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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 25th Nov 2019, 05:20
  #4141 (permalink)  
 
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Originally Posted by Rarife View Post
Well, maybe I misunderstood something but what is the difference? It would be like flying with poorly trimmed aircraft. Trimmed nose down, I'm pulling back and aircraft does not climb or descend. I just need more pulling or force to get it "up".
Under MCAS 1.0 the linearity would be preserved, because the system could fire a second time if the pilot continued to pull up. With MCAS 2.0 this repeated triggering can no longer happen during a single event (or so we are told). If the crew continues to pull up (for whatever reason) they are effectively entering test-pilot territory.

The response may be linear the first time, but definitely not for sustained maneuvers. This was true very early in the discussion, and remains so. No half-cooked computer system such as MCAS can be safe, effective and reliable, all of the time.

Loose rivets I think this is more a reaction of disgust than alarm. All the attention is implying that the system is almost ready, and we can soon move forwards.

The recent comments from across the spectrum seem to emphasise that nobody is truly happy with this band-aid/kludge/fix, and that the aircraft/manufacturer/regulator are permanently tainted in the view of inquiring minds.
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Old 25th Nov 2019, 05:22
  #4142 (permalink)  
 
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Cheeky, But Boeing Rolls Out the 737 MAX 10

https://aeronewsglobal.com/boeing-73...but-in-renton/
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Old 25th Nov 2019, 05:26
  #4143 (permalink)  
 
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Complex software, TC and lessons from history

Originally Posted by Zeffy View Post
The TC e-mail plays into the nagging fears and basic distrust that we all have about software, even when carefully designed and thoroughly tested. Is it possible to comprehensively test the new MCAS software for “gotchas” to the point where independent expert opinion warrants it 100% safe? Before you reply, remember the UK NATS SW bug that lay dormant in the Swanwick ATC system for 20 years until activated by the unique set of conditions that caused the system to fail.
https://www.itpro.co.uk/business-int...raffic-control

Last edited by Pinkman; 25th Nov 2019 at 05:37. Reason: Fat fingers
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Old 25th Nov 2019, 06:01
  #4144 (permalink)  
 
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Originally Posted by AerocatS2A View Post
The autopilot is a machine that will apply the required input to achieve the desired result. It doesn’t have expectations or past experience to fool it into doing the wrong thing. It doesn’t care if the “stick forces” change as it applies more elevator, it just does what needs to be done.
While humans can be fooled most competent pilots have been trained to ignore expectations or bodily sensations and control the aircraft by visual determination of attitude and performance to get the attitude and performance they desire. The MAX may have more problems than Boeing is attesting to but if it is only a non linear stick force/gradient at high g and AOA no competent pilot would not be able to compensate. If it is a more severe gradient that might allow you to pull up such as in a windshear escape maneuver and exceed the pitch attitude you wanted that might be a problem. If I am correct and the handling problem is a minor deviation from the Part 25 requirements in some rarely visited regimes then Boeing has let the tail wag the dog from day 1 and that dog ain't gonna stop wagging it's tail. We are just gonna have to say that's the dog waging it's tail and it's ok.
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Old 25th Nov 2019, 07:12
  #4145 (permalink)  
 
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Max 10 does look good ! But I too wonder if it is the right time to unveil it. People are going to wonder if it is safe and whether the first flight is going to be done with flaps extended (MCAS of course).

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Old 25th Nov 2019, 08:30
  #4146 (permalink)  
 
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Originally Posted by kiwi grey View Post
  • Will passengers' travel insurance exclude death and injury cover for carriage on an airframe cleared to fly by the FAA but not by EASA, CAAC, etc.?
In my country standard loan insurances exclude "flight in non-certified aircraft".
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Old 25th Nov 2019, 09:15
  #4147 (permalink)  
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Originally Posted by Fly Aiprt View Post
In my country standard loan insurances exclude "flight in non-certified aircraft".
But certified by who ? you can fly an aircraft certified in country A inside country A but not in B.
Example the Chinese MA-60 . Not certified in the US and EASA land , but in a few countries that bought it have certified it ( or more likely accepted the Chinese certification ) . ; e.g. Sri lanka, Laos, Cameroon ,etc.. . If you fly in one of those in those countries ( and you may have no other choice if you want to fly domestically) you are insured no ?
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Old 25th Nov 2019, 15:48
  #4148 (permalink)  
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Originally Posted by Pinkman View Post
Is it possible to comprehensively test the new MCAS software for “gotchas” to the point where independent expert opinion warrants it 100% safe?
No, of course not. And the same applies to all of the rest of the software and hardware -- on every . . . everything.

