737MAX Stab Trim architecture

[IFixPlanes]

hans brinker
Yes, English is not my first language.
Try to read my posting in correlation to:

Originally Posted by GordonR_Cape

...Apparently turning off power to the aircraft always resets MCAS to the captain's side AOA. The next flight carried out would then alternate the AOA side as you describe. Obviously during maintenance to replace the AOA sensor the technicians would have to turn off the power. This reset the AOA to the captain's side again...

BTW: what on my posting is not correct?

[GordonR_Cape]

Originally Posted by IFixPlanes

hans brinker
Yes, English is not my first language.
Try to read my posting in correlation to:


BTW: what on my posting is not correct?

Your post is correct in almost every respect. The difference hinges about the meaning of 'reset', which is not actually important in the context of this discussion. I did not go into details about the FCCs, since I was responding to a specific question by Arydberg .

As far as the pilots were concerned, they had no control over MCAS, and both they and the Lion Air maintenance technicians had no idea it existed.

[hans brinker]

Originally Posted by IFixPlanes

hans brinker
Yes, English is not my first language.
Try to read my posting in correlation to:


BTW: what on my posting is not correct?

Originally Posted by IFixPlanes

MCAS ...

If the value of an AOA improves, the FCCs stop their MCAS signal - t...

It is my understanding that if the value improves MCAS will take out the trim input it put in (if MCAS is working correctly, not in the broken AOA scenario)


As far as your post being a reply to Gordon, what he said is correct. If the power is turned off and than turned on again MCAS will always use AOA 1. It is definitely correct English to call that a reset.

[Loose rivets]

Probably best to define that as a Reset upon power up, or some such. The reset that has caused so much of the problem, MAY have been the fact that MCAS resets, and re-datumises following a function. The ratchet effect could be how the screw-jacks were at full travel.

[megan]

MCAS has a long history in Boeing. Was first proposed on the 767 to fix issues, but vortex generators came up trumps. Both the KC-767 and KC-46 have MCAS, the USAF is currently reviewing whether the -46 MCAS has hidden gotchas.

[GordonR_Cape]

Originally Posted by Takwis

They are, and they may find some, but the KC-46 still allows the control column cut-out switches to override MCAS inputs, by pulling back on the yoke. I would sure like to see that back on the MAX. Yes, the pilots could override the (what are we calling it?) stall avoider...which would be more appropriate than letting it drive the plane into the ground when it is activated erroneously.

The certification requirements for military aircraft are very different from passenger aircraft. MCAS may have been acceptable for the KC-46 tanker, but not for the closely related B767 passenger aircraft, which used aerodynamic fixes instead.

[Loose rivets]

I found this from 2016, of some interest, and indeed a link to a discussion about the cut out switches on the centre pedestal. It mentions the MAX but shows the right switch with a mention of the Auto Pilot. Misinformation abounds.

Both on R&N but this one has some historic discussion with gums et al.

https://www.pprune.org/archive/index.php/t-576817.html

https://aviation.stackexchange.com/q...-on-the-boeing

[infrequentflyer789]

Originally Posted by Loose rivets

Both on R&N but this one has some historic discussion with gums et al.

https://www.pprune.org/archive/index.php/t-576817.html

Thanks for the pointer. This bit jumped out for me:

Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.

So, that process would be:

1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;

Then repeat. Blip, relax, assess. Blip, relax, assess.

But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.

But it seems that trimming a large, THS aircraft that way can be fatal.

"Blip, relax, assess. Blip, relax, assess."

One wonders what the result of that trim technique would be with MCAS in play...

[737 Driver]

Greetings all!

New guy here, first post.

I'm still trying to get my head around why neither of the MAX accident crews simply applied sufficient nose up trim to neutralize the MCAS input. It could be an "airmanship" issue, but is there another possibility? Does anyone know if the 737 stab trim motor can actually stall in a high load environment? Is there an internal circuit breaker or thermal relief? It's a big electric motor, so there's bound to be some kind of protection if the stab was truly jammed.

