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

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

Old 27th Sep 2019, 06:43
  #2621 (permalink)  
 
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Originally Posted by calypso View Post
Simply not true. Manual trim is used regularly while conducting air tests. I have used it, it works fine. Obviously if you allow ANY trim to go to the stops you have a serious problem in your hands. Speed trim, aileron trim or rudder trim..... in a 737 or in most other aircraft.
For the avoidance of doubt and for the benefit of other readers - can you confirm then that the smaller trim wheel in the NG and the MAX works fine and that you have used it successfully in air tests? This would contradict reports dating to before the MAX issues that aircrew had had difficulty with it.
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Old 27th Sep 2019, 06:54
  #2622 (permalink)  
 
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Originally Posted by JPJP View Post



Your F.O. Blogs would reply -

“Which memory items Captain Austronaut ? There haven’t been any STAB TRIM Runaway memory items for years. We do have IAS DISAGREE and ALT DISAGREE. Should we do the UAS items first ?”

Boom !

Maybe you should get together with JDawg. He doesn’t know there are no “stick shaker memory items”. You could both invent some new procedures for Boeing. God knows, it couldn’t hurt.

love the peanut gallery.
There certainly is a memory item for stab trim runaway. Maybe your airline saw fit to remove it as a memory item but as soon as the lion air accident happened and the FAA put out the AD it should have been reinstated.

The stick shaker also has steps that need to be accomplished without reference to a manual. More of a procedure, but depending on semantics a stall recovery is a memory item. Boeing has defined steps to recover that will need to be completed from memory.

That said I agree with the overall message of your post. Where do you start with all the distractions.

Last edited by ARealTimTuffy; 27th Sep 2019 at 06:56. Reason: Add
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Old 27th Sep 2019, 07:40
  #2623 (permalink)  
 
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Originally Posted by Maninthebar View Post
For the avoidance of doubt and for the benefit of other readers - can you confirm then that the smaller trim wheel in the NG and the MAX works fine and that you have used it successfully in air tests? This would contradict reports dating to before the MAX issues that aircrew had had difficulty with it.
I think you guys are talking passed each other. There is a problem with using manual trim when the stab is grossly out of trim. There is not a problem when used “normally”, I.e, to adjust the trim as part of normal flying technique.
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Old 27th Sep 2019, 08:09
  #2624 (permalink)  
 
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Originally Posted by AerocatS2A View Post
I think you guys are talking passed each other. There is a problem with using manual trim when the stab is grossly out of trim. There is not a problem when used “normally”, i.e, to adjust the trim as part of normal flying technique.
This is the important trim wheel question in the present context.

Can the trim wheel be used in practice while close to VNE at low altitude, and simultaneously close to full nose down trim while simultaneously the crew are heaving back on the control column for all they are worth?

These are the circumstances in both of the crash cases and there seem to be rumours that under these circumstances the trim wheel cannot be operated by real-life crews. That's going to be a fun airtest.
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Old 27th Sep 2019, 08:44
  #2625 (permalink)  
 
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I think there will be zero doubt that the trim wheel will be tested by numerous regulators under the flight envelope conditions, if not by the FAA.

The results should the FAA certify the MAX first - would "can" the FAA as a credible regulator.

They already have trust issues, and just days ago seem to be caught lying or covering up events during initial certification.

It still seems neither the FAA or Boeing understand the gravity of the problems and still cling to "the best safety record" in a decade, to shield from the worst decade ever of a new model in a modern era of safe air transport.

The accountable persons need to step forward and or named - but that needs to be deferred
till the last court case.
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Old 27th Sep 2019, 08:58
  #2626 (permalink)  
 
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Please check the Mentour Pilot video from some NG sim to get an idea.

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Old 27th Sep 2019, 09:09
  #2627 (permalink)  
 
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Originally Posted by Less Hair View Post
Please check the Mentour Pilot video from some NG sim to get an idea.

https://www.youtube.com/watch?v=aoNOVlxJmow
Less Hair, If I recall the simulator control forces were not correct, they were less than actual forces.

At a guess they were based on the larger trim wheel diameter prior to the NG.
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Old 27th Sep 2019, 10:02
  #2628 (permalink)  
 
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Originally Posted by Bend alot View Post
Less Hair, If I recall the simulator control forces were not correct, they were less than actual forces.

