Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS • Hacker News Board

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#131

Post by manigandham » Sun, May 12, 2019, 1:49 AM UTC

hirowan wrote: post #100

manigandham wrote: post #80

Earlier quoted context omitted.

There's no fundamental design flaw, the aircraft is sound. The problem came from using software systems to artificially make the plane behave like older designs to avoid entirely new training and certifications. Even that would've been acceptable and similar systems are already in use in other planes but it was the inadequate info, redundancy, warnings and controls to disable the system that created the problem. It's…

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes.

There's a difference between software stability controls and specifically emulating behavior.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#132

Post by hirowan » Sun, May 12, 2019, 1:54 AM UTC

manigandham wrote: post #131

hirowan wrote: post #100

Earlier quoted context omitted.

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes. There's a difference between software stability controls and specifically emulating behavior.

I don't know which one you imagine MCAS to be but see [1]; specifically:

   MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation. ... This new location [of the engines] and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the [centre of gravity], this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

> Airbus

Airbuses operate in C* law (they are not positively stable) [2] and the stick is just that, a spring-loaded.... stick; it meets regulations by default.

[1] http://www.b737.org.uk/mcas.htm

[2] https://www.skybrary.aero/index.php/Fly-By-Wire

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#133

Post by CydeWeys » Sun, May 12, 2019, 1:55 AM UTC

skybrian wrote: post #121

CydeWeys wrote: post #106

Earlier quoted context omitted.

This isn't a valid way to put it into context. It doesn't matter which countries the planes crashed in; the causes were the same and lives lost are lives lost. And air travel is so globalized these days that one crash anywhere can kill citizens of dozens of countries.

Yes, it doesn't matter which two countries it was, but it's fine to point out that many airlines in many countries didn't see these problems. The planes flew in many places every day with no trouble. You are calling me out over nothing. This is uncharitable, to say the least. Why make trouble?

There were "only" two instances (which killed >300 people) before the plane was yanked from the air entirely. That means that, roughly speaking, it was only going to happen in two countries at all, or one really unlucky country. So there's no use in pointing out that it didn't happen in many more countries, because that would have required dozens more accidents with tens of thousands of deaths, which was simply never in the cards because the planes would've been yanked long before then. So there's no use in pointing out that it didn't happen in many countries, because that never would've happened.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#134

Post by ilaksh » Sun, May 12, 2019, 1:57 AM UTC

salawat wrote: post #113

stingraycharles wrote: post #41

Earlier quoted context omitted.

It's fair to recognize this is not entirely Boeing's fault either: it was the airline industries that don't want to re-train their pilots and wanted this type of plane (American Airlines was specifically keen on getting Boeing to manufacture this plane [1]), so they were kind of told "we want you to make exactly $this, otherwise we switch to Airbus". [1] https://www.vox.com/2019/4/5/18296646/boeing-737-max-mcas-so...

Given. However, in Engineering, it is totally expected that when you start getting into areas that sketchy, you say "No. The best I can do is this. I'm sorry." Instead, Boeing saw dollar signs, and threw caution and people's lives, and years of good will to the winds.

The people making the decisions weren't engineers. They were managers. And they just passed the buck to the engineers who actually had to go along with it to keep their jobs.

And I know people will try yo claim that it was the engineers fault and they should have just refused and gotten fired. But engineers have mortgages and have to work for a living. They can't afford to career suicide just because their boss is being unreasonable.

The people who made the decisions should be held accountable. That means the CEO should actually go to prison.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#135

Post by blendo » Sun, May 12, 2019, 2:00 AM UTC

hirowan wrote: post #100

manigandham wrote: post #80

Earlier quoted context omitted.

There's no fundamental design flaw, the aircraft is sound. The problem came from using software systems to artificially make the plane behave like older designs to avoid entirely new training and certifications. Even that would've been acceptable and similar systems are already in use in other planes but it was the inadequate info, redundancy, warnings and controls to disable the system that created the problem. It's…

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

As I understand it, previous 737s did maintain stick force on approach to stall. The MAX, due to different engines, has decreasing stick forces when the lightly loaded, and/or with an aft center of gravity.

My simple solution? Disable MCAS, keep the stick trim switches as on previous 737s, and require MAXs to only fly with a forward CG. 2000 kg of forward ballast might work. Even better, reduce aft weight by increasing economy seat pitch (20 rows rather than 30 or so), thereby reducing aft CG :-)

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#136

Post by manigandham » Sun, May 12, 2019, 2:01 AM UTC

hirowan wrote: post #132

manigandham wrote: post #131

Earlier quoted context omitted.