But, if MCAS failure/inadvertent activation have potentially catastrophic consequences, it should be certified at DAL-A.
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Old 25th Nov 2019, 16:33
  #4149 (permalink)  
 
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Originally Posted by OldnGrounded View Post
No, of course not. And the same applies to all of the rest of the software and hardware -- on every . . . everything.

But, if MCAS failure/inadvertent activation have potentially catastrophic consequences, it should be certified at DAL-A.
All DAL-A assures is that the software does what the software requirements say it should do, not that those requirements are themselves correct or complete. That's a SYSTEMS requirements issue not a software requirements issue - and indeed, can be thought of as applying to non-software aspects of design as well.
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Old 25th Nov 2019, 16:45
  #4150 (permalink)  
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Originally Posted by Mad (Flt) Scientist View Post
All DAL-A assures is that the software does what the software requirements say it should do, not that those requirements are themselves correct or complete. That's a SYSTEMS requirements issue not a software requirements issue - and indeed, can be thought of as applying to non-software aspects of design as well.
I understand that. And, if MCAS failure had been properly classified as catastrophic (which should be the case for any system with full authority over the H-stab), it would have failed certification at the DAL-A level. If MCAS 2.0 has similar authority, the same applies -- and may apply for other reasons, but we don't really know the 2.0 details, at this point.
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Old 25th Nov 2019, 23:38
  #4151 (permalink)  
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Can a piece of software fall into the 'catastrophic' category? I gather the vane can, (or more correctly the failure of) but that's a physical mechanism.
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Old 25th Nov 2019, 23:46
  #4152 (permalink)  
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Originally Posted by Loose rivets View Post
Can a piece of software fall into the 'catastrophic' category? I gather the vane can, (or more correctly the failure of) but that's a physical mechanism.
Yup, it (actually, the consequences of failure) sure can. See DO-178C . . . stand by . . . decent explanation here.
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Old 26th Nov 2019, 00:29
  #4153 (permalink)  
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Thanks. Popping open the diagram is revealing.

It's easy to see how self-certification happens in an industry that's become so complicated.
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Old 26th Nov 2019, 00:44
  #4154 (permalink)  
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Originally Posted by Loose rivets View Post
Thanks. Popping open the diagram is revealing.

It's easy to see how self-certification happens in an industry that's become so complicated.
Yes, and also to see why it's so easy for a culture that is constitutionally-hostile to regulation is able to convince itself and the world that it's best to just let the industry police itself.
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Old 26th Nov 2019, 01:59
  #4155 (permalink)  
 
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Originally Posted by OldnGrounded View Post
Yes, and also to see why it's so easy for a culture that is constitutionally-hostile to regulation is able to convince itself and the world that it's best to just let the industry police itself.
As Mrs Davies put it, industry was responsible for writing the regulations. In his interviews it was clear he didn’t always agree with them but he said something about “hey, you wrote these regs you need to comply”. In essence the industry did police itself by writing the regulations but after the fact they want to change the, to suite their individual needs.
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Old 26th Nov 2019, 22:04
  #4156 (permalink)  
 
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Originally Posted by jimtx View Post
There is a lot I don't know about the aero of the MAX but I don't think the original MCAS was put there for "unacceptable" stick force. It was put there for non linear stick force which would be unacceptable by the certification requirements. If you showed me how it responded in that flight regime I might say it was acceptable and of no consequence. Kinda like the T-38 I used to fly. Be careful around .9 Mach, it's more sensitive. I envision two cases where the MAX's stick force non linearity might come into play. It might make a steep turn easier if you fly the attitude you want to put the performance where you want. It also might be a problem in a clean off autopilot windshear escape. Would have to see that in the airplane or simulator.
737: The MAX Mess | bit-player

