 06:11
jimtx - I would like to help allay your confusion. What are the specific questions you would like to have answered?

jimtx
12th Dec 2018, 14:30
jimtx - I would like to help allay your confusion. What are the specific questions you would like to have answered?
Would MCAS activate in a 250 knot 45 degree bank steep turn. Would it activate in a slightly over bank 35 degree turn at a lower airspeed.
Can the autopilot be put in the MCAS envelope where it is not protected. I don’t think it would mind however.

FCeng84
12th Dec 2018, 14:50
Answers to questions from jimtx:

1. Would MCAS activate in a 250 knot 45 degree bank turn? Would it activate in a slightly over bank 35 degree turn at a lower airspeed?
- MCAS activates when AOA exceeds a threshold that is a function of speed/Mach number. MCAS activation is not dependent on bank angle.
- The 737 can be maneuvered to any bank angle at with AOA either above or below the MCAS activation threshold so it is not possible to determine whether or not MCAS would activate at either of the conditions proposed as there is no indication as to the associated AOA or the corresponding normal load factor.

2. Can the autopilot be put in the MCAS envelop where it is not protected?
- At some flight conditions it is possible to command the airplane when via the autopilot to an AOA high enough to put it into a condition where MCAS would activate if the autopilot were to be disconnected. MCAS will not come active with the autopilot engaged.

I hope these are helpful. As always, follow-ups welcome.

gums
12th Dec 2018, 16:03
Salute FC !

Yeah, AoA seems to be the driver for the pulses, and mach is a function that fades out the MCAS what? a) duration of pulse, b) rate of stab movement, c) amount of stab movement ( which necessarily appears a function of "a" and "b") or all of the above?
The available literature says the mach will limit MCAS cmds according to "some" function/equation until what? 0.68 M ? And then no more MCAS above that, if I read it correctly.
So AoA must not be as big of a factor at higher altitudes and normal cruise mach, huh?
Point being that "q" must be important, and Machinbird's question about linearity of the plane's static stability coefficients when near certain AoA and "q" values must be a player for certification.

Oh well, all very technical, but knowing what the plane is supposed to do and what the "help" provided by Hal is supposed to do is a good thing.

Gums asks...

infrequentflyer789
12th Dec 2018, 18:01
@FCeng84 I get most of what you've said, makes sense and moreover it matches my understanding of the high-AOA function in the NG, but this bit I don't get.

STS and MCAS are both functions implemented within the autopilot and thus use the same path to the stabilizer that the autopilot uses for autotrim when it is engaged and controlling the elevator.

If I understand the NG functional descriptions that I have acquired, Autopilot and STS take the same signal path which goes through the column switch module and then the console cutout switches, to the actuator. Now, we know MCAS doesn't honor the fwd/aft-column-cutout switches so there must, surely, be either a different signal path or some new bypass/override signal to the column switch module (unless MAX STS doesn't honor the fwd/aft-column switches either?) ?

Unless the column switch module has changed from an actual switch in the path (which is how I understand the NG docs - I may be wrong?) to merely a signal of switch position back to the FCC which then decides in software whether that "cutout switch" is effectively in the signal path or not?

Note: the above, would be a sort of red flag to me, because software is far more flexible than switches and relays, which is great until it goes wrong, by accident or design - it is very easy to change code such that a check/gate/switch point is unintentionally bypassed, and then Murphy decrees it will only be for a set of circumstances/inputs that aren't in the tests.:( Generally much harder to unintentionally bypass a relay or switch.

The "flaps up" signal from FCC to stab trim actuator must have changed in some way as well, because MCAS is apparently trimming at flaps-down speed when flaps are up?

Appreciate any clarification you can give, also fully understand if it goes beyond the system understanding that you can share - in which case I will remain with my speculation until more is published.

PS: If anyone wants me to post the NG details or diagrams that I am talking about then I can, I didn't want to make this post too large. Plus I get the impression FCeng84 doesn't need them!

FCeng84
12th Dec 2018, 18:08
Salute FC !

Yeah, AoA seems to be the driver for the pulses, and mach is a function that fades out the MCAS what? a) duration of pulse, b) rate of stab movement, c) amount of stab movement ( which necessarily appears a function of "a" and "b") or all of the above?
The available literature says the mach will limit MCAS cmds according to "some" function/equation until what? 0.68 M ? And then no more MCAS above that, if I read it correctly.
So AoA must not be as big of a factor at higher altitudes and normal cruise mach, huh?
Point being that "q" must be important, and Machinbird's question about linearity of the plane's static stability coefficients when near certain AoA and "q" values must be a player for certification.

Oh well, all very technical, but knowing what the plane is supposed to do and what the "help" provided by Hal is supposed to do is a good thing.

Gums asks...

The amount of airplane nose down stabilizer that MCAS commands is a function of both Mach number and the amount that AOA has exceeded the MCAS activation threshold (itself a function of speed/Mach). When MCAS commands the stab, it always moves at the MCAS rate of 0.27 deg/sec. The duration of the MCAS stab command is how ever long it takes to move the stabilizer at the MCAS rate the amount of incremental stabilizer commanded by MCAS.

FCeng84
12th Dec 2018, 22:53
InfrequentFlyer - When MCAS was introduced to address less than desirable Cm-alpha characteristics it was recognized that it would need to be able to apply airplane nose down stabilizer motion while the column was being pulled past its cutout switch activation point. I do not know all of the details, however I do know that some wiring was added to allow the MCAS command to get to the stabilizer even with a large column pull. This "path around the column cutout switch" is not activated for STS commands, rather only for MCAS commands. Note as stated earlier that MCAS commands take priority over STS commands.
MCAS is active only when flaps are up and its commands are a function of AOA regardless of speed. This allows MCAS to be active for flaps up stalls as well as higher speed high AOA maneuvers such as wind up turns.

jimtx
13th Dec 2018, 04:33
InfrequentFlyer - When MCAS was introduced to address less than desirable Cm-alpha characteristics it was recognized that it would need to be able to apply airplane nose down stabilizer motion while the column was being pulled past its cutout switch activation point. I do not know all of the details, however I do know that some wiring was added to allow the MCAS command to get to the stabilizer even with a large column pull. This "path around the column cutout switch" is not activated for STS commands, rather only for MCAS commands. Note as stated earlier that MCAS commands take priority over STS commands.
MCAS is active only when flaps are up and its commands are a function of AOA regardless of speed. This allows MCAS to be active for flaps up stalls as well as higher speed high AOA maneuvers such as wind up turns.

"such as wind up turns" Obviously no airline crew is going to do a windup turn, a test flight procedure, hopefully:) But they do or used to do steep turns in the simulator which should be a data point on the windup turn flight test info. Never have heard of anyone doing a real (on purpose) steep turn in airline flying. Can you relate that data point to a hazardous AOA and other envelope positions that would make Boeing and the FAA decide to protect the human from it's effects, which most probably they would never see in the airplane and only see it in the simulator if anyone still does steep turns in the simulator. Yet, the autopilot would not need any MCAS help. It would just move the stick back or forward, irrelevant of stick force, to command an attitude that complied with what it wanted the performance instruments to show. But we still have to deal with an autopilot that can be commanded to come close to or enter the straight and level MCAS envelope. MCAS won't help it but the autopilot will move the stick as required, uncaring what the stick force is, to command the required control inputs. But put a human in the loop and we have to help him. What about telling him what to be aware of as @Gums voodoo folks used to do?

infrequentflyer789
13th Dec 2018, 11:23
Obviously no airline crew is going to ...

Wonder how many people that phrase has killed in the history of aviation engineering. :(

For instance, obviously no airline crew is going to conduct a high AOA test at lower-than-specified level and go beyond the failure point... (XL888 - AOA sensor fail again).

Whenever there is human in the loop and someone says "obviously" I envisage Mr Murphy materialising dressed as the Joker (Heath Ledger style), and in aviation, strapping himself into the jumpseat.

@FCeng84 - thanks, clarification understood and appreciated.

