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Thread: 737 MAX
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04-04-2019, 05:58 PM #76Registered User
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Drove by Boeing Field this morning. On one side of the road is a tarmac filled with KC-46’s that the USAF won’t accept because Boeing isn’t cleaning their garbage out of the sealed compartments. The other side is a littered with Max’s that can’t fly.
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04-04-2019, 06:19 PM #77
Maybe this is the result of pissing on your workers for the last decade plus. Yay, America.
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04-04-2019, 10:44 PM #78
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04-05-2019, 12:11 AM #79
Overheard that trimming by hand from max position can take as long as 1-2minutes (many rotations of trim wheel), in a awkward´ish position. Add to that what Multiverse said, that it can be hard to do under high aerodynamic loads and one of the few (only way?) to lighten up the load is...to pitch nose down and trim then manually..
The floggings will continue until morale improves.
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04-05-2019, 12:16 AM #80
I haven't read the link, so this is just general info about the airplane. And I'll add the disclaimer that I've only flown the NG, so there might be some small differences wrt the max.
First, the trim/stab/elevators aren't as simple as they seem. The trim is mechanical in that there are manual trim wheels connected to cables that run to a drum connected to the horizontal stab actuator, that moves a jackscrew, which moves the leading edge of the stab. There's a single electric motor that sits on top of the actuator that runs when commanded by a pair of switches on each column, or by the autopilot (when engaged). If the flaps are in any position other than up, the trim moves in a fast mode.
There are two stab trim cutout switches located just behind the flap lever: 'main', which opens a contactor between the column switches and the electric motor, and 'auto', which opens a contactor between the auto pilots and the electric motor.
If a trim runaway happened, the procedure was to hold the column as hard as you could, and disengage the autopilot and auto-throttles. If the aircraft was flyable at that point you'd hand fly it to the airport (and you could use the main electric trim, because the fault in that scenario would've been one of the auto pilots).
If the runaway persisted, the procedure was to move both stab trim switches to the cutout position, and have the non-flying pilot grab the trim wheel and hold on for dear life.
But there's more... the stab is held by two brakes. If it wasn't, the aerodynamic force would overcome pilot input. So the force required on the control column could be really high to keep the thing flying (because the stab is out of position), while the force to move the trim wheel could be really high (from the brakes) to keep the stab from going even farther. IOW, you're having a really bad day.
And there's even more... when everything is functioning normally, the trim has a function called Speed Trim which will automatically use the autopilot circuit to trim the aircraft when hand flying to keep control column force "normal" for a given speed.
And finally, there are cam-operated cut-out switches that will stop electric trim movement when the control column is moved in the opposite direction (when hand flying).
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04-05-2019, 12:25 AM #81
Thank you for the explanation!
Originally Posted by blurred
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04-05-2019, 01:00 AM #82Registered User
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04-05-2019, 06:44 AM #83
So now some are saying that the trim system and software was implemented as an attempt to fix an aerodynamic flaw in the 737 Max. OK flyers, is there any truth to this.. that they added that system because the airframe was fucked?
https://www.nytimes.com/interactive/...x-8-flaws.htmlGo that way really REALLY fast. If something gets in your way, TURN!
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04-05-2019, 08:09 AM #84
Dominic Gates wrote a very comprehensive article in the Seattle Times that potentially explains why the Ethiopian pilots were unable to recover even after cutting off the automated system:
https://www.seattletimes.com/busines...iopian-flight/
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04-05-2019, 08:24 AM #85
The difference wrt the max is MCAS. The pilots disabled MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading (or per your explanation the brakes) prevented manual trimming using the trim wheels and they eventually turned electric trim back on which also re-enabled MCAS, further pitching the nose down:
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out.
At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try.
At 05:41:54, the First-Officer replied that it is not working.
At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough.
At 05:43:11, about 32 seconds before the end of the recording, two manual electric trim inputs are recorded in the Aircraft Nose Up direction.
At 05:43:20, approximately five seconds after the last manual electric trim input, an Aircraft Nose Down automatic trim command occurred and the stabilizer moved in the Aircraft Nose Down direction in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down.
