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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 10th Apr 2019, 01:25
  #3781 (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
  #3782 (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
  #3783 (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
  #3784 (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
  #3785 (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
  #3786 (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
  #3787 (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
  #3788 (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
  #3789 (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
  #3790 (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
  #3791 (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
  #3792 (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
  #3793 (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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Old 10th Apr 2019, 09:10
  #3794 (permalink)  
 
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Originally Posted by deltafox44 View Post
I know that, thank you

At low speed and with a limited amount of AND untrim, increasing speed may reduce or cancel aft column required, due to static stability of the aircraft. But in the case of ET302, even at 500 kt IAS and still both pilots pulling as much as they can, the aircraft has a -10° AoA and -2g ! No hope to reach any in-trim airspeed..

What I mean is that manual trimming was impossible due to loads on elevator (which vary as the square of airspeed) ; that the only thing that could have save the day is to slow down at a speed where loads would have permitted manual trimming.

Instead, they let full power up to VMO and above... I could not imagine why, now I know that it was what was recommended by Boeing...
my apologies. Misunderstanding. I was just referring to 737 needing AND trim all the time during acceleration.
all you say is correct. The NNP for runaway stab. Is predicated on staying ahead of trim requirements. To avoid the last ditch heave up , and unload- TRIM TRIM TRIM NU routine. Use trim switches NU until stab back in trim - not blip blip , but major NU input over several seconds. STAB OFF. TRIM Manually thereafter. In runaway stab in previous variants by the time you notice, the stab. will be a couple of divisions AND. Or more and requires a sustained ANU input.
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Old 10th Apr 2019, 09:39
  #3795 (permalink)  
 
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Originally Posted by fdr View Post


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
Well spotted. You trim nose down during acceleration because centre of lift moves forward. From memory.
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Old 10th Apr 2019, 09:53
  #3796 (permalink)  
 
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A0283

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 …
Way back on 27 Mar, the Seattle Times mentioned that Sinnett was trying to win back confidence. As VP of development, this is a double edged sword: https://www.seattletimes.com/busines...iginal-design/

Boeing details its fix for the 737 MAX, but defends the original design
Edit: As a computer programmer (not a pilot), those 3 key questions were ones I asked repeatedly after the "fix" was announced. It has taken a long time for details to emerge, so AvWeek needs to be thanked for that.

Last edited by GordonR_Cape; 10th Apr 2019 at 10:37.
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Old 10th Apr 2019, 11:18
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“In a situation where there is erroneous AOA information, it will not lead to activation of MCAS,” says Sinnett. He underlines that the entire speed-trim system, including the MCAS, will be inhibited for the remainder of the flight if data from the two vanes varies by more than 5.5 deg. If an AOA disagreement of more than 10 deg. occurs between the sensors for more than 10 sec., it will be flagged to the crew on the primary flight display.
Is there a reason for the different thresholds for AoA disagree and STS/MCAS inhibit?

So if an AoA vane sticks so that it's (say) 8 degrees offset from the other for a period, STS and MCAS will be disabled for the remainder of the flight, with a change to handling characteristics but no crew warning - or is there an STS/MCAS warning indication as well as AoA disagree?
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Old 10th Apr 2019, 11:40
  #3798 (permalink)  
 
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You trim nose down during acceleration because centre of lift moves forward
I think the larger effect is not so much any centre of pressure movement (which does contribute) but the fact that your overall lift is a V-squared function. When you accelerate you have to reduce AoA to maintain the same lift (for level flight), so lower nose attitude and more AND trim.
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Old 10th Apr 2019, 11:55
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Originally Posted by bsieker View Post
That is possibly the worst suggestion so far.

Assembly code is almost impossible to analyse for correctness in any meaningful way. It is far better (and provably so) to write in a well-specified (i. e. not C) language, prove the source code correct (for which scalable and practical techniques exist today), or define and prove correct a finite state machine and have code generated from it. That still leaves one with a need to have reasonable confidence in the compiler, but in many cases the service history for the most-used language core, and, in some recent cases, formally verified compilers, take care of that.

Just because you have one hero programmer who claims to have done it "Right" in assembly does not help you in any way because you need to demonstrate that it does what it is supposed to do (reliability), and never does what it is not supposed to do (safety), and ideally also never fails (availabilitiy). And this cannot be demonstrated by testing alone to the extremely high requirements needed in aviation. Assembly and machine code are avoided like the plague in safety-critical programming, and rightly so. Where some parts require it, extreme care must be taken to get it right, and the amount must be kept to a minimum.

Besides, as threemiles has pointed out, the implementation is not the problem (as far as we can tell, it may be flawless), but the specification. "Working as specified" can also mean that it did the wrong thing.

Bernd
Almost all the major 'programming errors' I have seen have been perfectly implemented errors in understanding due to poor systems analysis/design. Coding these days rarely has errors as there are many tools that can be used to verify the code it will also validate correctly as the code does just want the designer erroneously asked it to do. This is the shortcoming in mathematical approaches and formal proofs they do not find these errors in understanding but they do limit the avionics; it is one of the reasons FMCs are beasts of little brain as anything with any power is beyond formal proof.

.
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Old 10th Apr 2019, 12:12
  #3800 (permalink)  
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Originally Posted by Fortissimo View Post
I think the larger effect is not so much any centre of pressure movement (which does contribute) but the fact that your overall lift is a V-squared function. When you accelerate you have to reduce AoA to maintain the same lift (for level flight), so lower nose attitude and more AND trim.
section pitching moment, CsubM tends to increase on symmetric and cambered sections with increasing AoA. A section with a reflex camber may maintain low moments, which is why they are used occasionally on rotor sections, where they reduce pitch link loads. As Mach increases, pitching moment tends to increase. At very high Mach, (above MMO, approaching MD,), CsubL for a given AoA reduces, when a normal shock develops on the underside of the section. As washout results in a higher AoA inboard, and section T/C is greater normally inboard, the shock development is most pronounced inboard, and to maintain a constant total lift, the tips become more loaded, resulting in a shift rear wards of the total lift relative to longitudinal station, with increased lift distribution outboard. Separate to all that, the aircraft is required to have a nose down trim needed for increasing speed above trim speed (AoA) and vice versa, the result of inflow to the tail, tail volume and section moment design. A change in speed alters the normal force from the section as a function of the differences of the squares of the speeds, therefore level flight requires a lower AoA which is achieved by a lowering of attitude. To achieve that outcome the slope of CsubL to AoA, the a-slope need to be equal or the tail has to have a lower slope than the wing, when the inflow. Volume and section coefficients are considered. You can fly a plane with neutral static stability, they are fun to fly, they just are higher workload on instrument flying for extended periods, ask any Helo pilot ( helos are mildly statically stable, but are dynamically unstable in pitch in forward flight and hover and also in yaw in hover)
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