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Old 03-25-2019, 08:03 AM   #121
Thorsten
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After a few more bits of information have come to my attention and a nice discussion:

I don't think there's overly much wrong with the plane. The MCAS is a reasonable system intended to avois a dangerous situation, it can malfunction, then it can be switched off. The fault seems rather with the general circumstances surrounding it all.

* no redundancy in the MCAS AoA sensor - which is probably okay as long as it's halfway transparent to the crew when the system works properly and when not but

* the AoA sensor disagreement light which would alert the crew early on to a possible sensor failure is opt-in by the airline and costs extra, so not all planes have it

* ... which probably could be dealt with if the crew would otherwise be trained how it feels when the system malfunctions and what to do as a response

* but the existence of the system seems to have been played down by Boeing and the company did in fact all to avoid any additional training of the crews

* and to that end, they likely systematically understated the implications of a system failure

So the actual problem seemed to have been that the crews did not figure out in time what the real problem was (due to lack of information and training) and reacted the wrong way - in cases where the problem was recognized correctly by the crew and the system was switched off, no further problems occurred.

Which, well, is still a theme of saving money and compromising safety to do that unfortunately. But it rather seems to be an issue of the general culture and not so much a fault of the MCAS system itself or the engineering.
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Old 03-25-2019, 08:20 AM   #122
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One theory in the German newspaper Spiegel claims that blowback made it impossible to move the elevators far enough before the crash to prevent the final dive.

I am not sure about this, maybe somebody with 737 experience knows at least the piston area of the servoactuator and how much force it could maximally create. They should be similar between all current 737 models.


Another phenomena that I could rather imagine is transsonic effects, especially Mach tuck. But even then 400 knots is still just Mach 0.7 in 10000 ft altitude. High enough to cause a strong pitch down moment on WW2 fighter planes (the P-38 was notorious for it, started at Mach 0.65 there) but should be no issue on a modern airliner usually flying at Mach 0.95 in higher altitudes.

Also, I find it strange that the AoA sensors could be 22° apart each others in their readings on a new aircraft. And that without causing at least a warning in the PFD.

Last edited by Urwumpe; 03-25-2019 at 08:25 AM.
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Old 03-25-2019, 09:29 AM   #123
Thorsten
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Quote:
One theory in the German newspaper Spiegel claims that blowback made it impossible to move the elevators far enough before the crash to prevent the final dive.
I suspect it's plausible enough (I've read the same thing it seems...), but the root cause would not have been the MCAS pushing the nose down but the wrong crew understanding of the situation - if they suspected insufficient airspeed as their main problem, they would have caused the blowback basically themselves by increasing airspeed quite a lot in a high-pressure environment.

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Also, I find it strange that the AoA sensors could be 22° apart each others in their readings on a new aircraft. And that without causing at least a warning in the PFD.
Well, the disagreement warning was sold as an optional add-on. Go figure...

(I had the impressions these sensors are quite sensitive devices - basically a good splat of mud might already de-rail the reading...)
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Old 03-25-2019, 09:37 AM   #124
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Don't know the precise value of max Actuator force, but with a full nose down trim, it will be really really hard to pull back the stick without acting at least on the mechanical wheel trim if elec one was deactivated via cutout switches.

Runaway trim is really tricky and fast reaction is needed, even more with the increase in speed trim caused by MCAS :/


Max Speed on 37 is 340Kt/ .82
.95 is for top notch private jet usually, even Long Haul don't go above .88

For the Mach Tuch, there is a Mach trim system on 37 which is active after M 0.615 and adjust elevators with respect to the stabilizer when speed increases ( informations come from ADIRU).Though it is more a speed stability system at High Mach Numbers.


AOA disagree should show on PFD in amber when AOA differs by more than 10 ° . I don't recall if it was a main feature on NG as I always saw it in the QRH, or if it was already optionnal.

Last edited by Gingin; 03-25-2019 at 09:41 AM.
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Old 03-25-2019, 09:47 AM   #125
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Quote:
Originally Posted by Urwumpe View Post
 Another phenomena that I could rather imagine is transsonic effects, especially Mach tuck. But even then 400 knots is still just Mach 0.7 in 10000 ft altitude. High enough to cause a strong pitch down moment on WW2 fighter planes (the P-38 was notorious for it, started at Mach 0.65 there) but should be no issue on a modern airliner usually flying at Mach 0.95 in higher altitudes.
My understanding is that the combination of stabilizer trim with traditional elevators (as opposed to a stabilator) tends to give airliners more trim authority than elevator authority, because trimming out Mach tuck requires more authority than is strictly necessary for pitch control. So if you get runaway nose-down trim, plus a high airspeed, the situation may not be recoverable.

---------- Post added at 09:47 ---------- Previous post was at 09:38 ----------

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Originally Posted by Thorsten View Post
 I suspect it's plausible enough (I've read the same thing it seems...), but the root cause would not have been the MCAS pushing the nose down but the wrong crew understanding of the situation - if they suspected insufficient airspeed as their main problem, they would have caused the blowback basically themselves by increasing airspeed quite a lot in a high-pressure environment.


I've seen discussion of throttle manipulation actually being part of the procedure for dealing with runaway trim, as the engines are below COG and increasing/decreasing throttle will generate a pitch up/down moment. Of course, at low altitude, how much time you can run at full throttle before hitting Vne or starting to deal with blowback effects is going to be more limited than higher up.
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Old 03-26-2019, 02:15 PM   #126
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Default In Test of Boeing Jet, Pilots Had 40 Seconds to Fix Error

Excerpts from the New York Times article (emphasis added):

Quote:
During flight simulations recreating the problems with the doomed Lion Air plane, pilots discovered that they had less than 40 seconds to override an automated system on Boeing’s new jets and avert disaster.

The pilots tested a crisis situation similar to what investigators suspect went wrong in the Lion Air crash in Indonesia last fall. In the tests, a single sensor failed, triggering software designed to help prevent a stall.

...

The software, as originally designed and explained, left little room for error. Those involved in the testing hadn’t fully understood just how powerful the system was until they flew the plane on a 737 Max simulator, according to the two people.

Compounding the flaws, pilots received limited training about the system before the first crash. During the final minutes, the captain of the Lion Air flight flipped through a technical manual trying to figure out what was happening.

...

There are common procedures in place to counteract MCAS, as currently designed. If the system starts pushing the plane’s nose down, pilots can reverse the movement via a switch at their thumb, a typical reaction in that situation. In doing so, they can potentially extend the 40-second window, giving them more time to avoid a crash.

To fully neutralize the system, pilots would need to flip two more switches. That would shut off the electricity to a motor that allows the system to push the plane toward the ground. Then the pilots would need to crank a wheel to correct whatever problems had emerged.

The pilots, in the simulations, followed such procedures to successfully shut off the system and land safely. But they did so with a far better understanding of how it worked and prior knowledge that it would be triggered — benefits that the pilots of the fatal 737 Max crashes did not have.

If pilots don’t act hastily enough, attempts to disable the system can be too late. In the Lion Air crash, pilots used the thumb switch more than two dozen times to try to override the system. The system kept engaging nonetheless, most likely because of bad readings from a sensor, until the plane crashed into the Java Sea, killing all 189 people on board.
More information in the article here.
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Old 03-26-2019, 02:25 PM   #127
jedidia
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So the thing just flies the plane into the ground to prevent a stall? I understand the importance of encapsulated and independant systems, but this is taking it a bit too far...
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