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Comment by lawless_c | Hacker News Reader

The programmers are just trying to patch what once again is likely a hardware problem. If it's like the MAX the real issue is the flight characteristics

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Won quoted as, “The technical data required for type certification has not reached a point where it appears the aircraft type design is mature ..."

The 737MAX should have gone through a new type certification, but didn't to allow for less pilot retraining.

FAA failed to properly serve the public.

New type cert finds flaws and saves lives.

Basically every modern jet aircraft with relatively large engines has some sort of MCAS system. If you raise the nose of such a plane the balance does shift. This isn't a form of an aerodynamic error, it is a consequence of having large engines hanging off your wings (a lot of classic airplanes have the engines in or very close to their central axis). If you want the appropriate feedback in the steering column, you have to compensate for the shift. Airbus does it as well.

What is the advantage of the engines further from the central axis?

iirc that's another term for larger, more efficient engines hanging lower from the same wings

Time, money.

They don't need to redesign the wings to accommodate the newer, larger engines.

There's only so much room for the landing gear. Larger diameter engines require longer landing gear. Increasing the space given to landing gear requires changes to the wing box. Changes to the wing box is fundamentally a wing redesign.

Which is why the A320 is designed to be loaded with conveyors. This means it has no problem accommodating an even larger nacelle than the 737 in a regular position instead of the up and forward the 737 Max resorted to. Even the previous generation 737 was running into this issue and that's why they went with the distinct oval nacelles.

The main driver is the desire to make engines bigger, thus more fuel-efficient. The fuel savings of the newest engine generations are significant. If you hang engines from your wings (I assume that has practical reasons, it is easier to hang the engine than include it into the wing, it is also easier to maintain), larger engines mean engines further from the axis. That applies to all modern airplanes which share this configuration. A complete redesign of the 737 wouldn't have made this go away either. However the goal of keeping the same type rating made the especial design of MCAS necessary.

Non-engineer here. What's wrong with taller landing gear to provide more clearance?

In 737 case, it means redesign of the landing gear system, fuselage modifications, recertification, retraining, new support system requirements eg. Higher stairs, cargo support equipment etc.

You also can't put longer main gear: as they fold inward, they have to attached at points separated by over twice the height. If you move them apart, you have to make the center fuselage as well as the root of the wings much stronger. That's a much bigger change than anything else.

Taller landing gear makes loading the plane (especially with luggage) much harder.

I believe they altered the landing gear so it extends upon deployment, giving it a bit more height, but that's not enough. They can't just put longer legs, there isn't enough room. There's stuff there (cables, devices, bulkheads ...) and moving them requires a significant redesign. For the main gears, they would have to be attached further away from the center lest they bump into each other, which means the wings and fuselage would have to be stronger to accommodate for it. Very quickly you're just designing a new plane.

What about telescoping landing gear?

The thing is, this wouldn't have magically solved the issue. Larger engines hanging from your wing fundamentally affect the balance of an airplane. Take the thrust vector for example. The further you move the engine from the longitudinal axis of the plane, the stronger the angular momentum is, that is exercised from the engines onto the plane, especially when pitching up.

MCAS-like systems exist elsewhere, but MCAS specifically is worthy of criticism, as both an engineering failure and more importantly IMO a management failure.

Specifically with regard to the flight characteristics, MCAS is a band-aid on a problem created by cost-minimization -- the desire to avoid a hardware redesign (to properly accommodate the larger engines) and/or a re-certification.

Of course, the actual implementation of MCAS was a disaster. Especially, that they didn't deactivate it automatically on AOA disagree, that on top of that the screen signal for AOA disagree was not shown due to a separate issue and of course that late in development MCAS was given more control authority than initially planned.

My point just was, a complete redesign of the 737 still would have to deal with the same aerodynamic issues as the 737 MAX, as they are rather fundamental to this kind of jets. But indeed, it would have been easier to deal with it with a new design.

It is less mentioned, that shortly after the MAX crashes, Airbus mandated that in some planes the last rows were kept empty until some software updates were deployed. So they seem to have discovered some boundary conditions they were not happy with and decided to be rather safe till they augmented the software.

> late in development MCAS was given more control authority than initially planned

... without triggering re-evaluation of the seriousness/consequences of failure. That meant that failures of MCAS were considered unproblematic, which meant that lower levels of redundancy were deemed acceptable.

Quite a process/management failure.

Well, technically the failures of MCAS should be unproblematic, as the stabilizer runaway protocol is well trained for any 737 pilot and would cope with a MCAS misfunction. The problem is: in the two crashes the pilots either completely failed to follow the protocol (Lion Air) or to late and failed to recover (Ethiopian Airlines).

> a complete redesign of the 737 still would have to deal with the same aerodynamic issues as the 737 MAX,

No, it would have more ground clearance, so it wouldn't need flat bottomed cowlings nor MCAS.

That is wrong. First of all, the 737 MAX doesn't have flat bottomed cowlings, that was its predecessor, which did not have MCAS.

And the effect is fundamental to large engines. The effect which required MCAS would also happen if the machine had larger ground clearance and would be mounted more under the wings. Actually the way the engines are mounted on the MAX moves them closer to the longitudinal axis of the airplane and thus reduces and thrust-related tendency to pitch up.

First result on Google: https://fr.wikipedia.org/wiki/Boeing_737_Max#/media/Fichier:...

