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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 12:16
  #3721 (permalink)  
PJ2
 
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Most of this has been reviewed in the thread elsewhere, but to perhaps summarize from a B737-400 AMM describing the HS trim system:
The horizontal stabilizer trim control system provides longitudinal trim of the airplane by varying the angle of attack of the horizontal stabilizer. The horizontal stabilizer is moved through 17 degrees of travel by means of a jackscrew with ball nut. The main electric actuator and the cable drum on the jackscrew gearbox provide for trim control from three separate control systems. The normal control is an electrical system which actuates the jackscrew through the main electrical actuator. The main electric actuator and autopilot actuator, autopilot control is provided by the autopilot actuator on the gearbox. A manual control system drives through cables to the cable drum on the jackscrew gearbox. The manual system remains engaged at all times and is therefore back-driven by the main electric actuator or autopilot actuator during normal operation. Manual system operation will disengage both the normal electrical and autopilot actuators if these systems become jammed. A continuous indication of stabilizer position is provided by trim position indicators adjacent to trim wheels on the control stand. The indicators are positioned by the manual system. A takeoff warning system indicates any unsafe stabilizer position for takeoff.The control column actuated cutout switch located under the cockpit floor, stops electric trimming of the stabilizer when opposed by the motion of the elevator control.

Normal electrical trim control system consists of control switches, cutout switches, a column- actuated cutout switch, trim control relays, flap switch and relay, limit switches and an electric actuator. The electric actuator contains an electric motor, two electromagnetic clutches, a speed change relay, torque limiting clutch and output shaft to drive the jackscrew gearbox. The motor is operated by 3 phase AC power and the relay and clutches by DC power. The stabilizer actuator is controlled by the trim control switches, control cutout switch, limit switches and cutout switches. The control switches, located on the outboard horn of each control wheel, have two momentary positions, nose up and nose down, and are spring-returned to the center off position.The column- cutout switch located beneath the cockpit floor is actuated by forward and aft movement of either control column. Stabilizer limit switches are located on the bulkhead aft of the jackscrew gearbox and actuated by a striker to limit stabilizer leading edge up and down travel. The cutout switches on the control stand are used to remove power from the main electric actuator or the autopilot actuator.


Normal electrical trim control system consists of control switches, cutout switches, a column- actuated cutout switch, trim control relays, flap switch and relay, limit switches and an electric actuator. The PSTA contains an electric motor, two electromagnetic clutches, a speed change relay, torque limiting clutch and output shaft to drive the jackscrew gearbox. The motor is operated by 3 phase AC power and the relay and clutches by DC power. The PSTA is controlled by the trim control switches, control cutout switch, limit switches and cutout switches. The control switches, located on the outboard horn of each control wheel, have two momentary positions, nose up and nose down, and are spring-returned to the center off position. The column-cutout switch located beneath the cockpit floor is actuated by forward and aft movement of either control column. Stabilizer limit switches are located on the bulkhead aft of the jackscrew gearbox and actuated by a striker to limit stabilizer leading edge up and down travel. The cutout switches on the control stand are used to remove power from the PSTA.

Normal electric trimming of the stabilizer is done at one of two rates as controlled by flap position. Trim rate with flaps retracted is 1/3 the trim rate with flaps extended. The autopilot actuator also trims at one of two rates as controlled by flap position. High speed autopilot rate is equal to the normal electric low speed rate. The low speed autopilot rate is 1/2 the rate of the high speed autopilot rate.

Last edited by PJ2; 9th Apr 2019 at 14:34. Reason: clean up duplications
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Old 9th Apr 2019, 13:47
  #3722 (permalink)  
 
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Originally Posted by xyze View Post
Something I have never understood wrt this - is it possible for the a330 to be in the air with an airspeed of 60 knots or even 70 knots and NOT be stalled? If not then why inhibit the warning? Worrying about sensor accuracy seems to be missing the bigger picture wrt the purpose of the warning!
It is the design of the warning rather than the purpose. Invalidating the sensor readings (speed and AOA) at the ADIRU is not a bad design (and might have saved ET302). Failure to consider the possibility of known-invalid data further downstream is bad design.