The high-AoA instability of the MAX appears to be a property of the aerodynamic form of the entire aircraft, and so a direct way to suppress it would be to alter that form. For example, enlarging the tail surface might restore static stability. But such airframe modifications would have delayed the delivery of the airplane, especially if the need for them was discovered only after the first prototypes were already flying. Structural changes might also jeopardize inclusion of the new model under the old type certificate.
There were ways to fix the issue without MCAS. But they would have not allowed Boeing to use the existing B737NG Type certificate, meaning increased costs for CCQ training, new simulators instead of using NG simulators, ground school eyt

In the US, 737 pilots have just B737 on their licence. In the rest of the world, they have it like this:

B73A Boeing 737 Series 100 / 200 2T 2
B73B Boeing 737 Series 300 / 400 / 500 2T 2
B73C Boeing 737 Series 600 / 700 / 800 (NG Series) and -8 (MAX Series)

Using an aerodynamic fix for the problems related to the MAX would have likely turned the MAX into the B73D, which is not what Boeing had promised to its large Boeing 737NG customers like SouthWest..........

Last edited by Gilles Hudicourt; 27th Nov 2019 at 12:17.
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Old 26th Nov 2019, 22:14
  #4157 (permalink)  
 
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something not touched upon ( much) :
What changes did the new rear fuselage bring into the equation?
Why did Boeing do that rather involved change? ( and did not talk about it much)
Just for minor efficiency gains ( on a type that would by their own words "blow past the NEO" in performance?
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Old 26th Nov 2019, 22:29
  #4158 (permalink)  
 
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Originally Posted by Gilles Hudicourt View Post
There were ways to fix the issue without MCAS. But they would have not allowed Boeing to use the existing B737NG Type certificate
I think you mean Type Rating, not Type Certificate.

There was never any possibility of a 737 derivative being certificated under anything other than the legacy 737 TC A16WE.

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Old 26th Nov 2019, 23:15
  #4159 (permalink)  
 
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Originally Posted by Gilles Hudicourt View Post
Thanks for the link, which I had not seen before. It included a description of an issue that I raised recently (which elicited some skepticism):
That experience taught me just enough to recognize something odd about MCAS: It is way too slow to be suppressing aerodynamic instability in a jet aircraft. Whereas the X-29 controller had a response time of 25 milliseconds, MCAS takes 10 seconds to move the 737 stabilizer through a 2.5-degree adjustment. At that pace, it cannot possibly keep up with forces that tend to flip the nose upward in a positive feedback loop.
There’s a simple explanation. MCAS is not meant to control an unstable aircraft. It is meant to restrain the aircraft from entering the regime where it becomes unstable. This is the same strategy used by other mechanisms of stall prevention—intervening before the angle of attack reaches the critical point. However, if Brady is correct about the instability of the 737 MAX, the task is more urgent for MCAS. Instability implies a steep and slippery slope. MCAS is a guard rail that bounces you back onto the road when you’re about to drive over the cliff.
Which brings up the question of Boeing’s announced plan to fix the MCAS problem. Reportedly, the revised system will not keep reactivating itself so persistently, and it will automatically disengage if it detects a large difference between the two AoA sensors. These changes should prevent a recurrence of the recent crashes. But do they provide adequate protection against the kind of mishap that MCAS was designed to prevent in the first place? With MCAS shut down, either manually or automatically, there’s nothing to stop an unwary or misguided pilot from wandering into the corner of the flight envelope where the MAX becomes unstable.
My bold added.

Edit: I understand that it is an old article (Apr 2019), but the fundamental control theory issue remains the same, and still not clarified by Boeing or the FAA.

Edit: IMO the author Brian Hayes is the B737 MAX equivalent of Richard Feynman and the Space Shuttle Challenger. From Wikipedia:
He is a senior writer and regular columnist for the magazine American Scientist, and was editor in chief for the magazine from 1990 to 1992. He has also edited and written columns for Scientific American, as well as writing for Computer Language and The Sciences.

Last edited by GordonR_Cape; 26th Nov 2019 at 23:30.
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Old 27th Nov 2019, 00:19
  #4160 (permalink)  
 
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Originally Posted by Loose rivets View Post
One has to be careful what one reads. Just one nit-pick.
I agree to some extent, I noted dozens of nit-picks in the article. Many new facts have emerged since Apr 2019, but fundamental issues remain unanswered. Neither Boeing nor the FAA have clarified exactly what the pitch-up motions are, and EASA (and others) are right to be skeptical. Equally so, the insistence that MCAS is not an anti-stall device has become rather threadbare.
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