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Comment by breadwinner | Hacker News Reader

> Is it really that cheap?

737 MAX uses 20% less fuel.

Incidentally, this fuel-efficient design is what lead to the design defect. The fuel efficiency is thanks to larger engine (larger engines burn hotter and use less fuel). To make the larger engine fit under the wing without raising the fuselage, they had to reposition the engine forward. This forward-positioning of the engine causes the nose to pitch up when accelerating quickly (and this can cause stall) and they fixed this defect using software to push the nose down.

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> This forward-positioning of the engine causes the nose to pitch up when accelerating quickly (and this can cause stall) and they fixed this defect using software to push the nose down.

MCAS was not intended to be an anti-stall system.

The only purpose of MCAS was to have the 737 MAX exactly mimick the 737 NG's (previous gen) pitch dynamics so that airlines wouldn't have to retrain their pilots on the 737 MAX.

Commercial pilots are generally pretty good at not stalling planes and understanding flight dynamics, they don't need MCAS to avoid stalling even when the plane pitches up during thrust.

The biggest WTF to me is hiding the existence of this system from the pilots, even after the first crash. All in service of pretending that it’s exactly the same airplane as 737 NG.

The system was visible if the airline paid for the AOA disagree lights to be installed. Why such a basic feature was even an option is the WTF.

would that light be sufficient in the 1:30 minutes you have to make a difference, in between the cacophony of alerts and data feeds ?

I can’t find anything about that. As far as I can tell, the optional AOA indicator would also include the disagree alert which would tell the pilots something is wrong with the sensors, but that had nothing do to with MCAS per se. They still had no idea there’s this whole new system that uses only one of the sensors and repeatedly trims down. Even after the first crash they only went as far as recommend going through the runaway trim procedure without explaining or even acknowledging MCAS’s existence. The pattern of omission is downright criminal.

> recommend going through the runaway trim procedure

The EA crew did not follow the runaway trim procedure sent to them in an Emergency Airworthiness Directive. The procedure is:

1. restore normal trim with the electric trim switches

2. turn off the trim system

amazing how many seem willing to overlook this fact for their narrative

MCAS it is correcting the design defect due to put larger motors. Without MCAS that aircraft do not meets the certification requirements for to fly.

This video is about bledend-wing designs, but it touches on the 737 MAX problem with illustrations,

https://www.youtube.com/watch?v=59A8-rKRs-0&t=705s

at minute 11:45 , but it's also worth watching the part at 4:31 , where another illustration explains the forces at work in an aircraft, using a classic 737 as an example.

It is on a different subject but the whole video is very interesting.

Recent statements and speculation from FAA / EASA / Pilots suggests that the plane would've likely passed certification without MCAS: https://theaircurrent.com/aircraft-development/mcas-may-not-...

That said, it's true the MCAS was intended to address some comments from test pilots during development.

The reason they didn't want to raise the fuselage was to pretend it was actually a 737 and not a completely different plane, so that they could avoid all the testing required. If they'd just built it properly in the first place...

There was no way to raise the fuselage. That would require longer landing gear with no space in the wings to retract it, so it required new wings, leading to re-certifications of both the planes and the pilots and the associated costs.

This grand-fathering of 737 is the reason it still does not have fly-by-wire in 2024: it simulates the antique original 737.

In retrospect maybe they should have based the MAX on the 757. It's more or less a stretched 737 with longer landing gear. Sounds perfect for installing those enormous engines.

The primarily 737 airlies wouldn't have switched.

There’s a thousand reasons why this not true and not a good idea.

Does putting bigger engines more forward really have less impact on certification and training than higher landing gear and wings to fit them? Couldn't the effect of those wings have been corrected in a way similar to MCAS?

Or was misleading people on the invasive nature of the changes always the point? And MCAS is easier to hide than new wings?

> This grand-fathering of 737 is the reason it still does not have fly-by-wire in 2024

The 737 does not have fly-by-wire? Then how does MCAS work?

> larger engines burn hotter and use less fuel

I thought it was mostly due to the much higher bypass-ratio than burning temp?

The two are probably related. Intuitively, at least. For a given thrust level, a higher bypass ratio seems to imply less air going through the combustion section. Thus this smaller amount of air "works harder", probably getting hotter in the process.

It's both.

"The new two-stage High Pressure Turbine section adds advanced coatings for cooling metal parts that will enable the same metal temperatures as today’s engines despite hotter gas path temperatures."

https://web.archive.org/web/20150418191419/http://airinsight...

I learned in college that the limiting factor in turbine efficiency is how high the turbine temp can go before it fails. Engine designers get better and better at finding ways to get the temp up. Jet engine turbine blades are really miracles of design and metallurgy.

The same improvements are being made to electric utility turbines.

20% fuel savings is massive. At that point your safety evaluations need to consider the safety tradeoffs that arise because people decided to drive cars instead of flying, and died on the highway.

The 20% savings is compared to a 737 NG. On longer flights it still uses more fuel than an A320 NEO. Scroll down to page 23.

https://www.aircraft-commerce.com/wp-content/uploads/aircraf...

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