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Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

nytimes.com

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Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#111

Post by chimpburger »

piadodjanho wrote: post #91
tlb wrote: post #40

Earlier quoted context omitted.

They’re not expected to be that reliable. They’re small vanes sticking out the side of the nose, vulnerable to bird strikes. The article mentions hundreds of reported failures over the years. The way to make the system reliable is redundancy.

The article says it had 122 failure due bird strikes plus 85 unnamed problems in about 30 years of data. Considering the number of flights, that does sounds reliable to me. I still think two failures in the same sensor, in the same airplane, under the same condition, in less than one year did not happen by chance.

You have a very poor understanding of probability. At a mean failure rate of 0.66 per year, it is quite probable for 2 failures to be spaced 4.5 months apart.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#112

Post by leoc »

na85 wrote: post #107
nopzor wrote: post #104

Earlier quoted context omitted.

the airplane did not have stable flight characteristics because of its physical design. that’s a hardware problem. MCAS only exists because of that hardware problem. the fact that boeing also did not train or tell pilots about MCAS, in order to make the airplane more financially appealing by retaining the 737 type rating, is a separate (also bad) issue.

>the airplane did not have stable flight characteristics because of its physical design. This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable. At high alpha the aircraft has pitch problems. There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.

That's the devilish thing, I think. Making the plane unstable then patching things up with MCAS, or something else, isn't a fault in any straightforward way. But it is taking a risk, and the source of risk isn't simply the risk that eg. MCAS will fail on any given flight, it's also the higher-order risk that eg. the MCAS system will have a faulty design. (A bit like rewriting code in C for performance, or keeping on generally unloved code to satisfy a paying customer, presumably.) I assume that makes the ethical and safety decisions a lot more murky for the engineers than having a specific fault to point to, especially in an organisation afflicted with go fever.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#113

Post by sokoloff »

na85 wrote: post #107
nopzor wrote: post #104

Earlier quoted context omitted.

the airplane did not have stable flight characteristics because of its physical design. that’s a hardware problem. MCAS only exists because of that hardware problem. the fact that boeing also did not train or tell pilots about MCAS, in order to make the airplane more financially appealing by retaining the 737 type rating, is a separate (also bad) issue.

>the airplane did not have stable flight characteristics because of its physical design. This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable. At high alpha the aircraft has pitch problems. There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.

I believe the airplane is aerodynamically stable even at high alpha. What it fails in certification is the requirement to have continually increasing pitch feedback forces. I believe the pitch feedback forces are still in the stable region, just too low. This is not a flying wing or intentionally unstable airplane (as the F-16).

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#114

Post by cmurf »

nopzor wrote: post #104
na85 wrote: post #99

Earlier quoted context omitted.

The hardware isn't faulty. The problem is the way Boeing tried to achieve a zero training delta so pilots wouldn't have to get a second type rating.

the airplane did not have stable flight characteristics because of its physical design. that’s a hardware problem. MCAS only exists because of that hardware problem. the fact that boeing also did not train or tell pilots about MCAS, in order to make the airplane more financially appealing by retaining the 737 type rating, is a separate (also bad) issue.

Citation needed.

FAR 25 applies to all transport category aircraft. The section on stability (§§ 25.171 - 25.181). In exactly what manner is the airplane not stable, with or without MCAS?

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#115

Post by leoc »

sokoloff wrote: post #113
na85 wrote: post #107

Earlier quoted context omitted.

>the airplane did not have stable flight characteristics because of its physical design. This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable. At high alpha the aircraft has pitch problems. There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.

I believe the airplane is aerodynamically stable even at high alpha. What it fails in certification is the requirement to have continually increasing pitch feedback forces. I believe the pitch feedback forces are still in the stable region, just too low. This is not a flying wing or intentionally unstable airplane (as the F-16).

>> [...] In most of the flight envelope the aircraft is stable.

>> At high alpha the aircraft has pitch problems.

> I believe the airplane is aerodynamically stable even at high alpha. [...] This is not a flying wing or intentionally unstable airplane (as the F-16).

