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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 1st Nov 2019, 12:55
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PeterH,

I agree with your genuine question and have raised it before on these 737MAX threads, but with no concrete answers. It is not only about stall identification but also about altering the stick forces prior to the stall, which I would have thought would have been appropriate for the MAX.
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Old 1st Nov 2019, 14:49
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Originally Posted by PEI_3721
A technical point which I may have missed: with the stick full back, MCAS remained active, but the main electric trim is disabled by the lower stick cutout switch. In this condition would any pilot stick-top trim switch have any effect in moving the trim?.
The column cutout stops trim opposing the stick. If pulling up the cutout does not prevent ANU trim.
MCAS trims opposite the column (to make it harder to continue pulling up) which is why it had to bypass the column cutout.
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Old 1st Nov 2019, 15:42
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Originally Posted by ST Dog
The column cutout stops trim opposing the stick. If pulling up the cutout does not prevent ANU trim.
MCAS trims opposite the column (to make it harder to continue pulling up) which is why it had to bypass the column cutout.
Use the H-stab to modify elevator feel. Brilliant.

Edit: Just to be clear, doing what MCAS does is entirely different from ordinary trimming to relieve stick force.

Last edited by OldnGrounded; 1st Nov 2019 at 16:30.
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Old 1st Nov 2019, 16:28
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Peter H, ‘MCAS was used because it was already there’, and if implemented appropriately it would be a non issue - except.

Considering the CS definitions of stall warning (shake ~ 1.1Vs) and stall ident (push ~ Vs), and assuming that the Max non-MCAS approach to the stall characteristics were adequate, then for any nose-up tendency >1.1Vs the need would be for more stick force (trim down) and should be considered as a stability issue - a smooth change with Alpha, opposed to a step changes from a nudge, i.e. use MCAS.

Alternatively if the <1.1Vs non-MCAS pre stall characteristics were deficient (most likely), the use of the stability improving MCAS as a stall warning would be unusual; convention uses a shaker. I would disagree with Satcom’s views because of the differences between a step nudge and a smooth stability change, and the need to argue equivalence with a shaker.

However the objective was to mimic NG characteristics without a shaker, and the driver for all of the above was to avoid changes / training for the Max vice NG by using a background mechanism, not overtly visible to the crew (and thus not necessary to describe it).

As for cost, even in hindsight, who can tell. Avoiding the potential cost of training - Southwest, $1m / aircraft, could have provided an elegant alternative solution, but there were other drivers - timescale, competition.

Yet the concept of MCAS was an ‘elegant’ solution; the implementation was appalling, and it appears that the latter has to be improved to achieve recertification.

Last edited by safetypee; 1st Nov 2019 at 17:48.
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Old 1st Nov 2019, 17:46
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Originally Posted by Bergerie1
spronrad,

I think you will find that D P Davies required the stick nudger on the B707-320s registered in the UK (page 262, Handling the Big Jets). He also required a stick nudger on the 747s registered in the UK - it worked a treat.

I always wondered why Boeing did not fit one to the 737MAX.
After listening to all of his 4 part audio interviews he was critical of the FAA for certifying the 07. He said many of the manufacturers would complain about it costing millions, take years, etc to correct the deficiencies he noted. The FAA had already certified the 07 but he wouldn’t budge. His answer was “that’s rubbish” and sure enough they would be back in a short time with the fix and in the nudger case it was fast and cheap. I wish Mr Davies was involved in the Max because he seemed like a no nonsense guy. He probably would have hammered Boeing. He implied the FAA has been capitulating to Boeing for decades.



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Old 1st Nov 2019, 18:07
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Drc40,

I agree, I flew with DPD on several occasions and, indeed, he was a no-nonsense guy. Many airline pilots owe him a huge debt of gratitude.
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Old 1st Nov 2019, 18:52
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Originally Posted by Bergerie1
spronrad,

I think you will find that D P Davies required the stick nudger on the B707-320s registered in the UK (page 262, Handling the Big Jets). He also required a stick nudger on the 747s registered in the UK - it worked a treat.

I always wondered why Boeing did not fit one to the 737MAX.
Merci beaucoup!
From the Leeham article:In his book Handling the Big Jets (first published in 1967) D. P. Davies, formally head of the flight test department of the UK Air Registration Board (ARB), now the CAA, describes at length the different stalling characteristics exhibited by aircraft with different wing and tail configurations. He personally test-flew every new jet aircraft model that was destined to fly with UK operators, such as BOAC and BEA. Davies also describes the various control systems that were incorporated on both British and American aircraft that reduced the possibility of a line pilot entering a stall. Details of flying characteristics and the use of automated control systems at high angles of attack are also included.
Davies is best known for mandating the fin of the Boeing 707 be lengthened by approximately 36 inches in order to achieve published Vmcg speeds (minimum control speed on the ground). Boeing ultimately accepted Davies recommendation and, we understand, halted production of the 707 for 10-11 months, whilst a new fin was designed and installed. All existing 707s were then retrospectively modified. A ventral fin was also installed on some variants to prevent ‘fully stalled take-offs’ which the Comet was also susceptible to.
Whilst test flying the 707-400 series at Renton, Davies noted that with the first stage of flap selected, the aircraft had a tendency to pitch up just prior to entering the stall. This was determined to be caused by the inboard leading edge devices, peculiar only to the 707-3/400 series which, when extended, effectively retained lift to higher angles of attack, and moved the Center of Pressure forwards, causing the nose to pitch up. Davies was not comfortable with this pitch up tendency and insisted that all UK certified 707 series aircraft were modified with the installation of a ‘stick nudger’ system.
In his book Davies stated that “The stick nudger introduces a small force into the elevator circuit which imposes positive stick free stability and removed the otherwise self-stalling tendency”. He goes onto to say that “as its input is so small all the runaway cases are completely innocuous.”
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Old 1st Nov 2019, 21:45
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Originally Posted by OldnGrounded
Use the H-stab to modify elevator feel. Brilliant.

