U.S. FAA proposes requiring key Boeing 737 MAX design changes • Hacker News Board

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#131

Post by mgsouth » Tue, Aug 04, 2020, 1:53 AM UTC

Freaken wrote: post #8

Noob question here: Since, if I get this correctly, all these changes will require new pilot training, why is the MCAS still needed? My understanding was that this system was installed in order for the new plane to behave exactly like the old one therefore not requiring costly additional pilot training.

Some sort of corrective system is required to adjust the raw aerodynamics of the 737 MAX. It some conditions it has characteristics which are not allowed for commercial planes in the US. [0] This is not, in itself, a problem. Many (most? all?) modern airliners have handling corner cases. A common way to fix them is to have sensors, computers, and actuators which adjust the aerodynamics or "push the controls". Boeing chose to use MCAS to adjust the handling in the undesired conditions. Unfortunately, they tried to extend existing sensors and software which were not sufficient for the job. They also, critically, tried to make it transparent to the pilots; this was to avoid extra training, and to avoid creating making the MAX a separate type with all the certification and logistics issues that involves.

The proposed fix is to change the software to admit to itself that it can fail, to limit the amount of control that MCAS can exert, and to train pilots to recognize failure modes and how, in the event of MCAS failure, manually handle the undesireable aerodynamics. Boeing can't simply disable MCAS and say that pilots have to always manually handle it; the rules specifically say the plane cannot behave in that manner.

[0] In some uncommon-but-not-impossible situations, pulling back on the control yoke and putting the plane closer to a stall is easier than pushing on the yoke and getting further from stalling. That isn't allowed for commercial aircraft.

https://www.nytimes.com/2019/06/01/business/boeing-737-max-c...

https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1...

MCAS fixes this by detecting the condition and, in effect, pushing forward on a secondary control.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#132

Post by cmurf » Tue, Aug 04, 2020, 2:17 AM UTC

zaroth wrote: post #3

https://www.faa.gov/news/media/attachments/19_035n-R3-8-3-20... > "To address the unsafe condition, the FAA proposes to require four design changes: (1) installing updated flight control software (with new control laws) for the FCC operational program software (OPS), (2) installing updated MDS display processing computer (DPC) software to generate an AOA disagree alert, (3) revising certain AFM flightcrew operating p…

Yep. The flight characteristics of the 737 MAX in high angle of attack flight is either significantly different or it isn't, compared to prior 737's. Boeing said it is and created MCAS to mitigate/moderate that difference.

That MCAS can be disabled, means pilots need to know about, trained on, and demonstrate competency, on the ensuing natural (minus MCAS) flight characteristic.

And if the pilots are aware and demonstrate such competency, why even have MCAS? Rip it out.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#133

Post by chipsa » Tue, Aug 04, 2020, 2:25 AM UTC

Freaken wrote: post #8

Noob question here: Since, if I get this correctly, all these changes will require new pilot training, why is the MCAS still needed? My understanding was that this system was installed in order for the new plane to behave exactly like the old one therefore not requiring costly additional pilot training.

MCAS is needed to meet certification requirements. [0][1] Your understanding is common, but wrong, because the media commonly misunderstood what it was for. It's not a anti-stall system. It's not required to behave exactly like the old one. It is required that the stick forces monotonically increase until the plane stalls, which it does not without MCAS.

And it's required because they couldn't rip out all the old control system and replace it with a fly by wire system.

0: https://www.law.cornell.edu/cfr/text/14/25.173 1: https://www.law.cornell.edu/cfr/text/14/25.175

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#134

Post by linuxftw » Tue, Aug 04, 2020, 2:48 AM UTC

tgsovlerkhgsel wrote: post #129

linuxftw wrote: post #124

Earlier quoted context omitted.

This seems to be the part that everyone's missing. "Oh, this rarely happens" is ambiguous, we don't have any data on how often MCAS operated correctly during flights. Also, in events that 'rarely happen' one of which is approaching a stall (according to the FAA's report), is it safe to disable the system in the actual scenario when encountered? It doesn't seem so. I can't see how both "Need this system for extremely…

> I can't see how both "Need this system for extremely rare scenario" and "Disabling this system because scenario is extremely rare is okay" can both be true. The benevolent interpretation is: a) the plane being in a situation where the system is required is rare, but frequent enough that the risk wouldn't be acceptable without MCAS. b) MCAS breaking is rare, but frequent enough that we can't allow it to fly the plan…

These are fair points, but the FAA report used the wording "extremely rare" (or similar) instead of a concrete quantity, and I find that extremely reprehensible. As we all know, "extremely rare" needs to be quantified. There are 44k flights in the US alone every day (pre COVID).

> MCAS breaking is rare

This is the part I'm not particularly inclined to agree with. The scenario is no longer breaking, it's breaking OR disabling. There are a host of new conditions that will result in MCAS being disabled according to the report. Some of these conditions disable MCAS for the remainder of the flight, and the way that I read the scenarios, it's certainly possible for these conditions to stack together.

