tealnz
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MAX certification: new systemic issues identified

Sun Mar 17, 2019 11:28 pm

In a deeply reported story https://tinyurl.com/y49rxdo4 in today’s Seattle Times Dominic Gates presents important new information on the MAX certification process and the roles of FAA and Boeing. A focus of the article is Boeing’s System Safety Analysis for the MCAS and changes made apparently after flight testing. Gates’s take on the issues around the safety analysis has been widely quoted. He says the report:

    - Understated the power of the new flight control system, which was designed to swivel the horizontal tail to push the nose of the plane down to avert a stall. When the planes later entered service, MCAS was capable of moving the tail more than four times farther than was stated in the initial safety analysis document.
    - Failed to account for how the system could reset itself each time a pilot responded, thereby missing the potential impact of the system repeatedly pushing the airplane’s nose downward.
    - Assessed a failure of the system as one level below “catastrophic.” But even that “hazardous” danger level should have precluded activation of the system based on input from a single sensor — and yet that’s how it was designed.

Gates has chosen not to stray far beyond the information he was able to source from contacts in the FAA. But for those wondering how ready airlines and foreign regulators will be to accept a new Boeing-designed FAA-certified fix, and the extent of any legal exposure, there are some big takeaways:

- Plainly there are several engineers/technical experts currently or previously in the FAA who were directly involved in the MAX certification and strongly disagreed with aspects of the MAX certification process and some of the technical judgments that were made. If Gates’s report is any guide there will be FAA technical experts ready to testify to investigators that the MAX process, at least as it relates to MCAS, departed from good practice in important ways.

- There is a particular concern that FAA managers, under pressure from Boeing to accelerate the certification process for commercial reasons, cut corners on the technical assessment work, in some cases delegating assessment to Boeing engineers to an extent the FAA technical experts felt was inappropriate.

- Late changes were made to the MCAS software in response to findings during flight testing. The original Boeing safety analysis said that MCAS would be able to trim the stabiliser up a maximum of 0.6º (out of a physical maximum of 5º up). It came as news to Gates’s sources that the limit had been increased to 2.5º in the final MCAS configuration. The higher limit meant that “each time MCAS was triggered, it caused a much greater movement of the tail than was specified in that original safety analysis document”. It is not clear whether the higher limit was recorded in the final version of the safety analysis document.

- Both the FAA engineers and foreign regulators who received the safety analysis believed the aircraft believed the aircraft was designed to the 0.6º limit. The change appears not to have been communicated to all those who could reasonably have expected to be advised.

- The original safety analysis, with the 0.6º trim limit, treated an MCAS failure as a “major failure” - rather than a more critical “hazardous failure” or “catastrophic failure”. This lower categorisation of the risks associated with MCAS failure appears to have allowed MCAS to be configured with a reliance on a single sensor rather than requiring sensor redundancy.

There is much more in the article. Gates says Boeing and FAA were given an opportunity to comment on key points 11 days ago – before the Ethiopian crash but chose not to respond substantively.

One obvious question will be the extent to which the shortcuts and other flaws in the certification process in response to commercial pressures might have increased Boeing and FAA’s legal exposure. (From a legal point of view, can anyone help us understand at what point does the natural interest of an OEM in seeking an expeditious certification process cross a line a line and incur legal hazard?)

Looks as if we can also now infer that non-US authorities who moved to ground the MAX before the FAA might have had good reason to question the integrity of the information they had originally been given by Boeing and FAA on the basis for the MAX’s certification. If they were still working off the original Boeing safety analysis the new information Boeing provided after the Lion Air crash would have been disconcerting, to say the least. It is not hard in these circumstances to see why they might have been ready to ground the aircraft without waiting for the FAA. And it is easy to imagine that they might be reluctant to accept Boeing/FAA certification of a new fix without doing their own assessment.
 
747Whale
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 1:47 am

[quote="tealnz"
Looks as if we can also now infer that non-US authorities who moved to ground the MAX before the FAA might have had good reason to question the integrity of the information they had originally been given by Boeing and FAA on the basis for the MAX’s certification. [/quote]

It really doesn't.

I can understand that one might form such a conclusion if reading that information without any idea what it actually means, or if one has no knowledge of the aircraft, but otherwise, no.

"4X" the original stab movement target is only 2.4 degrees, the current limit.

The stab can be stopped through the standard procedure, and has cutoff switches. The aircraft can be manually flown with or without the stab trim change or input.

Groundings have been politically based on third world mishandling, not on a minor trim change mechanism.
 
tealnz
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 2:19 am

One of Gates’s sources says foreign regulators, like the FAA, believed that the MCAS could rotate the stabiliser by a maximum of 0.6º. It’s understandable they would do a double-take if they found out only after the Lion Air crash the the MCAS as implemented could move the stabiliser by 2.5º. It is very different in terms of aerodynamic effect. And if that was the first they heard of it, as Gates’s source implied, then there is a basic process fail.

The pilot failure theory still has to account for the fact that the ET pilots had been trained on MCAS and cutoff procedure. Clearly it’s not as simple as Boeing and some here would have us believe. Trouble is that the onus is on the manufacturer to anticipate and design for complex situations and failures. Seems that at 400knots and 3000 ft above ground level with the stabiliser jack screw up against the stops the “standard procedure” wasn’t adequate to the situation.
 
747Whale
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 3:37 am

tealnz wrote:
It is very different in terms of aerodynamic effect.


Not nearly so much as you seem to think. It's a very small stab change, actually.

tealnz wrote:
The pilot failure theory still has to account for the fact that the ET pilots had been trained on MCAS and cutoff procedure.


No, they hadn't.

They should have known the trim cutoff procedure, however, which works for disabling MCAS.

tealnz wrote:
Seems that at 400knots and 3000 ft above ground level with the stabiliser jack screw up against the stops the “standard procedure” wasn’t adequate to the situation.


No. Slowing the aircraft, however, does address the problem. Any time there's a trim issue, increasing speed will make it worse, in any aircraft. This comes back to a pilot issue.
 