I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes. There's a difference between software stability controls and specifically emulating behavior.

I don't know which one you imagine MCAS to be but see [1]; specifically: MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as requ…

I think we're talking past each other. MCAS is stability control which a lot of planes have and is allowable to pass certifications.

However MCAS also makes the MAX behave like the older design which specifically led to the gap in training and controls over the system. If they just had stability control but without the emulation of behavior then there would be new training and certification, which is something they wanted to avoid but would've prevented these accidents.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#137

Post by Gibbon1 » Sun, May 12, 2019, 2:09 AM UTC

manigandham wrote: post #131

hirowan wrote: post #100

Earlier quoted context omitted.

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes. There's a difference between software stability controls and specifically emulating behavior.

it's not an anti stall system it's to satisfy an aerodynamic requirement that the force needed to force the nose up increase progressively with angle of attack.

At least that was my understanding.

I don't think there is anything fundamentally wrong with that. But when you have active systems for that rather than passive ones you're in a different design domain. Boeing was trying to pretend they weren't.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#138

Post by kortilla » Sun, May 12, 2019, 2:10 AM UTC

JudgeWapner wrote: post #111

jhayward wrote: post #87

Earlier quoted context omitted.

> The engines are mounted incorrectly, so much so that Boeing believed pilots need the assistance of MCAS on take-off now. This overstates the issue. The engine mounting changes the flight characteristics enough that the MAX 8 needs to be flown differently in certain corners of the flight envelope. The MCAS was added to avoid having to re-train pilots or trigger a new type certification. The engine mounting certainly…

> The engine mounting certainly does not make the plane unsafe. I don't think this is a very...'precise' statement. what is "safe" in the context of aviation? If you had no MCAS, then this plane has a very bad propensity to stall . I hope we can both agree that's an unsafe airplane. You can't just say "don't do that [pull up too much] and then you're fine". The aircraft "wants" to pitch up too high and stall, which p…

No, it did not “have a bad propensity to stall”. That’s a very imprecise statement in the first place but if you take it to mean, it would try to stall during normal flight conditions, it’s definitely false.

Airbus aircraft are “safe” despite the fact that dual conflicting pilot input is averaged without stick feedback (see the Airfrance flight from Brazil). If electronics being used to fly the aircraft upsets you, you’re going to be in for a real shocker on any airbus.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#139

Post by cletus » Sun, May 12, 2019, 2:11 AM UTC

Obi_Juan_Kenobi wrote: post #130

cletus wrote: post #118

So a lot is known at this point: 1. The A320neo caught Boeing completely off-guard; 2. The threat of the likes of American Airlines (already a mixed Airbus/Boeing customer and the largest airline in the world) placing a large A320neo order for regional aircraft operations scared the bejesus out of Boeing management; 3. For airlines like Southwest that are pure 737, the prospect of adding a plane that didn't share a c…

> These engines moving made the plane more vulnerable to a stall situation. I don't think this is correct. A lot is being made about the engine move with regard to aerodynamic behavior, but it doesn't seem at all outside of the bounds of what is considered standard. Pilots report that a similar 'light stick feel' at high AoA is already present in aircraft like the 757. It's not that the aerodynamic change was worse o…

> It's not that the aerodynamic change was worse or better, it's simply that it was a change at all.

It seems you are correct [1]:

> MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

[1] http://www.b737.org.uk/mcas.htm

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#140

Post by cyberferret » Sun, May 12, 2019, 2:12 AM UTC

Wait, what? Reading this is turns out that flipping both switches on the centre panel not only deactivates MCAS, but also deactivates the thumb switch on the control wheel that the pilots routinely use to change the elevator trim.

So, MCAS trims the plan to a dangerous attitude, then when pilots follow Boeing's SOP to disable, they lose the ability to re-trim the way they have done it a million times before? I can see valuable second being eaten up when pilots flick the thumb switch towards them and sit there wondering why the back pressure isn't easing up.

And the report also says that the manual trim adjustment wheel would be harder to move due to the load on the tail when the aircraft is in a nose (death) dive? This is already on top of the fact that the pilots have to hold around 60Kg back pressure on the control wheel while leaning forward to crank the manual trim wheel?

Recipe for absolute disaster.