flybucky
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:16 pm

L1049L1011 wrote:
Quote from BOEING 737-7/-8 System Differences manual - see http://www.b737.org.uk/mcas.htm
... the pilots move the STAB TRIM PRI (primary) switch to the CUT OUT position. This removes power to the STAB TRIM B/U (backup) switch and these:
    - Captain and F/O stabilizer trim switches
    - R64 (de-energizes)
    - R1192 contact
    - Signals to the FCCs


Bingo, thank you! This exactly confirms what PixelFlight and Speedbored had said based on the schematic. In fact, this is the very page (p. 748) before the schematic page (p. 749). Cutting out either the PRI or B/U switch will remove power to the Capt and FO electric trim switches and their signals to the FCCs. I'm assuming the FDR data is recorded by the FCC.
 
dragon6172
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:23 pm

PixelFlight wrote:
SwissCanuck wrote:
speedbored wrote:
Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DFDR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.

I have published several times in this thread the link to the 737 MAX stab trim electric schematic. The 28V low power DC is used to power the circuits that pass through the various switches implementing the pilot interface and logic, because obviously the triple phase high power 115V is too dangerous and complex to wire everywhere into the cockpit. In the above discussion the "power" to the switches is the 28V DC low power. At one point, one of the 28V signal that pass trough the two cutoff switch is connected to the thumb switches to "power" them and the schematic very clearly show that there are no other power to that switches. Without electrical power source there is no physical way to sens the thumb switches. The same 28V low power DC signal energize the R64 replay that commute the high power triple phase feeding the stab trim electric motor. So if any of the two cutoff switches cut the 28V signal, the electrical source is removed from the thumb switches, preventing FDR recording, and from the stab trim motor (via R64 relay),

The drawing you refer to very clearly doesnt show where the FDR data regarding stab trim commands is taken from. Your idea that using the cutoff switches stops the recording of thumb switch position is a possibility, but it's also possible the data comes from elsewhere
Phrogs Phorever
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:25 pm

XRAYretired wrote:
Yes. Thumb Switch will undoubtably initiate a procedure in software (assuming other initiation conditions are met) as MCAS and STS are software procedures.

The FCC "MAIN ELEC TRIM INTLK" input 28V DC signal is feed by any of the thumb switches via the "R1193 MAIN TRIM ARM RELAY". This is certainly that signal that the FCC software probe to cancel the STS or MCAS automatic stab trim command actually executed.
 
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speedbored
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:50 pm

morrisond wrote:
flybucky wrote:
dragon6172 wrote:
During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction.

From the PR: "From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units."

This is disturbing. There were no Auto Trim commands (STS, MCAS, etc) or Electric Trim commands during that period. So that means either the pilots manually turned the trim wheel in the nose down direction, or the stabilizer was slipping on its own.

It's possible that if the pilots were not able to trim the wheel nose up, they might try if it will move nose down to see if the wheel is just stuck, or if they only could not overcome the nose up aerodynamic forces. However, if that were the case, I would expect to see a short jump from 2.3 to 2.1 units when they tried that, not gradually over 2.5 minutes.

Is it even possible for the stabilizer to slip gradually? With the elevator up, that puts pressure on the stabilizer to nose down. Could it gradually slip 0.2 units over 2.5 mins?


I would have to guess that when they were well over Vmo almost anything is possible.

I'm not convinced that 20-25kts, counts as "well over". Given the pressure altitude they were at, it is unlikely to have been overstressing the airframe. According to the report, they didn't exceed approximately 20-25kts over Vmo until they entered the final dive.
 
ikramerica
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:54 pm

dragon6172 wrote:
ikramerica wrote:
It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

I dont know about not understanding the system. I think the dire situation wasnt known though. During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction. Also during this time the target altitude and target heading settings were changed. As I said earlier these seem like things that aren't real important if you're struggling to control the aircraft

The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.
Of all the things to worry about... the Wookie has no pants.
 
ikramerica
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:58 pm

speedbored wrote:
morrisond wrote:
flybucky wrote:
From the PR: "From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units."

This is disturbing. There were no Auto Trim commands (STS, MCAS, etc) or Electric Trim commands during that period. So that means either the pilots manually turned the trim wheel in the nose down direction, or the stabilizer was slipping on its own.

It's possible that if the pilots were not able to trim the wheel nose up, they might try if it will move nose down to see if the wheel is just stuck, or if they only could not overcome the nose up aerodynamic forces. However, if that were the case, I would expect to see a short jump from 2.3 to 2.1 units when they tried that, not gradually over 2.5 minutes.

Is it even possible for the stabilizer to slip gradually? With the elevator up, that puts pressure on the stabilizer to nose down. Could it gradually slip 0.2 units over 2.5 mins?


I would have to guess that when they were well over Vmo almost anything is possible.

I'm not convinced that 20-25kts, counts as "well over". Given the pressure altitude they were at, it is unlikely to have been overstressing the airframe. According to the report, they didn't exceed approximately 20-25kts over Vmo until they entered the final dive.

Does anyone know the Vne for the 737NG? I cant imagine the margin over Vmo is less than 10%, but if you are approaching that speed it will start to feel like that scene in The Right Stuff where Chuck Y is approaching the sound barrier.
Of all the things to worry about... the Wookie has no pants.
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:05 pm

dragon6172 wrote:
That being said, the diagram on page 94 of this thread is not the actual circuit diagram and wouldn't be used to shoot wires during troubleshooting. Which means its possible there are parts missing.

Until someone publish an other schematic of the 737 MAX, this is what we have, and it shows many features like the details of each contacts of each switches that make me confident that there is no other circuit hidden into the thumb switches or stab trim cutout switches. If someone will disassemble those 737 MAX switches and take a picture we will certainly only see the wires that are already on the published schematic. I will happily revise my position if someone can prove me wrong on this.
 
kalvado
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:13 pm

ikramerica wrote:
dragon6172 wrote:
ikramerica wrote:
It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

I dont know about not understanding the system. I think the dire situation wasnt known though. During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction. Also during this time the target altitude and target heading settings were changed. As I said earlier these seem like things that aren't real important if you're struggling to control the aircraft

The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.

