> 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?
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.
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.
One hypothesis is that because the pilot was fighting the extreme nose down stabilizer with nose up elevator the jackscrew that moves the stabilizer was so heavily loaded that the co-pilot didn't have enough strength to manually turn the trim wheel which is connected to the jackscrew by cables.
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.
Here's the reality, the consequences, both financial and otherwise, for being wrong in China are a good deal more deleterious. (Ever lose that kind of a job for incompetence in China? You won't be getting another one.) So, not surprisingly, Chinese airlines, experts and regulators will more readily err on the side of caution. Simple as that. It's just basic human psychology.
By comparison, there are no consequences at all if you or I are incorrect in some assertion we make on Hacker News. Certainly no lasting repercussions of any great concern.
So making that comparison as you did is a bit non-sensical. Of course experts do things differently than some randoms on HN.
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
but you won't see it used heavily in the U.S. because of pilot union scope rules.
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.
Air France was criticized for not having high altitude training for the “vol avec IAS douteuse” procedure, in the Air France 447 final report. Pilots need to know all behaviors, natural and artificial, of an airplane.
I am very curious to what degree airlines knew of MCAS, and whether they instigated or were part of the conspiracy to withhold necessary training from pilots in order to avoid different a type certification for the 737 MAX, and thereby avoid pilots needing a type rating for it. The decision to deny training for MCAS is 180 degrees from the lesson that should have been learned from AF 447.
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.
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.
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.
