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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 09:07
  #3701 (permalink)  
 
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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)
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Old 9th Apr 2019, 09:13
  #3702 (permalink)  
 
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so why not have a separate cutout switch just for MCAS, keep the other two to cutout electric trim. But the new MCAS cutout switch would leave electric trim available.

I'm also unclear since the certification requirement was for force feedback on the stick during certain stall regimes why Boeing didn't just use some kind of motor/pneumatic on the stick to give this force, rather than connecting MCAS to control surfaces! Seems overkill, no pun intended. I mean the stick shaker no doubt uses a motor to shake the stick, it doesn't shake the ailerons in order to make the stick shake!

G
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Old 9th Apr 2019, 09:18
  #3703 (permalink)  
 
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Originally Posted by PerPurumTonantes View Post
I can't see any reason for the 'blips' in nose up trim other than pilot trying it and it not working/ not appearing to work. Why else would you release the switch when it's the one thing you're focussing on, the one thing you know you need?
I see on good reason : at very high speeds a long trim command would induce huge vertical acceleration, therefore you have to use it with caution, using actions of one second or so.

What I can't see is why they did not continue as many times as needed to trim back
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Old 9th Apr 2019, 09:24
  #3704 (permalink)  
 
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Originally Posted by groundbum View Post
so why not have a separate cutout switch just for MCAS, keep the other two to cutout electric trim. But the new MCAS cutout switch would leave electric trim available.

I'm also unclear since the certification requirement was for force feedback on the stick during certain stall regimes why Boeing didn't just use some kind of motor/pneumatic on the stick to give this force, rather than connecting MCAS to control surfaces! Seems overkill, no pun intended. I mean the stick shaker no doubt uses a motor to shake the stick, it doesn't shake the ailerons in order to make the stick shake!

G
There would be no force on the stick without the feel and centering system. It has been said that, for some reason, this system is not able to provide the extra force needed near stall. Of course it could have been modified to do so, but that mean a lot of money, whereas MCAS is just a software modification
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Old 9th Apr 2019, 09:29
  #3705 (permalink)  
 
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Originally Posted by DaveReidUK View Post
I don't think there is any dispute about the fact that grasping the trim wheel stops the stab moving. Reportedly, and depending on how positively you grab it, you may lose a bit of skin in the process.

Once the wheel is held, you are not fighting against the electric trim motor, which is disabled if resistance is detected at the cable drum.
Wasn't it the (unofficial) procedure to grab the copliot's leg and jam it as hard as you could against the wheel, then quickly unfolding your own
handle, shedding no skin at all !!
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Old 9th Apr 2019, 09:34
  #3706 (permalink)  
 
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Originally Posted by Deepinsider View Post
Wasn't it the (unofficial) procedure to grab the copliot's leg and jam it as hard as you could against the wheel, then quickly unfolding your own
handle, shedding no skin at all !!
Would this not require 3 hands?
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Old 9th Apr 2019, 09:41
  #3707 (permalink)  
 
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Originally Posted by Maninthebar View Post
Would this not require 3 hands?
Yes....that's how amazing captains are !

Sorry folks, just trying to lighten it up a bit,
but don't intend to imply any disrespect to
the victims of these crashes.

(It just happens that in my now ancient 737 times,
this was the joke about the runaway stab drill)

Last edited by Deepinsider; 9th Apr 2019 at 10:01.
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Old 9th Apr 2019, 09:59
  #3708 (permalink)  
 
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Originally Posted by ams6110 View Post
And yet Tesla, an automotive company which presumably follows this process, still has an "auto pilot" software function that on more than one occasion drove a car into a stationary object at 70mph.
Tesla used the same chip as Audi, BMW etc. from a company called Mobileye.
But Tesla called it AutoPILOT instead of Lane/TrafficASSISTANT and used the System far beyond it's clearly stated limitations (no cross-traffic detection, 0,4s driver reaction). All other OEMs limited the system to 30 sec. without driver interaction.
After the accident, the company stopped delivering their system to Tesla because they then knew that it was used beyond it's limits (like MCAS?)
Mobileye claimed Tesla was 'pushing the envelope in terms of safety'. I'm not allowed to post the link (Reuters).
In addition, Tesla was forced to prevent the forseeable misuse by stopping the car after a certain time without driver interaction (touching the steering wheel...)
All other OEMs ripped Teslas apart to learn all the modern new solutions and mostly considered the solutions not state of the art in terms of safety.