At a guess they were based on the larger trim wheel diameter prior to the NG.
And possibly on the smaller h/stab pre-NG.
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Old 27th Sep 2019, 10:21
  #2629 (permalink)  
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Originally Posted by ST Dog View Post
"While Boeing considered the possibility of uncommanded MCAS operation as part of its functional hazard assessment, it did not evaluate all the potential alerts and indications that could accompany a failure that also resulted in uncommanded MCAS operation,"

It's a problem with how FHAs are done. And that's following the process for them. Go read ARP4754 and 4761.
True words sir,

NASA/CR–2015-218982 Application of SAE ARP4754A to Flight Critical Systems is a good primer for the process, and indicates both the complexity and the potential for missing the plot.

Quantitative analysis is a flawed concept.

NASA post Challenger evaluated the failure probability estimates that the engineers had come up with for various system failures. The critical level for man carrying space vehicles was easily exceeded by orders of magnitude on all counts, an some cases with an extra 8 zeros behind the actual failure rate with the loss of STS-51L. A more robust solution arises from Monte Carlo simulation or similar iterative analysis, but at the end of it all, irrespective of the method employed, it is a guess of the likelihood of an improbable outcome. Same issue arises with the risk assessment of nuclear power stations, (sorry of you live downwind of any) of NASA up to Columbia... (...ooops, I did it again....) and.... the U.S. nuclear weapons safeties... Goldsboro, North Carolina, Damascus Titan II, and... Russian "nooks", bucket o' sushine, Chernobyl, Kyshtym (golly where did that town go...) Severodvinsk, H.E.N. "Hiroshima" class nuke boats, Yankee II (missile oops), Golf II (missile oops), Stanislav Yevgrafovich Petrov, who is singlehandedly the reason civilisation wasn't accidentally wiped out on 26th September 1983..., Japans Fukushima design (which is as risky as 50% of Europes hot tubs) Tokaimura Tokai, Mihama-3,

You can throw all the maths you want to at an analysis, but then the solution is fundamentally down to reading chicken entrails. Sometimes we get it done and the answers make sense, but say, NASA suggesting that the likelihood of a shuttle loss was better than a million to 1 doesn't seem rational from the design, and certainly from the stats of the operation.

As a broken record, I say again that our persistent assumption of linear or quasi linear causation is fundamentally flawed. It may make regulators happy, it makes QA auditors and FOIs/ASI's happy, but the problem is the world doesn't work that way. It is the exception not the rule that accidents occur from a simple linear chain of events arising from errors or violations of process rules; accidents happen as often from multiple interactions all achieving resonant outcomes that exceed a limiting boundary. The problem of living in a stochastic world and treating it as linear is we are effectively fighting the last war not the next, we apply bandaids to the issue, rather than seek the inner truth as to the zen of the problem. Every time we add a new sentence or component to a process, we alter the condition in expected and unexpected ways, and the problem is that a number of within limit components of the system acting within their expected tolerances can result in the wild ride.
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Old 27th Sep 2019, 11:29
  #2630 (permalink)  
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Originally Posted by Water pilot View Post
My understanding of this incredibly long thread is that "turn the trim off with those conveniently located trim switches" was exactly the wrong thing to do in this situation and was one of the reasons for the crash. The proper steps, as I understand this thread, are "use the electric trim to undo the nose down inputs from MCAS and then turn off the electric trim." How that would be the proper procedure for a true trim runaway is not something that I understand, if the electric trim is in fact misbehaving then you would want to disable it immediately rather than screw around on the off chance that there is something in the software logic that you don't understand.
I think that might be one of the clearest summaries in this (apparently endless) thread. The people who wrote B's emergency bulletin ("Just follow the runaway stab NNC.") should read it every morning.
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Old 27th Sep 2019, 12:08
  #2631 (permalink)  
 
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Response to above, in no particular order:

Critical action items, sometimes referred to as "memory items", are those steps that flight crews must be able to perform promptly either from memory or by reference to a a quick-reference card generally stored on the glareshield within easy reach of either pilot. There is no "stick shaker" memory item. Rather a stick shaker on takeoff should first be considered an indication of an impending stall (flaps mis-set, incorrect performance data, windshear, etc). If the aircraft performance is normal, then the stick shaker is erroneous and will immediately convert to an "Airspeed Unreliable" scenario for which there is a memory item. It appears that the fact that a bad AOA input can create immediate airspeed and altitude discrepancies was not well understood by average 737 crews. Even after the MAX accidents, I still come across 737 pilots who do not know this. I can only speak for my airline, but I have never seen an erroneous stick shaker on takeoff scenario in the sim, nor have I ever been given any unreliable airspeed problem during takeoff. I have never been taught to associate a bad AOA input with airspeed and altitude discrepancies. As I have mentioned before, whether one is in an NG or a MAX, an AOA failure during takeoff can present a cascading set of issues that can be just as challenging to manage as an engine failure. I would have thought by now there would have been some concerted effort by airlines to educate 737 crews in some of these lessons learned, but I haven't really seen any so far. This is a definite training and education issue.

Runaway Stab Trim has always been (well, at least for a few decades) a memory item on any Boeing aircraft. However, (and again, speaking only for my airline) it is a procedure that has been sorely neglected in training. Prior to the MAX, the 737 stab trim system was incredibly reliable and runaway stab events were rare. Accordingly, in an effort to use sim time more "efficiently," runaway stab problems and the possible issues related to manual trim use were rarely presented to aircrews. I suspect the same was happening at other airlines (other 737 operators, feel free to chime in here). I practice engine failure events during every sim session, and as a result I am well-versed in the immediate actions required. I will suggest here that if Runaway Stab Trim events had been presented to the accident aircrews on a regular basis, then their response to the MCAS malfunction would likely have been different.

As a side note, one would think that any procedure that has critical action items would be presented regularly during simulator sessions. It is not only important to know what steps are required, but to actually perform those steps so there is no question as which controls and/or switches need to be actuated and in what order. Unfortunately, that is not the case. For as long as I have been flying airliners, there has always been one or more "orphan" memory items that are rarely, if ever, presented once the pilot is through initial training.

I can confirm that the manual trim wheel works fine throughout the speed and altitude envelope as long as one keeps the aircraft in trim. Difficulties arise only when one is attempting to trim from an excessive out-of-trim state. The problem has nothing to do with airspeed or altitude, but rather with the amount of elevator deflection being held. The intent of the "roller-coaster" maneuver is to release elevator pressure, thus relieving forces on the jackscrew mechanism, which then decreases the force necessary to turn the manual trim wheel. As long as some nominal amount of climb can be maintained (for an excessive nose-down trim state) or sufficient margin against a stall (for an excessive nose-up state), then the maneuver can be performed successfully.

Last edited by Tomaski; 27th Sep 2019 at 12:29.
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Old 27th Sep 2019, 12:50
  #2632 (permalink)  
 
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It is the elevator deflection that results in the excessive force on the stab screw jack which then may defeat the torque available from the pilot in manual application of trim.
Has anyone actually calculated the torque required to move the stabiliser under maximum aerodynamic load? As the friction forces to be overcome on the threads of the screw-jack will increase exponentially as it moves towards maximum load, adding an electric servo as a solution may need to be done in conjunction with a redesign of the screw-jack mechanism to cope with this additional force.
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Old 27th Sep 2019, 13:34
  #2633 (permalink)  
 
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I wouldn't be surprised if Boeing had failed to actually measure those torques in flight.
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Old 27th Sep 2019, 14:27
  #2634 (permalink)  
 
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Originally Posted by Speed of Sound View Post


Has anyone actually calculated the torque required to move the stabiliser under maximum aerodynamic load? As the friction forces to be overcome on the threads of the screw-jack will increase exponentially as it moves towards maximum load, adding an electric servo as a solution may need to be done in conjunction with a redesign of the screw-jack mechanism to cope with this additional force.

Its not really the friction forces per se that are the problem- The jack screw uses a recirculating ball bearing ' nut' on the jakcrew which is then attached to the stabilizer. The the actual design- installation must take care to use a nut- screw thread combination to prevent being back driven ( eq ' nut' rotates even when jack screw does not ROTATE. ) . Cant post links - but look up ball screw calculator for example. Boeing has used the ball screw arrangement for decades- probably since the 707 for such actuation..