FCeng84
13th Dec 2018, 18:18
jimtx - I fully agree with you that wind-up turns are not a normal maneuver. They are fun to experience during flight testing, but I hope no fare paying passenger on a regular transport flight has to ride through one. Wind-up turns are the test maneuver most commonly used to collect data to demonstrate compliance with stick force vs. g requirements as specified in FAR 25. The overall objective is that increases in nose up controller forces as applied by the pilot will be required in order to command higher load factors (i.e., higher AOA at a given speed). This is considered necessary to provide the pilot with an airplane that enables control of load factor at elevated levels if the pilot chooses to command the airplane there and similarly that the airplane will promptly recover to more normal load factors if that is the intent of the flight crew.

In addition to wind-up turns, flight testing also often includes roller coaster maneuvers (wings level pull-ups and push-overs in succession). These are probably more relevant to line operation maneuvers such as vertical maneuvers to change climb/descent angle, avoidance maneuvers prompted by see-and-avoid or TCAS, and tight path control during an emergency descent. Those maneuvers are more dynamic than wind-up turns so the data collected during those handling qualities evaluations does not lend itself very well to showing compliance with the FAR force vs. maneuver requirements. For that compliance, wind-up turn data has been the standard.

CONSO
13th Dec 2018, 19:34
SLF here- looking at various articles/posts/analysis here and in aviation week, etc, and having spent many decades in aerospace- missile device testing - manufacturing /tooling of military and commercial aircraft, etc as a injun-ear - MY view of the MCAS ( Hal ) system fiasco boils down to the following simplified items

1) AOA mismatch notice/display an optional item- no doubt at a ridiculous cost- so pilots had no notice of that discrepancy
2) AOA single input with no matching- voting allowed to directly affect/move a critical flight control item ( stabilizer )
3) Pilots of 737 prior to Max ‘knew” that push pull of control column in opposition to autopilot would disengage autopilot and allow manual- direct control until autopilot manually reset/engaged
4) description of runaway stabilizer infers continuous movement despite column disconnect or no trim switch.- thus a positive action tripping circuit breaker is needed to stop
5) But MCAS- HAL on 737 MAX ignores both column and trim switch use- and moves stabilizer and then STOPS briefly- ALL dependent on ONE AOA value- with NO notice, NO display, and NO warning
6) Neither instruction manual or training mentions the absolute authority of MCAS- HAL- so pilots believing and trained that pull or push on control column, and/or disconnecting Autopilot gives them ABSOLUTE manual control at ALL times and that trim switch still works( briefly)
7) Pilots are now faced with a WTF conundrum. They are going too fast to lower flaps which stops MCAS-HAL and at a low altitude per training - “runaway” stabilizer seems to stop for a while- and autopilot is off so ????

8) Meanwhile stick shaker rattles, trim wheel starts and stops and ????

And only a few minutes to solve . . . " I'm sorry dave . . "

FCeng84
13th Dec 2018, 23:12
CONSO - I appreciate your summary list and agree with much of what you have presented. Below I have added my thoughts and a few points of clarification using italic text. The non-italic text below are the summary statements provided by CONSO.

1) AOA mismatch notice/display an optional item- no doubt at a ridiculous cost- so pilots had no notice of that discrepancy
- Agreed - neither did the engineers.

2) AOA single input with no matching- voting allowed to directly affect/move a critical flight control item ( stabilizer )
- Yes. Analysis determined that one increment of MCAS stabilizer motion by itself did not pose a safety issue. No second increment without pilot trim input. Design approach assumed pilot trim input would return airplane to trim prior to any subsequent MCAS activation.

3) Pilots of 737 prior to Max ‘knew” that push pull of control column in opposition to autopilot would disengage autopilot and allow manual- direct control until autopilot manually reset/engaged
- Push / pull on column does disengage autopilot (if engaged) and at all times provides direct control of elevator. Same for all 737 models.
- Autopilot box, however, continues to provide automatic stabilizer control even when autopilot function is disengaged:
- STS on earlier 737 models
- STS and MCAS on 737 Max

4) description of runaway stabilizer infers continuous movement despite column disconnect or no trim switch.- thus a positive action tripping circuit breaker is needed to stop
- Agreed from wording I have seen here in PPRUNE.

5) But MCAS- HAL on 737 MAX ignores both column and trim switch use- and moves stabilizer and then STOPS briefly- ALL dependent on ONE AOA value- with NO notice, NO display, and NO warning
- 737 Max does not ignore column or pilot trim switch. Column is sole control driver for elevator. Pilot trim has priority over automatic control of stabilizer. One difference is that pull column disables automatic airplane nose down STS command while pull column does not disable airplane nose down MCAS command.
- All 737 stabilizer motion involves turning of cockpit stabilizer trim wheels (one at each pilot's inboard knee) with ten revolutions of approximately 8" diameter, one inch wide wheels per degree of stabilizer motion. Wheel turn produces considerable noise as well (ranted not as significant when stick shaker is going.)

6) Neither instruction manual or training mentions the absolute authority of MCAS- HAL- so pilots believing and trained that pull or push on control column, and/or disconnecting Autopilot gives them ABSOLUTE manual control at ALL times and that trim switch still works( briefly)
- I cannot comment on instruction manual or training.
- No argument with this comment other than to restate that pilot stabilizer trim input via wheel mounted thumb switch always stops and overrides automatic stabilizer control whether coming from STS or MCAS functions.

7) Pilots are now faced with a WTF conundrum. They are going too fast to lower flaps which stops MCAS-HAL and at a low altitude per training - “runaway” stabilizer seems to stop for a while- and autopilot is off so ????
- Agreed. Design assumed that pilots would recognize improper stabilizer motion taking them repeatedly away from trim when flying relatively steady condition and take the action of cutting out stabilizer motors. I cannot speak to the associated manual nor training.

8) Meanwhile stick shaker rattles, trim wheel starts and stops and ????
- Agreed - quite the compound mess. Don't forget that they are also seeing air data miscompares (speed and altitude) and havd associated warning lights to potentially take their attention away from improper automatic stabilizer motion.

And only a few minutes to solve . . . " I'm sorry dave . . "
- Must continue to fly the airplane vertical/pitch axis with column and use control wheel mounted stabilizer trim switches to offload column forces while sorting out the situation. HAL never said "I'm sorry Dave", but HAL did continue to give errant nose down stabilizer increments that seem to have eventually overwhelmed Dave.

Let me add that with this and all of my entries on this topic I am not trying to place blame for this tragic accident on any one party. I am merely trying to shed some light on how the system involved functions so that all can better understand what this crew faced.

May all of those 189 souls rest in peace and everyone they left behind find the support and comfort that they need to face life after this horrific loss.

Respectfully submitted - FCEng84

CONSO
14th Dec 2018, 00:06
......
And only a few minutes to solve . . . " I'm sorry dave . . "
- Must continue to fly the airplane vertical/pitch axis with column and use control wheel mounted stabilizer trim switches to offload column forces while sorting out the situation. HAL never said "I'm sorry Dave", but HAL did continue to give errant nose down stabilizer increments that seem to have eventually overwhelmed Dave.

Let me add that with this and all of my entries on this topic I am not trying to place blame for this tragic accident on any one party. I am merely trying to shed some light on how the system involved functions so that all can better understand what this crew faced.

May all of those 189 souls rest in peace and everyone they left behind find the support and comfort that they need to face life after this horrific loss.

Respectfully submitted - FCEng84

Appreciate your explanations- like you not trying to blame any ONE.. but perhaps the ' committee ' approach needs revision to incorporate " murphy "

And correct me if the combination of mCAS, stick shaker, control column inputs ALSO increases the control force ( FEEL ) to some max limit. possibly requiring both hands (and planted feet ) to pull back but while in a dive everone becomes nearly weightless held only by harness ?

jimtx
14th Dec 2018, 04:54
jimtx - I fully agree with you that wind-up turns are not a normal maneuver. They are fun to experience during flight testing, but I hope no fare paying passenger on a regular transport flight has to ride through one. Wind-up turns are the test maneuver most commonly used to collect data to demonstrate compliance with stick force vs. g requirements as specified in FAR 25. The overall objective is that increases in nose up controller forces as applied by the pilot will be required in order to command higher load factors (i.e., higher AOA at a given speed). This is considered necessary to provide the pilot with an airplane that enables control of load factor at elevated levels if the pilot chooses to command the airplane there and similarly that the airplane will promptly recover to more normal load factors if that is the intent of the flight crew.

