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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 22:36
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Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Aviation Week: Boeing Expands MCAS Demos To Speed Lifting Of 737 MAX Grounding

Guy Norris Apr 9, 2019

Pilot feedback to the proposed software changes to the Boeing 737 MAX Maneuvering Characteristics Augmentation System (MCAS) flight-control law is positive, says Boeing. After demonstrations, pilots believe the potential for further flight-control problems from the system are a “nonissue,” the airframer says.

However, despite the positive response from pilots to the upgraded control system and associated training package, Boeing is gearing up for a prolonged international effort to reinstate the grounded MAX fleet. The embattled company, which first unveiled the proposed MCAS changes to a group of certification authorities and airline pilots in Seattle on March 27, is embarking on a global campaign to convince regulators that the updates will be sufficient to enable the aircraft to return to service.

The campaign encompasses a series of simulator demonstrations and briefings at multiple training sites throughout Europe, Asia and Australia and comes as Boeing attempts to handle a situation unprecedented in its history. Because the MAX was grounded first by China and other authorities around the world days before the FAA followed suit, the company says it is imperative to build support for an international caucus of regulators willing to reauthorize the MAX to return to flight.

The FAA, which in past years would have taken a lead role in such an effort, is similarly shifting gears and is now working alongside a broader group of international regulators to adjudicate the case. The agency says it expects to receive Boeing’s final package of its software enhancement over the coming weeks. Meanwhile the FAA has set up a Joint Authorities Technical Review (JATR) to conduct a comprehensive review of the certification of the aircraft’s automated flight-control system. Chaired by former NTSB Chairman Christopher Hart, the JATR is comprised of a team from the FAA, NASA and international aviation authorities.

Boeing, which on April 5 signaled a 19% production slowdown of its 737 line to ease the growing logjam of undelivered aircraft, is also providing more details about the changes to the MCAS functionality contained in the new P12.1 flight-control computer (FCC) software load that will replace the existing P11.1 software. Based on pilot reaction to date, the company says it is confident its software upgrade is certifiable.

The briefings continue to emphasize that the MCAS, which was added to the speed-trim system to standardize handling qualities with those of the 737 Next Generation, is “not a stall-protection function and not a stall-prevention function,” says Mike Sinnett, Boeing Commercial Airplanes vice president of product development and future airplane development. “It is a handling-qualities function. There’s a misconception it is something other than that.“

Added to ensure a linear relationship between stick force per G, “speed trim is a function of airspeed, so if you’re going fast, it’s a low angle-of-attack (AOA), and if you’re going slow, it’s at higher AOA,” he notes. “The thing you are trying to avoid is a situation where you are pulling back and all of a sudden it gets easier, and you wind up overshooting and making the nose higher than you want it to be.”

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.”

In the initial briefing sessions for pilots on March 27, “we didn’t talk specifically about either of the accidents, but we ran through MCAS scenarios,” says Sinnett. “From the accidents, we now know how MCAS can behave when there is an erroneously high AOA input, so we walked through scenarios where that could occur. We demonstrated those in the simulator.”

In these sessions, pilots and regulators were able to interact via intercom and a big screen with Boeing pilots in the 737 MAX engineering cab. Following the sessions, “we went back to the classroom and said, ‘Here are the things that concern us most when we look at the scenario of the two accidents we just experienced,’” Sinnett says. “Upon reflection on what has occurred, it appeared the system could present a high-workload environment—and that’s not our intention. So we looked at changing the design to compare values from multiple AOA indicators to essentially eliminate the unintended trigger condition that causes MCAS to activate."

Sinnett says pilots appear satisfied that the three main layers of protection now added to the MCAS will prevent any potential repeat of the circumstances involved in the Lion Air and Ethiopian Airlines accidents. “We answered a lot of questions during the discussion, and then we went back into the simulator and demonstrated a number of different scenarios to run against these changes,” Sinnett says. “And the most compelling thing is that the AOA failure case turns into a run-of-the-mill AOA failure case like you might have on any other airplane. We didn’t get any negative feedback. It was all very positive, and any of the pilots who got into the simulator and saw the before and after, it was like, ‘Yes, OK, this is now a nonissue.’”

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. AOA data continues to be fed from left and right vanes into their respective air data inertial reference units before being passed to the flight computers. However, the AOA data in both computers is now continuously compared. The change is made by software only and requires no hardware modification.

“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.

The second layer of protection is a change to the logic in the MCAS algorithm that provides “a fundamental robust check to ensure that before it ever activates a second time, pilots really want it to activate,” says Sinnett.

The change would have protected the system from continuing to activate in the case of the Lion Air accident, in which the left AOA vane was stuck in the 20-deg.-nose-high position. In that circumstance, the logic rechecked if the MCAS was required and, registering the apparent nose-high position from the errant vane, commanded more nose-down trim. “Now it sees you’re in the same spot, it says you’ve got a stuck vane and says, ‘I’m not going to activate again,’” Sinnett notes.

“That’s assuming it will activate in the first place, which it won’t because one AOA vane with a high value won’t activate,” he adds. “When you do defense in depth, you have to artificially fail one layer to make sure you adequately design and test the next layer—so that’s what we had to do.”

Explaining the background to the third layer of protection, Sinnett says: “We also made sure if the second layer of protection failed somehow in some weird way and allowed MCAS to activate multiple times, the system now ensures the sum total is command-limited.” The result is that the pilots always have maneuvering authority remaining with the control column. “Pilots will always have the ability to override—although they had that before in other ways, like with the trim switch, for example. But with the software update, the column itself will always provide at least 1.2g of maneuvering capability. So you don’t just have the ability to hold the nose level, you can still pitch up and climb.”
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Old 9th Apr 2019, 22:40
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Originally Posted by MurphyWasRight
To be clear the 737 does not use a ball recirculation screw Jack, it uses a acme nut and threaded rod hence irreversible .
I am not an aircraft mechanic (engineer across the pond) so I could be wrong but a quick search seems to confirm my understanding.
No, that's not correct.

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Old 9th Apr 2019, 22:41
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Originally Posted by Realbabilu
It was engaged in 05:39:20 with AP CMD A for 30 seconds [off about 05:39:50], or i was wrong?

Since it engaged, the pilot confidence to told the FO retracting the flaps. 05:39:45



FDR ET AVJ
Take a look at the blue line named "AP Warn CAPT".
That's the autopilot disconnect warnings they got.
You get that warning when
1. autopilot has disengaged.
2. an autopilot engage attempt is made and is unsuccessful.

The first two warnings were because of the unsuccessful attempts to engage the autopilot at around 400 ft.
The third warning was when it was disconnected after about 30 sek on autopilot.

But there is also a fourth warning at 05:43:15, right after they made those two quick trim inputs.
It certainly looks like they made a failed attempt to engage the autopilot here, doesn't it?


Was that the plan?
To turn the trim cutout switches back on so they could reengage the autopilot, hoping the automation would sort out their problems?
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Old 9th Apr 2019, 22:42
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Originally Posted by DType
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 9th Apr 2019, 23:11
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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 9th Apr 2019, 23:37
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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 9th Apr 2019, 23:37
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Originally Posted by DaveReidUK
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 9th Apr 2019, 23:38
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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 9th Apr 2019, 23:55
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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, 00:04
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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, 00:25
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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, 00:34
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Originally Posted by yanrair

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, 01:10
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Originally Posted by airman1900
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, 01:19
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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, 01:23
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Originally Posted by xyze



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, 01:37
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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, 02:07
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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, 02:20
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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, 04:01
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Originally Posted by Chris2303
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, 04:36
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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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