In some ways that's likely worse news than an airplane that's inherently unstable in general, no? A corner case, and one that evidently isn't actually all that uncommon. If you're building something like an F-16 you know that you absolutely have to make the fly-by-wire correct and robust, and the ground crews similarly know that if anything affects its performance the plane isn't fit to fly.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#116

Post by salawat »

mikeash wrote: post #4
aivisol wrote: post #2

Is this only me, or all this one-vs-two AoA sensor talk seems some kind of diversion from the real problem with this plane. I mean, if one-sensor based MCAS failed twice so early in the life span of the plane model, what is the probability that a two-sensor model will fail pretty soon as well? The math should be simple, we have all data needed: combined hours flown by all planes of the type and number of failures (at…

The problem isn’t failure, but detecting failure. If the sensor had just stopped responding, there wouldn’t have been any problem. The planes would keep flying, the sensors would get replaced, and everyone would be fine. What happened was that the sensor gave erroneous readings. The MCAS system reacted to those erroneous reading and crashes the plane. With two sensors, you can detect failure. It’s very unlikely that…

Increased redundancy in airborne systems is very unintuitive. Double redundancy can be more dangerous than having a single system (at least when you're talking engines or sensors anyway).

Triple redundancy is the norm for the specific reason that it's highly likely for symmetrically placed sensors to be prone to failing in the same way not long after each other, but having a third differently placed can keep you flying.

Although there's at least one instance where an Airbus plane had two AoA sensors malfunction at the same time and outvote the last remaining sensor.

This is why critical systems are built with higher degrees of redundancy and graceful degradation of operational envelope in mind.

Training on how to deal with unaided flight is also absolutely essential. Many Airbus accidents where pilot's were caught off guard when the automation that kept them from breaking out of the operating envelope failed.

Long story short; Boeing has put themselves in the unenviable position of having delivered a product in ways that are not only illegal, but deadly, and short of pilots accepting a significant burden in the form of being as good at or better than the MCAS system at this point; a lot of man hours and capital has been expended to end up in a situation where every MAX is in a not inconsiderable risk of being scrapped.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#117

Post by sokoloff »

soundpuppy wrote: post #108

Airplanes have become next to impossible to maintain, mostly software related, yet at the same time seem to be increasingly available to only the rich. Oh what a tragedy. And if you don't believe me, why are we doing this passport only to certain individuals I heard of?

> at the same time seem to be increasingly available to only the rich

I believe that airfares have never been cheaper than in the last decade.

In 1965, only 20% of Americans had ever flown in a plane. By 2000, 50% of Americans took at least one round-trip flight per year (with an average of 2 per year). Airfares are down by 50% in 30 years.

https://www.theatlantic.com/business/archive/2013/02/how-air...

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#118

Post by GoToRO »

kejaed wrote: post #27

I really see this as a failure of the Systems Engineering process. With so many people unaware of the impacts of the changes, it’s up to the systems types to have the big picture view and make sure these sorts of things are taken into account. Especially if as the article says a failure of the AOA sensor on the system would be Hazardous (looks like it was Catastrophic when paired with MCAS in retrospect), that would…

engineers overseeing this work

There the thing: nobody wants to pay engineers on par with managers. Maybe one can not manage something he/she can not understand.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#119

Post by gdavisson »

ficklepickle wrote: post #52

This whole plane sounds like any ugly hack. They slapped very different engines on an existing airframe. Then, when it inevitability exhibited undesirable behaviour, they tried to paper over the cracks. Then they hid this information from their customers, regulatory agencies and the pilots. It makes me wonder if there are other issues with the Max that the public doesn't know about yet. I hope a thorough review of Bo…

"Other issues" is what worries me most about this as well. We now know that MCAS is safety-critical, so it's going to be reviewed, but what else got missed/minimized during the design and review process? The fact that Boeing is still trying to minimize the danger of MCAS does not fill me with confidence in their willingness to raise and fix other problems.

They don't seem to have learned from this, which means it's likely to happen again.

Re: Boeing Built Deadly Assumptions into 737 Max, Blind to a Late Design Change

#120

Post by lutorm »

ficklepickle wrote: post #52

This whole plane sounds like any ugly hack. They slapped very different engines on an existing airframe. Then, when it inevitability exhibited undesirable behaviour, they tried to paper over the cracks. Then they hid this information from their customers, regulatory agencies and the pilots. It makes me wonder if there are other issues with the Max that the public doesn't know about yet. I hope a thorough review of Bo…

I think this is false thinking. The 737 is by now a very well tested airframe. Yes, you change some things and there are unexpected results, but the core is sound. Starting over on a completely new design, everything has to be debugged from scratch.

This reasoning reminds me of the tendency to argue "this code needs to be thrown out and rewritten from scratch" among software engineers. It's easy to see the flaws, but not so easy to all the things that have been fixed. See https://www.joelonsoftware.com/2000/04/06/things-you-should-...

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