Last edited by Grebe; 28th Sep 2019 at 02:48. Reason: changed jackscrew move to ROTATE for clarity
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Old 27th Sep 2019, 14:39
  #2635 (permalink)  
 
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Originally Posted by jimjim1 View Post
Can the trim wheel be used in practice while close to VNE at low altitude, and simultaneously close to full nose down trim while simultaneously the crew are heaving back on the control column for all they are worth?
It's 100% clear that the answer to that is no. And it is no for every variant of the 737. That's why there are notes about relieving aerodynamic forces.
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Old 27th Sep 2019, 14:50
  #2636 (permalink)  
 
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Originally Posted by fdr View Post
Quantitative analysis is a flawed concept.

As a broken record, I say again that our persistent assumption of linear or quasi linear causation is fundamentally flawed. It may make regulators happy, it makes QA auditors and FOIs/ASI's happy, but the problem is the world doesn't work that way.
Well, it's better than what was in use before and there's currently no viable better alternative.

Even with the current process, you can go down a rabbit hole of what-ifs and spend a lot of time and money for no added safety.

I wish there was a magic process, but there not. Only experience will find the missed cases. Normally they are found and mitigated with little fanfare, much like a patches CVE before it's exploited. But there will be the occasional issues found with spectacular results, like a zero-day exploit that affects millions.
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Old 27th Sep 2019, 15:01
  #2637 (permalink)  
 
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Originally Posted by Speed of Sound View Post
Has anyone actually calculated the torque required to move the stabiliser under maximum aerodynamic load?
Maximum? Within the normal flight envelope or at the edges (or even beyond) of the certified envelope?

Just to Vmo or all the way to Vne?
At what altitude? Since the aerodynamic effects differ with air density.

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Old 27th Sep 2019, 15:02
  #2638 (permalink)  
 
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Originally Posted by Fly Aiprt View Post

I wouldn't be surprised if Boeing had failed to actually measure those torques in flight.
The torque must have been measured (or calculated) when the original trim wheel gearing was designed. I assume that when the wheel diameter was reduced they left the rest of the system unaltered, hence the retractable handle to increase leverage.

Whether they calculated the torque required at maximum stabiliser deflection at Vne I don’t know, but what Boeing may not have done is looked at time taken as well as force required. When you are very fast and very low, how fast you can manually re trim is as important as how much force is needed which is why relieving that force by a gearing solution is not a complete solution.

Halve the force required through gearing and you double the time required to move the stabiliser through a set amount. That is why if the force needed is deemed unacceptable, then an electric servo is the only real solution.
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Old 27th Sep 2019, 15:24
  #2639 (permalink)  
 
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Originally Posted by Water pilot View Post
My understanding of this incredibly long thread is that "turn the trim off with those conveniently located trim switches" was exactly the wrong thing to do in this situation and was one of the reasons for the crash. .
I understand where you are coming from... Removing the stab cut-out switches made the situation worse.
The cut-out switches with 'relay logic' are more reliable than the current DAL-C flight control system, but I cant imagine that the 'relay logic' would fulfill DAL-A.
If DAL-A is needed since the runaway control forces can't be handled by the Pilots, then the functionality of 'relay logic' must be reimplemented in DAL-A software instead.
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Old 27th Sep 2019, 17:03
  #2640 (permalink)  
 
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Originally Posted by JPJP View Post



Your F.O. Blogs would reply -

“Which memory items Captain Austronaut ? There haven’t been any STAB TRIM Runaway memory items for years. We do have IAS DISAGREE and ALT DISAGREE. Should we do the UAS items first ?”

Boom !

Maybe you should get together with JDawg. He doesn’t know there are no “stick shaker memory items”. You could both invent some new procedures for Boeing. God knows, it couldn’t hurt.

love the peanut gallery.
LOL. Let me guess. You search through the manuals when you get a stick shaker right? Id suggest you go with your memory and recover from the stall. My point is with contradicting indications and warnings time is needed to sort the issue out. So if multiple memory items (and stick shaker/stall recovery) are needed you would hit the manuals for a bit right ?
welcome to Aviation. The western way. Be a pilot not a robot
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