Edit: Just to be clear, doing what MCAS does is entirely different from ordinary trimming to relieve stick force.
I am not disagreeing however I feel it might be clearer to state that what MCAS does is exactly opposite of ordinary trimming to relieve stick force. It 'trims' to add stick force.


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Old 1st Nov 2019, 21:59
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Originally Posted by jimjim1
I am not disagreeing however I feel it might be clearer to state that what MCAS does is exactly opposite of ordinary trimming to relieve stick force. It 'trims' to add stick force.
Yes, better description.
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Old 1st Nov 2019, 22:25
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Originally Posted by Bergerie1
spronrad,

I think you will find that D P Davies required the stick nudger on the B707-320s registered in the UK (page 262, Handling the Big Jets). He also required a stick nudger on the 747s registered in the UK - it worked a treat.

I always wondered why Boeing did not fit one to the 737MAX.
As the MAX was based on "old type" 737 certification, the addition of a stick nudger/pusher probably would have resulted in a "new type" certificate, which was why the MAX was meant to be a "common type rating" such as the B747-8 vs B747-400.
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Old 2nd Nov 2019, 00:58
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For my understanding, the original MCAS design would stop and return the stab to the original position
I remember this clearly from the early days of these pprune discussions. It seemed to fade away, as did the completely opposite 'redatumization' after each AND run.

I have never been clear about the software logic despite seeing the plots of what took place.
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Old 2nd Nov 2019, 01:21
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Originally Posted by 568
As the MAX was based on "old type" 737 certification, the addition of a stick nudger/pusher probably would have resulted in a "new type" certificate, which was why the MAX was meant to be a "common type rating" such as the B747-8 vs B747-400.
But the earlier 707-100s and -200s had no nudger/pusher, and yet it is a common type rating. I am certified to fly a 707-400, though I have never seen one. And also a 720 (which I have at least seen a few of), in the same type rating.

Last edited by Takwis; 2nd Nov 2019 at 01:34.
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Old 2nd Nov 2019, 01:47
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Quote:
For my understanding, the original MCAS design would stop and return the stab to the original position
Originally Posted by Loose rivets
I remember this clearly from the early days of these pprune discussions. It seemed to fade away, as did the completely opposite 'redatumization' after each AND run.

I have never been clear about the software logic despite seeing the plots of what took place.
What I read: A "normal' MCAS activation would unwind the applied trim when AOA returned to within range - unless it was interrupted by pilot input.
In the broken AOA case it would never unwind since the unwind is triggered by AOA returning to normal.



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Old 2nd Nov 2019, 02:28
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That sounds familiar. It spells out all too clearly just how potentially 'catastrophic' that single failure can be, inasmuch as it lets the AND inputs sum from the last stopping point.
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Old 2nd Nov 2019, 02:29
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Originally Posted by MurphyWasRight
What I read: A "normal' MCAS activation would unwind the applied trim when AOA returned to within range - unless it was interrupted by pilot input.
In the broken AOA case it would never unwind since the unwind is triggered by AOA returning to normal.
Amazing that nobody thought of that.
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Old 2nd Nov 2019, 02:38
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Originally Posted by MurphyWasRight
What I read: A "normal' MCAS activation would unwind the applied trim when AOA returned to within range - unless it was interrupted by pilot input.
In the broken AOA case it would never unwind since the unwind is triggered by AOA returning to normal.
This is correct.
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Old 2nd Nov 2019, 02:43
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To add: From the Lionair accident report. 1.6.5.4

The magnitude of the AND command was based on the AOA and the Mach. After the non-normal maneuver that resulted in the high AOA, and once the AOA fell below a reset threshold, MCAS would move the stabilizer ANU to the original position and reset the system.
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Old 2nd Nov 2019, 03:29
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AoA vane

A fundamental question: why was an Angle of Attack vane fitted to the NG at all?

To improve the altimeter accuracy for the vertical ANP of <125' required for RNP approaches? That's my best guess.

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Old 2nd Nov 2019, 03:43
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Stall warning.
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Old 2nd Nov 2019, 03:59
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Same as the 737-100
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