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Comment by Tuna-Fish | Hacker News Reader

A320 and the 737 were designed in entirely different worlds.

The 737 was designed using light tables and slide rules, to use low-bypass turbofans and direct controls with avionics only on board to optionally aid the pilots.

The A320 was designed in CAD and using CFD, with full digital fly-by-wire, and designed from the start for high-bypass turbofans.

Both designs have been updated plenty since, but because the basic design is much more modern, the A320 is much more amenable to being updated. There are elements of the 737 design that still exist on every new MAX coming off the line that would completely doom the certification chances of any new design, but are still there because they got grandfathered in for 737.

The wonder is not that the A320 finally caught up in sales, it's that the 737 can still be legally sold.

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> There are elements of the 737 design that still exist on every new MAX coming off the line that would completely doom the certification chances of any new design, but are still there because they got grandfathered in for 737.

Not only that, but Boeing is actually limited in how much they can "modernize" the 737, because doing too much might exceed the limits of the 737's type certificate. This is the reason behind the current engine inlet overheating worries, which has led to an airworthiness directive for the 737 MAX (https://aerospacenews.com/faa-airworthiness-directive_boeing...) and is also one of the reasons for the delay certifying the MAX 7 and MAX 10. This would be a complete non-issue for other planes, because all modern designs have a switch position that only turns on the engine anti-ice system when it's needed, but the 737 MAX can't have that because the 1967 737 didn't.

This sounds odd, they're able to certify the crazy MCAS but not a simple anti-ice switch?

I know that it's a complete nightmare to certify anything. However I apparently don't understand some underlying principle that allows to certify some things and not the others.

The constraint here (entirely self-imposed by Boeing's sales strategy) is that the newer models have to basically behave like the older models to avoid needing a new type certification. (See https://en.wikipedia.org/wiki/Supplemental_type_certificate) The aircraft behavior, and pilot procedures and training have to be substantially similar. That was the whole point of MCAS, to make the plane fly as-if it were an older model, despite the significantly different aerodynamics. Changing anti-icing procedures is apparently too big a difference to maintain the original type certification.

> That was the whole point of MCAS, to make the plane fly as-if it were an older model, despite the significantly different aerodynamics.

MCAS was implemented to make sure the control forces increase going into a stall, this is a requirement in the regulations. Without MCAS the control forces would drop on the way into a stall, which is an issue that would prevent certification of the aircraft.

> The wonder is not that the A320 finally caught up in sales, it's that the 737 can still be legally sold.

What’s wrong with the 737 design that it wouldn’t pass today as a new aircraft? (Ignoring the disaster that was the MAX.)

From my understanding, mostly based on Kerbal Space Program, the aircraft isn’t well balanced when equiped with modern engines.

So you have to constantly apply some controls to fly, done by software.

I love stupid car comparisons so imagine a car with a new engine that is more economical to run, but very heavy on the left so the car constantly want to turn left. But if you apply force to the steering wheel manually or the car does it for you with software, all good. Still a shit car though.

The main issue arose because Boeing wanted to install larger, more fuel-efficient CFM LEAP-1B engines without changing the aircraft’s landing gear height too much (which would have required expensive redesigns of the fuselage and systems, triggering a new certification process). On earlier 737s, engines were already mounted quite far forward under the wing because of the aircraft’s low stance. The larger MAX engines could not fit in the same place without scraping the ground. Boeing moved the engines further forward and higher on the wing. This changed the center of thrust and lift characteristics. At high angles of attack (nose up), the repositioned engines created extra nose-up pitching moments, making the aircraft more prone to stall. To make the MAX “feel” like older 737s (so pilots wouldn’t require expensive retraining), Boeing added software — MCAS. MCAS automatically trims the horizontal stabilizer nose-down if it detects a high angle of attack, countering that engine-induced pitch-up. The tragedy was that MCAS initially relied on a single angle-of-attack sensor, so a faulty reading could (and did) trigger repeated nose-down inputs, leading to the two fatal crashes (Lion Air 610 and Ethiopian 302).

I think they added redundant sensors which should theoretically prevent this in future. IMHO, I think several issues compound here. They should have redesigned the fuselage. The engineering compromise is bad, but if handled with care, could have been done relatively safely. They opted for no additional pilot training re MCAS. This was a fatal mistake, compounded by them relying on a single sensor. Nothing in avionics relies on a single sensor for remaining in the air. That was insane. There MUST have been engineers screaming about safety who were ignored.

That makes a very intersting read and adds the necessary context to the previous comment how that plan can actually be legally sold.

Boeing already had a angle of attack second sensor with the original design of the Max, but they ignored it and made decisions only using a single sensor.

How anyone thought making critical decisions on a single sensor reading made sense is beyond me.

> MCAS automatically trims the horizontal stabilizer nose-down if it detects a high angle of attack

I remember when I first heard about the MCAS issue and thought "Why didn't the pilots just pull up?" and someone explained to me that MCAS worked by trimming, and it would trim nose-down so hard that even with full elevation to try to nose back up, it wasn't enough to overcome the extreme amount of nose-down from the trim.

And since pilots weren't trained on the new MCAS system, they weren't aware that the trim would have been automatically moved.

For starters, the bicameral avionics.

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