Did it occur to you that manual drive directly translates force on the wheel into force on the stab, and (force*travel distance) product will be the same on two ends of a manual drive (actually a bit less on receiving end due to system inefficiency)?
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:18 pm

dragon6172 wrote:
The drawing you refer to very clearly doesnt show where the FDR data regarding stab trim commands is taken from. Your idea that using the cutoff switches stops the recording of thumb switch position is a possibility, but it's also possible the data comes from elsewhere

The STAB TRIM NOSE UP/DOWN signals are probed into ELECTRIC STAB TRIM MOTOR assembly that feed the FDR with others important parameters like the STAB TRIM position. Without the 28V power that pass through the two cutout switches those signals have no power source and consequently can be recorded. It's very simple.
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:34 pm

PixelFlight wrote:
dragon6172 wrote:
The drawing you refer to very clearly doesnt show where the FDR data regarding stab trim commands is taken from. Your idea that using the cutoff switches stops the recording of thumb switch position is a possibility, but it's also possible the data comes from elsewhere

The STAB TRIM NOSE UP/DOWN signals are probed into ELECTRIC STAB TRIM MOTOR assembly that feed the FDR with others important parameters like the STAB TRIM position. Without the 28V power that pass through the two cutout switches those signals have no power source and consequently can be recorded. It's very simple.


They have raised a very valid point. We are not privy to know exactly where the FDR is getting its information from. What has been suggested to me privately is that in the case of the LionAir accident the AOA probe that was removed has been though bench testing since the accident to suggest it was serviceable when removed. The replacement unit installed was overhauled in an avionics shop in the US, thus highly likely it was also serviceable. It has been suggested to me to look at where the FDR gets its AOA data from as a clue to the underlying problem, apparently it is not from the AOA probe directly. Likewise with this accident, we need to understand where the FDR trace data source is, and not make assumptions.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
flybucky
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:56 pm

PixelFlight wrote:
The STAB TRIM NOSE UP/DOWN signals are probed into ELECTRIC STAB TRIM MOTOR assembly that feed the FDR with others important parameters like the STAB TRIM position. Without the 28V power that pass through the two cutout switches those signals have no power source and consequently can be recorded. It's very simple.

I've been trying to study the schematic and description to make sense of how there was 1 MCAS command on the FDR with no change in Pitch Trim, whereas the next 2.5 mins no MCAS command was recorded.

I did notice that there are "A" and "B" blue circles. Only "B" feeds into the FCC Cutout Signal input. So my first thought was that if "B" is not cutout, then the MCAS unit is still sending commands, which would be recorded by FDR. (Of course, the commands would be useless, because the Stab Trim Motor would be disabled). This could explain the FDR graph.

The only problem with the above is that it seems if you cutout "A", it will also cut the power to "B". Or is there some way that "B" can still send a signal to the FCC Cutout input?

Which switch does "A" and "B" correspond to? And why is there an "FCC" input to "A"?
 
jollo
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:58 pm

PixelFlight wrote:


Interesting discussion about STS logic, but no mentions of the AUTO-PILOT STAB TRIM CUTOFF switch effect on STS. Thanks anyway.

hivue wrote:
MCAS does not function with AP on. I think AP cutout (the presence of which implies the possibility for automated trim other than MCAS) had to go due to the creation of MCAS. Two pieces of software in a food fight is pointless. Either all electric trim is gone or none.


MCAS is inoperative with A/P on, therefore there's no potential for conflict between the two "pieces of software" (control domains are not overlapping), and no incentive to change the STAB TRIM CUTOFF switches function.

On the other hand, there would be a very good reason for electric stab trim to be commanded only by thumb switch inputs, with no interference from any haywire automation!

Since getting to the bottom of whether AUTO-PILOT STAB TRIM CUTOFF disables STS on a 737 NG seems to be harder than expected, I'm going to try my luck in Tech/Ops and will report back if I get any useful info.
 
mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:59 pm

ikramerica wrote:
dragon6172 wrote:
ikramerica wrote:
It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

I dont know about not understanding the system. I think the dire situation wasnt known though. During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction. Also during this time the target altitude and target heading settings were changed. As I said earlier these seem like things that aren't real important if you're struggling to control the aircraft

The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.


I do not believe that the wheels are geared to move the trim slow. They are under normal circumstances not used and the electrical trim is pretty fast compared. They are geared to be able to move the trim. They seem even not to be geared enough, because when they are needed the forces can be to great to move them, but if geared even more, they would move the trim that slow to be nearly useless. Regarding the power needed to turn the wheels, they have been made smaller in diameter than in the 737-200, so it will be more difficult to turn them in a MAX (or NG).

I think they are kept as backup, so it is possible to show them as an added safety. You can end abnormal trim lists on: "and now you move the trim manually with the wheels" .
IMO it is a fail safe that works as well as a parachute that does not open most of the time, rather useless. In situation you would need it, the situation is already down the drain and manual trim that heavy that it will hardly be useful to you.
That this obsolete trim is not responsible for higher rate of accidents, may be a function of how often it needs to be used. In the NG I assume hardly ever
Last edited by mjoelnir on Mon Apr 08, 2019 10:01 pm, edited 1 time in total.
 
asdf
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:08 pm

lets say it clear:
the FAA directive to cut out the electric trim in case of MCAS failure was one of the big holes in the cheese for the ET crash
if manual trim is THAT much ineffectiv on the MAX one shouldnt have issued such a directive ...
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:19 pm

zeke wrote:
PixelFlight wrote:
dragon6172 wrote:
The drawing you refer to very clearly doesnt show where the FDR data regarding stab trim commands is taken from. Your idea that using the cutoff switches stops the recording of thumb switch position is a possibility, but it's also possible the data comes from elsewhere

The STAB TRIM NOSE UP/DOWN signals are probed into ELECTRIC STAB TRIM MOTOR assembly that feed the FDR with others important parameters like the STAB TRIM position. Without the 28V power that pass through the two cutout switches those signals have no power source and consequently can be recorded. It's very simple.


They have raised a very valid point. We are not privy to know exactly where the FDR is getting its information from. What has been suggested to me privately is that in the case of the LionAir accident the AOA probe that was removed has been though bench testing since the accident to suggest it was serviceable when removed. The replacement unit installed was overhauled in an avionics shop in the US, thus highly likely it was also serviceable. It has been suggested to me to look at where the FDR gets its AOA data from as a clue to the underlying problem, apparently it is not from the AOA probe directly. Likewise with this accident, we need to understand where the FDR trace data source is, and not make assumptions.