Related question: The 737 has a single stab trim motor that operates at two speeds. The yoke trim switches actuate the trim motor in the high-speed mode. The MCAS (I believe) uses the low-speed mode. Is there a difference between the available torque in high-speed vs low-speed modes? For example, I have a two-speed electric drill that can stall out in the high-speed setting, but will continue to operate at low-speed. If the stab motor suffers from a similar phenomenon, I could see how the MCAS could function while the pilot-commanded trim would not.

[Dave Therhino]

Originally Posted by 737 Driver

Related question: The 737 has a single stab trim motor that operates at two speeds. The yoke trim switches actuate the trim motor in the high-speed mode. The MCAS (I believe) uses the low-speed mode. Is there a difference between the available torque in high-speed vs low-speed modes? For example, I have a two-speed electric drill that can stall out in the high-speed setting, but will continue to operate at low-speed. If the stab motor suffers from a similar phenomenon, I could see how the MCAS could function while the pilot-commanded trim would not.

You have this backwards. My understanding is that manually controlled electric trim operates at high speed with flaps down and at low speed with flaps up. MCAS operates the trim at high speed.

Your drill (if it's like my battery powered Makita and my corded Milwaukee) actually has a gear ratio change when you go from low to high range. I don't think the stab motor speed difference is achieved that way, and instead is done electrically in some manner.

[DaveReidUK]

Originally Posted by Dave Therhino

You have this backwards. My understanding is that manually controlled electric trim operates at high speed with flaps down and at low speed with flaps up. MCAS operates the trim at high speed.

Do you have a reference for that ?

On the NG (which of course doesn't have MCAS), low-speed yoke trim (i.e. with flaps up) is the same rate as high-speed A/P trim. High-speed yoke trim is 3x that rate and low-speed A/P trim is half the rate of high-speed A/P trim. I would have thought that MCAS would trim at the A/P rate, but I haven't seen that documented anywhere.

There's no gearbox in the stab trim motor.

Good description here: B737NG Flight Controls

[DaveReidUK]

Originally Posted by 737 Driver

The MCAS trim speed (.27 degrees per second I believe) was set specifically for this function alone. This is faster than the original design spec, but for some reason it was increased before final release.

Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?

[737 Driver]

Originally Posted by DaveReidUK

Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?

Went back and checked, and the change in specs was the authority (from .6 to 2.5 degrees per activation). Article didn't mention speed. As noted elsewhere, the MCAS trim speed is faster than the flaps up main electric trim speed which is a problem. For all other trim functions, flaps extended trim speeds are faster than flaps up, and main electric trim speed is faster than automated inputs.

[infrequentflyer789]

Originally Posted by DaveReidUK

Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?

Authority (per increment / reset) was definitely increased from the original design of 0.6 degrees to 2.5, I don't know whether this was done by increasing speed or duration or both - the speed change wouldn't do it alone, but might have been required to "correct" the stick force fast enough.

Stab trim does go through a gearbox, between motor and jackscrew, and there are at least two clutches (nose up/down - not sure if ap/manual is also separate or is a speed change).

From NG AMM chapter 27:

When the flaps are up, the switch is open and low speed trim is engaged. Low speed trim moves the stabilizer at 0.2 units per second. When the flaps are not up, the switch closes and sends a signal to the stabilizer trim actuator to engage high speed trim. High speed trim moves the stabilizer at 0.4 units per second.
During autopilot operation, the stabilizer trim speed changes. When the flaps are up, the low speed trim is 0.09 units per second. When the flaps are not up, the high speed trim is 0.27 units per second

What is also known is that MCAS trims at the flaps down autopilot speed - i.e. the "High speed". I don't know if this was a change at some stage due to finding low speed was insufficient for certification - it might have been. I also know how it was implemented (which is as I guessed it might be) - by blocking the flaps-up signal on MCAS engage.

References - elec trim functional diagrams below. One is from NG AMM, one is from an internet source but appears to be the equivalent diagram from MAX AMM. A whole lot looks to have changed with the wiring to shoehorn the MCAS function in...

[737 Driver]

Thanks for that info. I stand corrected.

[1stspotter]

To summarize:
Manual electrim trim speed when flaps are up and autopilot disengaged: 0,2 units per second
MCAS trim speed when flaps are up and autopilot disengaged: 0,27 units per second