Thread by @monosynth

So the 737 MAX has this weird little glitch that causes it to actively accelerate and dive into the ground.

I’m not making this up.

https://twitter.com/alexinair/status/1104663346874404865

The MAX is a redesigned version of the 737 whose biggest selling point is greater fuel efficiency.

One of the design changes that led to this improved efficiency are larger engine nacelles that are positioned higher and farther forward.

Here’s a comparison, courtesy of AirlineSpotter

While this change allows the MAX to be more efficient, it also changes the flight characteristics of the aircraft in a few ways; the biggest of these is arguably the tendency of the nose to rise and the aircraft to climb

To counter this, Boeing engineers developed an addition to the avionics system to detect and counter these unwanted climbs — and the potential resultant stalls — by increasing the speed of the aircraft and pitching the nose down.

This system, known as the Maneuvering Characteristics Augmentation System (MCAS), was designed to work automagically, without pilots’ input or even awareness.

On its own, this isn’t particularly unusual. Fly-by-wire controls have long included little modifier bits to improve handling and to make aircraft feel more intuitive to pilots.

However, following the crash of Lion Air 610 last October — in which a 737 Max curiously took a powered nosedive minutes after takeoff — investigators started to suspect MCAS, specifically, may have had a few problems.

For starters, when MCAS detects a pre-stall event (real or perceived) it is supposed to begin the process of pointing the nose down only until a pilot manually overrides it — but it will try again every few seconds

The day before the Ethiopian Airlines crash, passengers and crew reported what sounds like a thoroughly horrifying ride in which the aircraft kept climbing and diving in a sort of rough sinusoidal pattern throughout the flight.

Based on preliminary reports from ground crew and investigators, this is believed to have been caused by the MCAS system responding to perceived stall events caused by malfunctioning Angle of Attack (AoA) sensors.

When pilots resisted MCAS’ automatic dive & accelerate routine, it would kick back on a few seconds later, resulting in those wild flight oscillations.

ｉｍｐｏｒｔａｎｔ
ｎｏｔｅｓ

❶ the comments about the flight oscillations on flights leading up to the crash pertained to the Lion Air aircraft, not that of Ethiopian Airlines (mea culpa)

❷ not only is it way too early to conclude what brought down Flight ET302, even the Lion Air Flight 610 crash is still formally under investigation and will likely continue to be so for several months

❸ I am — somewhat obviously — not an FAA investigator;
nor an avionics engineer,
nor a pilot

The major impetus behind this thread was me reading about MCAS and the abysmal way it was introduced to pilots (i.e., in some cases, not at all).

Those arguing whether any MCAS-linked crash should be chalked up to computer error or human failure are largely missing the point.

Those arguing that the Lion Air and Ethiopian Airlines crashes only happened due to poor training in those carriers’ respective countries are racist.

Sorry, I don’t make the rules.

¯\_(ツ)_/¯