The SMYD feed the AoA value to the FDR. https://www.pprune.org/10435549-post2885.html
The FDR is to probe the state of signals that have action on the aircraft. The FDR data sources are where there are assemblies that can collect such data and serialize them to feed the FDR that can be far away. Sure we can speculate on many different ways to implement a function but I don't expect that the reality is so different than the existing public information.
 
flybucky
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:23 pm

jollo wrote:
Since getting to the bottom of whether AUTO-PILOT STAB TRIM CUTOFF disables STS on a 737 NG seems to be harder than expected

Peter Lemme said that on pre-MAX 737s, the AUTOPILOT STAB TRIM CUTOFF would disable STS:

The cutout switch function was changed with the 737MAX from all prior 737 models. The legacy switch combination was one switch to cutout electric trim altogether, the other to cutout the autopilot trim commands. MCAS and Speed Trim System are both commands from the “autopilot”. With the legacy switch configuration, the flight crew can disable the autopilot commands and retain electric trim.

https://www.satcom.guru/2019/04/what-ha ... et302.html
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:30 pm

flybucky wrote:
PixelFlight wrote:
The STAB TRIM NOSE UP/DOWN signals are probed into ELECTRIC STAB TRIM MOTOR assembly that feed the FDR with others important parameters like the STAB TRIM position. Without the 28V power that pass through the two cutout switches those signals have no power source and consequently can be recorded. It's very simple.

I've been trying to study the schematic and description to make sense of how there was 1 MCAS command on the FDR with no change in Pitch Trim, whereas the next 2.5 mins no MCAS command was recorded.

I did notice that there are "A" and "B" blue circles. Only "B" feeds into the FCC Cutout Signal input. So my first thought was that if "B" is not cutout, then the MCAS unit is still sending commands, which would be recorded by FDR. (Of course, the commands would be useless, because the Stab Trim Motor would be disabled). This could explain the FDR graph.

The only problem with the above is that it seems if you cutout "A", it will also cut the power to "B". Or is there some way that "B" can still send a signal to the FCC Cutout input?

Which switch does "A" and "B" correspond to? And why is there an "FCC" input to "A"?

The "B" signal is the "STAB TRIM CUTOUT SW" probe for the FCC. I speculate that the STS and MCAS code prevent the creation of a new stab trim action if the "B" signal is not there, but that the STS or MCAS will terminate the execution of an ongoing stab trim action even if the "B" is not there. This could explain the last MCAS action without stab trim move and no repetitive MCAS pattern.
The "A" signal is more strange and there is no usable label attached to it on the schematic.
 
planecane
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:32 pm

mjoelnir wrote:
ikramerica wrote:
dragon6172 wrote:
I dont know about not understanding the system. I think the dire situation wasnt known though. During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction. Also during this time the target altitude and target heading settings were changed. As I said earlier these seem like things that aren't real important if you're struggling to control the aircraft

The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.


I do not believe that the wheels are geared to move the trim slow. They are under normal circumstances not used and the electrical trim is pretty fast compared. They are geared to be able to move the trim. They seem even not to be geared enough, because when they are needed the forces can be to great to move them, but if geared even more, they would move the trim that slow to be nearly useless. Regarding the power needed to turn the wheels, they have been made smaller in diameter than in the 737-200, so it will be more difficult to turn them in a MAX (or NG).

I think they are kept as backup, so it is possible to show them as an added safety. You can end abnormal trim lists on: "and now you move the trim manually with the wheels" .
IMO it is a fail safe that works as well as a parachute that does not open most of the time, rather useless. In situation you would need it, the situation is already down the drain and manual trim that heavy that it will hardly be useful to you.
That this obsolete trim is not responsible for higher rate of accidents, may be a function of how often it needs to be used. In the NG I assume hardly ever


You are very insistent that the "obsolete trim system" of the 737 must be completely redesigned. Perhaps the components of the trim system are just extremely reliable and the instances where it needs to be cut out (except when the terrible MCAS implementation was added) is exceedingly rare. Perhaps if the electric trim stops working somewhere near where the trim should be, the aircraft can still be controlled and landed using the elevators or, in that scenario, the manual wheel works.

Obviously runaway trim can't be very common at all on the pre-MCAS/pre-MAX 737's or the issue of being able to recover from it ending up impossible would have come up many years ago.

Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.
 
ikramerica
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 11:06 pm

planecane wrote:
mjoelnir wrote:
ikramerica wrote:
The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.


I do not believe that the wheels are geared to move the trim slow. They are under normal circumstances not used and the electrical trim is pretty fast compared. They are geared to be able to move the trim. They seem even not to be geared enough, because when they are needed the forces can be to great to move them, but if geared even more, they would move the trim that slow to be nearly useless. Regarding the power needed to turn the wheels, they have been made smaller in diameter than in the 737-200, so it will be more difficult to turn them in a MAX (or NG).

I think they are kept as backup, so it is possible to show them as an added safety. You can end abnormal trim lists on: "and now you move the trim manually with the wheels" .
IMO it is a fail safe that works as well as a parachute that does not open most of the time, rather useless. In situation you would need it, the situation is already down the drain and manual trim that heavy that it will hardly be useful to you.
That this obsolete trim is not responsible for higher rate of accidents, may be a function of how often it needs to be used. In the NG I assume hardly ever


You are very insistent that the "obsolete trim system" of the 737 must be completely redesigned. Perhaps the components of the trim system are just extremely reliable and the instances where it needs to be cut out (except when the terrible MCAS implementation was added) is exceedingly rare. Perhaps if the electric trim stops working somewhere near where the trim should be, the aircraft can still be controlled and landed using the elevators or, in that scenario, the manual wheel works.

Obviously runaway trim can't be very common at all on the pre-MCAS/pre-MAX 737's or the issue of being able to recover from it ending up impossible would have come up many years ago.

Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.

While I don't agree with a lot of what he is saying, there is still a case to be made that if airplanes were designed today, the controls would be very different. Same goes for cars.

The more we find out about the 737 system, the more we see how aircraft systems in general are designed so they "feel" like obsolete technology and use control mechanisms that mimic designs that should be outmoded. I'm not talking grandfathering exclusively. But the main paradigm of the feedback the aircraft gives is based on how much, much, much older aircraft with manual controls reacted and how they were controlled. Many of the paradigms go back to the piston era.

This happens because, even with a new type, the PILOTS are grandfathered in. To get experienced pilots, you need to have them already know how to fly the plane based on their experience. If you changed it up completely, you would have no one with experience to fly any of the planes you sell. Yes, you can train people, but getting enough people up to speed with enough experience would be astronomically costly and probably impossible.

So we have various aircraft of various vintages all trying to approximate how an older aircraft would behave just because. And as technology advances further and further, there will be more conflicts between how it was always done/needs to be done and how it really should be done.