Reading the report gives the impression that the FAA is completely incompetent. They didn't specify any rate of failure or disabling of MCAS, and the 'failure scenarios' that are proposed to be carried out in the near future are lacking in exposing these disabled scenarios.

Also, the FAA stated in the report that the plane handles similar to the 737 NG when STS is disabled; however, the MCAS automation disables MCAS and STS. There is no information about the rate at which STS has been disabled in flights in NGs, and no comparison on how disabling STS during the 'near stall' portion of a 'rare event' behaves in those planes.

They wrote this long-winded document that's almost entirely devoid of meaningful factual detail. It reads like a Boeing PR piece if I've ever seen one.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#135

Post by WatchDog » Tue, Aug 04, 2020, 2:56 AM UTC

Someone1234 wrote: post #87

imglorp wrote: post #66

Earlier quoted context omitted.

Triple is not needed, just double with a redundant "disagree" alarm. Once the pilot knows there's one bad AOA, they can lock out the bad one as an input to the autopilot/MCAS, or hand fly without automation. Of course this is all a bunch of new software and training though.

> they can lock out the bad one as an input to the MCAS The 737 MAX didn't support that as delivered. Even with the warning (which some US airlines paid for), you couldn't "lock out the bad one" even assuming you knew which one that was (you don't). All you could do is disable the electronic horizontal stabilizer completely, so that MCAS couldn't command it into a dangerous and unrecoverable state. MCAS has been fixe…

> Even with the warning (which some US airlines paid for)

Are you saying that a bad input warning was an optional(paid) extra?

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#136

Post by toast0 » Tue, Aug 04, 2020, 3:12 AM UTC

Androider wrote: post #75

onde2rock wrote: post #38

Earlier quoted context omitted.

Wow the report is very interesting ! In the paragraph about the mandatory AOA DISAGREE alert : > Some 737 MAX airplanes were delivered without this alert feature, by error. The FAA is useless ...

AFAIK the AOA disagree alert was an optional add-on sold by Boeing! https://qz.com/1608362/boeing-says-it-didnt-deactivate-stand...

Sure, but without knowledge of MCAS, what would a pilot do with an AOA warning?

They would still most likely get an airspeed disagree warning (standard) because AOA is a parameter in the airspeed calculation. Again, they wouldn't know that was why the stabilizer trim kept moving to nose down, because the MCAS system was not disclosed to pilots.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#137

Post by toast0 » Tue, Aug 04, 2020, 3:14 AM UTC

tgsovlerkhgsel wrote: post #106

inamberclad wrote: post #51

Their requirement is to make the AoA disagree light standard? Wow that's next to nothing. For those that don't remember, the AoA disagree light is an optional, costs-extra safety feature on this model. How about we add a big fat MCAS disengage button to the yoke instead, and make it separate from the trim cutout switch?

Or repurpose the existing second trim cutout switch... (AFAIK there are two, that used to serve different purposes, and are now both left in to keep the same type rating).

Yes! If this was available, at least the second crash seemed likely preventable given the timeline of events. I didn't read the timeline for the first one.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#138

Post by na85 » Tue, Aug 04, 2020, 3:26 AM UTC

frogblast wrote: post #84

imglorp wrote: post #66

Earlier quoted context omitted.

Triple is not needed, just double with a redundant "disagree" alarm. Once the pilot knows there's one bad AOA, they can lock out the bad one as an input to the autopilot/MCAS, or hand fly without automation. Of course this is all a bunch of new software and training though.

This is what I don't get. If the solution is "install a AOA Disagree light, and disable MCAS in that scenario", why has it taken two years to develop this solution?

It's not like pushing a JavaScript change to the website. Aircraft software changes are subject to the same level of design review as a structural modification.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#139

Post by RandallBrown » Tue, Aug 04, 2020, 3:28 AM UTC

thatswrong0 wrote: post #83

holler wrote: post #41

Earlier quoted context omitted.

> has big design flaws and a relatively bad safety record understatement...

Yeah 2 out of 387 hull losses over 2 years is a pretty awful record. I will stick to not 737 MAX thanks.

My understanding is that the crashes were due to properly functioning (but awkward and confusing) MCAS systems. I have to imagine that any pilot flying a 737 MAX is going to have a very solid understanding of how that system works and how to tell if it's malfunctioning and avoid the same situation.

Also, it was 2 crashes in 500,000 flights. It seems a little misleading to just compare it to hull losses.

Re: U.S. FAA proposes requiring key Boeing 737 MAX design changes

#140

Post by dmead » Tue, Aug 04, 2020, 3:28 AM UTC

valuearb wrote: post #108

bedhead wrote: post #13

Can we just scrap this thing and move on already? Isn’t all of this back-and-forth alone evidence that this thing shouldn’t be flying?? It’s over, Boeing, you lose.

Because it’s far cheaper to make them safe than to trash them. The problem isn’t the basic airplane, it’s the short shrift Boeing gave the safety systems.

that is not the commonly held interpretation of this my dude.