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Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

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Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

Old 1st Apr 2019, 13:10
  #501 (permalink)  
 
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Hello ! ! !
it happened more then one might think.
Remember, the issue on the flight BEFORE the Lion crash. No crash, but same issue.

Certification is on the same level as selling airplanes.
Deals are made at a good meal in the Moulins Rouge.(or equivalent)
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Old 1st Apr 2019, 14:58
  #502 (permalink)  
 
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Originally Posted by Vilters View Post
Hello ! ! !
it happened more then one might think.
Remember, the issue on the flight BEFORE the Lion crash. No crash, but same issue.

Certification is on the same level as selling airplanes.
Deals are made at a good meal in the Moulins Rouge.(or equivalent)
That was the same airframe with the same fault on its previous flight. The reason it didn't crash was the Batik Airlines jump seater apparently told the Lion Air crew to switch off the Stab Trim.
Just think Stab trim starts misbehaving it can be switched off in seconds and as shown by the Lion Air Flight the aircraft can continue and land - that is what Boeing expected _all_ pilots would do if there was a problem.
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Old 1st Apr 2019, 17:26
  #503 (permalink)  
 
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Originally Posted by Ian W View Post
That was the same airframe with the same fault on its previous flight. The reason it didn't crash was the Batik Airlines jump seater apparently told the Lion Air crew to switch off the Stab Trim.
Just think Stab trim starts misbehaving it can be switched off in seconds and as shown by the Lion Air Flight the aircraft can continue and land - that is what Boeing expected _all_ pilots would do if there was a problem.
Perhaps, just perhaps, the flight crew were being distracted by all the other stuff going on and to them the wheel trim didn't seem to be doing much wrong? Maybe a third pilot should be mandated in future in all non US locations since we all know US pilots are super experienced and knowledgeable!

More seriously I wonder whether Boeing thought through what the cockpit environment would be like for this type of failure with a number of different alarms and failures being indicated by systems.
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Old 1st Apr 2019, 18:48
  #504 (permalink)  
 
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Originally Posted by SamYeager View Post
Perhaps, just perhaps, the flight crew were being distracted by all the other stuff going on and to them the wheel trim didn't seem to be doing much wrong? Maybe a third pilot should be mandated in future in all non US locations since we all know US pilots are super experienced and knowledgeable!

More seriously I wonder whether Boeing thought through what the cockpit environment would be like for this type of failure with a number of different alarms and failures being indicated by systems.
If you have read my other posts, I have raised the issue of the human factors nightmare that the cockpits of modern aircraft are after a minor fault. The same happened in AF447 from 'just' an iced pitot. There should be more concentration on the cacophony of alarms from just one minor problem as the alarms themselves become a problem in their own right.

That does not alter the issue that the one failure that every 737 pilot worldwide was talking about for weeks was MCAS and switching off Stab Trim. A mandatory AD was issued, the entire break down of what could happen was in open discussion in even the tabloid press. MCAS was no longer a mystery. Yet after all that a 737 crew with the same issues _still_ did not switch off Stab Trim?
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Old 1st Apr 2019, 18:50
  #505 (permalink)  
 
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Originally Posted by ecto1 View Post
MCAS (a proper one) would need to be active at any pitch angle and vertical speed. One can pull to a stall while going up, level or down..
OK, that is reasonable.
How about this envelope protection cut-out: Turn off MCAS when airspeed is greater than 200 knots.
That is simple, and uses different sensor sources (pressure sensors on the nose) to disable MCAS which works off AoA vanes.
Ultimately the decision by Boeing to simply turn off MCAS when the dual AoA vanes disagree may be the best approach.
If it was me, I'd disable MCAS for those high (>200 knots should do it) airspeeds, and also compare the two AoA vanes to computed Inertial Alpha (pitch-gamma) as a sanity check to find the bad AoA vane when AoA disagrees happen.

Originally Posted by Ian W View Post
If you have read my other posts, I have raised the issue of the human factors nightmare that the cockpits of modern aircraft are after a minor fault. The same happened in AF447 from 'just' an iced pitot. There should be more concentration on the cacophony of alarms from just one minor problem as the alarms themselves become a problem in their own right.