If you compare this with the flight data graph, the stabilizer barely moves in response to the pilots commands but does move in response to MCAS, maybe because aerodynamic forces allow it move a lot easier in one direction but not the other. Meanwhile, the pilots are also pulling back hard on the yoke while all of this happening.
The big question is the inability to manually rectify trim, due to an aerodynamic, design issue, or something else after MCAS pitched the plane into a dive. Were the forces on the column such that the pilot couldn't assist in cranking and continue to pull back on the column at the same time?
Edit to add: The AD's article above gives more info about why the manual strategy might not have worked.
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04-05-2019, 08:56 AM #86
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04-05-2019, 08:58 AM #87
I'm guessing the "roller coaster" method mentioned in the article might not be a great strategy when you're ~1,000 feet AGL.
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04-05-2019, 09:24 AM #88
It's the most successful narrow body jet ever. There are more of them flying than any other type.
And that nyt article was weak. The correct answer is that the new engines have larger fans, necessitating larger nacelles. To keep them from dragging on the ground they mounted them further forward and higher than the engines on the previous model (the NG). At high angles of attack (think nose up), this created additional lift ahead of the c of g which runs afoul of the FAA's certification for stall characteristics (you're not supposed to make the airplane easier to stall), so the solution Boeing went with was a system to provide automatic nose down stabilizer input.
That's a far cry from the airframe being fucked.
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04-05-2019, 09:39 AM #89
737 MAX
No expert in planes, but why would the system be programmed to deal with a sensor discrepancy by performing a dangerous action such as nose downing? Seems like the logical action would be to abandon automatic actions and to warn the pilots if a malfunction and possible stall situation.
Instead the system trusted the most extreme of the readings and started aggressively nose-downing?
Seems like some stupid software design to me.
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04-05-2019, 09:45 AM #90
Well, the real root cause is no redundancy in the AOA sensors. A high AOA reading from just a single sensor would cause MCAS to activate, so if that sensor gave erroneous data it caused the airplane to dive. This is one of the things the proposed software change fixes. The remaining issue is if there's still even a very remote chance that MCAS could be activated erroneously is there an effective procedure to override the system and get the airplane back into trim?
It's an engineering challenge, but I don't think an overly difficult one. It's just sad it took two fatal crashes for these problems to be revealed.
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04-05-2019, 09:49 AM #91
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04-05-2019, 09:56 AM #92
Jeez
I thought that I read somewhere that there are two sensors - for the Ethiopian crash one registered 15 deg and the other (erroneously) 70 deg. So the MCAS system just used the one when two were on board?
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04-05-2019, 10:17 AM #93
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04-05-2019, 10:19 AM #94
Man, that seems really negligent
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04-05-2019, 10:39 AM #95
Thought Muilenburg's open letter issued yesterday was a little tone deaf.
We at Boeing are sorry for the lives lost in the recent 737 MAX accidents. These tragedies continue to weigh heavily on our hearts and minds, and we extend our sympathies to the loved ones of the passengers and crew on board Lion Air Flight 610 and Ethiopian Airlines Flight 302. All of us feel the immense gravity
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04-05-2019, 10:43 AM #96
Muilenberg is an engineer. Need I say anything more?
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04-05-2019, 10:59 AM #97
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04-05-2019, 11:05 AM #98Registered User
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The other problem is there should be a way to disable the MCAS system without also disabling the electric motor assist. It doesn’t look like that is covered by the proposed software fix, although the MCAS is now supposed to only kick in “once”. I’d need more clarification on what that means, too. Is that only once for the entire flight, or is there some timeout?
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04-05-2019, 11:44 AM #99
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04-05-2019, 11:45 AM #100
No. It's the FAA certification shortcut that's fucked. Read TS's post carefully: in order to certify the new model under the old they had to make it no easier to stall--compared to the existing airplane. That's where most clusterfucks start. Not to suggest throwing out the baby with the bathwater, but that cert method needs attention at the regulatory level.
A woman came up to me and said "I'd like to poison your mind
with wrong ideas that appeal to you, though I am not unkind."
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