In addition to wind-up turns, flight testing also often includes roller coaster maneuvers (wings level pull-ups and push-overs in succession). These are probably more relevant to line operation maneuvers such as vertical maneuvers to change climb/descent angle, avoidance maneuvers prompted by see-and-avoid or TCAS, and tight path control during an emergency descent. Those maneuvers are more dynamic than wind-up turns so the data collected during those handling qualities evaluations does not lend itself very well to showing compliance with the FAR force vs. maneuver requirements. For that compliance, wind-up turn data has been the standard.

So FAR part 25 has requirements for the ac aerodynamics (?) to provide the pilot with an airplane that enables him to control load factors with a linear stick pull and not have the the surprise of the ac continuing to pull load factor when the pilot is not requesting it? But FAR25 does not care about the actual aerodynamics, in the 737Max, as the autopilot is not protected from the stick force non linearity, rightfully so, as it doesn't care about sick force? So the ac's aerodynamics do not have to be adjusted. The only adjustment needed is to fool the pilot that he has to pull harder to get more g or AOA? And, if the pilot loses this MCAS, Boeing doesn't even tell him to be careful in any flight regime as he will not encounter it in normal operations. Although you alluded to some escape maneuvers that might approach the MCAS envelope. But the current abnormal runaway trim procedure, where MCAS will be disabled, that Boeing advertises as sufficient, does not caution about being careful in a TCAS maneuver or an escape maneuver. Because a Boeing exec said that pilots do not have to know about the nuances of the ac. But of course, nobody is going to get runaway trim and then get a TCAS alert or an escape situation on the same day.

PEI_3721
14th Dec 2018, 07:43
This Thanks for the clear and well considered resume, FCeng84 #58, :ok:
‘Design assumed that pilots would …’
Rule one for design; first list you assumptions; they are the basis of systems descriptions and pilot manuals.

jimtx you views, #60, muddy the ‘clearing waters’ with odd or misleading interpretations.
Beware of ‘not caring about aerodynamics’, or ‘AP view of force’. Those aspects are considered in high integrity FBW aircraft, where the pilots still care. The 737 MAX appears to be using low integrity AoA inputs (dual) to achieve a higher order function than may be available in such an ‘old’ design.

FCeng84 please correct or expand my comment to aid general understanding, and I hope in the fullness of time (soon) you might present some speculative thoughts on how the design of ‘weak’ systems might be re-engineered to meet the certification requirements and pilots’ perceptions expressed in Pprune; - what’s the fix.

Are certification requirements out of step with current piloting abilities, recognition, understanding. Is the industry, pilots, certification, and design, increasingly thinking like “FBW” systems, whereas in reality there are many ‘older’ systems requing previous levels of understanding and operation ?

gums
14th Dec 2018, 17:31
Salute!
After all the certification comments so far, I just had to read the stuff myself. You know me, heh heh.
And now I can see plenty of reason for discussion.

The relevant paragraphs I see deserving of the most disussion and interpretation and such are 25.671, in particular 25.672 Stability augmentation and automatic and power-operated systems. Of particular interest is (a) warning indications, and then (b) and (c) which outline system reaction to failures. This phrase is one that stands out to me; My bold...

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without
requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

My understanding is Boeing assumed the crew would interpret repeated down trim that religiously followed opposite trim by the crew, after waiting 5 seconds, to be "runaway trim". Sure enuf, disabling the trim system can be argued to satisfy Part 25 requirements. But with STS, I can see misunderstanding what is failing especially with no warning indication or even knowledge of the new system! GASP!

Gotta go now, poof!!

FCeng84
14th Dec 2018, 18:20
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.

megan
15th Dec 2018, 02:37
As an aside it seems they don't have the cash to recover the CVR.

https://www.avweb.com/eletter/archives/101/4211-full.html

ITman
15th Dec 2018, 04:16
AD NUMBER: 2018-23-51PRODUCT: All Boeing Model 737-8 and -9 airplanes.

https://ci6.googleusercontent.com/proxy/qfzu457B_BMhHpTmEgLmDSre5b-xfuS2s4VLzoGOXl8-v7SSNHBxuxdmFmPUK4c3BjFOzayBzbEHEY-dm411fuxaLCvBXTi9woJtJsx30IyfF9Ftr51FrF027GlQRGjE=s0-d-e1-ft#http://www.aero-news.net/images/content/commair/2014/737-MAX-8-9-1214a_tn.jpg (http://www.aero-news.net/#)

ACTION: Final rule; request for comments.

SUMMARY: This emergency AD was sent previously to all known U.S. owners and operators of these airplanes. This AD requires revising certificate limitations and operating procedures of the airplane flight manual (AFM) to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions.

This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer.

DATES: This AD is effective December 21, 2018 to all persons except those persons to whom it was made immediately effective by Emergency AD 2018-23-51, issued on November 7, 2018, which contained the requirements of this amendment. Comments must be received by January 22, 2019.

COST: The FAA estimates that this AD affects 45 airplanes of U.S. registry. Operators may incur the following costs in order to comply with this AD:
Revising the AFM - 1 work-hour × $85 per hour = $85

megan
15th Dec 2018, 04:29
AD link

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/fe8237743be9b8968625835b004fc051/$FILE/2018-23-51_Correction.pdf

CONSO
15th Dec 2018, 06:13
And the nitty gritty on the AD last page

Figure 1 to paragraph (g) of this AD — Certificate Limitations
Required by AD 2018-23-51
Runaway Stabilizer
In the event of an uncommanded horizontal stabilizer trim movement,
combined with any of the following potential effects or indications
resulting from an erroneous Angle of Attack (AOA) input, the flight crew
must comply with the Runaway Stabilizer procedure in the Operating
Procedures chapter of this manual:
• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light
• Autopilot may disengage.
• Inability to engage autopilot.

(h) AFM Revision: Operating Procedures(h) AFM Revision: Operating Procedures
Within 3 days after the Within 3 days after the effectiveeffective date of this AD, revise the date of this AD, revise the Operating Operating Procedures chapter of the Procedures chapter of the
applicable AFM to include theapplicable AFM to include the information in figure information in figure 2 to paragraph (h) of this AD. 2 to paragraph (h) of this AD.

So why is the AOA DISAGREE AN OPTION ???

DaveReidUK
15th Dec 2018, 08:06
(h) AFM Revision: Operating Procedures(h) AFM Revision: Operating Procedures
Within 3 days after the Within 3 days after the effectiveeffective date of this AD, revise the date of this AD, revise the Operating Operating Procedures chapter of the Procedures chapter of the
applicable AFM to include theapplicable AFM to include the information in figure information in figure 2 to paragraph (h) of this AD. 2 to paragraph (h) of this AD.

The aforementioned Figure 2:

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/597x769/figure_2_ad_2018_23_51_77b0a4e8e12ff7ce21fee3b2b6e7717f4ec4c b07.jpg

PEI_3721
15th Dec 2018, 08:51
The most important part is the phrase:-
’In the event of an uncommanded horizontal stabilizer trim movement, combined with…’
which rests on the assumption that movement is perceived, in combination with…

From what what has been deduced so far, the crew of preceding Lion flight did perceived unusual trim activity, but did not directly associate the combination as described in the AD (MACS was unknown at that time). Fortuitously an association with ongoing Air Data alerts and indications, and perhaps previous maintenance action linking with the known STS, the crew chose to inhibit the trim.

The supposition in the accident is the crew did not associate the Air Data indications with trim, perhaps biased by the situation before flap retraction where the indications were of unreliable airspeed. etc.

Therefore without appropriate perception and the linked association, the situation may not be fully understood and thus the electric trim not inhibited - residual risk.
The defence / mitigation of this depends on operators disseminating the AD knowledge so that crews might be aware of both the failure mode of MACS and the risk of misidentifying the situation.
Our safety remains, as ever in the ability of the pilots, on the day, in the situations they perceive.