That does not alter the issue that the one failure that every 737 pilot worldwide was talking about for weeks was MCAS and switching off Stab Trim. A mandatory AD was issued, the entire break down of what could happen was in open discussion in even the tabloid press. MCAS was no longer a mystery. Yet after all that a 737 crew with the same issues _still_ did not switch off Stab Trim?
Well, they somehow didn't get the memo. Seems impossible they didn't know about the trim cutout switches solving this issue.

About the Ian W's assertion of the "cacophony" of alarms, I do wonder if there should be one big fat red button (OK, with a shield over it) that turns ALL flight control surface automation OFF, one big master cut-out, which tells the pilots that when they hit it, they are now moving the control surfaces fully manually, trim and all..... Sort it out when you land, not diving toward the ground while flipping through checklists.
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Old 1st Apr 2019, 18:53
  #506 (permalink)  
 
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Originally Posted by Vilters View Post
Certification is on the same level as selling airplanes.
Deals are made at a good meal in the Moulins Rouge.(or equivalent)
And we have yet another farcical post regarding certification

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Old 1st Apr 2019, 22:43
  #507 (permalink)  
 
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Originally Posted by QuagmireAirlines View Post
How about this envelope protection cut-out: Turn off MCAS when airspeed is greater than 200 knots.
That is simple, and uses different sensor sources (pressure sensors on the nose) to disable MCAS which works off AoA vanes.
Ultimately the decision by Boeing to simply turn off MCAS when the dual AoA vanes disagree may be the best approach.
If it was me, I'd disable MCAS for those high (>200 knots should do it) airspeeds, and also compare the two AoA vanes to computed Inertial Alpha (pitch-gamma) as a sanity check to find the bad AoA vane when AoA disagrees happen.
I am not a pilot, but some of the suggestions on this thread seem like a contortionist trying too hard. It is perfectly possible to stall an aircraft at high speed and high altitude (ask the crew of AF447). Adding all sorts of flight envelope restrictions and inertial data to inhibit MCAS, does little to add safety, and creates the potential for new modes of failure. The proposed changes put forward by Boeing at least seem clear and coherent, even if not everyone agrees with them.
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Old 1st Apr 2019, 23:29
  #508 (permalink)  
 
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QuagmireAirlines
Quote:
Originally Posted by ecto1 MCAS (a proper one) would need to be active at any pitch angle and vertical speed. One can pull to a stall while going up, level or down..
OK, that is reasonable.
How about this envelope protection cut-out: Turn off MCAS when airspeed is greater than 200 knots.
That is simple, and uses different sensor sources (pressure sensors on the nose) to disable MCAS which works off AoA vanes.
Ultimately the decision by Boeing to simply turn off MCAS when the dual AoA vanes disagree may be the best approach.
If it was me, I'd disable MCAS for those high (>200 knots should do it) airspeeds, and also compare the two AoA vanes to computed Inertial Alpha (pitch-gamma) as a sanity check to find the bad AoA vane when AoA disagrees happen.
MCAS is not a stall protection system. It is there to meet the requirements of 14 CFR 25.173 that the pull force on the control column should be linear with speed and AoA. With the Max the pull force gets lighter as the AoA gets close to stall but it is not a stall prevention device. Apparently, Boeing was more concerned about handling problems at high AoA in steep turns somewhere most pilots would not take a 73.
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Old 2nd Apr 2019, 02:28
  #509 (permalink)  
 
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Confusion about MCAS anti-stall

Originally Posted by Ian W View Post
MCAS is not a stall protection system. It is there to meet the requirements of 14 CFR 25.173 that the pull force on the control column should be linear with speed and AoA. With the Max the pull force gets lighter as the AoA gets close to stall but it is not a stall prevention device. Apparently, Boeing was more concerned about handling problems at high AoA in steep turns somewhere most pilots would not take a 73.
Thanks Ian. This confusion is very widespread in the current general (outside PPRuNe) consensus of current MAX issues. Not only in the general media, but even in articles/content generated by commercial pilots and industry members (including current NG pilots), MCAS is almost ubiquitously described as an antistall system operating somewhat like a stick pusher operating on the stabilizer instead. This is required because of a putative dangerous pitch up tendency at high AoA.

We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.