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Francoflier
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 5:17 am

747Whale wrote:
tealnz wrote:
Seems that at 400knots and 3000 ft above ground level with the stabiliser jack screw up against the stops the “standard procedure” wasn’t adequate to the situation.


No. Slowing the aircraft, however, does address the problem. Any time there's a trim issue, increasing speed will make it worse, in any aircraft. This comes back to a pilot issue.


Fat load of good when you have no idea what speed you're doing. Both crews were likely dealing with unreliable airspeeds, immediate stick shaker activation and generally speaking a very confusing flight deck environment due to a complex instrument failure as soon as the airplane got airborne.
Hitting the stab cutout switches is easy enough when you know exactly what you're dealing with and have plenty of air between you and the ground...
Honestly, I'm not sure anybody could confidently say they could have saved the aircraft even with all the hindsight we have now.

Blaming the crew is easy from the comfort of one's armchair.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
747Whale
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 6:37 am

Why do you believe the crews had no knowledge of their speed? There's more than indicated airspeed available in the cockpit for that information, and in a complete loss of air data situation, the FMC/GPS/IRU derived data will give a groundspeed readout, which can also be used, and every cockpit crew is required to be aware of pitch/power combinations to fly in the event of loss of airspeed data.

Therefore, that argument doesn't hold water, except to point to the crew. The data IS available.
 
HaulSudson
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 7:07 am

Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.
 
HaulSudson
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 7:10 am

747Whale wrote:
Why do you believe the crews had no knowledge of their speed? There's more than indicated airspeed available in the cockpit for that information, and in a complete loss of air data situation, the FMC/GPS/IRU derived data will give a groundspeed readout, which can also be used, and every cockpit crew is required to be aware of pitch/power combinations to fly in the event of loss of airspeed data.

Therefore, that argument doesn't hold water, except to point to the crew. The data IS available.


Try harder, the world is about to believe your alternative facts.
 
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Francoflier
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 7:19 am

Again, easy to say if you're treating this as a 'simple' unreliable airspeed scenario.

Try this when it happens straight off the ground, with a stick shaker going erroneously (pilots are trained to trust the stall warning over their speed indication, as it is based on AoA signal which is considered more reliable than airspeed), bells and alarms ringing and a trim system trying to fly you into the ground while you're barely off it in the first place.

I have practiced plenty of unreliable airspeed exercises. They've become the flavor of the moment after AF447. They're never straightforward events, and require large amounts of crew concentration, CRM and adequate time to go through the checklist, which, by the way, first requires the crew to establish a stable flight path. They're also all practiced at altitude as the expectation is that the system would degrade in flight, not at takeoff.

I reiterate that this is very much an event that the crew did not train for and which required immediate action with little spare brain power to analyse the complex failure scenario they were dealing with.

Could they have kept the airplane airborne and brought it back? Maybe. Were they a sub-par crew? I see no indication of this.
Accidents are never caused by one factor. The crew is often one of them, to a varying degree, and I expect the final report to be no different in both these instances, but I very much doubt they will be outlined as the only or even main cause for the crash in light of the fact they were flying a critically faulty airplane.

At this stage, trying to blame the crew is a blatant and desperate effort to shift the attention away from the aircraft and to downplay the severity of the failure.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
WPvsMW
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 7:29 am

Riddle me this: MCAS activates, keeps 2.5 deg. nose-down for 10 seconds. Altitude lost. PF counters with elevator... but stab trim is still set at 2.5 deg. nose-down. MCAS activates again, adds another 2.5 deg. nose-down for 10 seconds. More altitude lost. PF counters with elevator, but stab trim is now 5 deg. nose down. repeat. Times up. Jack screw at max nose-down. Bang.

Corollary. The a/c is descending at max jackscrew nose-down, regardless of elevator command.

If that scenario is correct, IMO there is clear product liability, 300 souls worth.
 
WIederling
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 9:02 am

HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.


On another (German) site there is an issue with "NATO trolls".
You can see the call back times ( until those get active ) correlating with US office hours.
Topics posted late on Friday only ever get reacted to late on Monday ( Berlin time zone.)

Software to manage multiple identities ( what benign task could demand that feature )
came up first on the US side.
Murphy is an optimist
 
WPvsMW
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 9:51 am

whale is not a shill. whale is whale. He has opinions, like everyone on a.nut, but he also has experience that few others have, from cropdusting up, and as an instructor. He's partial to Boeings, and well trained pilots. You may not like some of his opinions, but you have more in common with him that you realize.

In a nutshell, I take whale's point to be that if the pilots had disabled MCAS, the ET jackscrew would not have been at max nose-down trim.

In a nutshell, my point is that MCAS should have disabled itself based on ground proximity, or other parameter (cumulative trim nose-down), and certainly should have had a 3-sensor network.
 
WNCrew
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 11:55 am

Can someone clarify: what I understand of MCAS is that you can turn it off with two switches on the center console, however in a NY Times articles it says the system can "reset itself".... does this mean that you cannot actually deactivate it?
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
kalvado
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 12:44 pm

HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.

I don't believe he's actually affiliated.
My impression a lot of it is a result of fairly rigid training, with a lots of questions pre-answered and put in procedures, and self-assurance being a big part of emergency response. There should be no doubts that hardware can perform, and you can solve the issue; panic in a cockpit is as counterproductive as it gets.
As a result, those making mistakes are deemed inferior, and design changes are seen as a crazy idea, until they come from the manufacturer. And, frankly speaking, equipment and procedures are highly refined and most times meet those expectations. Low hanging fruit, visible to those not deeply involved, is long since picked.
That is all good, until there is a real problem. Then the logic gets defied. There were other discussions on this site where literally crash by the rules was deemed a better option than a non-trained safe way out.
Last edited by kalvado on Mon Mar 18, 2019 12:46 pm, edited 1 time in total.
 
planecane
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 12:46 pm

WNCrew wrote:
Can someone clarify: what I understand of MCAS is that you can turn it off with two switches on the center console, however in a NY Times articles it says the system can "reset itself".... does this mean that you cannot actually deactivate it?