The really ironic part is that as aircraft are more automated, the pilots are doing less actual flying anyway and too often forget what they are supposed to know in a panic situation because they barely ever use it, so completely changing the paradigm shouldn't matter as much.
Of all the things to worry about... the Wookie has no pants.
 
mjoelnir
Posts: 8521
Joined: Sun Feb 03, 2013 11:06 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 11:43 pm

planecane wrote:
mjoelnir wrote:
ikramerica wrote:
The design of these wheels is purposeful. They require lots of rotations to do little for many reasons. Two reasons are to A. make sure that someone turning the wheel doesn’t overdo it too quickly, and B. because they are geared so they are easy to turn under great forces against them.
For A. Think of two types of plumbing valves. A quarter turn valve is hard to modulate and cuts things very quickly. A multi-turn style valve closes very slowly but can act against even super high pressures such as those one fire trucks or hydro dams.
For B. Think about a very low gear for riding a bike up a steep hill. It can feel as if you are going almost nowhere. Put a blindfold on a person and they may think they are not going up at all.
Without experience you might think its not working because of B, and if you don’t understand the trim system (never actually used those wheels in flight) you might think that because of the design, they aren't actually doing anything.
Or you can just completely not know what’sgoing on, try the disabled electric triggers and say they aren't working, then flip the cutoff back on because you would rather have some control than no control.


I do not believe that the wheels are geared to move the trim slow. They are under normal circumstances not used and the electrical trim is pretty fast compared. They are geared to be able to move the trim. They seem even not to be geared enough, because when they are needed the forces can be to great to move them, but if geared even more, they would move the trim that slow to be nearly useless. Regarding the power needed to turn the wheels, they have been made smaller in diameter than in the 737-200, so it will be more difficult to turn them in a MAX (or NG).

I think they are kept as backup, so it is possible to show them as an added safety. You can end abnormal trim lists on: "and now you move the trim manually with the wheels" .
IMO it is a fail safe that works as well as a parachute that does not open most of the time, rather useless. In situation you would need it, the situation is already down the drain and manual trim that heavy that it will hardly be useful to you.
That this obsolete trim is not responsible for higher rate of accidents, may be a function of how often it needs to be used. In the NG I assume hardly ever


You are very insistent that the "obsolete trim system" of the 737 must be completely redesigned. Perhaps the components of the trim system are just extremely reliable and the instances where it needs to be cut out (except when the terrible MCAS implementation was added) is exceedingly rare. Perhaps if the electric trim stops working somewhere near where the trim should be, the aircraft can still be controlled and landed using the elevators or, in that scenario, the manual wheel works.

Obviously runaway trim can't be very common at all on the pre-MCAS/pre-MAX 737's or the issue of being able to recover from it ending up impossible would have come up many years ago.

Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.


I have a very simple point of view, things that do not work, or work very badly need to be redesigned.

I completely uninterested in the data regarding the 737NG. We are here talking about the 737MAX, and there the system when needed failed the pilots and two aircraft of that type dropped out of the sky inside a rather short time period.
 
mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 11:45 pm

ikramerica wrote:
planecane wrote:
mjoelnir wrote:

I do not believe that the wheels are geared to move the trim slow. They are under normal circumstances not used and the electrical trim is pretty fast compared. They are geared to be able to move the trim. They seem even not to be geared enough, because when they are needed the forces can be to great to move them, but if geared even more, they would move the trim that slow to be nearly useless. Regarding the power needed to turn the wheels, they have been made smaller in diameter than in the 737-200, so it will be more difficult to turn them in a MAX (or NG).

I think they are kept as backup, so it is possible to show them as an added safety. You can end abnormal trim lists on: "and now you move the trim manually with the wheels" .
IMO it is a fail safe that works as well as a parachute that does not open most of the time, rather useless. In situation you would need it, the situation is already down the drain and manual trim that heavy that it will hardly be useful to you.
That this obsolete trim is not responsible for higher rate of accidents, may be a function of how often it needs to be used. In the NG I assume hardly ever


You are very insistent that the "obsolete trim system" of the 737 must be completely redesigned. Perhaps the components of the trim system are just extremely reliable and the instances where it needs to be cut out (except when the terrible MCAS implementation was added) is exceedingly rare. Perhaps if the electric trim stops working somewhere near where the trim should be, the aircraft can still be controlled and landed using the elevators or, in that scenario, the manual wheel works.

Obviously runaway trim can't be very common at all on the pre-MCAS/pre-MAX 737's or the issue of being able to recover from it ending up impossible would have come up many years ago.

Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.

While I don't agree with a lot of what he is saying, there is still a case to be made that if airplanes were designed today, the controls would be very different. Same goes for cars.

The more we find out about the 737 system, the more we see how aircraft systems in general are designed so they "feel" like obsolete technology and use control mechanisms that mimic designs that should be outmoded. I'm not talking grandfathering exclusively. But the main paradigm of the feedback the aircraft gives is based on how much, much, much older aircraft with manual controls reacted and how they were controlled. Many of the paradigms go back to the piston era.

This happens because, even with a new type, the PILOTS are grandfathered in. To get experienced pilots, you need to have them already know how to fly the plane based on their experience. If you changed it up completely, you would have no one with experience to fly any of the planes you sell. Yes, you can train people, but getting enough people up to speed with enough experience would be astronomically costly and probably impossible.

So we have various aircraft of various vintages all trying to approximate how an older aircraft would behave just because. And as technology advances further and further, there will be more conflicts between how it was always done/needs to be done and how it really should be done.

The really ironic part is that as aircraft are more automated, the pilots are doing less actual flying anyway and too often forget what they are supposed to know in a panic situation because they barely ever use it, so completely changing the paradigm shouldn't matter as much.


Airbus broke with that tradition. The joystick has no feedback. Even pilots trained on the 737 seem to be able to switch over to the A320 and fly those frames.
 
kalvado
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 12:53 am

planecane wrote:
Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.

It can be where another can of worms will be opened. There were some NG accidents mentioned which could be attributed to trim system.
If those are revisited, and actually found due to trim - very big if, of course - then the way to go would be a big retrofit AD with before-net-crash timing. Can easily become even worse news for the manufacturer than harsh MAX grounding.
 
morrisond
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 1:21 am

ikramerica wrote:
speedbored wrote:
morrisond wrote:

I would have to guess that when they were well over Vmo almost anything is possible.

I'm not convinced that 20-25kts, counts as "well over". Given the pressure altitude they were at, it is unlikely to have been overstressing the airframe. According to the report, they didn't exceed approximately 20-25kts over Vmo until they entered the final dive.

Does anyone know the Vne for the 737NG? I cant imagine the margin over Vmo is less than 10%, but if you are approaching that speed it will start to feel like that scene in The Right Stuff where Chuck Y is approaching the sound barrier.


