
737: The Max Mess - breadbox
http://bit-player.org/2019/737-the-max-mess
======
PaulHoule
A comparison to the A320 is in order.

The A320 (as well as modern Boeing Airliners like the 777 and 787) has a
comprehensive system of "flight envelope protection" which automatically acts
to keep things like a pitch runaway from happening.

In a modern aircraft, flight envelope protection is closely integrated with
the fly-by-wire system in normal operation. If sensors are degraded, the
system reverts to a "control rule" which has less protection.

In the case of the 737 they retrofitted a partial flight envelope protection
system onto an old aircraft. Thus it wasn't properly designed, tested, and
pilots not trained how to use it.

~~~
kevin_thibedeau
The aircraft is also uncontrollable when this system is disabled unless you're
a crack Boeing test pilot.

~~~
cmurf
Citation? This is a very significant claim if true, it would be a clear
violation of FAR 25.173. I don't see how a plane is certified to be airworthy
by software work around, in particular one that can be disabled instantly
rendering the plane not airworthy.

~~~
tyingq
Not exactly as strong as "crack test pilot", but from the article:

 _" after a few minutes of wrestling with the control yoke, the pilots on
Flight 302 did invoke the checklist procedure, and moved the STAB TRIM
switches to CUTOUT. The stabilizer then stopped responding to MCAS nose-down
commands, but the pilots were unable to regain control of the airplane."_

~~~
cmurf
That is due to the mistrim situation that a perturbed MCAS had already put the
plane into. Proposing that without MCAS (from the outset) the aircraft is
inherently unstable is what I'm questioning.

~~~
mike_john23
You can see the previous flight made by the same lion air plane that crashed
here[1]. They not only did not declare an emergency but they contained the
flight as normal and flew for over an hour with no MCAS and no electric trim.

[1]: [https://leehamnews.com/wp-
content/uploads/2018/11/Previous-f...](https://leehamnews.com/wp-
content/uploads/2018/11/Previous-flight.png)

I should point out they found the stab cut out procedure by trial and error
and were able to maintain control of the plane simply by counteracting the
errant downward trim. The reason the Ethiopian plane crashed is that they
didn't execute the airspeed disagree procedure and left the throttles on
basically maximum.

Yes MCAS is an issue, But since no one on this site has gotten this right I
might as well point that out

~~~
cjbprime
You're saying that IAS disagree and unreliable airspeed ought to have caused
them to throttle down? That seems counter-intuitive. If you don't know how
fast you're going, how are you supposed to avoid stalling?

Are you looking at a specific QRH entry? What does it say?

------
jayess
> By the time of the Ethiopian crash, 737 pilots everywhere knew all about
> MCAS and the procedure for disabling it. A preliminary report issued last
> week by Ethiopian Airlines indicates that after a few minutes of wrestling
> with the control yoke, the pilots on Flight 302 did invoke the checklist
> procedure, and moved the STAB TRIM switches to CUTOUT. The stabilizer then
> stopped responding to MCAS nose-down commands, but the pilots were unable to
> regain control of the airplane.

> It’s not entirely clear why they failed or what was going on in the cockpit
> in those last minutes. One factor may be that the cutout switch disables not
> only automatic pitch trim movements but also manual ones requested through
> the buttons on the control yoke. The switch cuts all power to the electric
> motor that moves the stabilizer. In this situation the only way to adjust
> the trim is to turn the hand crank wheels near the pilots’ knees.

Wow, so disabling MCAS also disables _all_ stabilizer commands?

~~~
KMag
My understanding is that the horizontal stabilizer position is determined by
the sum of the yolk ("steering wheel") position and the trim wheel position.
MCAS might actually spin the trim wheel. In any case, MCAS trimmed the plane
way out of whack, such that even with the yolk all the way back, it wasn't
flyable. The only way to fix the trim was to turn back on the electric motors
on the trim system (including its MCAS subsystem) or else crank the trim wheel
like mad by hand and hope their hands are fast enough to get the plane sanely
trimmed before the plane lost too much altitude.

Edit: I see inamberclad mentioned an additional detail that the trim wheels
get stuck when the yolk is pulled all the way back. I wasn't aware of this,
but it makes the pilots' actions make much more sense.

~~~
cjbprime
That doesn't sound right. It sounds like it might be correct on an Airbus, but
not a Boeing. On a 737, yoke movement mechanically controls the elevator and
the trim system mechanically (and with a motor to help) controls the
stabilizer. There is no connection between the yoke and the stabilizer on a
Boeing. Does that make sense?

It's not just that the trim wheels don't work when the yoke (not yolk) is
pulled, but that it can require too much torque to move them -- it's a purely
mechanical connection with the motor off -- when the elevator is deflected in
opposition to the stabilizer.

------
cjbprime
Great article! The only thing missing is a discussion of whether manual retrim
after disabling the stabilizer trim motor was actually possible given the
aerodynamic constraints. This theory's been discussed quite a bit here on HN
and pilot forums.

The theory is that, on the Ethiopian flight, after disabling the stabilizer
trim motor while there is mistrim, the aerodynamic load on the jackscrew was
too great for it be movable by the copilot using their crank. The pilot was
perhaps using their strength to pull the yoke back, which meant both that he
was unavailable to help crank, and that the aerodynamic load was increased by
the elevator directing airflow in opposition to the stabilizer.

There is an old “yo-yo maneuver” that stopped being mentioned in Boeing
manuals decades ago that describes having to relieve load on the stabilizer
before manually trimming, in this case by releasing the elevator and pushing
the nose down even further, which I’m sure would have been very unattractive
given their high airspeed and low altitude if the pilots even understood the
procedure, which is no longer part of simulator training.

This may explain why the trim motor appears to have been re-enabled towards
the end of the Ethiopian flight: because purely manual trim was impossible.

------
huslage
This is dangerous conjecture at best. I'm a little sick of reading that MCAS
shouldn't exist.

MCAS must exist.

The flight regime where MCAS would need to act should be exceedingly rare. The
bug isn't that MCAS exists. The bug is that there is a failure in the
code/inputs to the system which allow it to trigger in regimes where it
shouldn't.

~~~
cmurf
Are you asserting that MCAS must exist as a condition of its certification?
And that without it the airplane is not airworthy?

Can you reference any checklist item that when followed causes the airplane to
become unairworthy?

If MCAS is required for the airplane to be airworthy, following the FAA and
Boeing's directive to set stab trim to cutoff in the specific case of "MCAS
upset" under discussion, directs pilots to make the airplane unairworthy as a
work around for a perturbed system. I don't see how this is a defendable
regime.

------
gonvaled
It took China for once having an independent policy instead of "impartial"
agencies pushing their agenda onto a defenseless world.

May I remember that most HN jumped to defend Boeing and the FAA, without a
shred of evidence?

I welcome our Chinese overlords.

Competition is good.

~~~
PaulHoule
See

[https://en.wikipedia.org/wiki/Comac_C919](https://en.wikipedia.org/wiki/Comac_C919)

which is another boring narrowbody which is not technically ambitious at all
and could only compete with the long-in-the-tooth 737 and the slightly newer
A320.

I'd like to see somebody shake up the market with a plane that is small on the
outside and big on the inside like

[https://en.wikipedia.org/wiki/Embraer_E-
Jet_family](https://en.wikipedia.org/wiki/Embraer_E-Jet_family)

but you won't see it used heavily in the U.S. because of pilot union scope
rules.

~~~
Bendingo
What do you think of
[https://en.wikipedia.org/wiki/Irkut_MC-21](https://en.wikipedia.org/wiki/Irkut_MC-21)
?