Probable 737MAX behaviour versus typical media description


This seems to be a double edged sword for Boeing going forward. There is a lot of semi-informed consensus now that the re-engine was totally unacceptable because the unaugmented plane is dangerously unstable, and will remain so even with an improved MCAS2. But demonstrating that the unaugmented plane is fairly safe given pilot knowledge of it's decreased stability margin (over the NG) makes a lethal failure of MCAS1 look even worse, as it the problem it was needed to solve was more certification and sales critical than safety critical.
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Old 2nd Apr 2019, 02:57
  #510 (permalink)  
 
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Oops, the software fix has been delayed by several weeks again. Having been in the business, I know what that means. I think some anonymous tester/code reviewer just potentially saved hundreds of lives.

I'm glad that I never worked on aircraft or other life critical code.
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Old 2nd Apr 2019, 03:11
  #511 (permalink)  
 
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Originally Posted by Water pilot View Post
I think some anonymous tester/code reviewer just potentially saved hundreds of lives.
.
I think what is really going on is the FAA does not want to publicly test its new position in the world of regulatory authorities by seeing which CAAs will return the airplane to service based on an FAA approval alone. Instead, they will wait until at least EASA is also ready to approve it.
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Old 2nd Apr 2019, 03:21
  #512 (permalink)  
 
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It could be so, but the story is that (according to the Washington Post) Boeing has not supplied the software to the FAA and now expects it to take several weeks more. This is after that grand rollout they did last week.

I have been in that situation, one of the few times I ever had the grand pooh bahs of our company in my humble little office. Not comfortable at all but I'm glad that Boeing is doing the right thing apparently.
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Old 2nd Apr 2019, 03:22
  #513 (permalink)  
 
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Originally Posted by LEOCh View Post
We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.
.
The behavior shown in your left plot would be compliant with 14 CFR 25.203(a), which I believe was the requirement that necessitated MCAS. The slope going flat or positive at any point prior to the stall would be non-compliant.
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Old 2nd Apr 2019, 03:32
  #514 (permalink)  
 
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LEOCh
We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.
Thanks for your very informative comments.

The one quoted relates to something that puzzled me over the weekend. Several posters on social media virulently insisted that the MAX is a fundamentally unstable aircraft, which should never have been certified (MCAS notwithstanding).

Protestations to the contrary were met with incredulity, and describing the subtle details of elevator feedback and stabiliser trim deepened this reaction. It was inplied that we were shills, or knew nothing about aerodynamics, or that regulatory compliance was cheating.

I assumed this was just a handful of lunatics, but your comment indicates some widespread circulation of these ideas. The understandable silence from Boeing and the FAA does nothing to dampen the reaction, and they have a major uphill battle ahead.

I hope those 'conspiracy' theories do not extend further, since they seem immune to carefully reasoned discussion. This concern may be justified, given the limited aviation knowledge of the current Senate Transportation Commitee.

It may be that the only way to regain trust and move forward would be an unprecedented level of transparency, not seen in recent PR-driven aircraft launches.
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Old 2nd Apr 2019, 08:57
  #515 (permalink)  
 
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Originally Posted by Ian W View Post
Just think Stab trim starts misbehaving it can be switched off in seconds and as shown by the Lion Air Flight the aircraft can continue and land - that is what Boeing expected _all_ pilots would do if there was a problem.
The trouble is that the AoA issue presents itself to the aircraft in the form of a stick shaker (that is loud enough, in my experience, to mask the sound of the trim wheel) and indications on the PFD of a stall.

Training is now is emphasising:
  • At the first indication of a stall..
  • Reduce the AoA
The poor old pilots presented with this scenario have:

Stall indication -> stall recovery.
The stall detection and airspeed/pitch correlation is wrong. Which to believe?
The violent noise of the stick shaker masks the insipid nose down trimming.

Any fix by Boeing needs to be tested so that the average pilot can cope, given the scenario above. Boeing just accounted for "the trim runs away", which is not the same as stall/recovery/speed confusion/stick shaker cacophony. Boeing have been focusing on making the input to MCAS more reliable, but they really need to address what happens when the input is wrong, and the pilots are presented with a false stall indication.

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Old 2nd Apr 2019, 09:37
  #516 (permalink)  
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Originally Posted by LEOCh View Post
Thanks Ian. This confusion is very widespread in the current general (outside PPRuNe) consensus of current MAX issues. Not only in the general media, but even in articles/content generated by commercial pilots and industry members (including current NG pilots), MCAS is almost ubiquitously described as an antistall system operating somewhat like a stick pusher operating on the stabilizer instead. This is required because of a putative dangerous pitch up tendency at high AoA.

