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| Another deadly 737 Max control bug just found! |
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| djacobow:
--- Quote from: windsmurf on June 27, 2019, 08:25:53 pm --- --- Quote from: sokoloff on June 27, 2019, 01:25:21 pm --- --- Quote from: EEVblog on June 27, 2019, 12:58:16 pm --- --- Quote from: ptricks on June 27, 2019, 12:55:58 pm ---Too bad they can't add a toggle switch in the cockpit to cut power from the pin on the micro that controls the specific part the computer is trying to control , something like a auto/manual option. --- End quote --- It did/does actually have a switch that disables MCAS, and it could have saved those flights if they had been trained to use it. --- End quote --- There's a pair of switches in all 737s that cutout power to the stab trim and all 737 pilots have been trained to use that as a memory item (must be recalled without reference to a printed checklist) in the event of stab trim runaway. --- End quote --- They modified the switch functionality with the 737 max. https://www.seattletimes.com/business/boeing-aerospace/boeing-altered-key-switches-in-737-max-cockpit-limiting-ability-to-shut-off-mcas/ --- End quote --- No pilots were ever trained to flip just one of those switches. They weren't labeled with different functions, they weren't described with different functions, at least not in the QRH. It was a weird design change, to be sure, but I'm not sure it matters all that much. I've followed this whole saga pretty closely and I still am not sure I understand what happened. I mean, I get that single AOA + MCAS caused the machine to trim down hard. But that should have been manageable with the standard training. Knowing that MCAS existed would have helped pilots to have a picture of what was happening, but I don't think it would have been necessary. There was some evidence that once the plane was trimmed down and the pilots were pulling hard to recover, that the load on the stab made it very difficult to turn the trim wheel, at least without the pilots pushing the nose even further over to unload the stab. If that is the case, I think that's the more serious problem than MCAS, or at least MCAS alone. |
| sokoloff:
The trim is hard to run manually when the horizontal stab is biased aircraft nose down, the elevator is deflected aircraft nose up to compensate, and the aircraft flying at high indicated airspeed, that combination making for high loads on the stab jackscrew and following nut. The answer once you've let things evolve to that point is to either rollercoaster the airplane (to periodically unload the elevator and use those moments to manually trim), to take the thrust down and thereby reduce indicated airspeed and trim manually, or to power the electric trim and run the trim nose up electrically. I don't think that it's a distinct or Max-specific flaw. It's a matter of aerodynamics. While I'm generally critical of the Ethiopian crew, I find it hard to fault the crew for not inventing the rollercoaster solution on the fly [so to speak]. (In particular their decision to leave the engines at a very high thrusts is odd, but most disconcerting is the decision to turn the stab trim back on, momentarily command aircraft nose up trim, and then [inexplicably] leave the stab trim powered but without commanding continued aircraft nose up trim by thumb switch. I think that decision is very hard to understand.) |
| djacobow:
--- Quote from: sokoloff on June 28, 2019, 01:56:17 am ---The trim is hard to run manually when the horizontal stab is biased aircraft nose down, the elevator is deflected aircraft nose up to compensate, and the aircraft flying at high indicated airspeed, that combination making for high loads on the stab jackscrew and following nut. ... --- End quote --- I agree with all of that. If the Ethopian crew had gotten the plane trimmed, it's hard to understand why they left the system on. Well, actually, I could see that happening ONCE. But it happened several times, right? The media has made a huge deal out of the MCAS flaw, but I think most pilots just "get" (or should get) that every so often under the right (wrong) circumstances, the automation is going to try to murder you. That's just life managing a very complex system. That's not to say that such flaws are OK. They are obviously not OK. You probably shouldn't be sitting up front in an airliner if you don't think there's a chance it might actively try to kill you. The reason this got so deadly is that under the aerodynamics of trying to recover against the pitch, the trim is hard to move. In the heat of the moment, it might be hard to quickly reason out that you need to unload the stabilizer before you can move the wheel, especially if this regime of flight had not been part of your training. It's the combination that was really deadly, not either in itself. |
| BBBbbb:
--- Quote from: blueskull on June 28, 2019, 03:54:28 am --- --- Quote from: BBBbbb on June 27, 2019, 11:56:49 am ---well ordering 200 of something with such a hefty price tag, that has a very uncertain future, seems a bit irresponsible. --- End quote --- With such scrutiny, I would bet B737MAX will be the world's safest plane ever once things are ironed out. If you put the same amount of scrutiny on any other aircraft, you will find equally hilarious flaws. --- End quote --- Yes, for sure there are other very flawed aircrafts out there flying, but this is more to do with the lack of disclosure and mismanagement of all the attempts of a blame game that followed the series of accidents. Isn't it concluded that with a proper training and added redundancy this could be a flyable system? But is it still a competitive one after the fixes? BA (intent to) order does look like a gimmick, but unless there is a discount on other models that are getting ordered, or a restructuring of an ongoing payments to Boing, this really seems just like bad marketing for BA. |
| CiscERsang:
--- Quote from: blueskull on June 28, 2019, 03:54:28 am ---With such scrutiny, I would bet B737MAX will be the world's safest plane ever once things are ironed out. If you put the same amount of scrutiny on any other aircraft, you will find equally hilarious flaws. --- End quote --- Too late. The comprehensive scrutiny must have been performed before first commercial flight. Not today. |
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