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MCAS: How 737 MAX system gained power and lost safeguards

seattletimes.com

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Re: MCAS: How 737 MAX system gained power and lost safeguards

#11

Post by msbarnett »

etaioinshrdlu wrote: post #8

All of this to help avoid stalls. Such a pity because avoiding a stall seems to be one of the first things a new pilot learns. Every pilot should know how to detect and correct a stall. Can an actual pilot conform or deny this?

It’s not about avoiding stalls per se, it’s about the uncorrected aerodynamics of the 737 MAX being such that at a certain point it becomes easier to pull into a stall than push away from it.

While pilots learn to avoid stalls, commercial planes are designed deliberately such that at any point short of a stall it always takes less force to avoid one than create one.

Uncorrected, the 737 MAX’s behaviour is a dangerous and surprising deviation from norms. It runs counter to pilot training, and it makes the stall recovery they’re trained to perform significantly more challenging than it would otherwise be. That’s why the FAA rule about column forces exists.

Re: MCAS: How 737 MAX system gained power and lost safeguards

#12

Post by olliej »

cesarb wrote: post #7
olliej wrote: post #5

So my understanding following this: * they designed mcas with a g sensor and aoa sensor and a max pitch, to fix a problem in a high speed test. They provide safety assessments based on having two sensors, limited pitch changes, and it only triggering in specific rare situations. * they provided that to the FAA as justification no training was needed, and that there was no need to acknowledge the existence of the syst…

> the standard way to recover from the flight control computer triggering to much pitch was to flip a switch on the stick. Boeing made MCAS override that command, so the standard recovery could not work. My understanding of this part was different. The standard way to recover is a pair of switches in the central part of the panel (not in the stick), which cuts off all power to the trim motors; these switches still wo…

Ah, that seems entirely plausible (my misunderstanding I mean)

Re: MCAS: How 737 MAX system gained power and lost safeguards

#13

Post by thatswrong0 »

paulmd wrote: post #6
olliej wrote: post #5

So my understanding following this: * they designed mcas with a g sensor and aoa sensor and a max pitch, to fix a problem in a high speed test. They provide safety assessments based on having two sensors, limited pitch changes, and it only triggering in specific rare situations. * they provided that to the FAA as justification no training was needed, and that there was no need to acknowledge the existence of the syst…

And, Boeing's justification for increasing MCAS capabilities while reducing sensor redundancy needed was that these were changes in degree , not in kind. Nothing about the FAA certification actually depended on the number of sensors required, or the amount of command the system had, the FAA allowed pretty much anything to fly as long as it was only designed to operate when outside of the standard flight envelope. Thi…

The FAA situation is a difficult one to resolve - having full understanding of these systems requires a great deal of expertise, and that kind of expertise is found most easily at, obviously, the companies under regulation. More funding / regulators would certainly help, but I'm not sure if that would be sufficient.

> but the FAA let them get away with it

Well, the FAA thought the MCAS had a command limit of 0.6 degrees when it actually became 2.5 degrees after the changes mentioned in this article. From the other Seattle Times article about this:

> If the final safety analysis document was updated in parts, it certainly still contained the 0.6 limit in some places and the update was not widely communicated within the FAA technical evaluation team.

> “None of the engineers were aware of a higher limit,” said a second current FAA engineer.

To me it's less of a they "let them get away with it" and more of a "they had limited resources and things slipped through". Unfortunately this difference doesn't really matter when 300+ people are dead

Re: MCAS: How 737 MAX system gained power and lost safeguards

#14

Post by benatkin »

Props to the author or editor that came up with the title of this. To make it more embarrassing:

> MCAS: How 737 MAX system gained power to compete with Airbus and lost safeguards

It's really tragic. If only they'd seen how much of a blunder this would be before lives were lost. But it's hard to get a big picture view until after the fact, when it involves so many people. I think perhaps Southwest was the closest to having a big picture view, as they ordered additional equipment [1]. I wish they somehow blew the whistle on it.

1: https://theaircurrent.com/aviation-safety/southwest-airlines...

Re: MCAS: How 737 MAX system gained power and lost safeguards

#15

Post by olliej »

thatswrong0 wrote: post #13
paulmd wrote: post #6

Earlier quoted context omitted.

And, Boeing's justification for increasing MCAS capabilities while reducing sensor redundancy needed was that these were changes in degree , not in kind. Nothing about the FAA certification actually depended on the number of sensors required, or the amount of command the system had, the FAA allowed pretty much anything to fly as long as it was only designed to operate when outside of the standard flight envelope. Thi…

The FAA situation is a difficult one to resolve - having full understanding of these systems requires a great deal of expertise, and that kind of expertise is found most easily at, obviously, the companies under regulation. More funding / regulators would certainly help, but I'm not sure if that would be sufficient. > but the FAA let them get away with it Well, the FAA thought the MCAS had a command limit of 0.6 degr…

I feel that's also by design: Boeing went out of their way to minimize the opportunity that the FAA had to catch what they were doing. Providing the experts no doubt helped, but just omitting changes by lying and saying all the changes were minimal to a system that had already passed safety review was their main tools.