Vne is basically Vmo in Transport Category Aircraft.
 
planecane
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 2:15 am

kalvado wrote:
planecane wrote:
Using the history of the 737 series in service as the data, there is nothing wrong with flying around with the obsolete trim system. The problems with the trim system are with the algorithm that is commanding the trim motor. That's the most "state of the art" part of the whole thing. They could have put the 787 trim system on the MAX and it would have resulted in the same two crashes if MCAS was there (which it would have been).

It isn't your vision of modern technology for a cable to be running from the cockpit to the tail and physically turning a jackscrew but it works fine.

It can be where another can of worms will be opened. There were some NG accidents mentioned which could be attributed to trim system.
If those are revisited, and actually found due to trim - very big if, of course - then the way to go would be a big retrofit AD with before-net-crash timing. Can easily become even worse news for the manufacturer than harsh MAX grounding.

Can you list those NG incidents? I'd like to learn more to see if mjoelnir has a more valid argument.
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 3:02 am

PixelFlight wrote:
The SMYD feed the AoA value to the FDR.


Nice try, that is for the NG, the MAX integrates airplane data with the Onboard Maintenance Function (OMF).
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xmp125a
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 5:33 am

morrisond wrote:

Would you rather they take the manual out entirely? How would that make it safer? In the normal part of the Flight Envelope it is quite usable according to 7Boeing7

The A320 has one as well. I would have to guess it is pretty difficult to move outside the envelope as well. Should we remove that to make the 320 safer? What about complete Electric Failure?


I assume taking it out entirely would require finding another solution that would not depend on how much a pilot can bench press. Now that you mention total electrical failure - total hydraulics failure is death sentence as well (as I understand), so it would not be that extraordinary. But on the other hand, yes, I agree with you, manual backup is always better. Now, as "normal part of the flight envelope" goes I am not satisfied with this, the need for the wheel is apparently extremely extraordinary occurrence - so one can expect this rare need will be present where *it* really hits the fan, so outside "normal" part of the envelope. So perhaps it is a time to rethink it, maybe different gear ratio or something.If it is a last resort to trim a plane when electric motors cannot move the screw, then it should really be the resort, and not a trap.

Note that according to mentourpilot's deleted video transcript (sadly there are no timestamps), it was useless in Ethiopian case, even before the plane went out of the envelope.
 
jollo
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 7:51 am

flybucky wrote:
jollo wrote:
Since getting to the bottom of whether AUTO-PILOT STAB TRIM CUTOFF disables STS on a 737 NG seems to be harder than expected

Peter Lemme said that on pre-MAX 737s, the AUTOPILOT STAB TRIM CUTOFF would disable STS:

The cutout switch function was changed with the 737MAX from all prior 737 models. The legacy switch combination was one switch to cutout electric trim altogether, the other to cutout the autopilot trim commands. MCAS and Speed Trim System are both commands from the “autopilot”. With the legacy switch configuration, the flight crew can disable the autopilot commands and retain electric trim.

https://www.satcom.guru/2019/04/what-ha ... et302.html


Thanks a lot: I'll try to find additional confirmation out of an abundance of caution, but this pretty much clinches the case for me.

IMO, the chain of decision leading to the current MCAS design was something like:
  • aero analysis (or flight testing) highlights non-linear, non-monotonous response to pitch inputs in some flight envelope regions (flaps up, high speed, high AoA, etc.)
  • longitudinal stability augmentation system needed to comply with FAR
  • MCAS out of the KC-767A / KC-46 design is an "off the shelf", proven solution, and is also compatible with 737 "legacy" air data architecture (2 AoA channels, not 3)
  • MCAS is implemented as an "STS extension" run by the FCC; MCAS is disabled when A/P in engaged, so no interference; MCAS operates in rather different flight envelope regions vs. STS, so no interference there either
  • MCAS has stronger authority than STS (i.e. can command a higher stab trim angle at a higher rate of stab trim travel) and, running off a single AoA input channel like STS, introduces the potential for a "MCAS runaway" on a single sensor failure scenario that was simply not material with STS. However, disabling MCAS in an AoA disagree condition (logic also out of the KC-767A / KC-46 design), plus the possibility to disable MCAS manually via the existing STAB TRIM CUTOFF AUTO-PILOT switch is deemed sufficient to manage safely any "MCAS runaway" scenario.
  • Level D difference training will be needed to enable pilots to stay out of the relevant flight envelope corners with an inoperative/disabled MCAS. This does not prevent common type rating --> mission accomplished.

Up to this point, everything is cool. If things had been left at that, MAX would have been as safe as NG. Even marginally safer, in the vast majority of the flight envelope and, with an operative MCAS, across 100% of the envelope.

Note: switching only STAB TRIM CUTOFF AUTO-PILOT to CUTOFF while leaving STAB TRIM CUTOFF MAIN ELECT to NORMAL would have disabled all automations while leaving the electrical stab trim servo operative, to help pilots cope with aero loads on the stabilizer via thumb switches. Both Lion Air and Ethiopian crashes could easily have been avoided.

Enter the bad guy (circa 2011): the commercial imperative to avoid simulator training at all costs. Clients are clamoring for minimal transitions costs, and an over-eager sales manager, desperate to secure a mega-order out of Airbus hands, has promised to SW a 1M$/plane discount if any simulator training will be needed. That's a 280M$ penalty at stake. Who's gonna pay that kind of money, you? Then back to the design table and make that training requirement go away. Pronto: flight testing is just a few years away.

  • so no sim training
  • therefore MCAS has to be "always on",
  • therefore the AoA disagree flag needs to go --> compromise: it becomes an extra option, so it does not need to be incorporated into NNCs
  • therefore the STAB TRIM CUTOFF AUTO-PILOT switch needs to go, too. But removing a switch from the cockpit would be a Level D difference, while relabeling is just Level B (no sim training needed). So both STAB TRIM CUTOFF switches get relabeled rather ambiguously PRI and B/U, and both get the same functions - equivalent to the old STAB TRIM CUTOFF MAIN ELECT switch: operating either one will disable the stab trim electric actuator (so only manual trim wheel left). THIS IS A FUNCTIONAL CHANGE (Level C at least) that was hidden as "relabeling": this is the one and only criminal act in this sequence.
  • any "MCAS runaway" scenario needs to be managed through the existing Stab Trim Runaway NNC; luckily, the old checklist never calls for operating the cutout switches separately, so this does not give away the functional change on the cutout switches
  • but this is bound to generate awkward questions, so MCAS gets expunged from the FCOM (but not completely: someone forgets to purge it out of the acronyms reference list).
  • did some engineer raise concerns about this scenario creating a potentially catastrophic scenario on a single sensor failure event, mitigated only by pilot reactions? Probably, but we'll never know if he was silenced by management decision or if he was convinced by a risk analysis. We do know that nobody blew the whistle.

