MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures Mk II

[rog747]

Originally Posted by turbidus

Wonder if they kept the -800 and -900 jigs? Just start building those again.

A moot point - Airline accountants and all the airlines today (and in the future) just want fuel economy and $$savings, hence the fitting of the big new CFM LEAP engines to basically a 737-800, of which they are now the root cause of the unstable MAX air frame, necessitating the MCAS software requirement.

[Vendee]

Originally Posted by MechEngr

Did MCAS fundamentally fail? Not on the accident aircraft - the algorithm and resulting software did exactly what it was supposed to do with the information it was given.

Let me remind you that the "S" in MCAS stands for System.

MCAS did fail. The code may have done what it was designed for but the system (which includes the AoA vane) failed.

[martinhogbin]

Error in post corrected.

This is my first post here so I apologise for any errors in the way that I have done so. Advice welcome.

As I understand it the 737 Max cannot be certified to fly at all without MCAS because it does not meet the requirements of FAR 25.203(a) which says "No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall"

Is there any definitive confirmation of this fact?

If it is NOT true, it would seem to me that Boeing's best way by far to get the planes flying again would be to remove or completely disable MCAS and admit that it is a different aicraft from older 737s and agree to pay for all training required to fly it.

[Lake1952]

Re the Canadian plan, it is exactly what I had proposed sometime in some thread on this board. Kick MCAS out of the aircraft and train for the issues of stick force linearity or pitch up tendencies due to engine location or whatever the aerodynamic aberrations are. If that training requires a new type rating, then so be it.

My LA4-200 Lake Amphibian would pitch down with power and up when you decreased the throttle due to the rear facing engine located more or less at the CG. Exactly the opposite of every other aircraft that I had flown. That aerodynamic tendency was covered on day #1 of training.

[Bend alot]

Martin & Lake how do you propose to comply with the Grandfather rights?

[KCode]

"Seeking Alpha" provides an opinion on the cash flow of the Max program, and supports your comments on the balance of advanced payments versus production costs. Their analysis shows that the advanced payment revenue falls below the production costs (42/M) in Jan.

[clearedtocross]

The fact that B keeps the non-MCAS (raw) pitch-up characteristics a secret leads to the conclusion that those characteristics are much worse than just a mild non-linearity. That is probably also the reason why they multiplied the MCAS action by 4 without telling the FAA (and anybody else).
Drastical pitch up cannot be certified (thank God) as it is bl..dy dangerous even when pilots get told/trained. A commercial jet is not a Lake Amphibian and it is operated on instruments , not on seat of pants VFR operation where a pitch deviation up or down is easily identified and corrected.

[OldnGrounded]

Originally Posted by clearedtocross

The fact that B keeps the non-MCAS (raw) pitch-up characteristics a secret leads to the conclusion that those characteristics are much worse than just a mild non-linearity.

It certainly leads, at least, to serious skepticism. For some reason, despite lots of people repeatedly asking about this, the data has not been provided.

The Flyers' Rights lawsuit may pry some of it loose.

[Lake1952]

Here are two important parts of the story that we do not know as yet so far as I am aware.
1.) What are the aerodynamic characteristics or aberrations that led to MCAS?
2.) How many times (if any) has the MCAS been activated besides the two Lion and one ET flights? Has MCAS ever been activated when there was no malfunction in the AoA?

Bend a Lot, while I am aware of grandfather rights, I have no idea how much content on the MAX would be verboten or obsolete if a separate type rating was required. Or invent a "sub-type rating". And I have no idea what the type rating has to do with the type certification of the aircraft! Just because an aircraft requires a type rating, why would that affect the type certification of the aircraft?

[PEI_3721]

jimtx, #74 ‘just use the AP’.
This is similar to the calls for FBW; unfortunately the AP argument is flawed.

The Air Data Computers use AoA for corrections. The output of the two ADCs is used by the AP (Flight Guidance Computer) which compares values. If the values of speed / alt disagree ‘by xx’, AoA input error, then the AP will disengage - ‘invalid data’, and the relevant Disagree alerts given.
For Speed and Alt the crew can crosscheck the StBy systems: a checklist item. There is no third AoA, thus the consequential and distracting alerting remains on; no AP available: abnormal conditions #17 (https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10643705).

The ‘don’t pull too hard’ quip might represent residual problems relating to the design of the trim system and subsequent use without MCAS - see below.

[PHXPhlyer]

MCAS Annunciator

Has anyone proposed an annunciator to show when MCAS is active instead of trying to determine if it is a case of runaway trim or not?
Just a thought.