Does the FAA have issues? yes. Will this event reduce the influence of the FAA in the future? Probably - we're already seeing the European equivalent adopting a stance of "the FAA is working for the corporations they're meant to regulate". Honestly the various politicians who gutted the FAA are fairly directly responsible for causing significant long term harm to American influence (for good or bad, depending of who/what/where you are).

Is Boeing entirely responsible? Yes. The FAA is meant to be adversarial to prevent companies from choosing money over safety, but fundamentally it was Boeing executives that made the choice to favour hiding design flaws to preserve profit (and hence bonuses). The fact is that any repercussions are almost certainly going to be entirely financial, and only harm shareholders (dividend and stock price) and lower level employees (e.g non-managers: managers, and especially executives, don't get canned when cash is low).

Re: MCAS: How 737 MAX system gained power and lost safeguards

#16

Post by fwip »

runciblespoon wrote: post #10

Retrospective ass covering, Boeing installed MCAS to save money, and in the process took control of the airplane away from the pilots. They should be looking at jailtime for this.

Boeing still gives the pilots a lot of control over the airplane, compared to Airbus. Airbus has defaulted to fly-by-wire (i.e: the computer has control and the pilot makes suggestions) for planes made since the 1980's.

Re: MCAS: How 737 MAX system gained power and lost safeguards

#17

Post by supportlocal4h »

paulmd wrote: post #6
olliej wrote: post #5

So my understanding following this: * they designed mcas with a g sensor and aoa sensor and a max pitch, to fix a problem in a high speed test. They provide safety assessments based on having two sensors, limited pitch changes, and it only triggering in specific rare situations. * they provided that to the FAA as justification no training was needed, and that there was no need to acknowledge the existence of the syst…

And, Boeing's justification for increasing MCAS capabilities while reducing sensor redundancy needed was that these were changes in degree , not in kind. Nothing about the FAA certification actually depended on the number of sensors required, or the amount of command the system had, the FAA allowed pretty much anything to fly as long as it was only designed to operate when outside of the standard flight envelope. Thi…

Wouldn't it be interesting if FFA signoff included approval by a regulatory team from the competing manufacturer. I wonder what abuses would crop up there.

Re: MCAS: How 737 MAX system gained power and lost safeguards

#18

Post by salawat »

olliej wrote: post #5

So my understanding following this: * they designed mcas with a g sensor and aoa sensor and a max pitch, to fix a problem in a high speed test. They provide safety assessments based on having two sensors, limited pitch changes, and it only triggering in specific rare situations. * they provided that to the FAA as justification no training was needed, and that there was no need to acknowledge the existence of the syst…

>Because it activated at low speed a g sensor wasn’t possible (?) so it got removed. Now you only have a single (notoriously unreliable) AoA sensor controlling it. To compensate for poor aero

To clarify: The G load gate had to be removed to enable the low speed activation.

The increased authority wasn't due to poor aero per se; but the fact that aerodynamic forces are dependent upon velocity. The slower you go, the more control surface deflection you need to get the same deflection that same control surface can generate at a higher speed.

>So the FAA approved MCAS with a 0.6 degree pitch limit, occurring at high speed, triggered by two independent sensors

Incorrect. The FAA approved input from only a single sensor, and the G load gate. Double sensor would have required simulator time; so was a non-starter. Both AoA sensors could talk to the Flight Control Computer, and MCAS as a result; but only one sensor was feeding data to the controlling FCC at a time.

Re: MCAS: How 737 MAX system gained power and lost safeguards

#19

Post by salawat »

msbarnett wrote: post #11
etaioinshrdlu wrote: post #8

All of this to help avoid stalls. Such a pity because avoiding a stall seems to be one of the first things a new pilot learns. Every pilot should know how to detect and correct a stall. Can an actual pilot conform or deny this?

It’s not about avoiding stalls per se, it’s about the uncorrected aerodynamics of the 737 MAX being such that at a certain point it becomes easier to pull into a stall than push away from it. While pilots learn to avoid stalls, commercial planes are designed deliberately such that at any point short of a stall it always takes less force to avoid one than create one. Uncorrected, the 737 MAX’s behaviour is a dangerous…

The uncorrected behavior also violates a design directive encoded in the CFR's. The plane could not be certified as airworthy without MCAS active due to the non-compliant stick force response curves.

Re: MCAS: How 737 MAX system gained power and lost safeguards

#20

Post by jdc »

usrusr wrote: post #4
paulmd wrote: post #3

The thinking of "we don't want the system to not work if it's needed" thinking demonstrates a failure to understand the prior probabilities involved. Most of the time the flight conditions will not warrant activation of the MCAS, and apparently the system is sufficiently unreliable/easy to damage that the chances of the system inadvertently triggering is high. But Boeing wanted to be sure that MCAS would always activ…

Well, Boeing is multiple people. Business culture celebrates a manager who can push their project ahead of others. Maybe MCAS had a project manager who was a bit of an overachiever in that regard.

Surely I'm not the only one reading "overachiever" here as a wry euphemism.
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