MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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In the accident scenario, when MCAS was commanding the stab nose down the forces were increasing on the jackscrew mechanism. When the pilots were commanding nose up, the forces were decreasing. If the stab motor was going to stall under an aerodynamic load, it would have happened in the direction of increasing (MCAS), and not decreasing force (pilot trimming). On the contrary, MCAS had absolutely no problem using that one trim motor to drive against an ever increasing force until it hit the physical stops. There is no evidence that the motor was ever overtasked.
While I don't dispute your conclusion, I don't think that your description of forces on the horizontal stab are correct. I would agree that under normal circumstances the load would increase as you trim away from the previously trimmed airspeed, but that would assume no deflection of the elevator.
In this circumstance the pilots were applying substantial nose up elevator. This would tend to increase the force upwards on the front of the horizontal stab, and require the jackscrew to apply an opposite downward force to the front of the horizontal stab. If there were no screw and you applied a nose up elevator and the horizontal stab could slide around position at will, and you would likely see the stab go front up and by default AND which is the same direction the MCAS is driving the trim. The Alaskan Airllines MD80 accident even highlights this. Once the jackscrew let go the horizontal stab went full nose up with full after elevator (i recognize the MD80 uses tabs) and went beyond limits up creating a substantial nose down tendency of the aircraft which wasn't recoverable.
At the end of the day maybe it stalled out maybe it didn't, but from the FDR traces it certainly seems it was moving in the commanded direction.
From Alaskan Airlines Report
As the jam was overcome, the acme screw was being pulled upward through the acme nut by aerodynamic loads, causing upward movement of the horizontal stabilizer, resulting in greater airplane-nose-down motion. This upward pulling motion would have continued until the lower mechanical stop on the acme screw contacted the lower surface of the acme nut, preventing further upward motion of the horizontal stabilizer.
Release of the jam allowed the acme screw to pull up through the acme nut, causing the horizontal stabilizer leading edge to move upward, thus causing the airplane to pitch rapidly downward.
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I believe that you have this backwards. With sufficient positive elevator deflection, there is in fact a net downward force on the stab, resisting a return of the stab to a neutral position (hence the reason for the necessity of the roller coaster maneuvre described in the 737 original manual and taught to at least some pilots back in the day). Under such conditions, MCAS is assisted by aerodynamic forces, while pilot trimming is opposed by the same.
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As an engineer, not a businessman can anyone who knows about the contractual side of aircraft fleet purchases tell us whether the requirement to provide and pay for additional sim training for the ungrounded MAX be enough for existing customers to cancel orders without penalty? After all, one of the big selling points of the MAX was that further training wasn’t required for any current 737 pilot.
If so, could we see Boeing ending up having to foot the bill for that further training?
If so, could we see Boeing ending up having to foot the bill for that further training?
Salute!
Thank you Takwis
I was wondering the same thing since last November. Ditto for disabling the column switches if I really wanted to pull back hard so I could override HAL. You know, "I flew this thing for 30 years, and pulling back hard tiurned off the trims except the switches on my yoke"
My point is something lurks in the myriad of boxes, add-ons, etc that requires I turn off my manual trim switch power. I unnerstan that a roque "stuck" switch contact could be a problem, but if I have successfully beeped the switch over twenty times to get nose up trim, then I don't think that sucker is the problem.
Gums sends...
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Thank you Takwis
Why change from individual cutouts for manual and autoplot trim?
My point is something lurks in the myriad of boxes, add-ons, etc that requires I turn off my manual trim switch power. I unnerstan that a roque "stuck" switch contact could be a problem, but if I have successfully beeped the switch over twenty times to get nose up trim, then I don't think that sucker is the problem.
Gums sends...
.
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Question about trim motor speed
A couple of posters have mentioned both direction and speed as inputs to the trim motor. Is the motor speed really variable (I doubt it), or does the motor run at constant speed when activated (my bet)? If electric trim in the 737 is a variable speed system, then there must be another microprocessor or software routine somewhere that computes the PWM (pulse width modulation) pulse train.