PP

[PEI_3721]

Icarus2001, #76, following on Water pilot, #72,
The description of trim operation is based on normal operation; relating it to basic flying training. However, the 737 trim system differs from basic training aircraft . The 737 design can inadvertently create bad habits tending to fly the aircraft with trim (Boeing cautions against this).
In normal operation the pilot-aircraft-trim interaction is similar to any other system, the effects of the different design are not readily apparent. However, with large changes in stick position / force, or if the trim malfunctions then the pilot-trim interaction changes (https://www.skybrary.aero/bookshelf/books/2627.pdf).
…
The 737 Max pitch stability is like slice of stale bread, curled up at the edges. MCAS logic applies trim to flatten out the edge bits; small trim changes have a large ‘restorative’ effect, which works.
If large, unwarranted MCAS trim changes occur anywhere in the flight envelope then both aircraft control and restorative trim application is difficult - accidents.
After modification, comparison of sensor input, errors inhibit further MCAS trim movement after one small application; this value will place the aircraft ‘out of trim’. If this occurs in the ‘flat’, centre area of the bread slice, applying trim to flatten a non existent curl might dent or hole the bread causing further problems.
…
Before the Max can return to service the latter case must be assessed as satisfactory - within an agreed range of piloting competence. However, if the small amount of trim from MCAS ‘failure’ creates a false impression of pilot-aircraft-trim, via residual stick force, then the piloting task is increased. How this might apply to manual trim wheel operation is not clear, but evidence suggest its not beneficial.
Similarly application of corrective electric trim to reduce any offset could be masked by the higher stick forces. This situation could be further complicated if the aircraft is flown with trim, small trim blips inadvertently amalgamate to a longer application, applied instinctively in surprising situations - several alerts and distracting stick vibration (see trim system design link above, NB accidents - and recent FlyDubai).

Previous posts https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10642164 and https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10643566 assume that all electrical trim operations are inhibited with MCAS inhibit. The rationale is that if the dual FGC is used as the monitor for MCAS, and electric trim and AP trim are within the same box, then the most effective safeguard is to switch everything off. If so, could subsequent pilot evaluation via checklists determine which part of the system has failed and then some parts reinstated; particularly advantageous if electric trim is effective in restoring and continuing safe flight.

The above is influenced by not understanding why the Max trim-inhibit switches differ from the NG, separating elect trim and AP trim (extensive Tech Log discussion); did Boeing expect some aspects to be restorable - is the Max more dependent on electric trim than the NG, thus it is desirable to restore it to reduce workload in abnormal conditions; particularly where trim runaway still has to be considered.
It is in these areas which checklist drills and actions could be generating more certification discussion.
.

[GlobalNav]

Originally Posted by Bend alot

Martin & Lake how do you propose to comply with the Grandfather rights?

If by "grandfather rights" you are referring to the Changed Product Rule, I think service history, 2 catastrophic accidents, provides the necessary justification to impose the most current safety standards.

[OldnGrounded]

From Investor's Business Daily, yesterday:

Boeing 737 Max Suppliers, Customers Make Big Moves As Crisis Drags On

GILLIAN RICH 04:11 PM ET 12/20/2019

Boeing 737 Max suppliers Spirit AeroSystems (SPR) and General Electric (GE) are adjusting their production, and United Airlines (UAL) canceled 737 Max flights to the start of summer as the grounding drags on longer. Boeing (BA) stock fell.

"This suspension will have an adverse impact on Spirit's business, financial condition, results of operations, and cash flows," said Spirit AeroSystems, which gets about 50% of its revenue from the Boeing 737 Max.

Meanwhile, engine supplier General Electric will boost production of its Leap 1A for Airbus (EADSY) to help offset lost sales to Boeing during the 737 Max production halt, according to the Wall Street Journal.

Also Friday, United Airlines extended 737 Max cancellations to June, meaning the plane may risk missing another summer travel season. American Airlines (AAL) and Southwest Airlines (LUV) have canceled flights through April.

Late Thursday, S&P Global Ratings cut its score by one notch to A- with a "developing" outlook, citing uncertainty over the Boeing 737 Max's return to service, the production halt's supply-chain risks, and the potential long-term damage to the plane's competitiveness.

S&P also downgraded its assessment of Boeing's management to fair from satisfactory.

Late Wednesday, Moody's cut Boeing's debt one notch to A3 while giving it a stable outlook, saying the recently announced production halt will mean Boeing will have to provide additional financial support to many suppliers, increase risk in the production system, and boost customers' claims for compensation, which will lower 737 margins and cash generation "for years to come."

"Moreover, Moody's considers that Boeing's reputation can be adversely affected as the grounding extends and from its governance missteps with broadening social considerations related therefrom, which could have a more lasting impact on the company's business," Moody's added.

Shares fell 1.65% to finish at 328.00 on the stock market today. Boeing stock remains below its 50- and 200-day lines, according to MarketSmith analysis. Spirit AeroSystems lost 1%, while GE ended flat.

[MechEngr]

Originally Posted by clearedtocross

The fact that B keeps the non-MCAS (raw) pitch-up characteristics a secret leads to the conclusion that those characteristics are much worse than just a mild non-linearity. That is probably also the reason why they multiplied the MCAS action by 4 without telling the FAA (and anybody else).
Drastical pitch up cannot be certified (thank God) as it is bl..dy dangerous even when pilots get told/trained. A commercial jet is not a Lake Amphibian and it is operated on instruments , not on seat of pants VFR operation where a pitch deviation up or down is easily identified and corrected.