CONSO ‘why the AoA option’ … , - more associated with the optional AoA display on EFIS
What value would this alert contribute to a MACS situation; ‘a good idea’, or ‘it will help confirm the situation which can be deduced from several other features’.
Alternatively it could be a further distraction in a situation which might be overly biased towards AirData (especially flap down), and a continuing, mind sapping, situation biasing stick shaker.

jimtx
15th Dec 2018, 11:54
So if you have a malfunction where you apply the AD procedure you now have an aircraft with “unimproved” longitudinal handling characteristics and degraded trim capability. But FAA/Boeing does not think that would be a concern to warn you about because you will never approach the flight envelope where it would affect you?

PEI_3721
15th Dec 2018, 13:54
After reviewing the thread and other web descriptions of MCAS, it is not clear (to me) what effect the flap configuration has on an erroneous AoA input.

The most recent AD does not refer to flap (nor thrust or any other Air data inputs); thus ‘failed’ MCAS nose down trim could be active immediately after take off.
Is the flap configuration an input which could influence the conditions of the failure?
Is the lack of reference in the AD an oversight; or have I misread, or misunderstood some important aspect?

Is there any definitive description of the ‘MCAS’, purpose and operation, either from the manufacturer or the FAA?

Good points Jim #70

climber314
15th Dec 2018, 14:55
So if you have a malfunction where you apply the AD procedure you now have an aircraft with “unimproved” longitudinal handling characteristics and degraded trim capability. But FAA/Boeing does not think that would be a concern to warn you about because you will never approach the flight envelope where it would affect you?




The Engine Failure NNC does not specifically state to avoid high AoA or Steep Turns... ?
Flaps stopped MCAS inputs. The accident crew failed to notice that as well.

CATEGORY: Maintenance, Engineering, Flight Operations, Management, Safety
SERVICE REQUEST ID: 4-4298138108
ACCOUNT: Boeing Correspondence (MOM)
DUE DATE: No Action Required
PRODUCT NAME: Airplane
PRODUCT LINE: 737
PRODUCT: Several
ATA: 0000-57
SUBJECT: Information - Multi-Model Stall Warning and Pitch Augmentation Operation
REFERENCES: /A/ MOM-MOM-18-0655-018

A pitch augmentation system function called “Maneuvering Characteristics Augmentation System” (MCAS) is implemented on the 737-8, -9 (MAX) to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors in manual, flaps up flight. The system is designed to allow the flight crew to use the column trim switch or stabilizer aisle stand cutout switches to override MCAS input. The function is commanded by the the Flight Control Computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane AoA exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach numbers and greater at low Mach numbers. The function is reset once AoA falls below the AOA threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AoA condition persists, the MCAS function commands another incremental stabilizer nose down command according to the current aircraft Mach number at actuation.

The MCAS function is not incorporated on 737NG airplanes.



https://theaircurrent.com/aviation-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas-jt610/

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1560x1040/mcas_655823b351e02ef290ea526b292b8104a6d422c8.jpg

PEI_3721
15th Dec 2018, 15:17
c314, thanks for sharing.

So in terms of the human performance in identifying the failure: the AD assumes that the crew will notice the trim motion (after flap retraction) amongst all of the other alerts, but, ’Flaps stopped MCAS inputs. The accident crew failed to notice that as well.’

The safety case appears to be based on the publication of a new (interim?) drill via AD, and without further system description (flap, thrust, Air Data inputs) expect everyone, in all situations, to understand and detect failures.
For how long are we to be exposed to this risk ?

jimtx
15th Dec 2018, 16:52
The Engine Failure NNC does not specifically state to avoid high AoA or Steep Turns... ?
Flaps stopped MCAS inputs. The accident crew failed to notice that as well.

CATEGORY: Maintenance, Engineering, Flight Operations, Management, Safety
SERVICE REQUEST ID: 4-4298138108
ACCOUNT: Boeing Correspondence (MOM)
DUE DATE: No Action Required
PRODUCT NAME: Airplane
PRODUCT LINE: 737
PRODUCT: Several
ATA: 0000-57
SUBJECT: Information - Multi-Model Stall Warning and Pitch Augmentation Operation
REFERENCES: /A/ MOM-MOM-18-0655-018

A pitch augmentation system function called “Maneuvering Characteristics Augmentation System” (MCAS) is implemented on the 737-8, -9 (MAX) to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors in manual, flaps up flight. The system is designed to allow the flight crew to use the column trim switch or stabilizer aisle stand cutout switches to override MCAS input. The function is commanded by the the Flight Control Computer using input data from sensors and other airplane systems.





So the only MCAS flight envelope is steep turns? Following a TCAS climb command would not be a problem without MCAS? A windshear escape maneuver would not be in the envelope either, a rare occurrence anyway in the clean config?
But since steep turns are usually only done in the simulator, I think allowing a non linear pull would be good for your crosscheck. As you roll and pull to the bank and pitch required you might see the pitch force decrease but you're flying attitude and performance instruments and you would just allow the aircraft's pitch input to help you maintain attitude if you don't trim in steep turns. Unless the engines actually add so much pitch up that you would have to push. I doubt that based on the amount of down elevator that MCAS inputs.

climber314
15th Dec 2018, 17:42
From what I've read, Flight Global News has stated "Boeing added MCAS to the 737 Max because that aircraft has slightly different flight characteristics from the earlier-generation 737NG." They also described these flight characteristics as "interesting" in another article. The Air Current describes the addition of MCAS was “to compensate for some unique aircraft handling characteristics during it’s (sic) Part 25 Certification and (to) help pilots bring the nose down in the event the jet’s AoA drifted too high when flying manually…”

I'm sort of reading "between the lines" here, but that doesn't sound like these "characteristics" would lead to some sort of catastrophic failure in the absence of MCAS. Sounds more like a fix for something minimally disruptive if unsustained. IDK?

gums
16th Dec 2018, 02:13
Salute!

No need to "read between the lines", Climber.
I shall assert that without MCAS, the doggone plane would not have met the FAR 25 requirements for longitudinal stability, as well as the approach to stall, or even control in the stall.
One phase appears several times.
....average gradient of the stable slope of the stick force versus speed curve

It may be that as the Max approaches a critical AoA, not speed, that the stick forces lighten to the point that the plane eventually continues nose up with zero control input. Same as my VooDoo did 50 years ago ( but we didn't need Part 25 cert, and our control forces were way lower per gee and trim speed changes that I see in the Part 25 tables, e.g. about 4 pounds per gee, and this was same years later in the Viper, as our max force for 9 gees was about 35 pounds).

I suggest most here read the relevant FAR paragraphs, which are :
Parts 25.173, 25.175, 25.201, 25.203
I like "25.203 Stall characteristics", as it may provide a clue why MCAS was incorporated. The first paragraph requires:
No abnormal nose-up pitching may occur. The longitudinal control force must be positive up
to and throughout the stall.
.
All for tonight, but this should keep discussion going another week or so, huh?

Gums sends...

jimtx
16th Dec 2018, 06:52
Salute!

No need to "read between the lines", Climber.
I shall assert that without MCAS, the doggone plane would not have met the FAR 25 requirements for longitudinal stability, as well as the approach to stall, or even control in the stall.
One phase appears several times.


It may be that as the Max approaches a critical AoA, not speed, that the stick forces lighten to the point that the plane eventually continues nose up with zero control input. Same as my VooDoo did 50 years ago ( but we didn't need Part 25 cert, and our control forces were way lower per gee and trim speed changes that I see in the Part 25 tables, e.g. about 4 pounds per gee, and this was same years later in the Viper, as our max force for 9 gees was about 35 pounds).

I suggest most here read the relevant FAR paragraphs, which are :
Parts 25.173, 25.175, 25.201, 25.203
I like "25.203 Stall characteristics", as it may provide a clue why MCAS was incorporated. The first paragraph requires:

.
All for tonight, but this should keep discussion going another week or so, huh?