That refers to resetting itself after the pilot stops it with a trim switch input. The cutout switches trurn off all electric and automatic trim.
 
GalaxyFlyer
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 1:54 pm

747Whale wrote:
Why do you believe the crews had no knowledge of their speed? There's more than indicated airspeed available in the cockpit for that information, and in a complete loss of air data situation, the FMC/GPS/IRU derived data will give a groundspeed readout, which can also be used, and every cockpit crew is required to be aware of pitch/power combinations to fly in the event of loss of airspeed data.

Therefore, that argument doesn't hold water, except to point to the crew. The data IS available.


Also, the ISI presentation, independent of the main pitot tubes

GF
 
GalaxyFlyer
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 1:58 pm

GalaxyFlyer wrote:
747Whale wrote:
Why do you believe the crews had no knowledge of their speed? There's more than indicated airspeed available in the cockpit for that information, and in a complete loss of air data situation, the FMC/GPS/IRU derived data will give a groundspeed readout, which can also be used, and every cockpit crew is required to be aware of pitch/power combinations to fly in the event of loss of airspeed data.

Therefore, that argument doesn't hold water, except to point to the crew. The data IS available.


Also, the ISI presentation, independent of the main pitot tubes. Every unusual attitude exercise or unreliable exercise in the sim—look at the standby presentation to verify the main presentation.

GF
 
tealnz
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 4:13 pm

I started this thread on tech ops mainly because it seemed to me that the new information in Dominic Gates’s piece raised some big systemic questions around certification. Plus it looked as if Gates or his editors had chosen not to make too many inferences from the information set out in the report. But he reports as fact that foreign regulators were taken by surprise when Boeing published details of the MCAS after the Lion Air crash - implying that Boeing/FAA had not shared details of the final implementation of MCAS with other authorities. I think it’s a reasonable inference that there will have been informal communication between technical experts in FAA and eg their European and Australian counterparts which led to a loss of confidence. I’ve also seen comment from a Chinese regulator – sorry, can’t now remember where – pointing to frustration with FAA’s approach to information sharing around the MAX and MCAS.

The practical point – and here’s where, for once, I’m not sure Leeham have it right – is that we’re looking at more than a short-term exercise in validating and certifying Boeing’s MCAS rework and getting the MAX back in the air. Families of the foreign nationals killed on ET will be hounding their respective governments to investigate eg the EASA acceptance of the FAA assessment and certification of the MAX. Could drag out well beyond May.

And notwithstanding tit for tat risk between FAA and EASA I would expect we’ll see some questioning of current assumptions about de facto acceptance of FAA assessments on Boeings and EASA for Airbuses. Not to mention some deep scrutiny in House oversight committees of the FAA’s actions through the MAX certification process.
 
tealnz
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 4:28 pm

I guess we should also assume there will be closer scrutiny of the 77X certification process (both FAA and foreign regulators) in the wake of the MAX failures. There’s a new thread on civil aviation viewtopic.php?f=3&t=1418197#unread.
 
greendot
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 5:39 pm

Talk about faulty by design. They shouldn't be band-aiding systems on top of systems to make up for an aerodynamic limitation. Boeing needs to stop trying to expand a 50 yr old regional jet.
 
stratclub
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 5:55 pm

planecane wrote:
WNCrew wrote:
Can someone clarify: what I understand of MCAS is that you can turn it off with two switches on the center console, however in a NY Times articles it says the system can "reset itself".... does this mean that you cannot actually deactivate it?

That refers to resetting itself after the pilot stops it with a trim switch input. The cutout switches trurn off all electric and automatic trim.

To expand on that, the cut off switches remove all power to the stab trim motors so in effect MCAS cannot auto trim the aircraft making the manual trim wheels the only way to adjust stab trim. I would imagine that MCAS would still command stab trim inputs, but with the motors not receiving power nothing happens. Without the Wiring diagrams to study, this is my assumption that MCAS would still be trying to auto trim the aircraft but couldn't with the stab trim switches in cutoff.

Also if the flaps are in position one or greater, MCAS is not active so if the EA problem occurred shortly after rotation the flaps were not completely retracted yet and MCAS was not active yet.

On LA, if the PF had put flaps back to position 1, MCAS would have been deactivated. Didn't the problem become worse when the crew went to flaps zero?
 
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AirlineCritic
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Re: MAX certification: new systemic issues identified

Mon Mar 18, 2019 6:55 pm

The Seattle Times piece is indeed shocking.

tealnz wrote:
Late changes were made to the MCAS software in response to findings during flight testing. ...
Both the FAA engineers and foreign regulators who received the safety analysis believed the aircraft believed the aircraft was designed to the 0.6º limit. The change appears not to have been communicated to all those who could reasonably have expected to be advised.


My interpretation of this is that the MAX is essentially not certified as of today. It will have to go through the proper certification process, not just get out of the current grounding. The MAX as it is actually defined was not reviewed or certified by any of the agencies.

(Unless, of course, the current software changes get back to the same 0.6 limit but that was deemed insufficient in flight testing...)

tealnz wrote:
And it is easy to imagine that they might be reluctant to accept Boeing/FAA certification of a new fix without doing their own assessment.


That would certainly seem the sensible cause of action.

In addition, even if Boeing and FAA missed the differences earlier (mistakes happen), they had certainly heard from Seattle Times before the ET crash. And chose to do nothing, in fact, they chose to recommend flying even after the ET crash. In my book that means the management was on purpose holding back information that could have saved the ET passengers. That's certainly at least bad for the upcoming lawsuits, if not even otherwise legally bad.
 
tealnz
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 4:13 am

So the Secretary of Transport has asked DoT Inspector General to reviewnthe certification process. A Washington grand jury has issued a subpoena for documents. Canada and the EU have announced their own reviews. Now it has been reported that the FBI is supporting the DoT enquire. Meanwhile a congressional hearing has just been scheduled.