The AF447 stall warning problem is even more subtle though - invalid data was designed for, the problem is (it appears) that it was designed without state so known invalid data resulted in "no warn". This was arguably incorrect, but known invalid data shouldn't result in "warn" either. With a stateful design a transition from stalled-data to invalid-data would not take you out of the "warn" state, not complicated (he says, having forgotten most of his NFA/DFA and statemachine stuff), just wasn't done that way.

Ironically if AF had taken the optional (standard on later buses) A330 feature that they said their pilots didn't need, the stall warning design issue would have been fixed (BUSS, optional on 330, fixes it). Whether it would have saved them, I don't know for sure, very possibly irrecoverable by the time they got to that point.
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Old 9th Apr 2019, 13:56
  #3723 (permalink)  
 
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Originally Posted by Just This Once... View Post
So being unable to use the electric trimmers whilst pulling back on the column is a design feature, hard-wired into the system.

Not so keen on that idea.
You can always trim (within electric trim range) in the same direction as control column deflection:

Control column actuated stabilizer trim cutout switches stop operation of the main electric and autopilot trim when the control column movement opposes trim direction.
... and ...

Stabilizer Trim Override Switch
OVERRIDE – bypasses the control column actuated stabilizer trim cutout switches to restore power to the Stabilizer Trim Switches
NORM (guarded position) – normal operating position
So there's a "control-column-actuated-stabilizer-trim-cutout-switch override switch". In case you want to use the control wheel trim switches to trim opposite your control column input.

Bernd
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Old 9th Apr 2019, 13:58
  #3724 (permalink)  
 
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Originally Posted by TryingToLearn View Post
But this proves that electronic trim is less powerful than manual trim, right?
No, it simply proves that manual trim inputs outvote electric trim (and A/P trim) inputs by design.

Or can the motor distinguish where the force is coming from?
Yes, it can.

Originally Posted by Just This Once... View Post
So being unable to use the electric trimmers whilst pulling back on the column is a design feature, hard-wired into the system.
You can trim ANU while pulling back on the column, but not AND.
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Old 9th Apr 2019, 13:58
  #3725 (permalink)  
 
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Originally Posted by Just This Once... View Post
So being unable to use the electric trimmers whilst pulling back on the column is a design feature, hard-wired into the system.

Not so keen on that idea.
Only trim in the opposing direction is stopped, and all electric trim in the opposing direction is stopped. The only way to trim forward whilst pulling back would be manual trim wheel.

This is one of the simplest and neatest examples of Boeing philosophy of helping the pilot not do stupid but never by overriding the pilot's primary control inputs. This was a good system, but not any more. MCAS broke the system. Now there is another way to trim forward whilst pulling back - just let HAL do it for you, all the way down.
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Old 9th Apr 2019, 14:24
  #3726 (permalink)  
 
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Originally Posted by deltafox44 View Post
The 737 manuals says that incase of extreme nose-down out-of-trim, one should INCREASE speed to relieve elevator load and permit manual trimming.

That's insane !

(see https://www.satcom.guru/2019/04/what...-on-et302.html)
As you accelerate the plane pitches up due to lift being in front of cg and you need to trim nose down to fly level. And vice Versa. Lesson 1 of type rating course on sim covers this.
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Old 9th Apr 2019, 14:34
  #3727 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
Only trim in the opposing direction is stopped, and all electric trim in the opposing direction is stopped. The only way to trim forward whilst pulling back would be manual trim wheel.
Not all opposing electric trim is stopped. Only main electric (control wheel switch) and autopilot trim. STS is not the autopilot.

And you can also use the control-column-actuated-stablizer-trim-cutout-switch override switch.

This is one of the simplest and neatest examples of Boeing philosophy of helping the pilot not do stupid but never by overriding the pilot's primary control inputs.
So, the design philosophy is, "Don't let the pilot do anything stupid. Unless he does something stupid with the primary flight controls. Then, don't interfere."

This was a good system, but not any more. MCAS broke the system. Now there is another way to trim forward whilst pulling back - just let HAL do it for you, all the way down.
Well, the whole raison d'être for MCAS is to modify stick forces when pulling back, so obviously it needs to be able to trim against stick deflection.

And if you insist that something "broke" the column cutout switch design philosophy, STS did it first. It also trims against stick deflection, explicitly to "increase control column forces".

