MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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.
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.
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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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.
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.
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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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.
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.
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.
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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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.
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.
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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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.
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.
Psychophysiological entity
For my understanding, the original MCAS design would stop and return the stab to the original position
I have never been clear about the software logic despite seeing the plots of what took place.
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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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Quote:
For my understanding, the original MCAS design would stop and return the stab to the original position
For my understanding, the original MCAS design would stop and return the stab to the original position
In the broken AOA case it would never unwind since the unwind is triggered by AOA returning to normal.
Psychophysiological entity
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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Amazing that nobody thought of that.
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This is correct.
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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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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.
To improve the altimeter accuracy for the vertical ANP of <125' required for RNP approaches? That's my best guess.