It’s not looking like a quick and easy recovery from the MAX grounding, to say the least.
 
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Matt6461
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 3:57 pm

Is there another hazard/catastrophe around the corner?
If the MCAS patch corrects a tendency to pitch up towards stall in high-speed flight then don't we have an unstable aircraft?
Is there a danger that, absent 2.5 degree stab authority, the MAX will go into high-speed stall while a cruise captain isn't perfectly attentive?
If that's the case, then wouldn't the only possible fix involve redesigning the wing/engine interaction that created lift distribution changes underlying the problem? I.e. starting from scratch on the MAX?

Not saying that's the case, just wondering about the risk profile here.
 
WIederling
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 4:12 pm

Matt6461 wrote:
Is there another hazard/catastrophe around the corner?
If the MCAS patch corrects a tendency to pitch up towards stall in high-speed flight then don't we have an unstable aircraft?


From what has transpired: YES : the MAX has reduced nose down moment in (higher angle, but inside cert envelope) pitch up.
.. and every reasonable solution would kill the commonality to the predecessors and/or take away from the advantage the new engines can provide.
Murphy is an optimist
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 9:37 pm

WIederling wrote:
Matt6461 wrote:
Is there another hazard/catastrophe around the corner?
If the MCAS patch corrects a tendency to pitch up towards stall in high-speed flight then don't we have an unstable aircraft?


From what has transpired: YES : the MAX has reduced nose down moment in (higher angle, but inside cert envelope) pitch up.
.. and every reasonable solution would kill the commonality to the predecessors and/or take away from the advantage the new engines can provide.


Before entering this discussion, let me say my comments are based on past experience in commercial airplane flight test and handling characteristics certification. I have no knowledge of 737 MAX flight characteristics.

Swept wing airplane flaps up stall handling can be a bit squirrelly. Ideally, you want to have a linear stick force per g, commonly known as the stick force gradient. For instance, if the pilot pulls X lb, the airplane responds with 1.5 g's. For a linear stick force per g, if the input is 1.1X lb, the airplane responds with 1.65 g's (1.5*1.1).

The problem is that for swept wings, the tips tend to stall early due to span wise flow and boundary layer growth. As a result, the center of lift shifts inboard and for a 1.2X lb pull, the airplane may respond with 1.9 g's. A linear stick force would have produced 1.8 g's (1.5*1.2). This is commonly known as "stick lighting" as you get more g's than expected for given input. "Stick lighting" is an undesirable handling characteristic. The airplane may still be adequately stable and won't pitch itself into a stall but the change in handling characteristics is the issue.

Manufacturers deal with this issue in a variety of ways. Boeing typically uses wing vortex generators and the 767 has a stick nudger that gives the stick a gentle forward movement for various flight conditions. I believe that Airbus dealt with the issue electronically on the A300/A310. In extreme cases where "deep stall" was involved, the BAC1-11 had a stick pusher that forcefully moved the column forward. The UK-CAA required a stick pusher on the 727.

I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WPvsMW
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 10:20 pm

OldAeroGuy wrote:
... due to span wise flow and boundary layer growth. As a result, the center of lift shifts inboard ...


Thanks for pointing out the shifting of the center of lift, which is a topic in a parallel thread focused on whether the aerodynamic problem, for which MCAS was a partial solution, has been fully characterized. ("was a partial solution", since MCAS ver. 1 is now decertified)

viewtopic.php?f=5&t=1418079
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Thu Mar 21, 2019 10:54 pm

The difference is that the thread you're referencing describes the center of lift change as a function of the engine location.

I'm saying that it is an inherent issue with swept wings.

An engine cowl is in essence a ring wing. It has a lift curve (CL vs alpha) the same as any wing. The basic airplane lift curve and pitching moment slopes will include the effect of the nacelle. Before drawing any conclusions about the impact of the nacelle at high alpha, it would be useful to have wind tunnel data with nacelles on and off.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
stratclub
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Re: MAX certification: new systemic issues identified

Fri Mar 22, 2019 12:17 am

How would a change in angle of incidence of the engines effect the pitch up tenancy, or is lever moment because of the engine location the primary reason for pitch up when more thrust is applied?

My assumption is that if a change of angle of incidence of the engine could make a difference, it would cause some sort of penalty at climb and cruise.
 
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Faro
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Re: MAX certification: new systemic issues identified

Fri Mar 22, 2019 7:53 am

tealnz wrote:
So the Secretary of Transport has asked DoT Inspector General to reviewnthe certification process. A Washington grand jury has issued a subpoena for documents. Canada and the EU have announced their own reviews. Now it has been reported that the FBI is supporting the DoT enquire. Meanwhile a congressional hearing has just been scheduled.



Yes indeed, it looks like the grounding will last some time...not good new for B and the FAA...and the airline industry at large...Airbus is incapable of fulfilling global demand for narrow-bodies on its own...the industry is effectively a duopoly...this will mean that less efficient types will have to pick up the slack until the MAX is cleared...bad news for the industry and bad news for consumers...although safety is paramount so to a very great extent, it's also good news for consumers...

The direct costs for B are one thing, but don't discount legal action from B shareholders and consumer groups too...this may end up a very very expensive business...


Faro
The chalice not my son
 
WIederling
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Re: MAX certification: new systemic issues identified

Fri Mar 22, 2019 9:49 am

OldAeroGuy wrote:
I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.


Thanks for the explanation and background.
My understanding is that the moment curve has a region of negative slope before reaching a stall state
counter moment is strongly diminished. That reduces stick forces.
But IMU also would further push up the nose towards a stall. ( drag of the nacelles is above and ahead of the CG/regular CLift.)
going by that the MAX should "like" sitting in a stall. ?
Murphy is an optimist
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Fri Mar 22, 2019 3:34 pm

Do you have a source for saying there is a region of negative slope (ie pitch up)?