YYZjim
A couple of posters have mentioned both direction and speed as inputs to the trim motor. Is the motor speed really variable (I doubt it), or does the motor run at constant speed when activated (my bet)? If electric trim in the 737 is a variable speed system, then there must be another microprocessor or software routine somewhere that computes the PWM (pulse width modulation) pulse train.
YYZjim
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Salute!
Thank you Takwis
I was wondering the same thing since last November. Ditto for disabling the column switches if I really wanted to pull back hard so I could override HAL. You know, "I flew this thing for 30 years, and pulling back hard tiurned off the trims except the switches on my yoke"
My point is something lurks in the myriad of boxes, add-ons, etc that requires I turn off my manual trim switch power. I unnerstan that a roque "stuck" switch contact could be a problem, but if I have successfully beeped the switch over twenty times to get nose up trim, then I don't think that sucker is the problem.
Gums sends...
.
Thank you Takwis
I was wondering the same thing since last November. Ditto for disabling the column switches if I really wanted to pull back hard so I could override HAL. You know, "I flew this thing for 30 years, and pulling back hard tiurned off the trims except the switches on my yoke"
My point is something lurks in the myriad of boxes, add-ons, etc that requires I turn off my manual trim switch power. I unnerstan that a roque "stuck" switch contact could be a problem, but if I have successfully beeped the switch over twenty times to get nose up trim, then I don't think that sucker is the problem.
Gums sends...
.
That isn't to say MCAS is a good design, but if the intended goal is what it was, then it has to bypass that cutout switch.
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Question about trim motor speed
A couple of posters have mentioned both direction and speed as inputs to the trim motor. Is the motor speed really variable (I doubt it), or does the motor run at constant speed when activated (my bet)? If electric trim in the 737 is a variable speed system, then there must be another microprocessor or software routine somewhere that computes the PWM (pulse width modulation) pulse train.
YYZjim
A couple of posters have mentioned both direction and speed as inputs to the trim motor. Is the motor speed really variable (I doubt it), or does the motor run at constant speed when activated (my bet)? If electric trim in the 737 is a variable speed system, then there must be another microprocessor or software routine somewhere that computes the PWM (pulse width modulation) pulse train.
YYZjim
Flaps down: both manual electric and autopilot trim at a "fast" speed.
By all reports the MCAS trims at the fast speed even when flaps are up (using the autopilot trim function and the STS system) while the pilot trims back to neutral (in a failure mode scenario) at the slow speed.
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Thanks for the clarification, ARealTimTuffy
Two-speed control is something that can be handled quite nicely using relays and a passive component or two. If the original 737 also trimmed using just two speeds, it was probably done that way, and the original hardware implementation might have survived all the way into the MAX.
YYZjim
Two-speed control is something that can be handled quite nicely using relays and a passive component or two. If the original 737 also trimmed using just two speeds, it was probably done that way, and the original hardware implementation might have survived all the way into the MAX.
YYZjim
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The single trim motor was introduced on the NG. Previously the 737 used separate motors for A/P trim and pilot operated trim.
About how exactly speed of the motor is controlled, earlier I guessed it may use different voltages, but I'm probably wrong. I don't have much experience with motors.
I did find a document describing the NG trim motor upgrade between the B version and the C version (the max uses the D version). From that document:
The D version on the max is probably an evolution of the C version, so many of those features could apply to it as well.
http://www.eaton.com/ecm/idcplg?IdcS...ame=PCT_201019
About how exactly speed of the motor is controlled, earlier I guessed it may use different voltages, but I'm probably wrong. I don't have much experience with motors.
I did find a document describing the NG trim motor upgrade between the B version and the C version (the max uses the D version). From that document:
Eaton’s new Model 6355C Stabilizer Trim Motor features: • Brushless three phase motor design • Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control • Dual current limit (torque) control circuits • Power up built-in test • Continuous fault monitoring • Fault storage (non-volatile memory) • RS-232 test/maintenance interface • Investment cast housing • Two stage spur gear train • Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train
http://www.eaton.com/ecm/idcplg?IdcS...ame=PCT_201019
As an engineer, not a businessman can anyone who knows about the contractual side of aircraft fleet purchases tell us whether the requirement to provide and pay for additional sim training for the ungrounded MAX be enough for existing customers to cancel orders without penalty? After all, one of the big selling points of the MAX was that further training wasn’t required for any current 737 pilot.
