Earlier quoted context omitted.
Being capable of performing a glide without electricity does not necessarily imply that a no-power glide can safely be entered from extreme conditions like diving too fast with trim set to insane.
Why is it possible to set the trim to insane? Is there a legitimate scenario where that would be used?
Ethiopian Airlines 737 MAX Investigation Preliminary Report
161–170 of 347 posts
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#162Earlier quoted context omitted.
> > They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. > All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to ano…
I don't remember where I read it, but my understanding is that the MAX simulator does NOT simulate MCAS behavior. EDIT: Found it: https://www.reuters.com/article/us-ethiopia-airplane-simulat...
I would hope that Boeing internally had simulators for the MAX while they were testing it - along with a borrowed supply of pilots who were competent but had no inside information. Even if they didn't, a pencil and paper exercise might have been useful:
1. AOA sensor fails at takeoff (A), the pilots receive an unreliable airspeed warning and the stick shaker activates. The pilots now mistrust their sensors and want to keep their speed up and the nose down.
2. At about 1000 feet above ground the pilots level the aircraft, raise the flaps and start the unreliable airspeed checklist. MCAS starts trimming intermittently.
3. The pilot flying is having to pull further back on the stick and trim correctively while the crew work through their checklist. At this point the crew need to avoid any over-focus, task saturation or over-stimulus from the many warnings and indicators and ask themselves why the aircraft is trimming the way it is. They have a fairly narrow window of time in which to do this and then disable the trimming with the plane in a well trimmed state.
(A) That this happened twice in relatively quick succession might be due to a common problem. Even if such a problem is found and fixed the AOA vanes and pitot tubes are delicate sensors sticking out of the nose of the aircraft; the system should be robust to failures in these sensors.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#163Earlier quoted context omitted.
I think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input. I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane. I'm now wondering is the MA…
> Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down. Or, alternatively, the pitch up happens so quickly and severely the risk of stall is too great above a certain AoA and the pilot won't have adequate time to respond. If either is the case, it implies the aircraft is not safe to fly without MCAS operational, and possibly not at all.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#164Earlier quoted context omitted.
> they were only 7,000 feet above the ground No, it's worse: the origin airport is at 7000ft elevation. They only had around 1000ft height AGL for all of the flight. So I agree that releasing the elevator at all seems surely suicidal.
They're at 12,000-14,000 feet altitude at 05:43ish, when the co-pilot reports the non-functional manual trim. So ~6,000-7,000 height. Still damn close to the ground to be comfortable letting the plane dive.
I guess we will have to wait for the final report but the pilot's actions are perplexing here. Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? Why not enable it, trim to where you want, and then use the cut out switch again?
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#165Earlier quoted context omitted.
MCAS system is supposed to stop the plane from stalling. The faulty AoA sensor is causing the plane to think it’s about to stall and therefore MCAS works as designed to prevent the pilot from putting the plane into a stall. Of course the design is wrong because the plane is actually pointing toward the ground and MCAS thinks it’s pointing to the sky...
What about a single button to turn MCAS off and return to trim to a neutral position? Or a setting where the pilot is telling the system to ignore the sensor input. Maybe a bit like a self driving car planes will eventually have a system where they can use visual markers as a check on what sensors are reporting.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#166Earlier quoted context omitted.
this may be a naive question but with all of the advanced technology on these planes, why isn't there a check for altitude before any automated command runs which says "hey we just took off, are 400ft above ground, MCAS is saying nose down, we will crash if that command runs, CANCEL"
You'll 100% for sure crash if you stall at 400ft above ground. MCAS is designed to prevent stalls.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#167Earlier quoted context omitted.
The planes when setup correctly have a pretty good glide slope actually. When there is no power the pilots can deploy the apu to restore basic power for flight controls.
I assume you mean the Ram Air Turbine ( https://en.wikipedia.org/wiki/Ram_air_turbine )?
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#168Earlier quoted context omitted.
Why exactly? if Boeing did not updated the manuals and checklists how would pilots know that? from internet forums? Should a pilots ignore the checklists and try steps he read on the internet ? I could not find if Boeing updated their manuals/checklists after the first crash.
Pilots should have known that MCAS is inactive with flaps extended, per information from Boeing.
The Boeing/FAA directive re: MCAS does not instruct them to extend flaps.
Further, there's other things going on in the cockpit due to the malfunctioning AoA sensor that's the root of all this.
Specifically one of the effects is that they get an Unreliable Airspeed Indication, and run that checklist. That checklists memory items include maintaining the current flap configuration (in this case, flaps up).
The Boeing/FAA directive does not instruct them to deviate from this and extend flaps. They have a checklist memory item instructing them to in fact do the exact opposite, and not touch flaps.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#169Earlier quoted context omitted.
No, the wheel has an extendable crank handle and is meant for manual use precisely when the electronic stabilizer trim is cutout. But aerodynamic forces can conspire to make it very hard or physically impossible for human force to rotate it if the elevator is being used to combat the stabilizer at high speeds. See here for more discussion of the physics behind it: https://www.satcom.guru/2019/04/stabilizer-trim-loads…
Thanks, that makes sense. When the report says that the pilots found manual trim impossible after STAB TRIM CUTOUT, do you think it's referring to an attempt at manual electric trim (expected to fail) or both pilots manually moving the trim wheels with the handle?
So must be manual trim wheel movements.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#170Earlier quoted context omitted.
this video[0] from "mentour pilot" on youtube explains the stabilizer system quite well. it seems the trim wheels are connected manually to the trim mechanism in the back, but when the trim stabilizers are in an extreme position the wind loading on them makes it very hard/impossible to rotate the trim wheels manually. from what i understand, one way to relieve/lessen the wind loading so that the manual wheels can be…
Its possible and I think it was demonstrated in one of Mentour Pilot's videos that both pilots can work together to manually trim. Boeing put notes in the manual that this may be needed and it would not break the manual trim system to apply that much force to the trim wheels.
https://news.ycombinator.com/item?id=19575318 "option 2)" (on this same page) refers to a situation where a "dive" could be executed in a situation where manual strength is not enough to trim the aircraft.