Conjecture, yes, but IMO its chillingly fits.
 
WIederling
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 8:43 am

asdf wrote:
lets say it clear:
the FAA directive to cut out the electric trim in case of MCAS failure was one of the big holes in the cheese for the ET crash
if manual trim is THAT much ineffectiv on the MAX one shouldnt have issued such a directive ...


Who wrote that actually?
Boeing alone, FAA under Boeing's aid or FAA on their own?
Murphy is an optimist
 
XRAYretired
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 8:52 am

WIederling wrote:
asdf wrote:
lets say it clear:
the FAA directive to cut out the electric trim in case of MCAS failure was one of the big holes in the cheese for the ET crash
if manual trim is THAT much ineffectiv on the MAX one shouldnt have issued such a directive ...


Who wrote that actually?
Boeing alone, FAA under Boeing's aid or FAA on their own?

The FAA AD issued 7th November is a carbon of Boeing TBC-19 issued 6th November. However, suspect they colluded as to the instruction and its wording.

Ray
 
WIederling
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 9:08 am

XRAYretired wrote:
WIederling wrote:
asdf wrote:
lets say it clear:
the FAA directive to cut out the electric trim in case of MCAS failure was one of the big holes in the cheese for the ET crash
if manual trim is THAT much ineffectiv on the MAX one shouldnt have issued such a directive ...


Who wrote that actually?
Boeing alone, FAA under Boeing's aid or FAA on their own?

The FAA AD issued 7th November is a carbon of Boeing TBC-19 issued 6th November. However, suspect they colluded as to the instruction and its wording.

Thanks.
Murphy is an optimist
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 9:35 am

jollo wrote:
[
Note: switching only STAB TRIM CUTOFF AUTO-PILOT to CUTOFF while leaving STAB TRIM CUTOFF MAIN ELECT to NORMAL would have disabled all automations while leaving the electrical stab trim servo operative, to help pilots cope with aero loads on the stabilizer via thumb switches. Both Lion Air and Ethiopian crashes could easily have been avoided.


Those switches are not on the MAX, the MAX has PRI and B/U cutout switches.
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BaconButty
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 9:52 am

jollo wrote:
Conjecture, yes, but IMO its chillingly fits.


Very good read, just to throw into the mix the fact that the scope of MCAS may have changed at some point during flight testing, from an 0.6 degree max deflection to a 2.5 degree. People within the FAA have claimed that the system was certified based on 0.6 deg, and that was passed on to international bodies. Also the claim is the FAA did not to know about the increase until after the LionAir crash.

Sounds far fetched, but *if* that's true, it's possible that the initial approval of the design approach was legitimate, all failure modes analysed etc. based on that maximum deflection. However flight testing showed that in order to compensate for the lack of linear stick response in the all important approach to stall, MCAS would need to do much more. In that case the screw ups, and potential criminality, happen at that point.

https://www.seattletimes.com/business/b ... air-crash/
Down with that sort of thing!
 
Jetty
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:13 am

zeke wrote:
jollo wrote:
[
Note: switching only STAB TRIM CUTOFF AUTO-PILOT to CUTOFF while leaving STAB TRIM CUTOFF MAIN ELECT to NORMAL would have disabled all automations while leaving the electrical stab trim servo operative, to help pilots cope with aero loads on the stabilizer via thumb switches. Both Lion Air and Ethiopian crashes could easily have been avoided.


Those switches are not on the MAX, the MAX has PRI and B/U cutout switches.

Which is exactly what he said if you read carefully.
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:20 am

Jetty wrote:
Which is exactly what he said if you read carefully.


No he didn’t, the switches he refers to are on the NG and before, they have different names with a different architecture on the MAX.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
Jetty
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:27 am

zeke wrote:
Jetty wrote:
Which is exactly what he said if you read carefully.


No he didn’t, the switches he refers to are on the NG and before, they have different names with a different architecture on the MAX.

I know and the poster knew, as he said so himself in his post. :boggled: Pay attention to his entire post and you will understand.
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:32 am

The switches on the MAX do not have the same function as the NG, nor does the B/U have the same function as the PRI.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
XRAYretired
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:36 am

BaconButty wrote:
jollo wrote:
Conjecture, yes, but IMO its chillingly fits.


Very good read, just to throw into the mix the fact that the scope of MCAS may have changed at some point during flight testing, from an 0.6 degree max deflection to a 2.5 degree. People within the FAA have claimed that the system was certified based on 0.6 deg, and that was passed on to international bodies. Also the claim is the FAA did not to know about the increase until after the LionAir crash.

Sounds far fetched, but *if* that's true, it's possible that the initial approval of the design approach was legitimate, all failure modes analysed etc. based on that maximum deflection. However flight testing showed that in order to compensate for the lack of linear stick response in the all important approach to stall, MCAS would need to do much more. In that case the screw ups, and potential criminality, happen at that point.

https://www.seattletimes.com/business/b ... air-crash/

Well, not quite I think, since single sensor is not a legitimate design approach for a 'hazardous' classified system even if mitigated by trim cut-out for sensor failing high was considered legitimate at the time.

Ray
 
Jetty
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:37 am

zeke wrote:
The switches on the MAX do not have the same function as the NG

Indeed, just as he mentioned. Did you even read the post? :confused: If you skip the last part it can be misunderstood. Read all of it and it makes a lot of sense.

nor does the B/U have the same function as the PRI.

What’s the difference? Naming would suggest they do, and I haven’t seen any official source alleging otherwise yet.
 
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zeke
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:41 am

I did read it, he said “So both STAB TRIM CUTOFF switches get relabeled rather ambiguously PRI and B/U, and both get the same functions - equivalent to the old STAB TRIM CUTOFF MAIN ELECT switch”

They do not have the same function as the NG, nor do the have the same function as each other.

The rest of his post is factually incorrect. The NG was originally certified without the low speed stall function, that was added afterwards without the need for additional training. JAA insisted on the low speed stall function, it was added as a mod.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
Jetty
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 10:50 am

zeke wrote:
I did read it, he said “So both STAB TRIM CUTOFF switches get relabeled rather ambiguously PRI and B/U, and both get the same functions - equivalent to the old STAB TRIM CUTOFF MAIN ELECT switch”

They do not have the same function as the NG, nor do the have the same function as each other

What’s the difference in function between STAB TRIM CUTOFF MAIN ELECT on the NG and PRI and B/U on the MAX?