A reduction in a sable slope (ie stick lighting) might have been the sole reason for MCAS incorporation to pass stall handling requirements.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WIederling
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Re: MAX certification: new systemic issues identified

Fri Mar 22, 2019 3:49 pm

OldAeroGuy wrote:
Do you have a source for saying there is a region of negative slope (ie pitch up)?

A reduction in a sable slope (ie stick lighting) might have been the sole reason for MCAS incorporation to pass stall handling requirements.

Leehams Björn Ferm had a couple of diagrams to that effect.
Boeing would probably refrain from releasing that information without being forced.

The engines are quite a big "flap" ahead of the wing. and a "flap" that sticks up earlier than on the NG (, Classic, Jurassic )
( Just look at the top view outline of the plane.)

Q: "sable slope", "stick lighting" in this context?
Murphy is an optimist
 
dakota123
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Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 3:18 pm

WIederling wrote:
OldAeroGuy wrote:
Do you have a source for saying there is a region of negative slope (ie pitch up)?

A reduction in a sable slope (ie stick lighting) might have been the sole reason for MCAS incorporation to pass stall handling requirements.

Leehams Björn Ferm had a couple of diagrams to that effect.
Boeing would probably refrain from releasing that information without being forced.

The engines are quite a big "flap" ahead of the wing. and a "flap" that sticks up earlier than on the NG (, Classic, Jurassic )
( Just look at the top view outline of the plane.)

Q: "sable slope", "stick lighting" in this context?


Original spec was for max 0.6* would argue that at least initially it was thought to be a gradient concern. Where it ended up, with (supposedly) 2.5* max? Would that have argued for a pusher instead? At 0.27*/second, MCAS sure doesn’t move very fast, seems like you could be in real trouble before anything approaching full input could be achieved. That never made sense to me.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 4:05 pm

WIederling wrote:
OldAeroGuy wrote:
Do you have a source for saying there is a region of negative slope (ie pitch up)?

A reduction in a sable slope (ie stick lighting) might have been the sole reason for MCAS incorporation to pass stall handling requirements.

Leehams Björn Ferm had a couple of diagrams to that effect.
Boeing would probably refrain from releasing that information without being forced.

The engines are quite a big "flap" ahead of the wing. and a "flap" that sticks up earlier than on the NG (, Classic, Jurassic )
( Just look at the top view outline of the plane.)

Q: "sable slope", "stick lighting" in this context?


Do you mean this one from Björn?

https://leehamnews.com/wp-content/uploa ... -stall.png

It shows an unaugmented pitching moment curve with "stick lightening" and a "stable slope" entering stall. An airplane with the illustrated characteristics will not pitch up into stall on its own if the pilot holds a constant stick force.

The augmented airplane would not have "stick lightening"

Remember though, that this pitching moment curve is notional, very much like my earlier description of "stick lightening". It doesn't not reflect actual 737 MAX data. I believe Björn would agree.

In any case, the augmented airplane shown in the diagram would be certifiable, particularly if stall was accompanied with "deterrent buffet" that would discourage increasing AoA. See AC 25-7D, Section 8.1.

(sorry for the mis-spelled "sable" and "lighting" in the earlier reply)
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WIederling
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Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 5:18 pm

dakota123 wrote:
WIederling wrote:
OldAeroGuy wrote:
Do you have a source for saying there is a region of negative slope (ie pitch up)?

A reduction in a sable slope (ie stick lighting) might have been the sole reason for MCAS incorporation to pass stall handling requirements.

Leehams Björn Ferm had a couple of diagrams to that effect.
Boeing would probably refrain from releasing that information without being forced.

The engines are quite a big "flap" ahead of the wing. and a "flap" that sticks up earlier than on the NG (, Classic, Jurassic )
( Just look at the top view outline of the plane.)

Q: "sable slope", "stick lighting" in this context?


Original spec was for max 0.6* would argue that at least initially it was thought to be a gradient concern. Where it ended up, with (supposedly) 2.5* max? Would that have argued for a pusher instead? At 0.27*/second, MCAS sure doesn’t move very fast, seems like you could be in real trouble before anything approaching full input could be achieved. That never made sense to me.

Would you be better off with taking action on high positive dAoA i.e. pitch up rate and less on absolute pitch up.
( MCAS downside is the integral property working from a direct measured value. integral component without limiting accumulated value can be a problem.
Murphy is an optimist
 
dakota123
Posts: 233
Joined: Wed Aug 30, 2006 11:03 pm

Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 7:06 pm

WIederling wrote:
dakota123 wrote:
WIederling wrote:
Leehams Björn Ferm had a couple of diagrams to that effect.
Boeing would probably refrain from releasing that information without being forced.

The engines are quite a big "flap" ahead of the wing. and a "flap" that sticks up earlier than on the NG (, Classic, Jurassic )
( Just look at the top view outline of the plane.)

Q: "sable slope", "stick lighting" in this context?


Original spec was for max 0.6* would argue that at least initially it was thought to be a gradient concern. Where it ended up, with (supposedly) 2.5* max? Would that have argued for a pusher instead? At 0.27*/second, MCAS sure doesn’t move very fast, seems like you could be in real trouble before anything approaching full input could be achieved. That never made sense to me.

Would you be better off with taking action on high positive dAoA i.e. pitch up rate and less on absolute pitch up.
( MCAS downside is the integral property working from a direct measured value. integral component without limiting accumulated value can be a problem.


Really good question. Any kind of sophistication at all and a PID would be used (assuming still acting on stab and not elevator). Or at least switch to flaps down trim rate. If AOA rate of increase exceeds whatever. Again, at 0.6* max, maybe no need for anything other than fixed response, but 2.5* is not insignificant.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
strfyr51
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Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 7:52 pm

HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.

747 Whale is exactly RIGHT! if the system had cut out? The Pilot might have recovered the airplane. NO system should override the Pilot's input! And if that was the case?
Then WHY have a pilot at ALL?
 
strfyr51
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Re: MAX certification: new systemic issues identified

Sat Mar 23, 2019 8:06 pm

Francoflier wrote:
747Whale wrote:
tealnz wrote:
Seems that at 400knots and 3000 ft above ground level with the stabiliser jack screw up against the stops the “standard procedure” wasn’t adequate to the situation.


