
What is the Boeing 737 Max Manoeuvring Characteristics Augmentation System? - js2
https://theaircurrent.com/aviation-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas/
======
cperciva
I'm struck by the connection between this and the Air France Airbus 332 which
crashed en route from South America to France about a decade ago.

It seems that the 737MAX (at least, the one last year -- we don't know about
the latest one yet) crashed because a faulty sensor caused the computer to
automatically point the nose down in order to avoid a perceived stall risk and
the pilots weren't able (or didn't know how) to disable that.

In contrast, the 332 crashed because a faulty sensor misled the flight crew
into pointing the nose up and getting into a stall.

In both cases, crashes resulted from reliance on a faulty sensor -- but in one
case it was flight crew and in the other case it was software. Solving this
isn't going to be as simple as saying "the flight software should defer to the
pilot" or vice versa.

~~~
js2
I think you’re slightly mischaracterizing the cause in the AF447 flight in
that there was a lot more going on than just the airspeed indicator:

 _AF447 crashed because of a cyclical series of erroneous inputs based on
unreliable prompts from aircraft systems; a cycle that fed on itself to the
extent that control was never regained. The BEA’s final report attributes
multiple factors to the AF447 mishap: temporary and repeated inconsistencies
and loss of airspeed indication; inappropriate control inputs; failure to
identify unreliable indicated speed; the approach to stall; the full stall
state; and the inability to apply an appropriate response in these flight
regimes._

[https://sma.nasa.gov/docs/default-source/safety-
messages/saf...](https://sma.nasa.gov/docs/default-source/safety-
messages/safetymessage-2012-07-31-lossairfranceflight447.pdf?sfvrsn=e3ae1ef8_4)

Also, in the AF447 case the computer had to defer to the human because it
didn’t know what the air speed was. In contrast, one thing that’s surprising
to me in Lion case is that the MCAS seemed to be willing to trust a faulty AoA
sensor:

 _The DFDR recorded a difference between left and right Angle of Attack (AoA)
of about 20° and continued until the end of recording._

[http://avherald.com/h?article=4bf90724/0009&opt=0](http://avherald.com/h?article=4bf90724/0009&opt=0)

So it’s got a pair of AoA sensors that aren’t in agreement (split brain, as it
were) and it’s going to command nose down anyway?

(I’m sure there’s a reason there aren’t triple redundant sensors where two
have to agree or some such. I think the flight computer does require the
various different kinds of sensors to agree, but I don’t think there’s triple
redundancy of each sensor.)

(Edit: sorry folks, I edited this a bunch of times so the replies below may
not make perfect sense. I should really start typing my replies offline.)

~~~
carlivar
> the pilot’s action of pulling up the nose was an irrational thing to do

Actually the inexperienced COPILOT did this. The pilot realized that was a
problem eventually, but too late. And unfortunately the Airbus takes the
average of both pilot inputs.

~~~
learc83
>And unfortunately the Airbus takes the average of both pilot inputs

Really? That's the best way to handle that?

~~~
carlivar
Since I read that Popular Mechanics article, I think the exact same thing
every time I board an Airbus plane.

~~~
richardhod
Airbus has since updated its flight control feedback systems

~~~
foldr
Source for this?

~~~
richardhod
I may be mistaken; I had thought I had read that Airbus had acted, but on
checking up, I think not in ways which would make my brief implication useful.

------
samcheng
It's ironic that it's a Boeing with bad software (or sensors) interfering
between yoke and plane. After all, historically, Boeing has had the philosophy
of trusting the pilot with direct control of the plane. This is in comparison
with Airbus, where controls are by stick instead of yoke, and pilots are
taught to trust the flight computer and move the controls less. (Of course,
these days both manufacturers build fly-by-wire planes.) [1]

The addition of a 'maneuvering characteristics augmentation system' seems very
unlike Boeing. I guess it became necessary after adding too-big engines
without otherwise adjusting the airframe... [2]

This "we'll fix it with software later" attitude works well with CPU errata
and maybe Teslas, but it doesn't look like we're there yet for airplanes,
unfortunately.

[1] [https://www.pprune.org/tech-log/10321-stick-vs-yoke-
airbus-v...](https://www.pprune.org/tech-log/10321-stick-vs-yoke-airbus-v..).

[2] [https://theaircurrent.com/aviation-safety/what-is-the-
boeing...](https://theaircurrent.com/aviation-safety/what-is-the-boeing..).

