So the physical cutoff switches are still just inputs to software? WTF.
More likely is that the pilots turned the electric trim back on because as the aircraft gained airspeed (a normal thing when dealing with unreliable airspeed indications), the manual trim was not effective enough. This has been tried in the simulator:
https://leehamnews.com/2019/04/03/et302-used-the-cut-out-swi...
Generally, the manual trim and elevator can both overcome a nose-down trimmed stabilizer at the speeds seen just after takeoff. But if you don't rapidly trim back, and you let the airspeed build, you're now in a tricky situation. The manual trim is now too hard to move, increasing back pressure on the yoke is needed to counter the trim, and you don't want to throttle back, because the pitch up from the engines is all that's keeping you from diving. At this point, really your only course of action might be to re-enable electric trim.
This does lead to the worrying question as to whether a runaway nose-down trim in an older 737 just after takeoff could result in the same outcome.
Sure could but there are parts of the MAX that make it worse. The uncommanded stab trim movement on the MAX came about because of faulty angle-of-attack data. The bad AoA data meant that the pilots were faced with a "indicated airspeed disagree" warning (and on at least one of the Lion Air flights an "elevator feel system non-op" warning). The supposition here by MentourPilot is that the IAS disagree checklist dictated that the pilots increase their speed to the point where trimming manually is difficult or impossible. On an NG or Classic I don't think there's anything that would cause both IAS disagree and uncommanded stabilizer trim.
"[MCAS intervention] can be stopped by the pilot counter-trimming on the yoke or by him hitting the CUTOUT switches on the center pedestal. It’s not stopped by the pilot pulling the yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors. This would negate why MCAS was implemented, the pilot pulling so hard on the yoke that the aircraft is flying close to stall." [1] [my emphasis.]
So it seems we may have several issues here:
1) Prior to MCAS, trim stab cutout was almost the last-ditch option against trim runaway (there was one more: grab the trim wheels and physically prevent it), but it became the only way to deal with an MCAS-commanded runaway (other than grabbing the trim wheel.)
2) Boeing did not reveal this prior to the inquiry into the Lion Air crash, despite the fact that it introduced a new experience of trim runaway that differed from what pilots had been trained to expect.
3) The Boeing / FAA response to the Lion Air inquiry was an Airworthiness Directive that basically said "pay attention to what we said before about handling trim runaway" (though, whether or not Boeing / FAA made a point of it, pilots may well have become aware of how MCAS runaway differed from their prior training -- that seems to be the case for the Ethiopian Airlines captain, who did use the cutout switches.)
4) No one seems to have connected the dots to realize that one cause of MCAS failure (AofA sensor failure) would also cause other problems, for which the prescribed mitigation procedure made dealing with the consequent trim runaway very difficult, especially when close to the ground.
I would be interested in learning, from one of the 737 pilots here, how frequently the 'grab the trim wheel' situation is presented in simulator training.
[1] https://leehamnews.com/2018/11/14/boeings-automatic-trim-for...
That doesn't jive with the Indonesian Air crash or how MCAS works. MCAS makes repeated minor adjustments and the pilots can compensate for them. Which the Indonesian pilots did 20 some times before inexplicably stopping. You can see it pretty clearly in this image:
https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg
So I don't see how the Ethiopian air gets into a situation where they can't manually trim. They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there.
The Ethiopian flight was flying dramatically faster than a typical flight at that altitude would. If they followed the IAS disagree checklist that would explain the increased air speed as the general idea is to give the airplane enough thrust avoid a stall until you figure out which side has the correct air speed. Higher airspeed = more difficult to manually trim.
They should have been able to get back to a neutral stabilizer position, disabled electronic control, and manually trim from there.
They were barely 1,000 ft above the ground (if that). You could easily lose 1,000 ft fiddling with the elevator to unload the stabilizer to manually trim it into a safe position.
The Lion Air crew had far more altitude to work with (they were at 6,000-7,000 ft at their highest IIRC).
FWIW I think whoever's downvoting some of these comments is really hindering the discourse here. I think what you've posted is factually wrong, but not worthy of a downvote because it mostly seems to be wrong due to missing pieces from the puzzle (and not out of malice).
The point, which I think you missed, is that you don't need to manually trim. Look at the image I linked to. It's MCAS down 2.5 degrees. Pilots up 2.5 degrees. MCAS down 2.5 degrees. Pilots up 2.5 degrees. It's a repeated cycle of the pilots getting trim to where they want it and the MCAS kicking in to nose them down.
I suppose if you wait until the MCAS has trimmed all the way you might find yourself in an unrecoverable situation. But I'm not sure how that would happen and doesn't reflect what we saw in the Indoneisa Air crash.