Gums sends...
Based on the initial MCAS nosedown trim (referenced in previous posts) wouldn't you think that that amount of trim would not be to prevent an actual reversal of pitch input but just an adjustment to the stick force linearity. Or can you say that that amount of initial MCAS trim is to counteract an actual nose up pitch tendency? It doesn't seem like a lot of nose down trim but we are in the dark about what Boeing actually was doing with MCAS and they have not been forthcoming with info about it to us rank and file peasants as it should be. But right now, it appears to me that Boeing and the FAA addressed a glitch in the part 25 certification process, really not affecting the safety of real world ops, with a pencil whip software solution that unfortunately killed people when a flight crew and their maintenance people could not trouble shoot properly.

Machinbird
16th Dec 2018, 07:14
CONSO ‘why the AoA option’ … , - more associated with the optional AoA display on EFIS
What value would this alert contribute to a MACS situation; ‘a good idea’, or ‘it will help confirm the situation which can be deduced from several other features’.
Alternatively it could be a further distraction in a situation which might be overly biased towards AirData (especially flap down), and a continuing, mind sapping, situation biasing stick shaker.
Let me try to answer this differently.
When the aircraft rotated, the stick shaker activated. Invalid activation of the stick shaker is of itself an unnecessary hazard for a number of reasons and has caused accidents in the past.
Let me try to summarize what I believe I know about this AOA system
The 737 has two AOA vanes and does not do comparisons between sensors. Instead the port side vane controls the captain's stall warning and inputs to the ADIRU for the left side. The starboard AOA vane controls the right seat stall warning and inputs to the right side ADIRU.
The reference AOA sensor for MCAS on the MAX apparently swaps between sides based on WOW switch activation but the sequence can be thrown off by maintenance activity.
Problem number one is that the crew does not know that AOA mismatch is in play. They need a warning to know this critical piece of information. This warning should not be optional equipment.
Problem number two, once the crew realizes that AOA mismatch is in play, they need a means to deal with it right now.
Solution: Give each pilot a push button to switch the active stick shaker and MCAS to the opposite AOA vane and peace and quiet descends in the cockpit, (provided the switches are logically located and crews trained in their use.)
Then execute UAS procedures, and write the gripe up at destination.

PEI_3721
16th Dec 2018, 12:38
Thanks Machinbird; points for discussion.
Your system description is adequate, but if the crew are to have knowledge of an AOA system disagree, then at some point the systems (sensors) must be compared (I think you meant that).

Comparison, disagree alerts, direct the crew to errant systems for further consideration; speed / altitude have third systems for ‘manual’ comparison and choice, AoA has none, thus there is nothing further that crews can do.
AOA disagree indicates unequal values, but not which one is correct; so even with an EFIS display the crew can not determine which value should be used. (Some Operators / Unions think otherwise).

The Lion accident has associated the errant vane with the high value (from FDR - hindsight), but its possible to have a low-value errant vane. A less likely combination is a low-value AoA and a real stick shake; not impossible, the aircraft really is approaching a stall.
All that a crew can do in either of the undetermined situations is #1 fly the aircraft, ‘using all available (relevant) information’; compare speed systems and use the best 2 out of 3.
The Lion crews did just that (supposition), they flew the aircraft and considered UAS; operating just as would be expected.

MCAS was consequential, the trim only being a factor after flap retraction, and the failure presented in a manner not easily associated with ‘air data’ problems.
MCAS had a similar #1 ‘fly the aircraft’, and did so; but without any other means to check that the control input was required, MCAS generated a new problem for the crew.
MCAS should still operate normally with a errant low-value vane, trim would not be repetitive - uncommanded or runaway trim (thus an excluded failure combination which the crew might not differentiate).

Give each pilot a push button to switch the active stick shaker and MCAS to the opposite AOA vane and peace and quiet descends in the cockpit, ….
I do not agree; all that a switch might do is to add workload, confusion, be distracting, and a possible false conclusion. Also, a switch might only swap the side of stick shake - more puzzlement.

A fundamental problem of dual systems (dumb sensors - there are more intelligent ones), is that we can identify a difference between sensors, but not which one is correct.
Thus my view remains; - comparative information without the ability to change something is low priority, thus don’t display / distract crews from #1.
Without a third ‘voting’ system, the correct value of AOA cannot be determined, thus no point of a display.
New switches and associated procedures will not provide meaningful information for the crew to act on, thus focus on #1
And don’t distract from a real SS, or ‘other’ vane / SS abnormalities, which have been managed successfully in pre MCAS aircraft; for all intent and purpose, the same problem as managed by the the Lion pilots.


Thoughts from the dual system points above. In pre MCAS aircraft how does the STS differentiate between speed inputs, will the FCC shut down with a speed error, i.e. AP disengage logic - STS not available or erroneous operation ?
Would STS be reinstated with crew selection of air data to all 1/ 2 ?

And returning to MCAS how would the system (FCC) manage other dual inputs, speed as with STS, thrust value, flap position.
Are these other inputs ‘smart’, or are the ‘smarts’ within the FCC; if so how, then why not for MCAS ?
If automatic FCC switching / inhibition is not available, will errors in the other inputs also cause to MCAS to operate ?

Edit. The web page https://www.satcom.guru/2018/11/737-fcc-pitch-axis-augmentation-command.html addresses several of my questions; however neither these views or how MCAS works is substantiated.
Of interest, and also unofficial;-
The Mach trim, Speed trim, and MCAS commands should probably be inhibited while only one sensor or one FCC is available. In each case, pilot awareness of the loss of augmentation may be the safest course of action.
A decision to revert to a single channel mode, if dual channel is not available, must balance the benefit of augmentation against the potential for false commands, and where the false commands may be persistent.

gums
16th Dec 2018, 17:03
Salute!
@jimtx et al
from jim:

wouldn't you think that that amount of trim would not be to prevent an actual reversal of pitch input but just an adjustment to the stick force linearity. Or can you say that that amount of initial MCAS trim is to counteract an actual nose up pitch tendency?

Looks like it could be both, and I use a graphic of a real plane to show some stick position/force versus pitch moments. Ignore the order of magnitude of the envelope compared to a B737 or AB320/330 or any airliner.
Aero is aero, and the graph basically shows the position of the horizontal tail, which is one piece ( each side, and interchangeable), all-moving as this type of plane has used since Yeager discovered loss of elevator once supersonic. It is also used on various commercial planes. Extremely effective for pitch control compared to fixed stab and elevator configuration when subsonic.


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/654x334/pitch_moment_da785f324f59655f6958230a8698d1ef8b794217.jpg

As you can see, the neutral stick gradient is very linear from -40 deg to +50. But it passes into neutral moment territory and then proceeds to nose up command. As with the 'bus, we commanded gee, not AoA or actual stab angle. Hal moved what it had to in order to achieve our command or our trim cmd ( that's right, besidees the stick switch we had an old-fashioned pitch roller that commanded a gee). So as we got to the 20 deg territory our stab had to be trimmed more and more for nose down. If we beat the AoA limiter by climbing very steep and running outta air molecules that the stab could use, then we might "fall" into the dreaded deep stall area where we had no more nose down pitch authority, but still had nose up authority. Hal had already disconnected our stick as we were above 30 deg AoA, no kidding, and put in anti-spin rudder and kept wings fairly level as we descended at 10,000 ft/minute, heh heh.. They had to add a pitch override feature, which is what the 'bus has when it reverts to "direct" law. Could then use the up pitch and "rock the beast outta the stall.

My premise is that the neutrtal stick/yoke pitch crossover point is the "characteristic" I think Boeing was trying to deal with. Although I do not believe the plane has a true deep stall condition, it might be capablke of reaching the "deeply stalled" condition we saw on that 'bus when the crew kept pulling back on the stick(s) and Hal didn't crank the stab a bit to "help" due to the trim mechanization.

In short, the new motor mounts, length to the stab/elevator and maybe some other slight changes in stab position/size/camber affected the basic 737 aero pitch coefficient. Hence MCAS, which is like my old FLCS that moved things even when I just had the stick "neutral" ("hands free", no pressure).