If so, could we see Boeing ending up having to foot the bill for that further training?
If so, could we see Boeing ending up having to foot the bill for that further training?
The schedule was to build 60 737 MAXs a month, at near $100mm a copy. Current deliveries are zero and no resumption is as yet scheduled. So the locked in revenue shortfall is already tens of billions, ignoring the reputational impact and the competitive humiliation.
Any pilot training expense incurred will be petty change.
Hopefully this punch in the face from reality will wake up Boeing senior management, which seems to have lost sight of the priority need for quality, even ahead of low cost.
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motor overload ANU
In the accident scenario, when MCAS was commanding the stab nose down the forces were increasing on the jackscrew mechanism. When the pilots were commanding nose up, the forces were decreasing. If the stab motor was going to stall under an aerodynamic load, it would have happened in the direction of increasing (MCAS), and not decreasing force (pilot trimming). On the contrary, MCAS had absolutely no problem using that one trim motor to drive against an ever increasing force until it hit the physical stops. There is no evidence that the motor was ever overtasked.
The actual aerodynamic loads on the horizontal stabilizer in the configuration both accident aircraft were in, during the last minutes, would have strongly opposed ANU movements and strongly assisted AND movements. Excerbated further by significant NU elevator deflection.
The 'roller coaster' recovery technique discussed earlier sheds further light on this, as will a quick diagram of forces, if helpful.
KR
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Oops.
The 737 stabilizer trim motor is much more sophisticated than I had envisioned. I've attached a brochure from Eaton's plant in India. It's a three-phase motor, with an internal processor that monitors commutation and speed. It is a fully digital unit. Fault codes are stored in a non-volatile memory, and can be extracted using RS-232. It lools like there are two 9-pin plugs for signals. There is no way this motor and control system was used on the original 737s.
YYZjim
PS: It look like the manufacturer's testing has identified solder joints on the memory chip as the most likely point of failure, but that does not mean there have been any failures in use.
The 737 stabilizer trim motor is much more sophisticated than I had envisioned. I've attached a brochure from Eaton's plant in India. It's a three-phase motor, with an internal processor that monitors commutation and speed. It is a fully digital unit. Fault codes are stored in a non-volatile memory, and can be extracted using RS-232. It lools like there are two 9-pin plugs for signals. There is no way this motor and control system was used on the original 737s.
YYZjim
PS: It look like the manufacturer's testing has identified solder joints on the memory chip as the most likely point of failure, but that does not mean there have been any failures in use.
Last edited by YYZjim; 1st Jul 2019 at 01:15. Reason: I see MemberBerry already noted this.
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tackle the ball, not the man
The trim rim inputs are recorded separate from the THS movement. In both flights there were longer periods of manual inputs resulting in longer movements of the THS. In both flights towards the end there were short manual inputs followed by small THS movement. In both flights following manual inputs there was MCAS trimming AND. At no point in the FDR readout was there a prolonged manual input followed by no movement of the THS (the real indicator of an overpowered trim motor).
The balance of probability is there is an absence of knowledge on your part, and an overwhelming amount of long words to compensate for that.
The balance of probability is there is an absence of knowledge on your part, and an overwhelming amount of long words to compensate for that.
But I do thank you for the ad hominem :-)
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Yesterday afternoon I was stuck in traffic near Everett so I pulled off to the marina to wait it out. At exactly 4:30 PM, Isaw what I think was a 737 MAX not in livery (green primer or whatever) flying at a much more vertical attitude than I have seen any plane accomplish anywhere other than an airshow. It kept that attitude long enough for me to pull out my phone and get some video, although since it ended up flying over the top of my head the video does not do a good job of representing the angle of the plane relative to the ground.
So the good news to me is that at least Boeing is doing some real world testing. Tragically far too late, but real world and not some jumped up playstation. If there is interest and I can figure out how I will post the video but there is nothing really spectacular in it, perhaps an expert could spot something interesting. Basically the plane was flying level, pulled up in to a very steep (absurdly steep) flight angle, held for awhile, and leveled out. I got some of the steep angle and the overhead flight.
