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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 23:42
  #3781 (permalink)  
 
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Originally Posted by DType View Post
Acme nuts and threaded rods CAN be reversible, especially in a vibrating environment, as has been realised too late too many times.
But probably irrelevant in this case.
True, but they can be designed to reliably lock if the conditions are known.
There is a discussion of this in the Lion Air thread, I remember that the consensus seemed to be that it is unlikely to happen in the 737 design, certainly not enough to cause a noticeable issue.
I recall that one point is that Boeing would have designed it to not drift with vibration to eliminate need for an independent brake.
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Old 10th Apr 2019, 00:11
  #3782 (permalink)  
 
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So the Boeing software fix.

When you get a AOA disagree MCAS will not engage, and the "safety" requirement stick force feedback will not be there.

Is the feel of the stick there to assist the pilot not to over pitch? and is the feel of the feedback not best to have when you have falling items such as the AOA's.

I think this is not a satisfactory fix, as simply not having MCAS would be safer.
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Old 10th Apr 2019, 00:37
  #3783 (permalink)  
 
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A stupid question from a SLF, how many not FBW aircrafts needed a software patch to certified? I cannot understand why a main control surface has to be actuated by HAL on a not FBW aircraft, when the certification requirements could be fulfilled with a redesigned feel system, I understand that such will imply no grandfathering, but still, if the problem is pilot feel, adjust the feel, without moving control surfaces...
I apologize if the above sounds stupid, but would like to have a proper understanding ...
thanks for your patience
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Old 10th Apr 2019, 00:37
  #3784 (permalink)  
 
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Originally Posted by DaveReidUK View Post
No, that's not correct.

Thanks.
I stand corrected,looks like it is indeed a ball screw.

I was misled by more than one diagram referring to the above assembly as a 'nut', some even said 'acme nut'.
That and reading the Alaska Air report which did have a nut rather than ball screw.

This is a good illustration of the pitfalls of using other than detailed drawings/schematics for analysis.
So far in this thread I have seen only one what looks to be a true schematic, the yoke trim switches with wire# but unclear labels, everything else has been a conceptual logic illustration or worse.

Same thing applies to reading the fdr plots without access to the numerical data and other factors such as sampling rates.
I suspect that the sloped lines on binary events on the plots might represent sampling uncertainty but have no way of knowing.
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Old 10th Apr 2019, 00:38
  #3785 (permalink)  
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airman1900 #3824

Taken from Mike Sinnett's publication:

Underscoring the difference between the speed-trim system on the 737 Next Generation (NG) and the MAX, Sinnett says: “Mechanically, on the NG there is a column cutoff switch that stops any automatic trim when the column is back to a certain spot. On the MAX, we still needed automatic trim when you got to that spot.
The bee in my bonnet about that rear column cut-off switch having been removed on the MAX, is it seems, still open to interpretation. The inference is there, but not the positive statement.
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Old 10th Apr 2019, 00:55
  #3786 (permalink)  
 
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From the horse's mouth

Underscoring the difference between the speed-trim system on the 737 Next Generation (NG) and the MAX, Sinnett says: “Mechanically, on the NG there is a column cutoff switch that stops any automatic trim when the column is back to a certain spot. On the MAX, we still needed automatic trim when you got to that spot. MCAS differs from speed trim at elevated alpha because it bypasses that switch by design. To do that, it activates based on AOA rather than based on speed—which is what speed trim does. Speed trim is a function of airspeed, and MCAS is a function of angle-of-attack and Mach number, but it only triggers off AOA.”
The software is getting modified - should be good as long as both vanes don't get taken out by a flock and the algorithm to always leave 1.2 g authority in the elevator attains 10E-9 reliability.

What I haven't seen yet is a procedure to block MCAS in certain stick shaker cases, especially on rotation or early takeoff.
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Old 10th Apr 2019, 01:04
  #3787 (permalink)  
 
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9th Apr 2019, 23:38 #3831 (permalink) Loose rivets

airman1900 #3824 Taken from Mike Sinnett's publication:

Underscoring the difference between the speed-trim system on the 737 Next Generation (NG) and the MAX, Sinnett says: “Mechanically, on the NG there is a column cutoff switch that stops any automatic trim when the column is back to a certain spot. On the MAX, we still needed automatic trim when you got to that spot."
@LooseRivets - The bee in my bonnet about that rear column cut-off switch having been removed on the MAX, is it seems, still open to interpretation. The inference is there, but not the statement.
The article also states:

On the MAX, we still needed automatic trim when you got to that spot. MCAS differs from speed trim at elevated alpha because it bypasses that switch by design.
Bypassing could also mean the switch is still there for use by other systems. So no removal?