But right now, it appears to me that Boeing and the FAA addressed a glitch in the part 25 certification process, really not affecting the safety of real world ops, with a pencil whip software solution that unfortunately killed people when a flight crew and their maintenance people could not trouble shoot properly.

I agree completely, jimtx, and would like to know what other pilots feel about this view. I would especially like to see what the 'bus drivers feel.

Gums sends...

gums
18th Dec 2018, 23:49
Guess the thread has gone "thread bare".
PUNT!
Gums....

KRUSTY 34
19th Dec 2018, 08:31
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
19th Dec 2018, 14:39
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
15th Mar 2019, 11:46
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
15th Mar 2019, 21:14
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
16th Mar 2019, 03:40
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
16th Mar 2019, 08:58
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
16th Mar 2019, 10:24
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
16th Mar 2019, 10:51
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
16th Mar 2019, 21:15
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
16th Mar 2019, 22:54
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
17th Mar 2019, 16:00
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
17th Mar 2019, 16:24
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
17th Mar 2019, 16:43
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
17th Mar 2019, 16:46
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 (https://www.seattletimes.com/business/boeing-aerospace/faa-evaluates-a-potential-design-flaw-on-boeings-737-max-after-lion-air-crash/), 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
18th Mar 2019, 02:56
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
18th Mar 2019, 05:35
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
18th Mar 2019, 05:41
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
18th Mar 2019, 08:34
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
18th Mar 2019, 11:14
@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.

flyingfalcon16
18th Mar 2019, 22:08
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.



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.

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.


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.

flyingfalcon16
18th Mar 2019, 22:17
@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.

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?

flyingfalcon16
19th Mar 2019, 02:38
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..

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?

FGD135
19th Mar 2019, 04:25
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...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.

Or put very shortly: MCAS effects the pitch attitude of the plane.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.

flyingfalcon16
19th Mar 2019, 06:15
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.
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.

scifi
19th Mar 2019, 16:37
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..

GordonR_Cape
19th Mar 2019, 16:52
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.

FCeng84
19th Mar 2019, 18:30
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.

FGD135
20th Mar 2019, 07:31
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.No, the MCAS is absolutely not there for the stall scenario. It is to improve manual handling characteristics at flight near the stall. It is not for stall recovery. If Boeing had wanted that, they would have installed a “stick pusher”, which is a simple system, common to many airliners.

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.No, it is not trying to pull the nose down. It is trying to change how the plane feels when manually flown at speeds near the stall.

It can pull the nose down, as we now know, but that was never Boeing’s intention.

flyingfalcon16
20th Mar 2019, 10:01
No, the MCAS is absolutely not there for the stall scenario. It is to improve manual handling characteristics at flight near the stall. It is not for stall recovery. If Boeing had wanted that, they would have installed a “stick pusher”, which is a simple system, common to many airliners.

No, it is not trying to pull the nose down. It is trying to change how the plane feels when manually flown at speeds near the stall.

It can pull the nose down, as we now know, but that was never Boeing’s intention.
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.

FGD135
20th Mar 2019, 13:30
MCAS is qualified in many sources as fundamentally a stall prevention system.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.

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".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 (http://www.b737.org.uk/mcas.htm)

​​​​​​​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.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.

Arydberg
20th Mar 2019, 20:18
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

flyingfalcon16
20th Mar 2019, 20:20
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:

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.

"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.

gums
20th Mar 2019, 20:51
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...

flyingfalcon16
20th Mar 2019, 20:53
Why would Boeing have adopted the new and unproven MCAS when they could have simply used a stick pusher?

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

scifi
20th Mar 2019, 22:56
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.

gums
20th Mar 2019, 23:38
Salute!

from scifi:

Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.

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...

flyingfalcon16
20th Mar 2019, 23:47
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.

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.

john_tullamarine
21st Mar 2019, 00:27
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 ....

GordonR_Cape
21st Mar 2019, 00:44
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 have been told more than once that this kind of statement contains several misperceptions. I leave it to others to provide the correct answer.

HighWind
21st Mar 2019, 06:42
Is a software fix sufficient? Is the B737M flight controllers reliable enough for the task?
The software changes to the B737M made the detection of a ’trim runaway’ failure mode much more difficult since it is behavior is changed, and masked by other faults and noises.
It may also have increased the frequency of the trim runaways.

If the original DFMEA/Design Risk Assessment had the conclusion that a ’trim runaway’ is something that is easy for the pilots pilots to handle, then the safety case is limited to providing reliable ’cut-out’ switches (And some training).

If the conclusion in the new DFMEA/Design Risk Assessment is changed since it can’t be expected that the pilots reliable can detect and isolate the fault, then this drive a significant change to the hardware (and software) requirements.
It is not enough to have redundant sensors as the voting between flight controllers can also fail. The actuator and its electronics as well as the network may also fail.To me it seems that the THS control is moving in to the realm of a software controlled primary control surface (since if can overpower the muscle strength of pilots if not isolated fast enough). In essence requiring a FBW like system with full byzantine fault tolerance.
What are the capabilities of the existing THS control system:
-Fail safe by means of lock-step operation?
-Voting between fail-silent replicas with byzantine fault tolerance?

jan99
21st Mar 2019, 12:19
I would suggest that attempting to comply with a 'feel' requirement by moving a powerful control surface is bad engineering from the start.

GordonR_Cape
21st Mar 2019, 14:23
Forgive my curiosity as a complete outsider. Do pilots ever refer to the trim scale on the pedestal, other than before takeoff? Is is part of any scan, or checklist?

FGD135
22nd Mar 2019, 03:31
I would suggest that attempting to comply with a 'feel' requirement by moving a powerful control surface is bad engineering from the start.Agreed. Terrible engineering, and Boeing would have known that. The Boeing engineers would be the first to admit that.

But, there was no other way open to them for creating a pitch-down moment, without major airframe modifications. It must have been a miserable time at Boeing when, after realising the MAX needed pitch-down moment(s) at high alpha, it was decreed that they would have to come from the stabiliser. The investigations into certification will surely reveal the opposition within Boeing to taking this route. Will be interesting!

gums
22nd Mar 2019, 04:47
Salute!

Some good aero stuff being discussed.
I have wondered since November if using some variation of aerodynamically loaded slats ( F-100 and others) on the nacelles or even the leading edge of the wing could have moved the center of pressure further aft at high AoA to keep the pitch moments O.K.
I realize that the goal is to have the trim drag at a minimum for cruise AoA, but once up there near stall you would not be worried a lot about fuel economy, ya think?

Gums wonders....

FGD135
22nd Mar 2019, 06:38
I realize that the goal is to have the trim drag at a minimum for cruise AoA, but once up there near stall you would not be worried a lot about fuel economy, ya think?Agreed, gums.

Something that would give the pitch-down moment at high alpha, but would not add drag in the cruise. I haven't come up with anything yet. The closest I have come is the idea of some surfaces protruding from the new tail cone. Having such a long arm would mean they wouldn't have to be too big and ugly.

WHBM
22nd Mar 2019, 12:41
If I can ask a hopefully not trivial question ... for a long time there have been not only stick shakers but, by extension, stick pushers. Was the Trident the first with this ? And how does MCAS differ in its principle to a stick pusher, apart from operating on a separate control, the trim rather than the yoke ? And if it's established, why did Boeing do it on a different control rather than push the yoke forward ?

There's an interesting account of dealing with this from 55 years ago here. Compare and contrast with what Boeing have done. "3,500 test stalls" !!!

https://www.flightglobal.com/FlightPDFArchive/1964/1964%20-%200246.PDF

safetypee
22nd Mar 2019, 13:32
WHBM,
MACS relates to stability.
Stick push relates to stall, identification and initial recovery motion.
The 737 stall characteristic is identifiable and has satisfactory recovery action.

gums
22nd Mar 2019, 15:46
Salute!

Granted, Safety, MCAS is not stall prevention or stall recovery. But media and many folks on these very forums still feel that it is.

