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seagull967
15th Mar 2019, 16:43
From the below linked article, I am trying to understand the system architecture here. Perhaps I am reading it wrong, but I am not seeing why this scenario would cause an increase in stick force?

"If it’s a too high AoA signal causing the unreliable airspeed, we can assume the stall warning will be on for the affected pilot from liftoff. This will cause the artificial Yoke feel system to increase the Yoke force. The flying pilot will feel he needs an increased back force on the Yoke to hold the aircraft level."

https://leehamnews.com/2019/03/15/bjorns-corner-the-ethiopian-airlines-flight-302-crash/#more-29660

FCeng84
15th Mar 2019, 19:04
From the below linked article, I am trying to understand the system architecture here. Perhaps I am reading it wrong, but I am not seeing why this scenario would cause an increase in stick force?

"If it’s a too high AoA signal causing the unreliable airspeed, we can assume the stall warning will be on for the affected pilot from liftoff. This will cause the artificial Yoke feel system to increase the Yoke force. The flying pilot will feel he needs an increased back force on the Yoke to hold the aircraft level."

https://leehamnews.com/2019/03/15/bjorns-corner-the-ethiopian-airlines-flight-302-crash/#more-29660

737MAX elevator feel system pressure is a function of stabilizer position, impact pressure, and AOA. The AOA dependency provides increased feel stiffness when AOA is near stall AOA. This has been added to provide one of the characteristics associated with stall identification "nose down pitch that cannot be readily arrested".

safetypee
15th Mar 2019, 19:57
FCeng84,
Thanks for that description. However would the stick forces attributed to the feel system change with an error in ADC input, due to a faulty AoA input, which changes the ADC - ‘feel diff pressure’. Could this add to any offset stick force applied by MACS ?

seagull967
15th Mar 2019, 20:03
737MAX elevator feel system pressure is a function of stabilizer position, impact pressure, and AOA. The AOA dependency provides increased feel stiffness when AOA is near stall AOA. This has been added to provide one of the characteristics associated with stall identification "nose down pitch that cannot be readily arrested".

Thanks, same question as sp, also, does that not describe MCAS also, or is this in addition to MCAS?

FCeng84
15th Mar 2019, 20:57
FCeng84,
Thanks for that description. However would the stick forces attributed to the feel system change with an error in ADC input, due to a faulty AoA input, which changes the ADC - ‘feel diff pressure’. Could this add to any offset stick force applied by MACS ?

Feel system will respond to faulty AOA input along with stick shaker, MCAS, and static pressure correction logic. The larger the column displacement at the time of the feel shift, the more pronounced the increase in column force will be.

gums
15th Mar 2019, 23:04
Holy molely, Batman!

Now we find out that stick pressure increases to “help” the pilot control the plane like the one he used to fly, Somehow, the FCOM did not advise us that we could expect increased control pressure at high AoA when flying “manual”. I would have thot that was an established characteristic of the beast, as with most planes flying today.

So MCAS is the aero solution, with computer help and not basic aero laws of stability and control without “help” And we have also tried to help the clueless pilot by changing the feel system.

I am concerned that there are several systems on this plane that contribute to, maybe conflict with, each other using various sensors in different manners, and finally wind up commanding nose down trim when you least expect it. SHEESH!


Gums takes a break.....

seagull967
16th Mar 2019, 05:03
Feel system will respond to faulty AOA input along with stick shaker, MCAS, and static pressure correction logic. The larger the column displacement at the time of the feel shift, the more pronounced the increase in column force will be.

Can you provide an example to ensure clarity? Let's say the AoA fault shows an abnormally high AoA, will the stick force just go up, or is it also dependent on where the column displacement is? I am trying to understand how it would manifest to the pilot. Thank you!

FGD135
16th Mar 2019, 06:44
Surely the only inputs to the stick force would be the elevator/stab "misalignment" and the dynamic pressure (q). I cannot see how AoA could make a useful contribution to the stick force calculation.

When I say "elevator/stab misalignment", I am referring to the angular difference between the stab and elevator positions. When those two are aligned, the angle is zero and the stick force is zero. Push or pull the column slightly, however, and there is then a slight misalignment - with consequent opposing stick force (of magnitude equal to the product of the angle and the dynamic pressure (q)).

Note that the stick force resists the movement of the column away from the centered (neutral) position.

This is my understanding of how it all works. I cannot see any other way that it could work.

CONSO
16th Mar 2019, 06:59
Surely the only inputs to the stick force would be the elevator/stab "misalignment" and the dynamic pressure (q). I cannot see how AoA could make a useful contribution to the stick force calculation.

When I say "elevator/stab misalignment", I am referring to the the angular difference between the stab and elevator positions. When those two are aligned, the stick force is zero. Push or pull the column slightly, however, and there is then a slight misalignment - with consequent opposing stick force (of magnitude equal to the product of the angle and the dynamic pressure (q)).