Bernd

Last edited by bsieker; 9th Apr 2019 at 16:16. Reason: Struck out incorrect portions. Thanks to infrequentflyer789 and 737 Driver!
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Old 9th Apr 2019, 14:39
  #3728 (permalink)  
 
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Salute!

AvWeek has a summary of the thjings Big B is doing, and also iterates the purpose and function of MCAS. If it's paywalle I'll come back and cut-and-paste.

https://aviationweek.com/commercial-...-max-grounding

I wonder why Boeing didn't separate the two stab trim cutout switches. One for only the wheel electric trim switches and the other for all of HAL's stuff -STS, A/P and MCAS.

That would seem to have saved both planes and it also allows the "revert to manual" procedure that many of the 737 drivers here have bragged about. 'cause I do not see using electric trim switches to reduce loads as being much removed from cranking a wheel, just less tiring and faster.

Gums sends..

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Old 9th Apr 2019, 14:40
  #3729 (permalink)  
 
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Originally Posted by PJ2 View Post
... The normal control
is an electrical system which actuates the jackscrew through the main electrical actuator. The main
electric actuator and autopilot actuator, autopilot control is provided by the autopilot actuator on the
gearbox. A manual control system drives through cables to the cable drum on the jackscrew
gearbox. ...

Normal electric trimming of the stabilizer is done at one of two rates as controlled by flap position.
Trim rate with flaps retracted is 1/3 the trim rate with flaps extended. High speed autopilot rate is
equal to the normal electric low speed rate. The low speed autopilot rate is 1/2 the rate of the high
speed autopilot rate.
From this excerpt it seems that there were two discrete actuators for autopilot and for electric manual (hence the need for two cut out switches).
This would also suggest that there could be two electric motors fighting each other when AP and manual el. trim are applied the same time?
Which one would succeed? I would also expect the rate switch to have some impact on available torque. So the outcome may be dependent on AC coniguration.

From what has been posted here it seems very likely, that this has been changend on the MAX.

So unfortuneately this actually raises more questions to me.
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Old 9th Apr 2019, 15:05
  #3730 (permalink)  
 
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Originally Posted by gums View Post
I wonder why Boeing didn't separate the two stab trim cutout switches. One for only the wheel electric trim switches and the other for all of HAL's stuff -STS, A/P and MCAS.
737s prior to MAX worked like this.
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Old 9th Apr 2019, 15:06
  #3731 (permalink)  
 
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Originally Posted by bsieker View Post
Not all opposing electric trim is stopped. Only main electric (control wheel switch) and autopilot trim. STS is not the autopilot.

.......

And if you insist that something "broke" the column cutout switch design philosophy, STS did it first. It also trims against stick deflection, explicitly to "increase control column forces".
Incorrect. STS function is stopped by the control column cutout switch. MCAS is the only system that bypasses this function.
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Old 9th Apr 2019, 15:10
  #3732 (permalink)  
 
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Originally Posted by BDAttitude View Post
From this excerpt it seems that there were two discrete actuators for autopilot and for electric manual (hence the need for two cut out switches).
This would also suggest that there could be two electric motors fighting each other when AP and manual el. trim are applied the same time?
Which one would succeed? I would also expect the rate switch to have some impact on available torque. So the outcome may be dependent on AC configuration.
There is only one stab trim motor on the 737.

Yoke trim wil outvote A/P trim.
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Old 9th Apr 2019, 15:23
  #3733 (permalink)  
 
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It looks as if the STAB CUT-OUT switches were set to OFF at around 05:40:39 and remained OFF until around 05:43:06. Presumably, during this period the crew tried to use the manual trim crank wheels to counteract the ND trim which had been applied by MCAS before the STAB CUT-OUT was switched to OFF. The pitch trim trace shows no sign of reduced ND trim using the trim crank wheels so, presumably, the force needed to turn the trim wheels was too great. Switching the manual electric trim back on must have been a last-ditch attempt by the crew to save their aircraft. I'm wondering if the two short manual electric trim inputs just before the aircraft entered its final dive are an indication that manual NU electric trim becomes impossible once ND trim reaches a certain point.

