7 ms·
So the physical cutoff switches are still just inputs to software? WTF.
by keyme 8y ago
So the physical cutoff switches are still just inputs to software? WTF.
- bilbo0s 8y agoIf that's true, it's inexcusable. But we should wait to find out more information.
- ams6110 8y agoNo, the working theory I've heard is that the hand-crank trim wheels may have been overwhelmed by the air load on the stabilizer given its "nose down" configuration, probable full "nose up" pull on the elevators, and the very high speed they were flying. Speculation is they may have turned the electric trim back on to use the manual trim switches to trim up. By that time possibly too late or even the electric trim was overwhelmed by the load, and of course this would have brought MCAS back into play also.
- bilbo0s 8y agoAgain, instead of relying on speculation, we should wait to get more information.
- ams6110 8y agoAgreed. Hard not to speculate though. I personally find this stuff fascinating from a technical and human factors perspective -- the deaths are tragic of course.
- msbarnett 8y agoThis isn't rumor mongering. This is straight out of Boeing's own documentation on the 737: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra... 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.
- inferiorhuman 8y agoBoeing documents possibilities, not what actually happened.
- msbarnett 8y agoYes, and? 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?
- cjbprime 8y agoAnd the news article we're commenting on raises the possibility that this is exactly what happened. If that doesn't make the hypothetical germane to discuss here, what can we talk about?
- bbojan 8y agoWhy? I tend to enjoy informed speculation, that's one of the reason I visit this website. Isn't speculation one of the things that characterizes us as humans?
- keyme 8y agoThis makes more sense, thanks. 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?
- weaksauce 8y agoprobably mostly due to needing to have a larger operation margin for different center of gravity as load changes depending on the current flight passenger/cargo loadout.
- ncallaway 8y agoMentour Pilot (as others have mentioned in various other comments) does a great job explaining this. 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.
- inferiorhuman 8y agoI 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.
- phire 8y agoRedundancy too. 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.
- inferiorhuman 8y agoPresumably you could have a redundant drive system for the combined stabilizer/elevator. The L-1011, for instance, had a safety record Boeing should be envious of.
- thekingofh 8y agoSo they'd have to let the plane nose down towards the ground for a bit to then be able to trim back up. This sounds like a cascading failure to me.
- cjbprime 8y agoExactly. And Ethiopian was less than 1000ft above ground level at all times. They didn't have the altitude to trade.
- morganw 8y agoAfter watching Mentour Pilot's demo correcting for runaway trim in a 737 simulator, it seems like a switch that cuts out computer (autopilot, auto-speed, MCAS) control of the elevator trim motors, but keeps the column switches working would be nice. Consider how cumbersome & slow the manual wheels are: https://youtu.be/xixM_cwSLcQ?t=1129 https://youtu.be/xixM_cwSLcQ?t=1129 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).
- js2 8y agoThere are switches for auto pilot and auto speed. MCAS is disabled with flaps extended. It's not procedure but had they set the flaps to a single degree it would have saved them. I think the Lion flight experimented with flaps briefly.
- cjbprime 8y agoThis is sensible, but recall that Boeing is still claiming that the MAX flies like any other 737 and requires no special training. Adding a new button would require training in the use of that button. (In case it's unclear, I don't support Boeing's position here.)
- mhandley 8y agoThe cutoff switches actually disconnect the power to the trim motor, so this is unlikely unless Boeing changed something else they've told no-one about. 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-switches-to-stop-mcas/#more-29790 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.
- inferiorhuman 8y agoThis 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.
- mhandley 8y agoI agree that on the NG, IAS disagree and an uncommanded stabilizer trim are much less likely to occur together. But if you did have an uncommanded nose-down trim close to the ground, you still wouldn't want to reduce throttle because that would cause a pitch-down, so it's possible you'd still end up in the same place. You'd have fewer distractions though, I agree. Anyway, as far as I know, it's never happened, and the NG has a generally good safety record.
- WalterBright 8y agoOn the 757, they bypass the software and cut off the electrical signal to the drive system.