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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 16th Jul 2019, 00:16
  #1401 (permalink)  
 
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A properly designed aircraft shouldn’t require all these aerodynamic add ons, get it right in the first place and you won’t need to go sticking pieces of metal everywhere.
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Old 16th Jul 2019, 02:05
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Originally Posted by SamYeager View Post
As far as MCAS is concerned my gut feeling is that Boeing will stick with a software solution supplemented as necessary by training. However should they go down the route of aero changes I'm curious how much that might affect the claimed fuel savings for the MAX. Any ideas?
Micro VGs on the outboard slats would add a slight drag rise in the cruise, but that could also be offset if acting on the cross flow instabilities near the winglet-wing transition. A couple of biz jets have found benefit in that area.

Strake mods could go either way as well, Reducing or removal of the strakes would adversely affect TODR, but may actually improve cruise drag, if strut-nacelle-wing interference is worked on near the rear of the nacelle.

Tabs and wedges can improve cruise drag, however as the use of those would be to alter Cm at high AOA, the residual effect at low AOA would add to a cruise drag penalty from trim drag. That is able to be offset in part or fully by the flow improvements that occur from the transonic effects of the device itself.

Aft strakes have been used successfully on numerous aircraft, the Lear is the poster child for those. Cruise drag can be improved by their use dependent on design. They alter both directional and longitudinal stability. As a modern device, they are as recent as the DH-82 Tigermoth, would look right at home on a canary yellow B737 with roundels.

Bottom line is, make lemonade from lemons.



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Old 16th Jul 2019, 04:29
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Originally Posted by Takwis View Post
It's best to get it as right as possible without the aerodynamic band aids, of course. But I'd stake my life on metal bits over lines of code any day.
And then you'd only be left with the manual trim wheel issue.....
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Old 16th Jul 2019, 04:57
  #1404 (permalink)  
 
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Ryanair 737 8200

Looks like Ryanair is shying off the "MAX" name, wonder about others...

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Old 16th Jul 2019, 06:24
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Originally Posted by john_tullamarine View Post
Is not any longitudinal SAS a pitch feel modifier ? ..
Yes but that's a side effect, not necessarily beneficent, especially if the SAS is installed to deal with dynamic (oscillatory) stability issues.

Originally Posted by john_tullamarine View Post
inadequate pitch forces being the reason for SAS in the first place ?
Not the reason. A symptom. Now, what could be the underlying cause for either manual or powered-cum-synthetic-feel pitch controls to go flabby?

Originally Posted by john_tullamarine View Post
I concur with the inference that the anti-stall call was a media mis-hype mistake.
I'm actually inferring that there is good chance the media got it right this time
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Old 16th Jul 2019, 09:28
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Originally Posted by krismiler View Post
A properly designed aircraft shouldn’t require all these aerodynamic add ons, get it right in the first place and you won’t need to go sticking pieces of metal everywhere.
It is a very rare airplane that doesn't need some fixes here or there. VG, vanes, strakes, fences, leading edge "dog tooth" or droop are very common to provide adequate behavior within the whole flight envelope.
For reasons of their own, Boeing chose the quick-and-dirty software patch way.

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Old 16th Jul 2019, 11:06
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I presume that all work done on an aerodynamic solution during development of the MAX, was tempered by a constant consideration of whether or not a new certification would be required, which would certainly hamper a full and frank investigation of all options. Add to that, potential negative effects on fuel economy, and you can start to see why Boeing was strongly drawn to MCAS as a solution.

As far as the trim wheel issue is concerned, this will probably need to be resolved by added redundancy in any ‘powered trim’ system. Retrofitting larger diameter trim wheels or placing a minimum strength requirement on flight crew are both non-starters.
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Old 16th Jul 2019, 11:09
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Originally Posted by Speed of Sound View Post

As far as the trim wheel issue is concerned, this will probably need to be resolved by added redundancy in any ‘powered trim’ system. Retrofitting larger diameter trim wheels or placing a minimum strength requirement on flight crew are both non-starters.
I suspect you are right, though note that this solution will need to be applied to the NG also, and that all the checklists that require use of the trim wheel will need to be rewritten and training provided.
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Old 16th Jul 2019, 11:29
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This. Exactly this. Now that Boeing has publicly stated that they won't go the aerodynamic fix route, they have dug themselves a deep hole.
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Old 16th Jul 2019, 11:30
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Originally Posted by Maninthebar View Post
I suspect you are right, though note that this solution will need to be applied to the NG also, and that all the checklists that require use of the trim wheel will need to be rewritten and training provided.
Rewriting two checklists and half an hour in the simulator will be nothing compared to installing 7,000+ trim servos to the existing NG fleet. I presume that this will be phased in and mandated to be completed by a set date rather than seeing the entire NG fleet grounded which would be political and economic dynamite in the US..
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Old 16th Jul 2019, 12:19
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Originally Posted by Speed of Sound View Post


Rewriting two checklists and half an hour in the simulator will be nothing compared to installing 7,000+ trim servos to the existing NG fleet. I presume that this will be phased in and mandated to be completed by a set date rather than seeing the entire NG fleet grounded which would be political and economic dynamite in the US..
Interesting to note that in previous posts on many of the MAX threads - The B737 manual trim wheel was "marginal" on the classics. They said stab trim was a issue back then and basically never let it get away or your in trouble.