We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.

This seems to be a double edged sword for Boeing going forward. There is a lot of semi-informed consensus now that the re-engine was totally unacceptable because the unaugmented plane is dangerously unstable, and will remain so even with an improved MCAS2. But demonstrating that the unaugmented plane is fairly safe given pilot knowledge of it's decreased stability margin (over the NG) makes a lethal failure of MCAS1 look even worse, as it the problem it was needed to solve was more certification and sales critical than safety critical.
LEOCH, you are absolutely correct.... For the record:

MCAS normalises a stick force gradient that does not fully meet the applicable certification standard § 25.173 Static longitudinal stability, sub paras a through d. It is not a stall prevention mechanism. Stall warning requirements are specified in § 25.207 Stall warning and for handling in § 25.203 Stall characteristics. Compliance with the latter is exhibited in demonstration compliant with § 25.201 Stall demonstration. The lifting body effect of the engines is a non linear effect, at a modest relative inflow angle, they will develop lift, at high angles that lift increment will not occur, and inertial forces will dominate the aircrafts behaviour, weight is still forward of the Cp, the plane will pitch down at the break #.

The non linear stick force gradient issue is not permitted to be so significant that the failure of the augmentation system precludes flight within the operational envelope, and specifically up to the stall. § 25.672 Stability augmentation and automatic and power-operated systems.

In simple terms, the control force in part of the operating envelope, outside of normal flight conditions experienced by the RPT pilot, but within the full flight envelope did not meet the standard that was set half a century ago, in a time where the automation and instrumentation would have made it unacceptable to fly for a period of time an aircraft that had say the same control forces as a Lancair 360, and which are still more applicable to IFR operation than a Pitts or an Extra. The Lancair, Pitts and Extra can easily be flown by instruments, it is just undesirable for long term comfort, and therefore the system safety. To remove the issue, Bill Boeing added the MCAS, which is a variant of the STS that has been there for years on the SLUF, dealing with a similar issue in a small part of the envelope around retracting flaps and initial acceleration, e.g., 3rd segment.

Bingle 1.0 is understandable in part, the guys didn't have much heads up other than the info in the tech log, unless there was a side bar discussion by engineering or the preceding sectors crew in what they found. Bingle 2.0 highlights the fact that a crew that had been briefed on the issue, still had the same sort of outcome. Crew 1.0 did contain the problem for a period, with the Captain frequently retrimming the aircraft against the MCAS input. On handing over the plane to the FO apparently, that trim intervention got lost and the trim ran down under MCAS to a bad outcome.

When we understand why the first crew could not reconcile the stab motion with an effective run away and flick the cut out switches, we will have a better knowledge of humans making decisions under uncertainty, and with cognitive and temporal stressors. When we understand how a crew briefed on the problem emulate the same outcome, then maybe we will be able to have a safer flight deck than we apparently have now.

The stability issue is an irritant, the cure as implemented led to 2 trained crews losing it.

For the news media etc, please stop referring to MCAS and stall prevention, it has precious little to do with stall.

Reference:
§ 25.672 Stability augmentation and automatic and power-operated systems.
If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:

(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.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplane configurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]

# A nacelle develops lift from a mix of normal circulation theory lift, and some vortex structure lift. The main lift effect will arise from the vortex structure, and that is non linear, with a startup at modest AOA of the nacelle, to a peak around 15 AOA or thereabouts, and a slow drop off of lift to AOA around 25. At higher AOA, the vortex structures fail and lift drops off, with drag being the main component affecting the resultant force couples to the aircraft structure. Normal circulation related lift is limited in total derived force. As an aside, the vanes often placed on the nacelles affect the shedding structure off the nacelle with slat/Kruegers extended at modest to high AOA. They result in a substantial recovery of CsubL for the section of the wing behind the nacelle, and give something nice to watch on humid days.

Last edited by fdr; 2nd Apr 2019 at 10:00. Reason: Note on Nacelle Lift
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Old 2nd Apr 2019, 12:06
  #517 (permalink)  
 
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Seems to me that that there is a bit of confusion regarding what constitutes an aircraft's normal envelope of operation. There's what the airframe hopefully gives you - basic flying and stability. Then there's the extended envelope that FBW gives you.