The MAX does have flattened cowlings, though the effect is less pronounced than on the NG. Compare the MAX (right) to the completely round cowling on the A320neo's LEAP-1A:

https://leehamnews.com/wp-content/uploads/2020/11/737NG-vs-M...

https://aerocorner.com/wp-content/uploads/2020/01/Aegean-Air...

The bonkers thing seemed to me to be having the notification and/or override as an opt-in feature you could buy. Southwest didn't want to train pilots and newly visible behavior requires new training. No surprise that airlines elsewhere in the world also skimped on the "optional" features to the doom of the passengers and crew.

The opt-in feature you mention was for a disagree alert for the plane’s angle of attack sensors.

But, MCAS was only ever using a single angle of attack sensor as its source of truth, so if there was a fault in that single sensor, MCAS could activate at normal attitudes.

But the real problem was that Boeing hadn’t documented MCAS, to avoid the requirement for pilots to obtain type certification for the MAX (so any existing 737 pilot could to fly it without additional training - and it wasn’t just Southwest objecting to this training), and to avoid a full FAA certification process for the MAX.

So on an MCAS activation, even if they had had an AoA alert, pilots would not know how to instinctively deal with it. (I don’t doubt an AOA disagree alert would give invaluable information allowing the pilots to rule out most failures, which when only a few thousand feet high would maybe have saved both planes.)

On the previous day to the Lion 610 crash, the incident aircraft suffered an MCAS activation and fortunately a pilot in the jump seat realised what was happening and what was needed to deactivate it.

It seems being sat behind the trim wheels (so he could see they were moving) and not wrestling to keep the plane in the sky at the same time was needed for that to happen. It must have been terrifying.

737 trim wheels are very visible and very loud. The noise and motion they create have been a joke within the 737 community for at least 20 years.

Yes, it's the aerodynamic lift from the big engine fans that was the 737 Max issue, according to Aviation Week. Fans have gotten bigger as engines have improved and the bypass ratios and fuel efficiency has gotten better.

Think of the engines as a big horizontal stabilizer but it's at the front, in front of the center of gravity. Normally it doesn't do much anything since the angle of attack is zero. But if you increase pitch, it will increase nose-up momentum.

For some reasons it's not common to put the engines further back. DC-9 style, on the sides of the rear fuselage might work. Or maybe over the wing and slightly to the rear, like Hondajet.

The difference is that all Airbus planes since the 1980's A320 are built around computerized flight commands, so the code paths involved are exercised constantly. MCAS only triggers occasionally, meaning if it has a bug, it only shows up at the worst possible time. Airbus planes also have double redundancy (three inputs and computers) for all those systems, where MCAS had none by default.

> If it's like the MAX the real issue is the flight characteristics

The real issue was the reliance on one sensor coupled with too much authority give to MCAS and inadequate failure analysis.

And pilots who did not read, remember, or follow the 2 step recovery process in the Emergency Airworthiness Directive sent to all MAX pilots.

I always get a negative reaction for mentioning the Emergency Airworthiness Directive sent to all MAX pilots. Media accounts never mention it, either.

The Boeing Emergency Airworthiness Directive says:

"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

https://theaircurrent.com/wp-content/uploads/2018/11/B737-MA...

For those who are skeptical of mainstream media fake news on the topic, here's the official final report on the Lion Air crash:

2018 - 035 - PK-LQP Final Report http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/2018%20-%20...

I also recommend the recent Aviation Disasters episode on it. While shallow, because it has to fit in 40 minutes, I was shocked that it was surprisingly accurate. Kudos to the Smithsonian channel. I wouldn't be surprised if AD waited until that report was released before doing the episode, so it would be using verifiable facts.

What are you trying to say, exactly?

Not sure what you find confusing about it.

Are you saying the pilots should have been able to handle it? Training wasn't needed? Obviously you're trying to make a point but you aren't saying it, you're just presenting supporting details. So I'm asking you to clearly state what the take away from all that is supposed to be.

> The Boeing Emergency Airworthiness Directive...

One out of 313 active AD's [1], 12 with no effective dates that seems to signify a different class of AD's because their issue dates are all over the map. I'm okay with 313 patches that I apply and the computer executes it from then onwards. Above a certain limit of number of AD's that should be memorized by pilots at all times, instead of entered into maintenance tracking systems for example, I'd like to hear from pilots why safety-wise that doesn't trigger a type re-certification.

I know that is expensive and time-consuming, but I certainly don't expect for example, my most senior ops team members to recall off the top of their heads more than a handful of critical procedures to execute on-the-spot without referring to the runbooks. Why does it appear that AD's are expecting pilots to apply a seemingly unbounded number of these "patches" like computers without re-training?

[1] https://www.faa.gov/regulations_policies/airworthiness_direc...

Your link is to Airworthiness Directives. The one I linked to is an EMERGENCY Airworthiness Directive. This EMERGENCY AD was about an actual crash of the same airplane, and contained a 2 step procedure from recovering from the MCAS malfunction.

It's a whole two steps:

1. trim back to normal with the electric trim switches

2. turn off the trim system

Not only that, this is what they're supposed to know anyway. Recovering from stab trim runaway is supposed to be a memory item, meaning memorized.

If you were a MAX pilot, wouldn't you pay attention to this?

People make mistakes, especially in high pressure situations.

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