No. Slowing the aircraft, however, does address the problem. Any time there's a trim issue, increasing speed will make it worse, in any aircraft. This comes back to a pilot issue.


Fat load of good when you have no idea what speed you're doing. Both crews were likely dealing with unreliable airspeeds, immediate stick shaker activation and generally speaking a very confusing flight deck environment due to a complex instrument failure as soon as the airplane got airborne.
Hitting the stab cutout switches is easy enough when you know exactly what you're dealing with and have plenty of air between you and the ground...
Honestly, I'm not sure anybody could confidently say they could have saved the aircraft even with all the hindsight we have now.

Blaming the crew is easy from the comfort of one's armchair.

And? Please explain HOW the crew wouldn't know the Airspeed? Was the Pitot-Static system inop? And if it WAS? Then they killed Themselves. There is a manual trim switch on the yoke, And a trim cutout switch on the pedestal.. Both the Captain and the First officer have independent Airspeed indicators fed from independent Pitot and Static sources. That neither could assess the airspeed? Preposterous!! You're making this up! Or? You have No IDEA what you're talking about. I've got 42years total in commercial Aviation. Unless the Pitot heat went totally inop? Then they have at the worst? 2 known forms of speed information. So what you espouse? Didn't Happen!!
 
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Starlionblue
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Re: MAX certification: new systemic issues identified

Sun Mar 24, 2019 12:09 am

strfyr51 wrote:
HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.

747 Whale is exactly RIGHT! if the system had cut out? The Pilot might have recovered the airplane. NO system should override the Pilot's input! And if that was the case?
Then WHY have a pilot at ALL?


I'm perfectly fine with systems that override pilot input in some situations. All Airbus FBW fly around with control system features that can override the pilots, e.g. Alpha Max and bank angle protection. These systems have a positive impact on safety.

The problem is not an "override" system per se. The problem is an "override" system that is:
- Poorly implemented.
- Lacking in sensor redundancy.
- Non-transparent to the pilots.
- Not covered in training.
- Intermittent.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
HaulSudson
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Re: MAX certification: new systemic issues identified

Sun Mar 24, 2019 3:42 pm

strfyr51 wrote:
HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.

747 Whale is exactly RIGHT! if the system had cut out? The Pilot might have recovered the airplane. NO system should override the Pilot's input! And if that was the case?
Then WHY have a pilot at ALL?


My math skills are insufficient to calculate the number of times you are contradicting yourself - in merely six sentences with randomly dropped exclamation and question marks.

But perhaps that's exactly the idea behind derailing conversations. Keep up the good work.
 
Amiga500
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Re: MAX certification: new systemic issues identified

Sun Mar 24, 2019 4:18 pm

OldAeroGuy wrote:
I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.


You know as well as, if not better than, me, that you don't correct a divergence between two gradients with a single discrete input.


If the thing is aerodynamically stable far enough up the AoA regime to be certifiable - then they'd be better off putting a stick pusher on it than this.
 
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Starlionblue
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Re: MAX certification: new systemic issues identified

Sun Mar 24, 2019 11:39 pm

Amiga500 wrote:
OldAeroGuy wrote:
I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.


You know as well as, if not better than, me, that you don't correct a divergence between two gradients with a single discrete input.


If the thing is aerodynamically stable far enough up the AoA regime to be certifiable - then they'd be better off putting a stick pusher on it than this.


A stick pusher would have been a much better solution in all respects. Unfortunately, that might mean breaking the common type, or at least requiring more differences training.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
IADFCO
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 3:35 am

The cm(alpha) curve for which OldAeroGuy posted a link is exactly the type of chart that would greatly help assess the root cause of the problem (although I am not sure I understand it: if the bare airframe reaches Cm limits due to stall, how can any kind of augmentation increase it?). It would be interesting to compare the same curves for the MAX and the NG, as well as the MAX's for different x- and z-positions of the engines. Too bad we are not very likely to see those curves (the real curves, not notional ones), assuming that Boeing has them, unless someone leaks them from inside. Their legal office must have built a triple wall with moat and alligators around this kind of technical data. I wonder whether FAA (the real FAA, not the Boeing people who play FAA) gets to see them.
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 2:55 pm

Amiga500 wrote:
OldAeroGuy wrote:
I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.


You know as well as, if not better than, me, that you don't correct a divergence between two gradients with a single discrete input.


If the thing is aerodynamically stable far enough up the AoA regime to be certifiable - then they'd be better off putting a stick pusher on it than this.


Stabilizer input will shift the Cm-Alpha curve. The stabilizer pitching moment change would not go in as a step input but as a function of stabilizer rate. Depending on stabilizer rate at the trigger point, from a stick force input standpoint, the airplane could feel like the augmented curve shown in the referenced notional Cm-Alpha curves. The stabilizer rate would make it feel sort of like an alpha proportional stick nudger.

After the maneuver or stall approach recovery, the pilot would need to trim out the stabilizer input.

Please appreciate this is all speculation on my part as I have no knowledge of 737 MAX flight testing. Just pointing out some possibilities.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Amiga500
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 3:46 pm

OldAeroGuy wrote:
Amiga500 wrote:
OldAeroGuy wrote:
I suspect that the 737 MAX stick force gradient near stall flaps up is less than the 737 NG and the MCAS was added to make the MAX stick force gradient better match the NG.


You know as well as, if not better than, me, that you don't correct a divergence between two gradients with a single discrete input.


If the thing is aerodynamically stable far enough up the AoA regime to be certifiable - then they'd be better off putting a stick pusher on it than this.


Stabilizer input will shift the Cm-Alpha curve. The stabilizer pitching moment change would not go in as a step input but as a function of stabilizer rate. Depending on stabilizer rate at the trigger point, from a stick force input standpoint, the airplane could feel like the augmented curve shown in the notional Cm-Alpha curve.