In all honesty, having the stick get "lighter" due an aerodynamic effect at high AoA can be conducive to entering a stall. But I do not believe the 737 pitch moments would be like the VooDoo I flew 50 years ago that got a "light" stick , then went end over end, heh heh.

Gums sends...

Bergerie1
22nd Mar 2019, 17:18
WHBM,

safetypee and gums are absolutely correct.

When we did CofA air tests on VC10s, we always had a third AoA vane mounted on a plug that replaced the glass of one of the cabin windows and an AoA indicator connected to it mounted on the pilot's instrument panel coaming. During stall tests, one crew member was required to watch it like a hawk and shout if it approached (I think) 16degs before the stick pusher pushed.

Arydberg
23rd Mar 2019, 00:09
Could anyone tell me the difference between the MCAS system and a runway horizontal trim condition.

FCeng84
23rd Mar 2019, 05:24
Could anyone tell me the difference between the MCAS system and a runway horizontal trim condition.

Are you meaning MCAS with the AOA signal it is using failed high? Try searching on my username. I have made a number of posts that describe how MCAS works nominally and if its AOA signal is failed high. Good related info in both the Rumors and News and the Tech Log sections of PPRUNE

flyingfalcon16
23rd Mar 2019, 06:52
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.

So I can affirm that this forum is heavily curated. I've sent multiple posts days ago. I'd say 80%+ of my posts never get posted. They are relevant and I really have no idea why they are not posted. They certainly are not "left field inappropriate posts by new folk and, more importantly, spammers". Not sure if this post will make it but users should know that this is certainly a heavily curated forum. I believe gums was also concerned about heavy moderation (another thread) and a mod clarified the policy.

flyingfalcon16
23rd Mar 2019, 08:33
In fact, I would word it this way. Trim effects the pitch of the aircraft, which, in turn, effects the speed.

MaverickSu35S
23rd Mar 2019, 10:12
... AI avoids that problem. For good or evil, no?


You said it well when you said: "For good or evil". In fact we will see that it's NOT the "good" that will win in this AI (artificial intelligence) fantasy, but the evil itself. We just had 2 awful answers (the 2 accidents of the MAX) from our subject regarding how does the plane try to automatically (apparently intelligent) trim the nose down continuously until "it believes" that the stall has been eliminated. One A330 had a cruise incident while another A330 and a belly (crash) landing due to "automation gone wild" encounters! This case showed one more glimpse of what it's like to lay your belief in automation.

If safety is still to be no.1, then automation should only be there to relax the pilot, NOT turn him into a donkey that doesn't know how to fly anymore after so much "auto" in his head. The pilot remains a pilot, if he still wants to be called that way anymore.

Regards!

FCeng84
23rd Mar 2019, 10:59
You said it well when you said: "For good or evil". In fact we will see that it's NOT the "good" that will win in this AI (artificial intelligence) fantasy, but the evil itself. We just had 2 awful answers (the 2 accidents of the MAX) from our subject regarding how does the plane try to automatically (apparently intelligent) trim the nose down continuously until "it believes" that the stall has been eliminated. One A330 had a cruise incident while another A330 and a belly (crash) landing due to "automation gone wild" encounters! This case showed one more glimpse of what it's like to lay your belief in automation.

If safety is still to be no.1, then automation should only be there to relax the pilot, NOT turn him into a donkey that doesn't know how to fly anymore after so much "auto" in his head. The pilot remains a pilot, if he still wants to be called that way anymore.

Regards!

Automation and Artificial Intelligence are not one and the same. With automation control systems automatically respond in a predetermined manner to a combination of airplane state and pilot input. With Artificial Intelligence the control system is designed to “think” on its own and come up with control actions that have not necessarily been deterministically defined.

There are plenty of examples of automation in the current state of the art in commercial aviation. Many of the performance improvements we have realized over the past generation would not have been possible without automation. That ship has sailed and there is no turning back to pre-automation days in aviation.

On the other hand, while I have no doubt that AI will find itself playing a greater and greater role in commercial aviation over the coming years, we need to proceed with caution down that road. It is one thing to design an AI system to drive a car where you always have the option of slowing and stopping on the side of the road if something unexpected comes up that the AI determines it is not ready to sort out. In an airplane it is quite another story when the unexpected happens. There are no pull over and stop options between takeoff and landing.

Mr Optimistic
23rd Mar 2019, 11:03
Pax. It's been a while since I had anything to do with fault trees but presumably part of the technical evidence underpinning the design solution to MCAS would have started by the need to show that a failure outcome for the system was less than some low, and acceptable, probability threshold. This logic would have a starting event which represented the need for the system to intervene, the aircraft being in the dynamic state that challenged it's stability for which help was needed. That logic sequence must have given the right numbers even when subevents like aoa sensor failure were included.
However the next page of the analysis, and a new fault tree, would be considered, this to cover eventualities on the system intervening when it shouldn't. AOA faults would be one initiating event and would be ascribed a probability of occurrence. To my layman mind, this would be more probable than the starting event in the first tree, ie AOA failure is more likely than the aircraft finding itself in a bad dynamic state.
What I can't understand is if this were true then the rest of the fault tree after AOA failure must also get to an acceptably low probability despite the fact that it is the more likely event chain to start with. Subsequent mitigations, such as the intervention of a trained crew, would have to have a correspondingly low failure rate to make up the numbers.
Can't see the numbers for that working out.
Would love to see the workings out but don't suppose we ever shall.

john_tullamarine
23rd Mar 2019, 12:05
this is certainly a heavily curated forum

One needs to accept that it can take a little while for a mod to review and pass a new post to the thread. Once again, I think I can say that we don't unduly constrain posting to this or any other thread in TL.

Alchad
23rd Mar 2019, 12:31
I posted this a couple of days on the Rumours thread where it was suggested it might be more relevant and of interest here, so in case it is.....

Ethiopian ET302 similarities to Lion Air JT610Reports from Ethiopian investigators have implicated the same Angle of Attack (AoA) sensor malfunction that was observed on Lion Air. Lion Air captain AoA sensor read about 22 degrees higher than the First Officer AoA sensor (a large bias error). Initial assessment of Lion Air AoA failure modes did not reveal any obvious electrical malfunction that could create the bias. The simplest explanation was that the AoA vane had been bent, causing a gross aerodynamic offset in the readings. If ET302 encountered the exact same offset, with the likelihood of it being bent exactly the same way not being conceivable, some other factor must be in play. For example, the ARINC 429 representation of AoA uses two's complement fraction binary notation (BNR). It is interesting to note that bit 26 represents 22.5 degrees which would be the bit "flipping" between the Captain and F/O AoA values (all other bits would match). Is it possible that the ARINC 429 word is getting corrupted (software defect)? If the ET302 offset was something like 20 or 24, this theory falls apart.

Full post

https://www.satcom.guru/2019/03/ethi...lion.html#more (https://www.satcom.guru/2019/03/ethiopian-et302-similarities-to-lion.html#more)

Alchad
23rd Mar 2019, 12:47
Link..

https://leehamnews.com/2019/03/22/bjorns-corner-the-ethiopian-airlines-flight-302-crash-part-2/#more-29712

The article is not really relevant to the precise topic of this thread as it speculates on the possibility of elevator "blowback" being a factor in the crashes. HOWEVER, I thought the following exchange of comments on the article might be of interest, particularly the emboldened bit in the reply by poster Transworld. As someone who has nothing to do with the industry it was a fascinating piece of information, apologies if it's something professionals knew from day 1.

Knuffi
March 22, 2019 (https://leehamnews.com/2019/03/22/bjorns-corner-the-ethiopian-airlines-flight-302-crash-part-2/#comment-261377) Does this mean that – when the FDR shows the left AoA sensor having 25deg and the right 5deg – the AoA vanes really where in this position? Or would you still consider a failure in how the data was processed by the flight computer yielding the difference?