AFAIK a different function would trigger training requirements which is exactly what Boeing wanted to avoid and why there is the buggy implementation of MCAS in the first place.
 
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PW100
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 12:18 pm

MSPNWA wrote:
mjoelnir wrote:
Barely turning the trim wheel can also be based upon it being impossible to turn the trim wheel.

The points to the crash are very simple. Boeing laid a deadly trap for pilots with designing MCAS. Boeing offered a solution, a checklist ending on an impossible task, turning the trim wheel when there is a load on the tailplane. Boeing compounded all of the above by sabotaging the possibility for training or even only trying out the failure mode of MCAS on a simulator.


Can also be based? Yes. Supported by the evidence we have so far? No. The evidence - very incomplete at this stage - points the other way.

It's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.


It is obvious that the evidence we have so far is quite incomplete. But that it points the other way is incomprehensible. I won't dispute that mjoelnir is overdoing it, but the indications we have so far are overwhelming pointing in a direction that doesn't seem to fit your agenda.

So, to use your own words: it's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.
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mjoelnir
Posts: 8521
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 12:34 pm

PW100 wrote:
MSPNWA wrote:
mjoelnir wrote:
Barely turning the trim wheel can also be based upon it being impossible to turn the trim wheel.

The points to the crash are very simple. Boeing laid a deadly trap for pilots with designing MCAS. Boeing offered a solution, a checklist ending on an impossible task, turning the trim wheel when there is a load on the tailplane. Boeing compounded all of the above by sabotaging the possibility for training or even only trying out the failure mode of MCAS on a simulator.


Can also be based? Yes. Supported by the evidence we have so far? No. The evidence - very incomplete at this stage - points the other way.

It's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.


It is obvious that the evidence we have so far is quite incomplete. But that it points the other way is incomprehensible. I won't dispute that mjoelnir is overdoing it, but the indications we have so far are overwhelming pointing in a direction that doesn't seem to fit your agenda.

So, to use your own words: it's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.


I do not overdo it. That a system like the manual trim system in the 737 is aloud to pose as a backup, is a scandal, nothing less. If you show me that that system is possible to operate over the whole flight envelope of them 737, as a backup system should be able to do, I will accept that I overdo it.

The argument that it has been like that for 60 years, not even true, because it has been degraded since by making the wheels smaller in diameter, is about the worst argument anybody cam bring. It simple does not work and should be thrown out and I assume that should have happened 30 to 40 years ago.
 
User avatar
SomebodyInTLS
Posts: 1728
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 12:39 pm

XRAYretired wrote:
BaconButty wrote:
jollo wrote:
Conjecture, yes, but IMO its chillingly fits.


Very good read, just to throw into the mix the fact that the scope of MCAS may have changed at some point during flight testing, from an 0.6 degree max deflection to a 2.5 degree. People within the FAA have claimed that the system was certified based on 0.6 deg, and that was passed on to international bodies. Also the claim is the FAA did not to know about the increase until after the LionAir crash.

Sounds far fetched, but *if* that's true, it's possible that the initial approval of the design approach was legitimate, all failure modes analysed etc. based on that maximum deflection. However flight testing showed that in order to compensate for the lack of linear stick response in the all important approach to stall, MCAS would need to do much more. In that case the screw ups, and potential criminality, happen at that point.

https://www.seattletimes.com/business/b ... air-crash/

Well, not quite I think, since single sensor is not a legitimate design approach for a 'hazardous' classified system even if mitigated by trim cut-out for sensor failing high was considered legitimate at the time.


I've thought for a while now that it was like BaconButty described - the original intention was to be an upgrade to STS to maintain the NG "feel" close to stall (not safety-critical). However, flight testing revealed worse than anticipated characteristics close to stall, hence the pumped-up MCAS to act as a safety-critical anti-stall measure. Then management leans on the engineers not to allow any changes to reach the pilots, their training and other documentation... and at that point questions need to be answered about how much was made known to the authorities, how much was overlooked in haste and how much was deliberately repressed.

Part of the "it's not anti-stall", "it's not about safety" BS may be down to it having changed purpose during flight testing and the same questions re. culpability need to be asked about the people spreading this argument as well.
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
jollo
Posts: 381
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 1:00 pm

zeke wrote:
I did read it, he said “So both STAB TRIM CUTOFF switches get relabeled rather ambiguously PRI and B/U, and both get the same functions - equivalent to the old STAB TRIM CUTOFF MAIN ELECT switch”

They do not have the same function as the NG, nor do the have the same function as each other.


Please, do share. Looking at the functional diagram I wasn't able do discern a difference in function between PRI and B/U switches (they are wired in series, and opening either will cut power to the R64 stab trim control relay, therefore disabling the electric stab stab trim). BTW on the diagram the switches are labeled A and B: do you happen to know which is PRI and which is B/U?

zeke wrote:
The rest of his post is factually incorrect. The NG was originally certified without the low speed stall function, that was added afterwards without the need for additional training. JAA insisted on the low speed stall function, it was added as a mod.


I am sorry my post did not get the meaning across clearly enough: I was conjecturing about the "genesis" of the MAX, not of the NG.

BaconButty wrote:
Sounds far fetched, but *if* that's true, it's possible that the initial approval of the design approach was legitimate, all failure modes analysed etc. based on that maximum deflection.


Just to be clear: I personally do not think that this hypothetical "initial" design iteration would have been legitimate. An airliner certified in 2017 should *not* give up all flight envelope protections on a single sensor failure. However, it would still have been much better (and safer) than an airliner vulnerable to a catastrophic control runaway on a single sensor failure, unless pilots with exceptional deductive skills and physical strength react within seconds with a specific sequence of actions.

I can't figure where exactly in the sequence the step-up in MCAS control authority from 0.6°/cycle to 2.5°/cycle happened, and how it got "lost" in the communication with FAA. Could be as much a smoking gun as the hidden functional change to STAB TRIM CUTOFF switches. IMO either would be enough to warrant a full revision of the certification process.
 
morrisond
Posts: 1395
Joined: Thu Jan 07, 2010 12:22 am

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 1:09 pm

SomebodyInTLS wrote:
XRAYretired wrote:
BaconButty wrote:

Very good read, just to throw into the mix the fact that the scope of MCAS may have changed at some point during flight testing, from an 0.6 degree max deflection to a 2.5 degree. People within the FAA have claimed that the system was certified based on 0.6 deg, and that was passed on to international bodies. Also the claim is the FAA did not to know about the increase until after the LionAir crash.