Next to that you would like to know things like if we are talking column or columns. How many switches are there. Are they NG or MAX only. Etcetera, etc… Is there an official Boeing press release with this information? … stilll many questions…

So, Mr Sinnett’s statements add something but are not clear enough and not deep enough. Many would welcome more.

I posted earlier in the thread that my impression was that only a detailed public presentation and publication by the ‘chief engineer (with FAA delegated certification signature authority)’ of the MAX would do. I wonder if Mr Sinnett fits that bill? The text has a ‘nice and sunny taste’, rather more commercial than tech savvy …
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Old 10th Apr 2019, 01:25
  #3788 (permalink)  
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Waaaaaaaaaay back I questioned the removal of the rear column switch(es) on the MAX - several times. It was in the same page as the change of cut-out switch wiring.

The statement was unequivocal but having read in for months without making many notes, I fear it was lost in the haze.

I try to filter out my source material and missives from Seattle usually catch my eye.
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Old 10th Apr 2019, 01:34
  #3789 (permalink)  
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Originally Posted by yanrair View Post

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.
back on planet earth, where Newtonian physics still reigns, you may find that for an aircraft to be statically stable, mass is always ahead of the center of pressure. Now your B2 may be unstable but pretty much no airliner is unstable, including the Max8. It has a reduced stick force gradient in part of the envelope, which MCAS addresses, just as STS deals with another gradient issue in a different part of the envelope.

Nect time you you are in a pub with a dartboard, throw a dart feathers first and observe status stability play out.

The acceleration issue as quoted before this comment, where in the excessive out of trim AND case is so that the aircraft approaches the speed that it has been miss-trimmed to, and near that point, the stab forces between section Cm and the elevator countering load will approach a minimum, allowing for movement of the stab with less force.

Now the the non linear longitudinal stability arises from the component of lift arising from the engine nacelle, which happens to be forward of the center of mass, so as additional lift is generated, that results in a pitching moment being added to the total body pitching moment. The fuselage is a lifting component, as are the wings. The stabiliser normally generates a downforce to counter the pitching moment of the total aircraft.

end
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Old 10th Apr 2019, 02:10
  #3790 (permalink)  
 
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Originally Posted by airman1900 View Post
Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding
No offense to those here but I would feel a hell of a lot better if Boeing were demoing the system to engineers rather than (or in addition to) pilots. Perhaps Airbus engineers? The best testers are your competitors.

I have no experience with aircraft but their solution does not give me the warm fuzzies. One thing that I find extremely weird is that it silently disables MCAS if the AOA sensors differ by five degrees. So do you need MCAS or not?

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Old 10th Apr 2019, 02:19
  #3791 (permalink)  
 
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So do you need MCAS or not?
Only if you change the aerodynamics of the aircraft by moving the engines forward and up and making them bigger in order to burn less fuel in order to compete with your competitor and at the same time avoid paying for certifying a new aircraft. ie If you want to have your cake and eat it too.
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Old 10th Apr 2019, 02:23
  #3792 (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!
I guess you then run into the other failure situation, false stall warnings when a sensor is broken, possibly overloading the crew similar to the stick shaker here.

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Old 10th Apr 2019, 02:37
  #3793 (permalink)  
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DaveReidUK #3825 Showed a clear picture of the screwjack motor but presumably not the gearboxes and clutches.

Apropos it having a 'continuous rating' or not, judging by the size of the code plate, it's not an extraordinarily large unit though the 3 phase supply is an indication of its power.

It was stated somewhere that there's one motor, and the clutches and gears are selected for the differing modes. More dredging memories. One statement said the motor can keep running while functions change - implying there might be a time where neither clutch was engaged. I would assume a time-out, on the running, but have failed to find positive details of the processes. I read the very detailed modes with relay control etc., but not how that motor operates inside. i.e., there must be a huge reduction even prior to the other gears/clutches. I base this on the shaft size.