Reply (https://leehamnews.com/2019/03/22/bjorns-corner-the-ethiopian-airlines-flight-302-crash-part-2/#comment-261377)TransWorld
March 22, 2019 (https://leehamnews.com/2019/03/22/bjorns-corner-the-ethiopian-airlines-flight-302-crash-part-2/#comment-261518) Presently there is no way to tell.If the flopped to full up on less than takeoff speed, both should be flopped the same.How these work, what portions they go to when no significant airflow is all unknown to other than ATR pilots. Possible to even have a test command drifting around in the systemThe two comput3ers are programed by two different teams (its a method of ensuring no code is written duplicated to the computer that is identical so their is not a dual failure under the same circumstances) that said, it seems the two issue were both pilot side so there may be something in that coding on that side. Swap to the other side for control and it should go away but Lion it did not, so just more questions.

Alchad

Le Flaneur
23rd Mar 2019, 13:31
Attached are some notes I made in mid-Jan 2019 regarding the MCAS architecture and Lion Air crash

gums
23rd Mar 2019, 14:45
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...

scifi
24th Mar 2019, 00:09
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 (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.

jimjim1
24th Mar 2019, 02:50
Attached are some notes I made in mid-Jan 2019 regarding the MCAS architecture and Lion Air crash

I am converted - we need a Like button.

flyingfalcon16
24th Mar 2019, 08:57
this is certainly a heavily curated forum

One needs to accept that it can take a little while for a mod to review and pass a new post to the thread. Once again, I think I can say that we don't unduly constrain posting to this or any other thread in TL.

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.

flyingfalcon16
24th Mar 2019, 09:02
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...

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?

john_tullamarine
24th Mar 2019, 10:00
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.

flyingfalcon16
24th Mar 2019, 10:09
Would you happen to know the max nose down trim angle on the 737 NG and MAX?

Alchad
24th Mar 2019, 12:55
I am converted - we need a Like button.

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

alf5071h
24th Mar 2019, 13:37
ff16, #148,
in my days (a long time ago), after the initial exploratory test flying, subsequent flights were jointly crewed with tps and training captains. This involved systems evaluation, normal and emergency procedures and handling, performance, cg range, auto-flight, instruments, etc. Some flights would have been to collect data specific to the design and manufacture of training simulators.
Later, training captains could captain production test flights depending on experience and the aircraft’s certification status.

In addition to CAA tp validation tests, operations inspectors and training staff flew on flights to evaluate workload, and practicality in service.

DaveReidUK
24th Mar 2019, 16:38
Would you happen to know the max nose down trim angle on the 737 NG and MAX?

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.

scifi
25th Mar 2019, 10:26
.
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.
..

Alchad
25th Mar 2019, 14:11
"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.org-vbulletin/530x800/ngpanel_e14aa53b5a020842d6c9596ce6f807b7978fc4da.jpg

737 MAX


https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/530x800/maxpanel_ae917efad0540f26d3c00e125bf3694ae6cdebb5.jpg

infrequentflyer789
25th Mar 2019, 17:10
"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!

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:
confusion regarding switch function [...] related to ‘poor training and even poorer documentation

The First Officer offered to hit the SEL function in flight, to test it out, but I thought something irreversible or undesirable might happen (not knowing what we were actually selecting), so we did not try it out in flight.

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.

GordonR_Cape
25th Mar 2019, 17:14
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.

Alchad
25th Mar 2019, 18:01
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 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 (left=https://www.pprune.org/members/429011-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.

GordonR_Cape
25th Mar 2019, 18:55
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.

BobM2
26th Mar 2019, 15:58
1. On the NG there are only two inputs for electrical trim, the autopilot and the yokes. Each has their own switch, wired in parallel..
Where does STS come from?

GordonR_Cape
26th Mar 2019, 16:42
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?p=10303984&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).

SteinarN
26th Mar 2019, 21:52
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.

Loose rivets
27th Mar 2019, 02:52
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.

FCeng84
27th Mar 2019, 03:02
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!

jimjim1
27th Mar 2019, 06:53
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 :-
https://www.pprune.org/tech-log/615709-737max-stab-trim-architecture-9.html#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.

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:).

jimjim1
27th Mar 2019, 07:13
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.org-vbulletin/771x395/post999999_e3991ac68448a206e1ed93a6958fd45f36332e15.png

jimjim1
27th Mar 2019, 14:28
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:-
10430909
or
Your desired label!

Looks like this:-
10430909
or
Your desired label!

jimjim1
27th Mar 2019, 14:40
Lastly - phew!
You can see what is actually in any post by quoting it and pressing "<> Source" in the Edit tool that opens.

ATC Watcher
27th Mar 2019, 16:11
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. .

infrequentflyer789
27th Mar 2019, 18:33
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/619326-boeing-advice-aerodynamically-relieving-airloads-using-manual-stabilizer-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...

ATC Watcher
28th Mar 2019, 08:32
infrequentflyer789 (https://www.pprune.org/members/214043-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.

Loose rivets
29th Mar 2019, 00:25
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/ethiopian-et302-similarities-to-lion.html

Loose rivets
29th Mar 2019, 02:32
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.

Arydberg
29th Mar 2019, 04:45
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.

IFixPlanes
29th Mar 2019, 06:55
...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.

GordonR_Cape
29th Mar 2019, 07:50
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.

IFixPlanes
29th Mar 2019, 08:23
I do not think...
https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1278x850/b737_max_columnswitchingmodule_2cf8055695fc24da4521cb26ab661 afac41f93a3.jpg

GordonR_Cape
29th Mar 2019, 12:54
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.

GordonR_Cape
29th Mar 2019, 16:39
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).

Arydberg
29th Mar 2019, 16:42
But I thought that defective AoA was replaced before the second flight.

IFixPlanes
29th Mar 2019, 17:19
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...

hans brinker
29th Mar 2019, 17:50
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.

IFixPlanes
29th Mar 2019, 18:00
hans brinker
Yes, English is not my first language.
Try to read my posting in correlation to:
...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: what on my posting is not correct?

GordonR_Cape
29th Mar 2019, 19:54
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.

hans brinker
29th Mar 2019, 21:02
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?

MCAS ...

If the value of an AOA improves, the FCCs stop their MCAS signal - t...

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.

Loose rivets
30th Mar 2019, 01:40
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.

megan
30th Mar 2019, 04:46
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.

GordonR_Cape
30th Mar 2019, 07:00
They are, and they may find some, but the KC-46 still allows the control column cut-out switches to override MCAS inputs, by pulling back on the yoke. I would sure like to see that back on the MAX. Yes, the pilots could override the (what are we calling it?) stall avoider...which would be more appropriate than letting it drive the plane into the ground when it is activated erroneously.

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.

Loose rivets
31st Mar 2019, 01:31
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/questions/58798/why-doesnt-a-full-yoke-deflection-also-change-the-stabilizer-trim-on-the-boeing

infrequentflyer789
1st Apr 2019, 17:21
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


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

Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.

So, that process would be:

1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;

Then repeat. Blip, relax, assess. Blip, relax, assess.

But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.

But it seems that trimming a large, THS aircraft that way can be fatal.

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

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

737 Driver
6th Apr 2019, 16:43
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.

Dave Therhino
11th Apr 2019, 05:28
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.

DaveReidUK
11th Apr 2019, 08:11
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.

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 (http://www.737ng.co.uk/B_NG-Flight_Controls.pdf)

DaveReidUK
11th Apr 2019, 15:44
The MCAS trim speed (.27 degrees per second I believe) was set specifically for this function alone. This is faster than the original design spec, but for some reason it was increased before final release.

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

737 Driver
11th Apr 2019, 16:16
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.

infrequentflyer789
11th Apr 2019, 18:10
Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?

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:

When the flaps are up, the switch is open and low speed trim is engaged. Low speed trim moves the stabilizer at 0.2 units per second. When the flaps are not up, the switch closes and sends a signal to the stabilizer trim actuator to engage high speed trim. High speed trim moves the stabilizer at 0.4 units per second.
During autopilot operation, the stabilizer trim speed changes. When the flaps are up, the low speed trim is 0.09 units per second. When the flaps are not up, the high speed trim is 0.27 units per second

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...

737 Driver
11th Apr 2019, 18:45
Thanks for that info. I stand corrected.

1stspotter
12th Apr 2019, 13:25
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