Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

737MAX Stab Trim architecture

Tech Log The very best in practical technical discussion on the web

737MAX Stab Trim architecture

Old 14th Dec 2018, 06:43
  #61 (permalink)  
 
Join Date: Mar 2006
Location: England
Posts: 774
This Thanks for the clear and well considered resume, FCeng84 #58,
‘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 ?
PEI_3721 is offline  
Old 14th Dec 2018, 16:31
  #62 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 76
Posts: 936
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!!
gums is offline  
Old 14th Dec 2018, 17:20
  #63 (permalink)  
 
Join Date: Feb 2009
Location: Seattle
Posts: 362
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.
FCeng84 is online now  
Old 15th Dec 2018, 01:37
  #64 (permalink)  
 
Join Date: Mar 2005
Location: N/A
Posts: 2,561
As an aside it seems they don't have the cash to recover the CVR.

https://www.avweb.com/eletter/archiv...4211-full.html
megan is offline  
Old 15th Dec 2018, 03:16
  #65 (permalink)  
 
Join Date: Feb 2001
Location: Singapore
Posts: 89
Boeing AD

AD NUMBER: 2018-23-51

PRODUCT: All Boeing Model 737-8 and -9 airplanes.



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
ITman is offline  
Old 15th Dec 2018, 03:29
  #66 (permalink)  
 
Join Date: Mar 2005
Location: N/A
Posts: 2,561
AD link

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/fe8237743be9b8968625835b004fc051/$FILE/2018-23-51_Correction.pdf
megan is offline  
Old 15th Dec 2018, 05:13
  #67 (permalink)  
 
Join Date: Mar 2014
Location: WA STATE
Age: 73
Posts: 1
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 ???
CONSO is offline  
Old 15th Dec 2018, 07:06
  #68 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 9,631
(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:

DaveReidUK is offline  
Old 15th Dec 2018, 07:51
  #69 (permalink)  
 
Join Date: Mar 2006
Location: England
Posts: 774
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.


CONSOwhy 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.
PEI_3721 is offline  
Old 15th Dec 2018, 10:54
  #70 (permalink)  
 
Join Date: Sep 2018
Location: Laredo, TX
Posts: 84
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?
jimtx is offline  
Old 15th Dec 2018, 12:54
  #71 (permalink)  
 
Join Date: Mar 2006
Location: England
Posts: 774
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



PEI_3721 is offline  
Old 15th Dec 2018, 13:55
  #72 (permalink)  
 
Join Date: Jun 2014
Location: Chocolatetown
Age: 58
Posts: 83
Originally Posted by jimtx View Post
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-s...em-mcas-jt610/

climber314 is offline  
Old 15th Dec 2018, 14:17
  #73 (permalink)  
 
Join Date: Mar 2006
Location: England
Posts: 774
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 ?




PEI_3721 is offline  
Old 15th Dec 2018, 15:52
  #74 (permalink)  
 
Join Date: Sep 2018
Location: Laredo, TX
Posts: 84
Originally Posted by climber314 View Post
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.
jimtx is offline  
Old 15th Dec 2018, 16:42
  #75 (permalink)  
 
Join Date: Jun 2014
Location: Chocolatetown
Age: 58
Posts: 83
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?

Last edited by climber314; 15th Dec 2018 at 22:43.
climber314 is offline  
Old 16th Dec 2018, 01:13
  #76 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 76
Posts: 936
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...

Last edited by gums; 16th Dec 2018 at 01:19. Reason: grammar, mainly
gums is offline  
Old 16th Dec 2018, 05:52
  #77 (permalink)  
 
Join Date: Sep 2018
Location: Laredo, TX
Posts: 84
Originally Posted by gums View Post
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.
jimtx is offline  
Old 16th Dec 2018, 06:14
  #78 (permalink)  
 
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 76
Posts: 1,453
Originally Posted by PEI 3721
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.
Machinbird is offline  
Old 16th Dec 2018, 11:38
  #79 (permalink)  
 
Join Date: Mar 2006
Location: England
Posts: 774
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-...n-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.









Last edited by PEI_3721; 16th Dec 2018 at 15:37.
PEI_3721 is offline  
Old 16th Dec 2018, 16:03
  #80 (permalink)  
 
Join Date: Jun 2009
Location: florida
Age: 76
Posts: 936
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.




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 is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.