The point has been raised, but the hammering that system got, i.e. heaving against stalled (as in mechanically stalled) stabilizer, might just have made those inert last moments come about by failure rather than just the excessive (aerodynamic) loads.
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Old 10th Apr 2019, 03:07
  #3794 (permalink)  
 
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SLF here

Could the pitch up problem caused by the new engine position be ameliorated with the use of strakes such as those on the CF6 attached to the DC10?
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Old 10th Apr 2019, 03:20
  #3795 (permalink)  
 
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Loose rivets
It is entirely possible to change speed with a properly wound 3 phase motor by electrical means only, without resorting to clutches and alternate gear paths.
When motors are powered by 400 cycle power, you can make them much smaller for the same horsepower output.
No doubt, there is a gear reduction between the motor and stabilizer trim to match torques properly, but that is all that is necessary to do the job.
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Old 10th Apr 2019, 05:01
  #3796 (permalink)  
 
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Originally Posted by Chris2303 View Post
SLF here

Could the pitch up problem caused by the new engine position be ameliorated with the use of strakes such as those on the CF6 attached to the DC10?
Probably, but then you have the issue of understanding what that will do to the rest of the performance of the aircraft - will almost certainly increase fuel consumption, and would require major testing of what the altered airflow may impact in other stages of flight
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Old 10th Apr 2019, 05:36
  #3797 (permalink)  
 
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Taken from Mike Sinnett's publication:
The first main layer of protection provided by the update is a cross-channel bus between the aircraft’s two FCCs, which now allows data from the two AOA sensors, or alpha vanes, to be shared and compared.
Is this a hardware change, it certainly reads like it?
If it is,
  • what are the certification implications? and
  • what are the retrofit requirements - minutes / hours / days per aircraft?
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Old 10th Apr 2019, 06:40
  #3798 (permalink)  
 
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The most important tech question on the Lion are and Ethiopian crash investigations!

Ref.: 737-7/8/9 Training Manual 22-11-00, Dated 19.Sep.2016 Pages 165, 166 and 167

Ref.: THE MOST IMPORTANT QUESTION ON THE LION AIR AND ETHIOPIAN 38M AIRCRAFT CRASH INVESTIGATION maybe!

A simple version here.

Will the PRI Toggle when moved to CUT OUT, IS IT 100% GUARANTEE to remove -all- electrical signals to the Stab Trim Motor?

Why do I ask? "The FCC supplies MCAS signal to enter high speed mode on the stab trim motor and bypass the aft column cutout switches for trim down commands.". As we know also in error.

THUS if the B/U is in CUT OUT, then MCAS still has authority but what about PRI in CUT OUT?"

The trim commands from the FCC is processed in the "autopilot section of the motor". That means besides the FCC A in these cases, there is another controlling Software either part of the MCAS programing (thus active when a Fault like is being considered part a chain of errors in these two accidents) or a separate sub routine / program influencing the Stabilizer Motor into moving the Stabilizer that has never been mentioned before.

I'm thinking like a chicken with its head cut off, the nerves can still allow it to run around.

So the PRI in CUT OUT, does it still allow impulses from the "auto pilot section of the motor" however created, to move the stabilizer via 28VDC thru the motor un-commanded when the operating crew thinks they've isolated that electric trimming (CUT OUT), thus do not expect further electric trimming (non-pilot induced)?

To keep it simple - PRI and B/U toggles in CUT OUT there is no possible way the stabilizer movement (NU or ND) can be activated unless done manually by the crew using the Trim WHEELS OR a failure of the stabilizer mechanics i.e. excessive speeds beyond VMO, thus possibly breaking or stressing / stretching the components moving the stabilizer itself i.e. for example into the full AND position which is then fatal as non-recoverable?

Any Source I can contact, kindly PM? I do not have access to Boeing Customer Service any more.

Thanks in advance for all PPRUNER's efforts here.

CP Bernd von Hoesslin

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Old 10th Apr 2019, 06:41
  #3799 (permalink)  
 
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The MCAS was needed for certification, because the large engine nacelles produce lift, because they are large and in a forward position to CG. That’s for any nose up situation.
I ask myself if the opposite is true in a severe nose down situation with negative AoA, then a larger nose up force by the stabiliser/elevator will be needed to counteract those additional nose down forces than for the non MAX versions.

I would assume that that was a part of the MAX certification test flights.
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Old 10th Apr 2019, 07:41
  #3800 (permalink)  
 
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Originally Posted by kiwi grey View Post
Taken from Mike Sinnett's publication:


Is this a hardware change, it certainly reads like it?
If it is,
  • what are the certification implications? and
  • what are the retrofit requirements - minutes / hours / days per aircraft?
"The change is made by software only and requires no hardware modification." Is what the article says.
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