Yeah that's what the Lion Air crew did, and it worked until it didn't (lost control in a turn?). But counter the trim with the push buttons and then hit the cutout is not in any Boeing QRH (yet?). As a non-pilot this seems like a reasonable option, but there may be something glaringly obvious as to why it's a bad idea. Even if this is the ultimate solution it's quite different to how you'd react in an NG and will come with extra training. Basically a bad situation all around.
If I've parsed everything appropriately the Ethiopian crew did try to turn the stab trim motors back on presumably to regain control over the stabilizer and we saw how that worked out. There's just not a lot of room for diagnostic work at that altitude.
IIRC the pilot handed control over to the copilot so that he could investigate the issue. The copilot stopped counteracting the trim.
Part of the reason that the MCAS got passed regulators is that it was not given enough authority to override the pilots. It can send you on a wild ride, but it can't crash the plane unless the pilot doesn't counter act the trim when the MCAS is paused.
From what I saw on that graph someone was trimming up as the plane nosedived. Take a look starting at the 23:30:53 mark. Someone was hitting the trim up button and MCAS was continuing to input trim down until the crash. The big problem with the preliminary report is the X axis, IMO. It's different on the graphs between the two flights, and there's not enough precision... but to me it looks like either hardware failure or the FO was simply not inputting enough trim up. They fought that plane all the way into the ground.
Part of the reason that the MCAS got passed regulators is that it was not given enough authority to override the pilots.
From what I've read MCAS as described to the FAA (relatively low limit on the down trim) was different than MCAS as implemented (about 4x larger movements allowed). Additionally it's not clear if the repeated down trim was by design or by bug.
I think that even MCAS in its current state is probably something most pilots could recover from as long as they know what to expect. The way MCAS works now (both in terms of the amount of authority as well as the misdirection from "unrelated" warnings) combined with zero documentation is a recipe for additional crashes.
Yes. But that's not what the checklist in Boeing's MCAS bulletin said.
It said, disable the electric trim motor then trim manually.
In hindsight, prehaps it should have said "return stabilizer to neutral then disable electric trim motor".
I wonder if Boeing did that deliberately, because a bulletin with a modified checklist would have undercut Boeing's implied narrative that a procedure already existed and it was the Lion Air pilots fault for not using the existing runaway stab trim checklist. Or did Boeing simply not test the procedure in a simulator?
https://spectrum.ieee.org/image/MzE4Mjg2OQ.jpeg
They were successfully doing trim up commands for at least 21 cycles to counter the MCAS trim down commands. Then after the last few activations, they try pressing trim up again, but the button presses are shorter and the stabiliser barely moves up.
Did MCAS push the stabiliser down past a point the aerodynamic forces were so strong that even the electric trim motor was having problems trimming up?
That could explain why the last 4 trim up commands were so short, if the pilots heard a large electric motor stuck noise they might have stopped holding the button.
When the stabilizer is extended and is being counteracted by elevator inputs, the forces on the control column make manually trimming stabilizers difficult or impossible. Boeing instructs that elevator input on the stick must be eased back in order to manually adjust the stabilizers.
If you object to people discussing the technical details of a plane w.r.t. the observed behaviour of that plane in an accident prior to the final report on the accident being released might I suggest....not clicking on the comment section, rather than trolling the comments on news items complaining that people are using the website for its stated purpose: to discuss the news being linked to?
Isn't speculation one of the things that characterizes us as humans?
I still don't understand why the trim is "stronger" on these planes than a fully pulled back column. Is this necessary on large planes?
On the 737 the trim adjusts the entire rear stabilizer (the entire horizontal structure of the tail), while the elevator is a sub-component of that stabilizer.
So, when you adjust the trim, it's a much larger surface area that is moving. Ultimately, the stabilizer just has more aerodynamic control that the elevator, due to the larger surface area.
I have no idea why the design is that way, though.
Cost, maintenance, weight, etc., probably. The only jet airliner I can think of that uses an "all-flying tail" is the Lockheed L-1011. Nearly every other plane you'd be riding on will have a big stabilizer and a smaller elevator.
If both redundant hydraulic systems to the elevator fail, or the elevator gets stuck for other reasons, the emergency procedure is to control pitch with just the stablizer trim alone.
It would be one more layer of complexity, but perhaps when the column trim switches are operated, all computer control could stop until it (autopilot, etc) is re-enabled by a human. The cutout switches in the center are still there if two column switches short out (used to be a single up-down switch, but it was single point of failure to runaway so now there are separate enable and direction switches).
(In case it's unclear, I don't support Boeing's position here.)