~~~
goodcanadian
_This "we'll fix it with software later" attitude works well with CPU errata
and maybe Teslas, but it doesn't look like we're there yet for airplanes,
unfortunately._

I'm sure I'm preaching to the choir here, but I'm absolutely against the "fix
it with software" attitude. I'm working with embedded systems, now, and while
we can often make it work by tweaking the software, it always results in a
less than optimal solution. Fix the hardware; get it right, preferably before
manufacturing a few thousand units.

~~~
iSnow
Doesn't work in the current speed of development any more. Time to market is
more critical than maturity, sadly. That's why every system can be fixed in
software now, and that's also why every system can be pwned now.

------
bronco21016
Lots of great technical discussion about MCAS and potential improvements in
here.

I think the bigger issue though that needs to be addressed is the
certification and training process. What factors, other than the obvious
engineering factors, were driving the design process? Why were those factors
allowed to also influence the training administered to pilots? After the Lion
Air crash the response from US pilots unions seemed to speak volumes. Pilots
were not informed of this system on their new aircraft and as such were
treating the aircraft the same as all other aircraft falling under the 737NG
type rating.

Airlines and Boeing worked together to minimize training costs to help the
aircraft sell easier and as a result critical systems knowledge was left out.
Knowledge that likely could have saved Lion Air.

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toufka
Here's a pilot describing the MCAS system on the 737 Max:

[https://youtu.be/zfQW0upkVus?t=220](https://youtu.be/zfQW0upkVus?t=220)

~~~
lostsock
Super interesting, he's really great at explaining things in laymens terms.
Thanks for sharing

~~~
halfmatthalfcat
I follow Mentour's channel pretty regularly. For anyone with any flying
anxiety, it's great to see how seriously they take their jobs, how regimented
they are and how really safe you are flying.

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ChicagoBoy11
The big issue here is how much of the existence of this system, the desire to
NOT make OTHER changes to the plane that would inhibit the pitch-up
tendencies, and the lack of necessity for another type rating were all driven
by simply an attempt to skirt existing regulations and save Boeing and
Southwest a ton of money.

The reality is that this was the whole deal with the Max. All of these
decisions were made in a context were avoiding the requirement for a new
certification or type rating for pilots was a must. We can't avoid the reality
that had the necessity to skirt regs not been there, Boeing would have
definitely come up with a different solution from an engineering perspective.

~~~
rpiguy
To add to your hypothesis - making a change to the fly-by-wire system was
probably much cheaper than making airframe changes to make the aircraft more
neutral in flight (airframe changes which would likely also negatively affect
fuel economy).

So Boeing took the cheap rout and decided to have the computer handle the
pitch-up tendencies of the aircraft...

This kind of fix is quite common on fighter aircraft which are purposely
designed to not be neutral/stable in flight for better handling
characteristics (among many other reasons).

I have no idea how common this kind of fix is on a passenger aircraft.

~~~
PaulHoule
It is a violation of the brand expectations pilots have for Boeing.

Boeing introduced fly by wire after Airbus and it did it in such a way to
preserve the feel of flying a plane, in particular if you have a fight with
the system you could pull harder and win.

Some pilots have strong feelings about this and are loathe to switch from
Boeing to Airbus or vice versa.

~~~
ChicagoBoy11
As a pilot (sadly not a Boeing, or anything close to that side), I think this
is a non-trivial point. Every second professional airline pilot you will talk
to will talk about that philosophy difference between Airbus and Boeing, and
99.999% of them will take Boeing's side.

Sadly, pilots really don't have much of a say in those discussions. They'll
fly what's put in front of them, but I can assure you that all feel safer
being given a plane not trying to actively fight them.

------
avar
On the subject of why Boeing didn't think to tell pilots about the MCAS:

> "Since it operates in situations where the aircraft is under relatively high
> g load and near stall, a pilot should never see the operation of MCAS"

This seems to me to indicate a much deeper issue with the aviation industry
than the 737 MAX issue. Aren't pilots as part of their simulation time testing
the limits of the flight envelope of the aircraft they're flying? If so surely
they'd be expected to encounter the MCAS system.

~~~
plt11
I think Boeing claimed that no retraining of existing 737 pilots was necessary
for the new model.

This attitude has led to crashes in the past as well.

~~~
alkonaut
If airlines would need to invest in a lot of training for existing 737-non-MAX
pilots, that would probably have made the plane a lot less attractive. Being
"a regular 737" is part of the appeal of the 737 MAX.

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djsumdog
Going back a few generations, the earlier 737s were plagued by crashes as
well, relating to rudder issues:

[https://en.wikipedia.org/wiki/Boeing_737_rudder_issues](https://en.wikipedia.org/wiki/Boeing_737_rudder_issues)

~~~
BorRagnarok
From that article: "leading the manufacturer to suspect and insist that the
pilot had responded incorrectly"

After a few years of course it was concluded that it wasn't the pilots fault.

I find it appalling that Boeing is always and every time trying its damndest
to blame the crew, even before the investigation has concluded. This is a
pattern, and imho should be penalized by law. Its disgusting to blame people
who are dead and cannot defend themselves against the allegations, when
there's no conclusive evidence. I wonder if they apologize to the crews family
when they do that and are later found out the be in the wrong. I suspect they
couldn't care less though.