It should be possible to calculate the turning moment about the HS pivot point for a range of ND trim units at various aircraft speeds and elevator positions. This should determine the range of forces which has to be resisted by the HS trim jackscrew. A ground test rig capable of applying this range of forces to a jackscrew could then be used to check the ability of the manual trim (both electric and crank wheel) to apply NU trim under all circumstances.
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Old 9th Apr 2019, 15:31
  #3734 (permalink)  
 
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Originally Posted by bsieker View Post
Not all opposing electric trim is stopped. Only main electric (control wheel switch) and autopilot trim. STS is not the autopilot.
[...]
And if you insist that something "broke" the column cutout switch design philosophy, STS did it first. It also trims against stick deflection, explicitly to "increase control column forces".

Bernd
Sorry, with the greatest respect, I think you are mistaken.

STS is not the autopilot, and it only operates with autopilot off, but it operates using the autopilot trim signal. It is therefore cutout by (a) the "autopilot" console cutout switch on NG, and (b) the column cutout switch (if in opposing direction). I have multiple references all clearly stating this and functional diagrams showing it, but I'll quote just one, from the NG AMM 27-41-00:

The column cutout switches stop the stabilizer trim actuator when the pilot moves the control column in a direction opposite to the trim direction.
Stop the actuator, for opposing trim, period, regardless of signal source.

But not on the MAX - the cutout is bypassed when MCAS enabled (and that new bypass wouldn't be needed if STS already bypassed it).
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Old 9th Apr 2019, 15:39
  #3735 (permalink)  
 
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Avionista

It should be possible to calculate the turning moment about the HS pivot point for a range of ND trim units at various aircraft speeds and elevator positions. This should determine the range of forces which has to be resisted by the HS trim jackscrew. A ground test rig capable of applying this range of forces to a jackscrew could then be used to check the ability of the manual trim (both electric and crank wheel) to apply NU trim under all circumstances.
I sincerely hope that Boeing engineers are doing this as we speak, otherwise they won't have a leg to stand on if this ever comes to trial. Saying that it was designed and calculated to do X and Y back in 1968, is not going to impress a jury...
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Old 9th Apr 2019, 15:40
  #3736 (permalink)  
 
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Originally Posted by weemonkey View Post
" the whole raison d'être for MCAS is to modify stick forces when pulling back"

Is it? That seems to be in contradiction with everything I have read on the media.
Just about everything you have read in the media is wrong. MCAS is not stall protection. MCAS does not counter the additional thrust of the more powerful engines. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA longitudinal stability and stick force certification standards.

The easiest fix was to automatically apply a little nose down trim at high angles of attack.


25.173 Static longitudinal stability.

Under the conditions specified in 25.175, the characteristics of the elevator control forces (including friction) must be as follows:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight.

(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in § 25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in § 25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.

(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
*

Last edited by Lost in Saigon; 9th Apr 2019 at 15:52.
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Old 9th Apr 2019, 15:53
  #3737 (permalink)  
 
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Originally Posted by Lost in Saigon View Post
Just about everything you have read in the media is wrong. MCAS is not stall protection. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA stick force certification standards.
I fear you are fighting a losing battle in your campaign.

For the mainstream media, and even some of the industry press, knowing that aircraft can stall and that it's generally considered a BAD THING represents about the limit of their understanding.

Start talking about FARs and stick force gradient certification standards and their eyes will start to glaze over ...
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Old 9th Apr 2019, 15:55
  #3738 (permalink)  
 
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The easiest fix was to automatically apply a little nose down trim at high angles of attack.
But a little [0.6] became a lot [2.5], semi-persistently/repetitively?
Am I reading that correctly?
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Old 9th Apr 2019, 16:01
  #3739 (permalink)  
 
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Originally Posted by DaveReidUK View Post
I fear you are fighting a losing battle in your campaign.

For the mainstream media, and even some of the industry press, knowing that aircraft can stall and that it's generally considered a BAD THING represents about the limit of their understanding.

Start talking about FARs and stick force gradient certification standards and their eyes will start to glaze over ...
Heard BBC Radio4 news the other day saying that MCAS was a system to prevent the ENGINES stalling. I wrote them a stiff email...!
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Old 9th Apr 2019, 16:06
  #3740 (permalink)  
 
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Originally Posted by MPN11 View Post
But a little [0.6] became a lot [2.5], semi-persistently/repetitively?
Am I reading that correctly?
Yes, and Boeing has admitted that it was a little too much: “Upon reflection on what has occurred, it appeared the system could present a high-workload environment—and that’s not our intention."

It appears as though they have now remedied the situation.
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