Shouldn't the guiding principle embraced by all relevant FAA /CAA rules be that no commercial aircraft flies , whether passenger or freight, unless it is proven, by documentation and demonstration, that their airframe has been designed to be inherently stable within the design flight envelope and not require "Outside" assistance, such as by the use of FBW, to maintain stability in the basic flight envelope

What's going on at the moment with Boeing suggests that there is a gaping great hole in this basic principle as regards this aircraft, suggesting, if the FAA/CAA rules were properly configured, that the airframe would need to be re-designed so as to enable the aircraft to be fully controllable in normal flying modes and conditions.

If designers wish to add further features which enhance the control and stability, then so be it, provided it can be proved that, in the absence of these external controls, that a normal crew can adequately control the aircraft within its stated design envelope.

N
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Old 2nd Apr 2019, 14:04
  #518 (permalink)  
 
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HundredPercentPlease #522
A perceptive post. An example of point focus on a modification, also the risk of introducing a new hazard due to change: - equipment / training.

However, beware (with others) of introducing an average pilot. The many, unresolvable problems of such a view are deeply rooted in the ‘wishy washy’ language in certification requirements.
The bottom line involves judgement; currently well founded and balanced within joint certification requirements, and spanning many different authorities.

If there is any concern about pilots being able to manage a given scenario, then the system which generates it must be changed.
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Old 2nd Apr 2019, 14:18
  #519 (permalink)  
 
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fdr,

Thank you for the best and most comprehensive description to grace this thread. Like you, I do wish people who post here, and the media, would stop referring to the MCAS as an anti-stall system, when most definitely IT IS NOT.
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Old 2nd Apr 2019, 17:33
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Boeing's MCAS Software Fix

Boeing’s new MCAS software fix will compare the inputs from both AOA sensors. Specifically, the AoA signals coming from the sensor to the ADIRU (Air Data Inertia Reference Unit) on each side of the aircraft will be compared before sending it on to the Flight Control Computer (FCC) to trigger the MCAS function.
• If the AoAs disagree by more than 5.5° when the flaps are retracted, MCAS will be disabled for the flight.
• if the AoAs disagree by more than 10° for over 10 seconds, MCAS will also be disabled for the flight.
• if MCAS is activated, it will only provide one nose down stabilizer command for each elevated AOA event.

This MCAS fix will not stop the stick shaker from being activated on the side where an AoA signal exceeds the threshold for a stall. Therefore, if one of the AoA signals exceeds the threshold for a stall during takeoff, the stick shaker’s vibration and annoying sound will remain active until the aircraft has landed, making it more difficult for the pilot to control the aircraft. The MCAS fix will also not prevent the display of different air speeds and altitudes on the displays of the pilot and F/O as a result of the correction of these values by the AoA signals. This may also cause pilot confusion in control of the aircraft. Finally, if the MCAS function is triggered as a result of one of the above conditions, then MCAS is disabled, leaving control of the aircraft’s pitch attitude unprotected. This can increase the possibility of a stall that MCAS was intended to prevent.

It seems to me that a better fix would be the following. If the stick shaker activates on either side, then the pilot or F/O should immediately de-activate the ADIRU on the side that the stick shaker is active, causing the ADIRU on the non-activated side to be used for both displays, the Flight Control Computer (FCC), and for the MCAS function. The MCAS function should then operate according to the new software fix. This change will assure that the stick shaker is turned off, that the pilot and F/O both receive the same known good display information, and that the MCAS function remains active until it issues its first nose down command. This switching of ADIRU functions should be possible because the ADIRU’s send their data to the displays and to the Flight Control Computer (FCC) to trigger the MCAS function. It may even be possible to automate the switching of the ADIRU’s on the condition that one of the AoA signals exceeds some value indicative of an impending stall. Finally, if both AoA signals exceed some value indicative of an impending stall, then both ADIRU’s and MCAS will be switched off, allowing the pilot to take manual control of the aircraft.

Now, I am not a pilot. But I would like to hear the reaction of B737 MAX pilots to this enhanced MCAS fix. Perhaps it can’t be implemented as soon as Boeing’s latest MCAS fix, which is intended to get the B737 MAX fleet back in the air as quickly as possible. But perhaps it can be implemented in a later software fix which improves upon Boeing’s latest MCAS fix to further help the pilot maintain control of the aircraft.
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