Note that the stick force resists the movement of the column away from the centered (neutral) position.

This is my understanding of how it all works. I cannot see any other way that it could work.

Consider when the horizontal stabilizer has bdeen tileted leading edge up say 10 degrees at 200 mph. This via aa long ( fuselage ) leverr arm pushes the nose down. to get the nose to go up, one must overcome the air pressure at 100 mph ( stick your hand out the window at 50 mph and feel how hard to tilt with fingeers closed ) to move the stabilizer down to zero ( in line to airflow and then perhaps a further 10 degrees down so as to push nose UP. Now remember the plane has a systtem to make it harder to move the stabilizer further up or down ...

The above is a very ovder simplified description of the problem

have a good evening

Station Zero
16th Mar 2019, 11:15
From some notes of mine.

The AOA input is into the Stall Management and Yaw Damper Computer (SMYD), it only appears to have an effect on the elevator feel when close to a stall. In these circumstances, the SMYD provides an input to the elevator feel shift module which provides 850 psi system A pressure to the elevator feel computer and dual feel actuator. This causes the elevator feel force in the feel and centering unit to increase and to counteract or resist elevator up movement at the control column. Activation of this occurs at an AOA of 8 to 11 degrees depending on flap position.

Kerosene Kraut
16th Mar 2019, 13:32
They got two AoA probes and two computers but seem to only use one of those systems on one side at a time, changing them after each flight. Why is that?

seagull967
16th Mar 2019, 19:22
From some notes of mine.

The AOA input is into the Stall Management and Yaw Damper Computer (SMYD), it only appears to have an effect on the elevator feel when close to a stall. In these circumstances, the SMYD provides an input to the elevator feel shift module which provides 850 psi system A pressure to the elevator feel computer and dual feel actuator. This causes the elevator feel force in the feel and centering unit to increase and to counteract or resist elevator up movement at the control column. Activation of this occurs at an AOA of 8 to 11 degrees depending on flap position.

Thanks, that clarifies it!

CONSO
17th Mar 2019, 04:39
A pressure to the elevator feel computer and dual feel actuator. This causes the elevator feel force in the feel and centering unit to increase and to counteract or resist elevator up movement at the control column. Activation of this occurs at an AOA of 8 to 11 degrees depending on flap position.

But what happens if AOA >>11 degrees due to bad sensor- and one cuts out all electrical - does it then resist both push and pull ?

Station Zero
17th Mar 2019, 08:01
But what happens if AOA >>11 degrees due to bad sensor- and one cuts out all electrical - does it then resist both push and pull ?

Not sure what you mean by cutting out electrical? If your talking cutouts due to the recent incident they would have no affect on this functionality as its on the elevator side of things not trim.

To me it looks like you would have to pull CBs to isolate the SMYD to prevent the input to the EFSM.

In the evenet of high airspeed then both up and down resistance with be generated as per normal. Function of airspeed and not through the SMYD

vmandr
17th Mar 2019, 18:38
I have a few questions regarding AOA sensor, if anybody is kind enough to comment...

a. is the position of the sensor(s) arbitrarily selected ?
b. what is the benefit of being near the nose ?
c. as its sole purpose is to relate AOA to stall, is it not more logical to be on top of the wing and/or stab trim ?
d. any other sensor types ever considered and if have been rejected on what basis ?

thanks

seagull967
19th Mar 2019, 11:30
Not sure what you mean by cutting out electrical? If your talking cutouts due to the recent incident they would have no affect on this functionality as its on the elevator side of things not trim.

To me it looks like you would have to pull CBs to isolate the SMYD to prevent the input to the EFSM.

In the evenet of high airspeed then both up and down resistance with be generated as per normal. Function of airspeed and not through the SMYD

for high AoA the force is just increased, so it would effect both push and pull also, correct?

Station Zero
19th Mar 2019, 16:19
for high AoA the force is just increased, so it would effect both push and pull also, correct?

Would have to look at the hydraulic schematic to see how the EFSM supplies the elevator dual feel actuator to confirm. But I believe it only affects one way as it provides the additional system A pressure to just one side of the actuator.

seagull967
20th Mar 2019, 03:42
Would have to look at the hydraulic schematic to see how the EFSM supplies the elevator dual feel actuator to confirm. But I believe it only affects one way as it provides the additional system A pressure to just one side of the actuator.

Do you have access to it? I would be interested to know.

infrequentflyer789
20th Mar 2019, 11:06
Would have to look at the hydraulic schematic to see how the EFSM supplies the elevator dual feel actuator to confirm. But I believe it only affects one way as it provides the additional system A pressure to just one side of the actuator.

According to AMM diagrams this is correct. Also AMM wording on function (my emphasis) is "to increase elevator feel pressure to counteract or resist control column elevator up movement" which appears to confirm .

However, big caveat that is all for the NG systems and may have changed for the MAX.