Before I begin, let me say that I got most of this information from news reports and science videos on Youtube, along with bits and pieces that I looked up on Google. I am not an expert in anything, and this summary should on no account be considered definitive. It is merely what I’m pretty sure I know.

To understand what’s going on with the Max 8 and 9 variants of the Boeing 737, it is necessary first to understand turbojet engines. In simplest terms, a turbojet engine, (hereafter a “jet”), is two sets of turbines connected by an axle and covered by a cylindrical wall that keeps pressurized air inside it. As the axle spins, the first turbine stuffs air into a combustion chamber. There, it is mixed with fuel and burned, which increases the pressure a lot more. The burned air next flows through the second turbine, which turns the axle, which turns the first turbine. Finally, the hot, burned air flows out of a nozzle at high speed and with high force. This, in agreement with Sir Isaac Newton’s third law of thermodynamics, causes an equal and opposite reaction which pushes the whole thing forward along with anything attached to it, (like an airplane.)

It’s like a gun, when you shoot it, pushing back against you, except that instead of one hard shove, it’s a constant push.

The problem with this is that a lot of the energy in the burned air gets thrown away as heat. That’s why jets are powerful, but not very efficient. What engineers do about that is make the second turbine bigger. This makes it take more power from the nozzle and add it to the power turning the axle. It’s more power than the first turbine needs, but then they add a third turbine on the axle in front of the first one. This makes better use of the exhaust energy.

The third turbine can be an old fashioned propeller and “turboprop engines” are very efficient. (They’re usually quieter, too.) But a turboprop can’t push an airplane as fast because propellers add drag. What engineers do instead is add a “bypass fan” inside an outer cover. This turbine doesn’t help make the jet go, it just adds thrust, (pushes the airplane), so it makes the whole thing a lot more efficient. The more energy from the jet that goes into bypass thrust, the more efficient it is.

The Max 8 and 9 variants of Boeing’s 737, (hereafter the Max 8), use a new jet engine with the highest bypass yet created. It makes the Max 8 about 14% more fuel efficient than the previous version, (series 800.) At current fuel prices, each flight would be a lot cheaper. This is good, (for somebody.)

But they come with a problem. The new engines are big. They’re so big that, mounted on series 800 engine pylons, (the parts that connect the jets to the plane), their bottoms come within seven inches (18 cm) of the ground, close enough that they could bump it. That’s not good.

There are a couple of things that the engineers could have done about this. They could have made the landing gear taller. That solution would have been “sub-optimal,” as they say, because then the gear would have taken more space in the nose and wings, weighed more and crowded either the fuel tanks or other important things. Instead, they changed the engine pylons. Making them shorter wouldn’t have helped, (they’re already short), so the engineers made them sweep up and forward. This gave the engines enough clearance from the ground for safe take-offs and landings.

But that created another problem. Now, I’m not an aerospace engineer, so it seems to me that the new pylons would make the plane a bit nose heavy, so that the nose would tend to tip down. In fact, the new pylons make the plane tail heavy, so that the nose tends to tip up. Neither is good, but a tail heavy plane can stall, especially taking-off; the wings tilt so far up that they don’t lift the plane anymore and it falls down tail-first. That’s very bad.

There are a few things that the engineers could have done about this. They could have added dead-weight ballast to the nose, lead plates or something. This would have been sub-optimal because it would make the plane heavier and take away from the number of passengers and their luggage that it could carry. They could have used live ballast to adjust the trim, for instance by putting a fuel tank in the nose and using that tank last. That would still have been sub-optimal, because fluids shift as the plane moves, which would have made it harder to control. (That’s bad.) The best solution would probably have been to move the wings back a little. The problem there is that a lot of things attach to the wings, wires and fuel pipes and hydraulic lines and what all else. Moving the wings, even a little, would have meant re-designing a lot of the plane’s systems and taken a lot of time.

Added to this, Airbus was selling a new version of their A320, which had the same new enginess. It was competing with the Max 8. So Boeing wanted to get their new plane out the door ASAP!

What the engineers did was install their Maneuvering Characteristics Augmentation System, MCAS for short. That’s an extra on-board computer connected to a pair of sensors as input and the systems that control the tail flaps as output. The sensors measure the plane’s “angle of attack,” how much the nose is up or down compared to the tail. If it’s getting close to the angle at which the plane would stall, it makes the tail flaps move to bring the nose back down. (It can do this because modern airplanes are all “fly-by-wire.” A computer watches what the pilot and co-pilot do to the controls and make the plane’s wings and tail and engines and so forth do what they want. It has to be that way because flaps and things are too heavy to operate the old fashioned way, you’d have to be the Incredible Hulk!®) And this solution does work.

But... surprise... there’s a problem. There are situations in which the two sensors give different readings. (I don’t know what those situations are.) When that happens, MCAS becomes unpredictable. It can make the nose point down, even though the plane is already flying straight and level. What’s worse, MCAS works in the background, so pilots don’t even know why the plane is trying to crash itself. And this is what happened to Lion Air Flight 610 and Ethiopian Airlines Flight 302.

Now there are several things that the engineers could have done to prevent this. They could have written the software that the MCAS computer runs to account for the possibility that the two sensors would disagree. And they’re doing this, now that people have died. They could have gone with just one sensor, although two gives the system back-up in case one fails. They could have used three sensors and had MCAS pick the two that agree. Sure, the other one might be the one that’s right, but it’s a lot less likely. And of course, they could have fixed the nose-heavy engine pylon problem in the first place. Then MCAS wouldn’t have been necessary.

In fact, the engineers did supply safety systems that would help when MCAS goes wrong. They put a couple of switches to disable MCAS beside the throttle controls. They also added an angle of attack indicator to show the pilot what MCAS thinks that is, and a “Disagree” light to warn when the two sensors are giving different readings, so that the crew are warned when MCAS is about to go wrong. There are two problems with this solution.

- Boeing offers the angle of attack indicator and disagree light as optional extras. Buyers can “cheap-out” and not get them. (Neither Lion Air nor Ethiopian Airlines bought them.)
- MCAS is not even mentioned in Boeing’s training, either for pilots new to the 737 line, or for 737 pilots new to the Max 8. (Only Boeing knows why.) The Lion Air pilot might never have known what the MCAS switches were for.

But it gets worse. Black box data from the Ethiopian Airlines crash showed that the pilot did figure out what the switches were for. The voice recorder showed that the co-pilot desperately searched the manual for the correct procedure and apparently found it. But MCAS seemed to keep turning itself back on! That is most definitely and emphatically Not Good!

TL:DR
So the proximate cause of the two crashes was the MCAS. The cause-in-fact, however, was big jet engines on a (relatively) small plane. The motive for wanting the big jets was reasonable, but it created a chain of solutions causing more problems that ended with an unsafe airplane. And what can we learn from this? In the words of Mick Jagger, you can’t always get what you want. Boeing should have put smaller engines on the Max 8, even though they were less efficient.