Sounds far fetched, but *if* that's true, it's possible that the initial approval of the design approach was legitimate, all failure modes analysed etc. based on that maximum deflection. However flight testing showed that in order to compensate for the lack of linear stick response in the all important approach to stall, MCAS would need to do much more. In that case the screw ups, and potential criminality, happen at that point.

https://www.seattletimes.com/business/b ... air-crash/

Well, not quite I think, since single sensor is not a legitimate design approach for a 'hazardous' classified system even if mitigated by trim cut-out for sensor failing high was considered legitimate at the time.


I've thought for a while now that it was like BaconButty described - the original intention was to be an upgrade to STS to maintain the NG "feel" close to stall (not safety-critical). However, flight testing revealed worse than anticipated characteristics close to stall, hence the pumped-up MCAS to act as a safety-critical anti-stall measure. Then management leans on the engineers not to allow any changes to reach the pilots, their training and other documentation... and at that point questions need to be answered about how much was made known to the authorities, how much was overlooked in haste and how much was deliberately repressed.

Part of the "it's not anti-stall", "it's not about safety" BS may be down to it having changed purpose during flight testing and the same questions re. culpability need to be asked about the people spreading this argument as well.



If you read the best decription I've seen on MCAS from 737.org.uk

"MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up."

Please note that it is a slight pitch up effect - the controls get lighter. If you hold the controls steady the Nacelles will not generate enough lift to pull it into a stall - it just becomes easier to pull it into a stall if the pilot ignores the AOA indicator - the stick shaker - the stall horn - the frame buffeting , etc..etc...

The control response is non-linear which is not allowed by the FAR's.

BTW - Air is not a constant medium - you can fly through pockets which totally unload the controls as well - that is why there is an AOA indicator as you can't totally depend on the feel of the controls - you must be constantly scanning your instruments (along with looking ouside) - especially when you are in high AOA attack situations while turning.

I would have to guess that the .6 Degree change wasn't fast enough to counteract the feel and that's why they changed it to 2.5 in flight test.
 
MSPNWA
Posts: 3437
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 3:39 pm

PW100 wrote:
It is obvious that the evidence we have so far is quite incomplete. But that it points the other way is incomprehensible. I won't dispute that mjoelnir is overdoing it, but the indications we have so far are overwhelming pointing in a direction that doesn't seem to fit your agenda.

So, to use your own words: it's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.


Decades of certification, testing, and real-world use is incomprehensible? There's no proof that the manual system doesn't work in the flight envelope. And there's absolutely no concrete evidence showing us that it positively didn't work on ET302. We don't even have proof they tried the manual system, let alone it didn't work as designed.

I know your statement about my beliefs is false because I've come to no conclusions on the function of the manual trim system. I'm in search of all the evidence, which we clearly don't have. Coming to a conclusion without all the evidence is foolishness, and the only reasons to do so aren't noble. That needs to be called out lest people get more misinformed.

mjoelnir wrote:
I do not overdo it. That a system like the manual trim system in the 737 is aloud to pose as a backup, is a scandal, nothing less. If you show me that that system is possible to operate over the whole flight envelope of them 737, as a backup system should be able to do, I will accept that I overdo it.

The argument that it has been like that for 60 years, not even true, because it has been degraded since by making the wheels smaller in diameter, is about the worst argument anybody cam bring. It simple does not work and should be thrown out and I assume that should have happened 30 to 40 years ago.


The burden of proof is on you to prove that the system cannot be used over the range of the flight envelope.
 
User avatar
zeke
Posts: 13990
Joined: Thu Dec 14, 2006 1:42 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 3:54 pm

jollo wrote:
Please, do share. Looking at the functional diagram I wasn't able do discern a difference in function between PRI and B/U switches (they are wired in series, and opening either will cut power to the R64 stab trim control relay, therefore disabling the electric stab stab trim). BTW on the diagram the switches are labeled A and B: do you happen to know which is PRI and which is B/U?


The switches are not labelled A bad B, they are the placeholders for the other A and B on the motor control and and the FCC input.

The primary has very similar function to the autopilot switch on the NG, it will remove power from the FCCs.

The backup switch does not remove power from the FCCs. Much like the stab cutout on the NG.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
User avatar
PixelFlight
Posts: 650
Joined: Thu Nov 08, 2018 11:09 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 5:17 pm

zeke wrote:
PixelFlight wrote:
The SMYD feed the AoA value to the FDR.


Nice try, that is for the NG, the MAX integrates airplane data with the Onboard Maintenance Function (OMF).

The OFM is an other box that get data from the DFDAU. The OMF don't feed AoA value to the FDR.
 
User avatar
zeke
Posts: 13990
Joined: Thu Dec 14, 2006 1:42 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 5:32 pm

PixelFlight wrote:
The OFM is an other box that get data from the DFDAU. The OMF don't feed AoA value to the FDR.


Which is a totally different box with a lot more capability than the NG (2 orders of magnitude). The MAX Enhanced DFDAU can be retrofitted to the NG, but the NG equipment cannot be used on the MAX. The ONS QAR stores 75 hours of QAR data.

Fact is you have not been able to demonstrate where the MAX data in the FDR is coming from. The MAX and NG are not the same.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
User avatar
PixelFlight
Posts: 650
Joined: Thu Nov 08, 2018 11:09 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Tue Apr 09, 2019 5:33 pm

zeke wrote:
jollo wrote:
Please, do share. Looking at the functional diagram I wasn't able do discern a difference in function between PRI and B/U switches (they are wired in series, and opening either will cut power to the R64 stab trim control relay, therefore disabling the electric stab stab trim). BTW on the diagram the switches are labeled A and B: do you happen to know which is PRI and which is B/U?


The switches are not labelled A bad B, they are the placeholders for the other A and B on the motor control and and the FCC input.

The primary has very similar function to the autopilot switch on the NG, it will remove power from the FCCs.

The backup switch does not remove power from the FCCs. Much like the stab cutout on the NG.

The FCC -> CUTOUT -> A -> ELECTRIC STAB TRIM MOTOR signal function is still largely unknown. I have read speculation but no more about it.
And he is the _ONLY_ difference between the two cutout switches. Now, aside of a very corner failure case and manipulation outside of the documented pilot procedure, I really don't see how this signal can make any difference on a ELECTRIC STAB TRIM MOTOR that is not powered anymore.
As I said before, the electrons have no 737 history. The two switches have the same function for the pilots: remove the power to the stab trim motor. Over interpreting a signal that goes to a non powered motor is a cognitive bias.

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