Say the aircraft starts out in this thought experiment at a fairly high AoA - just below whatever the trigger for MCAS is.

<1> If the pilot held the column steady at - say for a number - 20% way from nominal elevators level and fully forward, but the aircraft continued to pitch up[1] then MCAS kicked in - the weighting to the column and the aircraft pitching moment - will change, without the pilot moving the column, while the change may be introduced at whatever the MCAS rate is (~0.3 deg/sec? for whatever seconds as a function of Mach number). Force back through the control column would be greater and pitch down would be greater.

<2> If the pilot relaxes back to an equivalent force position to the starting point - then the aircraft level Cm will be somewhat different to their original starting point. All fine and dandy as that is the aim of MCAS.

But if we go back to para <1> and assumes the pilot holds the column as would be conventional - that means as the MCAS stabilizer inputs, the control column is being forced backward through a mechanism where the pilot has had no input. Highly, highly disconcerting.


If MCAS continually adjusted the stabilizer through flight, I could accept it - but not if it jumps up and activates during a very particular phase of flight (and given the scenario it would exist in), one where the pilots would already be on edge and working hard to get their aircraft into a more benign part of the flight envelope.


[1]As per the reasoning that the elevator needs significant input to push the nose down at higher AoA.


OldAeroGuy wrote:
The stabilizer rate would make it feel sort of like an alpha proportional stick nudger.


Except the loading on the stick (and any displacement of the stick if held with iso-Force) would be the opposite to the actions of a conventional stick nudger.

MCAS trimming the stabilizer down would increase the load on a nose down elevator - which would increase the backward force on a forward pushed control column.


OldAeroGuy wrote:
After the maneuver or stall approach recovery, the pilot would need to trim out the stabilizer input.


Yep, I think. Dunno if MCAS has an auto untrim or not.


OldAeroGuy wrote:
Please appreciate this is all speculation on my part as I have no knowledge of 737 MAX flight testing. Just pointing out some possibilities.


Of course - I'm running essentially on educated speculation too!
 
BoeingGuy
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 5:15 pm

HaulSudson wrote:
747Whale wrote:
Why do you believe the crews had no knowledge of their speed? There's more than indicated airspeed available in the cockpit for that information, and in a complete loss of air data situation, the FMC/GPS/IRU derived data will give a groundspeed readout, which can also be used, and every cockpit crew is required to be aware of pitch/power combinations to fly in the event of loss of airspeed data.

Therefore, that argument doesn't hold water, except to point to the crew. The data IS available.


Try harder, the world is about to believe your alternative facts.


No they aren’t. Whale is exactly right. What “alternative” facts do you think he’s stating? Everything he stated is required to be memorized as part of the Airspeed Unreliable procedure, which I’m guessing you have no clue about.

You’ve attacked Whale in several threads but you have no factual basis to do so. I think he is the one who knows what he is talking about.
 
BoeingGuy
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 5:20 pm

HaulSudson wrote:
strfyr51 wrote:
HaulSudson wrote:
Wonder how much people like 747whale get paid by Boeing to spam and derail conversations.

747 Whale is exactly RIGHT! if the system had cut out? The Pilot might have recovered the airplane. NO system should override the Pilot's input! And if that was the case?
Then WHY have a pilot at ALL?


My math skills are insufficient to calculate the number of times you are contradicting yourself - in merely six sentences with randomly dropped exclamation and question marks.

But perhaps that's exactly the idea behind derailing conversations. Keep up the good work.


He’s right and Whale is right. They are stating factually correct info. Given that A.net is starting to turn into a tabloid like board where people just post factually incorrect statement, I suspect some people would think that posting factually correct stuff is detailing a thread.
 
747Whale
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 6:40 pm

Matt6461 wrote:
Is there another hazard/catastrophe around the corner?
If the MCAS patch corrects a tendency to pitch up towards stall in high-speed flight then don't we have an unstable aircraft?


Negative.

All aircraft with underslung engines pitch up when applying power, and this is not instability. In fact, the issue of stability is another subject.

There's a lot of inaccurate information and wild ideas being tossed around in this thread, and every other one regarding the 737 Max, and MCAS.

MCAS is not installed to prevent a pitch up in high speed flight. If it's high speed flight, except for an accelerated stall, then stalling is not an issue. In other words, it's irrelevant.

The 737 Max is not unstable. We can certainly start a very lengthy thread to discuss static and dynamic stability, but it will take that long before you understand the subject, and you can't really discuss it until you understand it. Simply put, the 737, like any other transport category airplane, when disturbed from steady state trimmed flight, will return to it's trimmed state.

MCAS is an auxiliary function added to the Max to address a change in location of engines (farther forward), and an increase in thrust. It's there to assist the pilot, and was designed to be transparent; other aircraft utilize systems which provide control input, adding trim or providing control column movement; all transport category airplanes, for example, use stick shakers and pushers. Autopilots trim without pilot input. The MD11 uses LSAS which provides a variety of different control actions independent of pilot input, and so on. What each of those systems and subsystems and functions have in common is that they can all be overridden and stopped by the pilot. This INCLUDES MCAS.

The Lionair event had a similar problem the day before. It's well known that the pilot riding in the jumpseat informed the crew to use the cutoff switches, per the manufacturer procedure, and they did, ending the problem. That does introduce two important questions that most don't seem to be addressing: the crew should have known that and done that automatically based on their systems knowledge, training ,and basic flight procedures (the stab trim cutoff procedure applies to all situations involving unwanted trim movement, when not stopped by other means. This is a crew training issue, then. The other question which should be asked long and loud is if the situation occurred in the Lionair airplane the day prior, why was it flying the next day? This points to a company culture issue, and a maintenance issue.

Similar questions can be addressed to the Ethiopian event.