~~~
nisa
Yes, their reaction to crashes are terrible. Just look at what happened to
Lauda Air Flight 004 that came up in a thread about this yesterday:
[https://en.wikipedia.org/wiki/Lauda_Air_Flight_004#Investiga...](https://en.wikipedia.org/wiki/Lauda_Air_Flight_004#Investigation)

"Lauda attempted the flight in the simulator 15 times, and in every instance
he was unable to recover. He asked Boeing to issue a statement, but the legal
department said it could not be issued because it would take three months to
adjust the wording. Lauda asked for a press conference the following day, and
told Boeing that if it was possible to recover, he would be willing to fly on
a 767 with two pilots and have the thrust reverser deploy in air. Boeing told
Lauda that it was not possible, so he asked Boeing to issue a statement saying
that it would not be survivable, and Boeing issued it. Lauda then added, "this
was the first time in eight months that it had been made clear that the
manufacturer [Boeing] was at fault and not the operator of the aeroplane [or
Pratt and Whitney]."[18]"

------
andy_ppp
I mean just to give an anecdote that is slightly worrying; I was about to land
in Grenoble and it was quite rocky the turbulence and incredibly windy, you
could tell we were coming in too fast and the pilot decided to back out of the
landing and take off again with the wheels nearly on the runway.

Suffice to say the angle of attack was extreme and the power produced by the
engines quite impressive/terrifying. I would say anything that decided to
change the express wishes of the pilot in that situation would have been
_hugely_ dangerous.

Can someone explain why ever overriding the pilot rather than warning them
would be a good thing?

~~~
sean-duffy
I can't find a source right now but I remember reading that there had been a
few cases where there had been an emergency situation and pilots had ignored
stall warnings, resulting in the plane stalling and crashing. This was why
Boeing had elected to make the system on the 737 Max 8 override the pilots
inputs.

I think having an emergency system that overrides the pilot is fine, but it is
crucial that the pilot can easily and quickly disable such a system when they
know the plane is doing the wrong thing.

~~~
tarabanga
> a few cases where there had been an emergency situation and pilots had
> ignored stall warnings

I remember that being about pilots' becoming over-sensitized to visual (red
lights) over audio ("stall warning" audio) warnings during a crisis, often
ignoring the audio inadvertently (i.e. not hearing it).

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coryfklein
Two planes full of passengers destroyed because the pilot couldn't get the
nose to point upward and we find a secret computer system that overrides
manual controls and automatically points the nose downwards?

Yeah, sounds like a real possibility to me.

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spookyuser
Interesting that this website prevents you from copying text

~~~
doodlebugging
You can use the option in your browser to view the page source and grab it
from there.

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oasisbob
Another few good technical articles on MCAS, and other preexisting automatic
stabilizer trim adjustments on the 737:

[http://www.b737.org.uk/mcas.htm](http://www.b737.org.uk/mcas.htm)
[http://www.b737.org.uk/flightcontrols.htm#Stab_Trim](http://www.b737.org.uk/flightcontrols.htm#Stab_Trim)

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raverbashing
To me, the biggest contradiction (as it seems on the design of the system) is
that it seems to rely on very fickle measurements to command big movements on
the aircraft. So, in the case of Lion Air, you have 2 contradictory inputs and
you're basing an automated decision on it?

Also, it seems aviation hasn't caught up with 20th century fluid measuring
techniques. Speed of fluids in pipes can be measured non-invasively. With the
biggest caveat that doing things inside a pipe on the ground is easier it
seems we're still using 19th century techniques to measure fluid speeds and
direction. Which didn't matter too much when their noisy/unreliable inputs
were simply ignored or compensated by pilots and mechanical devices.

No real world scenario (at least not a surviveable one) can produce the sensor
readings that happened in AF447 (sudden drop in airspeed) and in the Lion Air
case. But this is being taken at "face value" (there is some filtering) and
acted upon it (with no contradictory input from other systems - for example an
accelerometers - and yes it's not so obvious how to put everything together).

~~~
dreamcompiler
The AoA sensor doesn't measure fluid pressure in a pipe; it measures fluid
_direction_ with a small mechanical wing that rotates to align itself with
airflow [0]. Thus it has to move, unlike a pitot tube. If it can't rotate for
some reason, it gives faulty data.

[0] [https://aviation.stackexchange.com/questions/2317/how-
does-a...](https://aviation.stackexchange.com/questions/2317/how-does-an-
alpha-aoa-vane-work)

~~~
raverbashing
Yes, I'm aware of the differences, but you could measure air speed in two axis
then calculate AoA from there, that's what I mean.

~~~
burfog
It's done. Here you go, only a few hundred dollars:

[https://www.dynonavionics.com/aoa-pitot-
probes.php](https://www.dynonavionics.com/aoa-pitot-probes.php)

~~~
dreamcompiler
Nice! Finds AoA through vector addition. Now I'm curious why Boeing uses a
moving sensor. Seems like having two different kinds would be ideal.

