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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.
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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... |
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... |
Originally Posted by gums
(Post 10333403)
Salute Vess!
Sounds like the L39 has only a coolie hat switch on top of the stick, heh ? 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) |
gums said - You can attribute my quotes, but I recognized it right away |
Originally Posted by FCeng84
(Post 10333381)
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.
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jimtx - I would like to help allay your confusion. What are the specific questions you would like to have answered? |
Originally Posted by FCeng84
(Post 10333681)
jimtx - I would like to help allay your confusion. What are the specific questions you would like to have answered? Can the autopilot be put in the MCAS envelope where it is not protected. I don’t think it would mind however. |
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. |
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... |
@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.
Originally Posted by FCeng84
(Post 10333381)
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.
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! |
Originally Posted by gums
(Post 10334085)
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... |
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. |
Originally Posted by FCeng84
(Post 10334393)
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. |
Originally Posted by jimtx
(Post 10334490)
Obviously no airline crew is going to ...
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. |
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. |
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 . . " |
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 |
......
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 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 ? |
Originally Posted by FCeng84
(Post 10335004)
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. |
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 ? |
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. Gotta go now, poof!! |
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. |
As an aside it seems they don't have the cash to recover the CVR.
https://www.avweb.com/eletter/archiv...4211-full.html |
Boeing AD
AD NUMBER: 2018-23-51PRODUCT: All Boeing Model 737-8 and -9 airplanes.https://ci6.googleusercontent.com/pr...9-1214a_tn.jpg 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 = $85 |
AD link
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/fe8237743be9b8968625835b004fc051/$FILE/2018-23-51_Correction.pdf |
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. |
(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. https://cimg0.ibsrv.net/gimg/pprune....7f4ec4cb07.jpg |
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. |
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? |
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 |
Originally Posted by jimtx
(Post 10336224)
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? 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/ https://cimg6.ibsrv.net/gimg/pprune....04a6d422c8.jpg |
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 ? |
Originally Posted by climber314
(Post 10336322)
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. 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. |
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? |
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 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... |
Originally Posted by gums
(Post 10336645)
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... |
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. 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. |
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. |
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? 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....ef8b794217.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. Gums sends... |
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