So Boeing have known about it for along time, but chose never to fix it but make it more "marginal" to control on the NG & MAX.
30 years+ to fix, or ground all for 2 years? Guess that's a management call.

Personally ground them if the wheel can not be turned (and Boeing knew) within the envelope.
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Old 16th Jul 2019, 13:07
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Ryanair will stop flying to some airports because of the Boeing 737 Max crisis

London (CNN Business)Ryanair is taking the knife to its operations in Europe as it prepares for two years of disruption caused by Boeing's 737 Max debacle.
Europe's biggest low-cost airline said Tuesday that it was planning to cut back operations at some airports and abandon others entirely because regulators may not return the grounded 737 Max to service until December."We are starting a series of discussions with our airports to determine which of Ryanair's underperforming or loss making bases should suffer these short term cuts and/or closures from November 2019," it said in a statement.

More
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Old 16th Jul 2019, 13:07
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Originally Posted by Bend alot View Post
.
Personally ground them if the wheel can not be turned (and Boeing knew) within the envelope.
This isn’t simply a matter of physical strength of an individual pilot.If a scenario exists where both flight crew members need to have a hand on each wheel/wheel handle to re-trim the aircraft, this will seriously degrade cockpit resources at a time of already high workload.
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Old 17th Jul 2019, 01:10
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Originally Posted by Speed of Sound View Post


Rewriting two checklists and half an hour in the simulator will be nothing compared to installing 7,000+ trim servos to the existing NG fleet. I presume that this will be phased in and mandated to be completed by a set date rather than seeing the entire NG fleet grounded which would be political and economic dynamite in the US..
For the NG there is probably an out by revising the checklist to re-incorporate the use of 2 (auto and all) cutout switches.

Can probably make the case that non auto instigated trim runaway is very unlikely (common english, not attempting to use certification levels) so having manual electric trim available would suffice.
The service history would also support this.

For MAX this would also require modifying the cutout switches to revert to NG functionality. Although a HW mod it would be trivial compared to adding extra power assist or even mechanical tweaks (foldout ratchets etc) to wheel.

Have to say I do like the image of the battery operated drills coming to the rescue
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Old 17th Jul 2019, 02:09
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Originally Posted by MurphyWasRight View Post

Have to say I do like the image of the battery operated drills coming to the rescue
So do some of us ;-)
For some reasons the relevant posts were deleted.

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Old 17th Jul 2019, 17:03
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If the fix goes beyond software, which looks increasingly likely, they also have to explain why the.NG (same stabiliser, same trim wheel) is OK to very sceptical regulators.....
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Old 17th Jul 2019, 22:10
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Originally Posted by SLF3 View Post
If the fix goes beyond software, which looks increasingly likely, they also have to explain why the.NG (same stabiliser, same trim wheel) is OK to very sceptical regulators.....
There was some consensus on here a couple of months ago that any ‘ungrounding’ would be co-ordinated between major regulators. This was before the difficulties with the trim wheel operation and the microprocessor overload issues came to light. I still believe that some form of joint ungrounding will take place but will be after some interesting discussions which will inevitably prolong the process.
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Old 17th Jul 2019, 23:49
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Originally Posted by Speed of Sound View Post
I still believe that some form of joint ungrounding will take place but will be after some interesting discussions which will inevitably prolong the process.
Maybe it is not the discussion that will prolong the process, but rather fixing the issues raised by "other" agencies.
Bad habits die hard, and the FAA might be tempted to overlook some of them, for the sake of national industry.
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Old 18th Jul 2019, 08:49
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Originally Posted by MurphyWasRight View Post
For the NG ....
Can probably make the case that non auto instigated trim runaway is very unlikely (common english, not attempting to use certification levels) so having manual electric trim available would suffice.
No doubt that the operational statistics on the NG prove that this is very unlikely.
My concern is that the system have over achieved in the sense that the low runaway rate is not a result of the Design Assurance Level (DAL C instead of A), but undocumented good engineering :-) This can have unexpected consequences that a minor design update (e.g. the trim motor drive electronics, or implementing MCAS) can change the statistics drastically, despite being within specified limits.
The risk of a trim runaway to it’s stops, is the combined risk of trim system falling, and the pilots falling to react correctly in time.

CS 25 paragraphs 671/672 does not include the possibility of including the pilots reaction time into the equation.
Either the system by design have a low failure rate.
Or the pilots must be able to handle a full runaway without requiring exceptional piloting skill or strength.
It does however not exclude the solution of flipping switches to engage a standby/alternate system.
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Old 18th Jul 2019, 08:54
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The problem is not so much how often it has been required but that it is used as a mitigation in the hierarchy of risk for the airframe.

If it has been demonstrated that the device cannot be used to perform the risk control as stated then EITHER the risk has to be entirely reassessed, another control put in or the trim wheel re-engineered.

If the risk assessment for the NG (and MAX) is unchanged from previous versions for the elements of flight for which the manual trim wheel is a control then the assessments for these will also have to be redone or another control put in.

Either way a big big task for Boeing and regulatory authorities.

Last edited by Maninthebar; 18th Jul 2019 at 10:30.
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