It's the same story, management wanted something impossible. In one case a new aircraft without additional training, in the other case being in the news as first autonomous car manufacturer with affordable electric cars...
In one case it may have been forseeable that pilots go into the stall memory item (more speed), in the other one that people may take a nap while driving.
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Old 9th Apr 2019, 10:02
  #3709 (permalink)  
 
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Originally Posted by YRP View Post
Jus' sec here. AF447 had plenty of stall warning. It only stopped after the crew held it into the deep stall. The crew didn't register the warning or believed it spurious.

On the 60 kt criteria, I find it hard to criticize that design decision. It would get increasingly difficult to get reliable meaningful data from the sensors at that point (i.e. IAS vs CAS spread). And who would have thought an airliner would actually manage to be flown to that point.

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!
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Old 9th Apr 2019, 10:03
  #3710 (permalink)  
 
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Originally Posted by PerPurumTonantes View Post
A motor which is forcibly stopped can pull a huge amount of current. Depends on motor type but for old fashioned non-electronic controls can be 20x full speed current. Of course we don't know the motor type or controlling electronics. Anyone?
Yes, so it will heat up, and eventually the wiring will burn up / melt /otherwise fail. But that will take a while. Being essential, that motor will be dimensioned rather larger to avoid such problems. And we know that on the Lion Air flight, the trim motor worked much more than in Ethiopia, and there was no question about it overheating or being disabled.

Also DaveReid's post above about motors stopping if they meet resistance.
No, that's not what he said. He said that the motor will be "disabled if resistance is detected at the cable drum.", which is different from just meeting any resistance to its motion, e. g. resistance from the jackscrew nut.

I can't see any reason for the 'blips' in nose up trim other than pilot trying it and it not working/ not appearing to work. Why else would you release the switch when it's the one thing you're focussing on, the one thing you know you need?
We don't know what they were focusing on at that time. Possibly not trim, but just pulling together. Just before the blips the "Captain asked the First Officer to pitch up together and said that pitch is not enough." And pitch trim did work, it moved from 2.1 to 2.3 units. I cannot comment on how much that would be felt in the control column.

Bernd
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Old 9th Apr 2019, 10:18
  #3711 (permalink)  
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Quote:
Originally Posted by yanrair
The Tristar taught flying GPS G SPEED approaches in serious headwind landings - none of this is new, but it is being forgotten. So here we have big jets using this technique when it is the very opposite of still air!
Surely the Tristar predated GPS by some years.
I looked that up just to see how many were still flying. One thing I found is that you can apply a GPS package to almost any Flight Sim aircraft.
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Old 9th Apr 2019, 10:29
  #3712 (permalink)  
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LR, Yanrair, we used IRS GS for monitoring headwind correction. Did that on the DC8 too using the INS, from the early 70's on, as a formal, (book) procedure. The Lockheed, (100 & 500), didn't have GPS when I flew it in the late '80's..
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Old 9th Apr 2019, 10:41
  #3713 (permalink)  
 
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Originally Posted by DaveReidUK View Post
I don't think there is any dispute about the fact that grasping the trim wheel stops the stab moving.
Once the wheel is held, you are not fighting against the electric trim motor, which is disabled if resistance is detected at the cable drum.
But this proves that electronic trim is less powerful than manual trim, right? Or can the motor distinguish where the force is coming from?

To me as an engineer everything lines up if one adds this clutch/force limit:
Pilots try manual trim: Too much force, no chance (and free hands)
Pilots reactivate electric trim: Short movement and the clutch/force limit stops it. (FDR Trace...)
Pilots try a few times: Small blips... (FDR Trace...)
Pilots give up: MCAS kicks in again...

If you find people at Boeing with lot's of bandaid, you know they had the same idea recently...
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Old 9th Apr 2019, 11:05
  #3714 (permalink)  
 
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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. With the 737MAX, the flight crew lose both electric trim and autopilot trim with the cutout switch.

The Flight Standardization Board (FSB) took notice of the nomenclature change of the cutout switches but did not make any mention of the difference in responding to autopilot stab trim runaway. On the 737NG, the flight crew retain electric trim; on the 737MAX the flight crew must use manual trim. It turns out, this is a significant difference in pilot workload and pilot capacity to fly the airplane.
Was this taught in the difference powerpoint "training"?
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Old 9th Apr 2019, 11:16
  #3715 (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 13:34. Reason: clean up duplications
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Old 9th Apr 2019, 12:47
  #3716 (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, 12:56
  #3717 (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, 12:58
  #3718 (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, 12:58
  #3719 (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, 13:24
  #3720 (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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