I would almost guess that they had pulled it into a stall (intentionally) but I am in no way qualified to judge that. Shortly after an unmarked white helicopter with video equipment flew past, may have been news for the traffic jam (although that is not news in Everett!) or perhaps Boeing recording the flight.
Shortly after
So the good news to me is that at least Boeing is doing some real world testing. Tragically far too late, but real world and not some jumped up playstation. If there is interest and I can figure out how I will post the video but there is nothing really spectacular in it, perhaps an expert could spot something interesting. Basically the plane was flying level, pulled up in to a very steep (absurdly steep) flight angle, held for awhile, and leveled out. I got some of the steep angle and the overhead flight.
I would almost guess that they had pulled it into a stall (intentionally) but I am in no way qualified to judge that. Shortly after an unmarked white helicopter with video equipment flew past, may have been news for the traffic jam (although that is not news in Everett!) or perhaps Boeing recording the flight.
Shortly after
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occam's razor
Certainly worthy of further consideration IMHO.
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Returning to the "They are among us" theme...
I was browsing another thread on the 787 and shall we say, ahem, issues with its production when I stumbled across the following.
[Quote]
Ironically, just a couple of days after those claims of "lack of evidence", evidence came out that it may have indeed been possible, since such a condition was discovered in a simulator test performed by FAA pilots.
Let's be fair here. The "evidence" you cite involved upgraded software/firmware that had not yet been certified for flight, much less installed on the accident aircraft, during a test that intentionally created a malfunction that was unrelated to MCAS. Until there are further details to the specifics of the test and the particular failure mode, there still is no evidence that there were any problems with the Main Electric Trim system on the accident aircraft. As you note, after many months of perhaps one of the most intense investigations in modern aviation history, not a single authority has suggested such a problem or called for any redesign work associated with this system.[Quote]
Question 1: The preceding post was written by:
a) Boeing's Lawyer
b) An employee of Boeing with three kids to support
c) A Boeing corporate manager
d) A disinterested third party
e) Brittney Spears
Oh man, just hiccuped with beer- headed to an early demise-
dce
I was browsing another thread on the 787 and shall we say, ahem, issues with its production when I stumbled across the following.
[Quote]
Ironically, just a couple of days after those claims of "lack of evidence", evidence came out that it may have indeed been possible, since such a condition was discovered in a simulator test performed by FAA pilots.
Let's be fair here. The "evidence" you cite involved upgraded software/firmware that had not yet been certified for flight, much less installed on the accident aircraft, during a test that intentionally created a malfunction that was unrelated to MCAS. Until there are further details to the specifics of the test and the particular failure mode, there still is no evidence that there were any problems with the Main Electric Trim system on the accident aircraft. As you note, after many months of perhaps one of the most intense investigations in modern aviation history, not a single authority has suggested such a problem or called for any redesign work associated with this system.[Quote]
Question 1: The preceding post was written by:
a) Boeing's Lawyer
b) An employee of Boeing with three kids to support
c) A Boeing corporate manager
d) A disinterested third party
e) Brittney Spears
Oh man, just hiccuped with beer- headed to an early demise-
dce
But your proposition requires that
a) the trim motor was stalled due to aerodynamic forces; and
b) simultaneously there was an unrelated software/processor issue that prevented the pilots' trim inputs from being recorded on the FDR
As opposed to the stab not moving simply because the pilots hadn't commanded it to.
Occam again ...
True, there is very little that we can be absolutely sure about.
But your proposition requires that
a) the trim motor was stalled due to aerodynamic forces; and
b) simultaneously there was an unrelated software/processor issue that prevented the pilots' trim inputs from being recorded on the FDR
As opposed to the stab not moving simply because the pilots hadn't commanded it to.
Occam again ...
But your proposition requires that
a) the trim motor was stalled due to aerodynamic forces; and
b) simultaneously there was an unrelated software/processor issue that prevented the pilots' trim inputs from being recorded on the FDR
As opposed to the stab not moving simply because the pilots hadn't commanded it to.
Occam again ...
For this reason full stab trim exercising in pre flight checks has in the past on another aircraft type been discontinued. Subsequently only a short stab run in each direction was practised.