I don't follow that. Sure, if one sensor goes back, it's detected, and MCAS is disabled. But that's my point. If MCAS is needed for airworthiness, how big a deal is it when it goes inop in flight? If I was an xAA, it's a question I'd be asking.

Some quick mental arithmetic says the MAX fleet had accumulated ~1 million flight hours when grounded (within a factor of 10). So AoA sensor failures are happening every 500K flight hours (actually more since sometimes it's the inactive one and nobody notices). When there are 5000 aircraft out there, that's a failure every 20 days or so. If MCAS is necessary... what happens when an aircraft has a failure and then, say as a result of an RA, enters the condition MCAS is supposed to handle? And it's one of these low-time minimally-trained crews that are the reality of airlines today - maybe the resurrection of the PF on AF447?

As has been said many times, there are going to be a lot of red faces if there is another hull loss because of MCAS, or the underlying aerodynamic problems which prompted it. I would sure want to be careful not to be one of them.

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

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.

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...



.

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

The disabling of the cutout switches is relatively straight forward to understand. They only don't cutout during MCAS activation. They will cutout during STS. But it makes sense (given what they were trying to accomplish) to not have the MCAS cutout by the internal column switch. If that switch did in fact also cutout the MCAS, the MCAS wouldn't be able to do what it was designed to do. It needs to be able to trim Nose Down despite further back pressure. That is how it works. If it did cutout MCAS wouldn't do anything.

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.

Not sure how it is accomplished, whether variable voltage via resistors or software processing or something, but the trim does indeed function at 2 different speeds. Flaps up: both manual electric and autopilot trim at a "slow" speed
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.

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

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

At this point, I think Boeing would be delighted to put this debacle behind it at any price, even if it costs multiple billions.to resolve.
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.

motor overload ANU
 

I admire your persistency in refusing to even remotely consider it a possibility that the motor could have been overloaded towards the very end of the accident sequences, whilst attempting to move the stab ANU, I am afraid that you have (perhaps inadvertenly) presented an entirely incorrect/reversed description above.

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

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.

tackle the ball, not the man
 

With respect, we do not know with any certainty that there was no manual ANU input without corresponding stab movement. I am merely highlighting that known evidence at this time suggests otherwise.

But I do thank you for the ad hominem :-)

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

occam's razor
 

You may well be correct, time will tell, however surely the simplest of all explanations is that during the finals stages both crews were unable to trim ANU. It fits all known evidence at this time. And the simplest reason for that is due to overwhelming forces acting in the opposite direction.

Certainly worthy of further consideration IMHO.

The Lion Air flight preliminary report clearly shown stabiliser ANU trim movements that appear to correspond to ANU trim switch operations. It seems unlikely that the motor was unable to move the stabiliser. Each ANU pulse results in ANU movement.

https://cimg7.ibsrv.net/gimg/pprune....9102c162c4.png

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

Carefully disclosing what is in the public already, e.g. wiring diagrams on Lemme's site, but not one yota more.

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 ...