MCAS runs very slowly; a third of a degree of stab trim movement per second, and is limited to 2.4 degrees of travel; easily controlled, easily handled, and does not happen quickly. It can be immediately stopped by use of the control wheel trim switches. It can be easily shut off completely with the stab trim cutoff switches. Trim in motion has an audible warning, and wheels on both sides of the control column, conspicuously painted to ensure they're seen in motion, are available for use and provide visual indications of trim movement. This isn't something that blindsides the crew; it's easily seen, and the crew can see the direction of trim, and feel it.

Several posters keep inexplicably returning to an airspeed issue. Why they keep introducing this is a mystery; it's irrelevant. Airspeed isn't the issue here.

The two most salient points of MCAS are that it does not function with the autopilot; it functions when the autopilot is not engaged. It can be stopped with the normal control wheel trim switches, but if stopped, it will delay for several seconds, reset, and begin trimming again. This is easily handled by cutting off the trim, if the crew feels it to be a problem. The panic response, illustrating no understanding of the aircraft, basic core flying procedures, or systems or the use of systems (cutoff switches) is at the heart of both losses. I very much doubt either of these events would have happened in a safety culture environment with top level training. Both happened in third world environments, and with relatively inexperienced crews (one of them very inexperienced). When an airline has experienced the situation that the Lionair airplane experienced, the airline should have been proactive in addressing the matter, rather than stuffing another crew in the same airplane and sending it right back into the field. Therefore, the airline is also to blame.

This isn't a blame game; it's about identifying cause and effect, and crew and company approach in both cases are squarely at the heart of the matter.

Amiga500 wrote:
Except the loading on the stick (and any displacement of the stick if held with iso-Force) would be the opposite to the actions of a conventional stick nudger.

MCAS trimming the stabilizer down would increase the load on a nose down elevator - which would increase the backward force on a forward pushed control column.


You have that precisely backward, actually.
 
OldAeroGuy
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Re: MAX certification: new systemic issues identified

Mon Mar 25, 2019 7:17 pm

Amiga500 wrote:
Say the aircraft starts out in this thought experiment at a fairly high AoA - just below whatever the trigger for MCAS is.

<1> If the pilot held the column steady at - say for a number - 20% way from nominal elevators level and fully forward, but the aircraft continued to pitch up[1] then MCAS kicked in - the weighting to the column and the aircraft pitching moment - will change, without the pilot moving the column, while the change may be introduced at whatever the MCAS rate is (~0.3 deg/sec? for whatever seconds as a function of Mach number). Force back through the control column would be greater and pitch down would be greater.


If I understand you, you're saying that the pilot is holding enough elevator that the airplane is pitching up toward stall. Correct?

That's what a pilot would do if he/she was attempting to stall the airplane or pulling g's in a turn.

If MCAS kicks in at the appropriate alpha and the pilot holds steady, the airplane will probably pitch down. That's what will happen for a stable stall break and it's considered desirable stall behavior as the airplane is headed to an unstalled condition. If the stick shaker is going off, so much the better.

Amiga500 wrote:
<2> If the pilot relaxes back to an equivalent force position to the starting point - then the aircraft level Cm will be somewhat different to their original starting point. All fine and dandy as that is the aim of MCAS.

But if we go back to para <1> and assumes the pilot holds the column as would be conventional - that means as the MCAS stabilizer inputs, the control column is being forced backward through a mechanism where the pilot has had no input. Highly, highly disconcerting.


I don't see why you consider that this is disconcerting.

First, MCAS is not doing anything to the column. The pilot is pulling back if an AoA increase is desired but it requires additional force for her/him to do so, as it would on any stable conventional control airplane.

Second, the definition of stall recognition from AC 25-7D says:

8.1.3 Stall Demonstration—§ 25.201.

8.1.3.1 The airplane is considered to be fully stalled when any one or a combination of the characteristics listed below occurs to give the pilot a clear and distinctive indication to cease any further increase in angle-of-attack, at which time recovery should be initiated using normal techniques.

8.1.3.1.1 The pitch control reaches the aft stop and is held full aft for two seconds, or until the pitch attitude stops increasing, whichever occurs later. In the case of turning flight stalls, recovery may be initiated once the pitch control reaches the aft stop when accompanied by a rolling motion that is not immediately controllable (provided the rolling motion complies with § 25.203(c)).

8.1.3.1.2 An uncommanded, distinctive, and easily recognizable nose down pitch that cannot be readily arrested. This nose down pitch may be accompanied by a rolling motion that is not immediately controllable, provided that the rolling motion complies with § 25.203(b) or (c), as appropriate.

Normal MCAS operation is meant to operate at high angle of attack when the flaps up airplane is approaching stall. Its operation would tend to promote the stall recognition features described above.


Amiga500 wrote:
If MCAS continually adjusted the stabilizer through flight, I could accept it - but not if it jumps up and activates during a very particular phase of flight (and given the scenario it would exist in), one where the pilots would already be on edge and working hard to get their aircraft into a more benign part of the flight envelope.


You're describing MCAS operation with a failed AoA indicator. Normal MCAS operation is helping the pilot get out of an edge of the envelope situation. In the Lion Air case, MCAS did not jump up suddenly. It was erroneously operating continually due the AoA vane failure.


OldAeroGuy wrote:
The stabilizer rate would make it feel sort of like an alpha proportional stick nudger.


Amiga500 wrote:
Except the loading on the stick (and any displacement of the stick if held with iso-Force) would be the opposite to the actions of a conventional stick nudger.

MCAS trimming the stabilizer down would increase the load on a nose down elevator - which would increase the backward force on a forward pushed control column.


No, you're thinking about this in the wrong way. MCAS moves the stabilizer nose up which is airplane nose down. A stick nudger or stick pusher makes the airplane move nose down. The pilot has to pull back on the stick to stop the airplane from going nose down for either MCAS or stick nudger/stick pusher activation.

Keeping MCAS from activating when it shouldn't means having better AoA vane reliability and/or shutting down AoA signals to MCAS in case of a disagree. Stick nudgers/stick push would have exactly the same issue.

Again, please appreciate this is all speculation on my part as I have no knowledge of 737 MAX flight testing. Just pointing out some possibilities.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

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