Hacker News Reader

Comment by manigandham | Hacker News Reader

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 easily fixable but still a tragedy of greed and oversight that led to the situation in the first place.

View on HN

> There's no fundamental design flaw, the aircraft is sound.

From what I have read, the new aircraft design had a habit of raising it's nose when engine power was applied, which put the plane into a situation where it might stall.

The MCAS was added to automatically detect this behaviour and stop the nose lifting.

So basically, without the MCAS, under certain scenarios the plane had a tendency to want to stall.

That does suggest the aerodynamic design of the plane is less than ideal and this could even be considered a design fault.

Lots of airplanes do weird undesirable things at certain points in the flight envelope. Airplane design is all about tradeoffs. There is nothing fundamentally wrong with the aerodynamic design of the Max.

There are serious flaws with a safety system (MCAS) which was added to prevent the need for more training and a new type rating for pilots who are already rated to fly the 737. This needs to be fixed.

All planes with under mounted engines pitch up under power. Some more than others. The MAX just happens to do it more than a traditional 737.

I don't think the problem is pitching behaviour.

The problem is the fact the MCAS is required to fix the pitching problem.

Boeing tried to fix the design issue with a workaround solution only to then find they had created another, even bigger problem.

>There's no fundamental design flaw, the aircraft is sound.

You can claim this because now all the FAA approvals for this planes, all tests need to be redone by somebody that we can trust. Sure they but larger engines, did they improved the supports, the bolts ,FAA "checked" this updates but can you trust them? the idea that MCAS is the only subsystem that must be fixed and checked is flawed, everything needs to be tested.

The fault here is complex and involves the architecture of the system, the sensors, and how the behavior mimicked the older designed which reduced the training required which meant pilots were not as aware of both the changed flight characteristics and the MCAS itself.

Sure the FAA definitely needs to be questioned as to how they let this through without enough redundancy and pilot awareness but the engines are not the problem and certainly not the supports and bolts. Those are things that are well-tested before it even gets to the FAA and having extreme paranoia about the industry doesn't help.

Why should I trust you or FAA that the engine, the engine mounts and all the new updates related to this larger engines were tested when we know from previous articles that hings were rushed and stuff like the fire suppression system was not OK and the engineer was changed because was objecting.

I imagine that putting a larger engine is not as easy, isn't a larger engine heavier? the forces larger? Don't you need to test the engine supports, the wings, all the related mechanical support structure.

Your theory that only MCAS is an issue is already false since it was found that without autopilot 2 humans don't have the physical force to control the plane , so probably a new requirement in the FAA should be added that with autopilot off 1 single pilot should be able to control the plane. You may say this is related to MCAS but6 it is obvious that this item was not in any checklist for things to test so even with a working MCAS but other emergency where autopilot would be turned off you would get in the same situation.

Part of the problem is that Boeing changed the trim increment AFTER the FAA certification without FAA knowledge. I'm surprised a configuration change like that was allowed and wonder what level they were analyzed by safety engineers.

So there's not just the design workaround but there seems to also be process gaps that contributed to the situation.

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

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.

There is no other modern airliner with a software bodge for static stability. No, Airbuses are not comparable (Airbus FBW control laws are _not_ for stability control) and they are (to the best of my knowledge) positively stable throughout the flight envelope in direct law.

Emulation is incidental. Redesigning the 737 with FBW is not something that Boeing would've considered, even for a moment.

It was certified though so it seems we agree that the stability control system was not robust enough and the training for it was not sufficient enough?

Is the cause for that just oversight then? And further exacerbated by similar behavior leading to even less training requirements?

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.

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 :-)

Wouldn't both of those solutions undermine the advantage of the airframe, which is the fuel cost savings of the larger engines?

That said, I think seating pitch and spacing is definitely a conversation that needs to be had. I'm an average-sized guy, and I have a hard time with today's airline seats. I would hate to see what it's like for someone taller. There definitely needs to be more stringent regulation of airline seating to mitigate ever-vanishing personal space.

This is pretty far from my area of expertise but I if it was behaving as the older designs did then why did it crash?

As far as I can tell there are definitely some flaws, which seemed to have crept in as a result of the attempt to make them behave like older designs, but it seems optimistic to assume you can simply remove the flaws by sacrificing on that point.

It crashed because the system that was making it behave that way failed. The MCAS did not get correct data, did not react well, was not easily deduced to be the problem, and was not easily disabled.

Training and technical improvements can resolve these problems and it's ready to fly. If that's still deemed too risky then Boeing can remove the behavior modifications with training and certification as a completely new design. It was this cost that they were trying to avoid through emulation but clearly not done in a safe enough manner.

The engines we placed in a far from ideal position because of low ground clearance making the plane not fly close to neutral that is a fundamental design flaw

Normal flight is flight is fine. The issue with the new engines was that the nacelles generated lift in specific scenarios at high-angle-of-attack which changed the pitch.

This doesn't mean the aircraft is fundamentally unsound, it's just a flying characteristic and can be managed just fine if the pilots knew about it and what the MCAS system was doing to counter it.

A aeroplane that NEEDS an added control system just to make it APPEAR to fly neutral is a design flaw.

Again it's only in specific scenarios, not level flight. And if control systems are not allowable then how was it certified at all?

Citation needed

Ok, start with Boeings page: http://www.boeing.com/commercial/737max/737-max-software-upd...

And here's a good overview: https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...

Reply on news.ycombinator.com