There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports. The size increases the coupling of fuel burn to air movement for an increase in thrust specific fuel consumption, aka, fuel efficiency. So, they keep the thrust the same and burn less fuel.

I'm sure the change in factor was not yelled from the roof tops because no one had a fault-tree analysis linking the AoA system to the MCAS output to a definitive emergency outcome. Had they passed along the change in rate without anyone having done the analysis, it would have not raised any questions, just like it did not in the first place. The more aggressive change should have acted to draw the attention of the flight crew to the more rapid pitch trim change and then to the cause of it, rather than being a slower and less obvious one; it appears in the first and second MCAS events that this is exactly what happened. In the third event, the pilots seemed unaware of the change in trim for a relatively long time and did not notice they were left with a large increment.

So far no one has volunteered just how the control forces for pitch mis-trim can be so low so the pitch trim error is unrecognized and so high they cannot be overcome.

[jdawg]

Originally Posted by MechEngr

There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports. The size increases the coupling of fuel burn to air movement for an increase in thrust specific fuel consumption, aka, fuel efficiency. So, they keep the thrust the same and burn less fuel.

I'm sure the change in factor was not yelled from the roof tops because no one had a fault-tree analysis linking the AoA system to the MCAS output to a definitive emergency outcome. Had they passed along the change in rate without anyone having done the analysis, it would have not raised any questions, just like it did not in the first place. The more aggressive change should have acted to draw the attention of the flight crew to the more rapid pitch trim change and then to the cause of it, rather than being a slower and less obvious one; it appears in the first and second MCAS events that this is exactly what happened. In the third event, the pilots seemed unaware of the change in trim for a relatively long time and did not notice they were left with a large increment.

So far no one has volunteered just how the control forces for pitch mis-trim can be so low so the pitch trim error is unrecognized and so high they cannot be overcome.

Don't think of it as an issue of "thrust line" nor "thrust to weight" but rather nacelle lift farther forward and higher than the NG. This in fact causes a pitch up moment force behind the curve.

[Fly Aiprt]

Originally Posted by MechEngr

There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports.

I'm not sure I'll fully agree with the above.
Small changes in configuration *can* lead to drastic change in flight qualities. Heavy tail buffet in a transport plane has been known to be due to windshield/windshield wipers perturbations, and had to be cured with VG's on top of the cockpit.
Some very little variations in airfoil, washout, dihedral, sweep or mechanical details can trip unwanted flow behaviour.
Boeing said the stability issue was due to the airflow over nacelles and adjacent wing portion rather than thrust line.
Not sure there is a way to foresee mach tuck etc. just by comparing 3-view drawings so I'd say there is more to the 737 stability issue than is officially published.

[OldnGrounded]

Originally Posted by MechEngr

There's no drastic pitch-up.

We don't know how pronounced the pitch-up tendency may be. We haven't seen data from testing of the bare airframe.

How does this get started and why is it repeated?

It got started because both Boeing and the FAA have indicated that the size/shape/positioning of the LEAP engines creates additional lift at some attitudes/corners of the envelope and that as a result of that, MCAS is required to provide linear stick force to discourage pilot input bringing the aircraft dangerously close to stall. There is a fair amount of skepticism about the assertion that a system with full authority over the horizontal stabilizer was truly developed and implement merely to control stick force.

Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports.

I don't think anyone has suggested (at least in many months) that thrust is the issue here. The issue, as far as we can tell, is increased lift due to the engine nacelles and their position.

[Drc40]

Originally Posted by OldnGrounded

It certainly leads, at least, to serious skepticism. For some reason, despite lots of people repeatedly asking about this, the data has not been provided.

The Flyers' Rights lawsuit may pry some of it loose.

You’re right in it might provide insight but when? Considering how long it takes lawsuits to get moving I can’t see it happening before the FAA makes a decision on the MAX. My guess is most of the speculation here is 100% dead on about the aerodynamic issues with the MAX sans the exact specifics/numbers. I think the best hope we have for answers is the FAA. I would hope they publish a detailed analysis of MCAS and why it’s needed, how it engages, etc when the commentary period opens.

Maybe I’m overly optimistic about the FAA but at this point they have to think 100% transparency is paramount to regaining the lost trust.

On another note, the announcements of shutdowns is now directly impacting many more thousands of workers. Their livelihoods are being compromised, families will have to endure cuts and the number will only increase. This will result in enormous political pressure. Might even become an election issue if this drags on and the out of work numbers continue to rise. The economic impact of Boeing and its ancillary businesses is far and wide. Time is now becoming a factor and I hope it doesn’t cloud the eyes of regulators.

[Phugoidinator]

OldnGrounded, could it also be that higher and more forward position of the nacelles can cause turbulence over the wing at higher angles of attack? Difficult to model or predict what turbulence/loss of laminar flow can do to lift generated by wing.