Does MCAS do anything other than drive the trim servo?
I don’t quite get this. It is possible to overpower the trim servo. There are lots of YT videos showing how to do this. Whether a pilot with “normal” training would be doing that is another matter.
Plane pitched down after trim cut out and sic reporter manual trim was not working. And although speed mode was engaged, plane did accelerate over vno and mcas continue to pitch down…
The report didn’t mention if manual trim is yoke buttons or manual action on the wheel
After the pilots called out “stab trim cutout”, there was a trim-down command from the MCAS system that didn’t change the pitch trim so at this point the automatic trim-down was apparently disabled.
BUT just before control was lost, there were two short manual trim-up commands that did cause a slight change of pitch trim in the up direction, shortly followed by another MCAS trim-down command which did cause a downward pitch trim change
So from reading between the lines and looking at the FDR readout it appears that the stabiliser trim was re-enabled at some point – either by pilot action or some other mechanism.
Exactly; the auto trim must have been re-enabled and the trim wheel was not physically blocked.
Looking at one of the videos, using the first officer’s boot is easier than fingers 
Can’t say I’ve quite got the full sequence of events either.
However, and seems obvious, the elevator (with its relatively small size) is not capable of physically overriding a badly errant stabilizer.
If a last resort is to ‘lock’ the trim wheel with your boot, then that falls into the realms of desperation.
I can’t see the FAA Certifiers saying “that’s okay – the captain just ‘puts his boot in’”.
WarleyAir wrote:
If a last resort is to ‘lock’ the trim wheel with your boot, then that falls into the realms of desperation.
I can’t see the FAA Certifiers saying “that’s okay – the captain just ‘puts his boot in’”.
I’ve noticed that the trim wheel in Boings is geared to be moving very fast. I guess the reason is that only a small force on the wheel should be enough to move it or to hold it against a runaway servo. The flip side is that the speed makes it difficult to get a grip on the wheel while it is moving.
Indeed, for those that know about mechanical gearing, gearing certainly could come into it’
I’m not sure what in the stabilizer / elevator department is controlling what?
Trim Button on yoke under thumb.
Trim Wheel
Autopilot
MCAS
anything else that decides to have an input.
Which of these inputs are working what (stabilizer and / or elevator)?
That stabilizer jackscrew is a massive piece of kit and it can’t be ‘back driven’ due ‘gearing’ – in the same way that a ‘worm & wheel’ will only drive oneway.
To move the nut on the screw, the screw (jackscrew) must be turned by either its main electric motor or some other means – what?
I’ve noticed that the trim wheel in Boings is geared to be moving very fast. I guess the reason is that only a small force on the wheel should be enough to move it or to hold it against a runaway servo. The flip side is that the speed makes it difficult to get a grip on the wheel while it is moving.
The procedure appears to be to grip it quickly and firmly, otherwise it can take the skins off the fingers.
the screw (jackscrew) must be turned by either its main electric motor or some other means
Only the servo (motor) or by hand.
I suspect the pilots in these crashes didn’t know about the MCAS auto trim feature and didn’t just grab the trim and hold it.
Another factor I understand is that movement of the stabiliser manually is not possible, when there air loads are very high. I understand the procedure is to reduce back pressure, then trim the stabiliser up. I think they are taking things too far, in pretending the MAX is “same” aircraft that was certified in the sixties.
I would not be surprised that the outcome of all this is that regulators take the same view. The dash 10, has even got a “clever” telescoping undercarriage that is longer but fits in the same wheel well, so they can extend the fuselage even further.
The stabiliser in effect is an all flying tailplane.
At high airspeed and increased angles, it will indeed take some moving.
However, it is driven by a jackscrew which has a huge ‘mechanical advantage’.
i.e. the torque required to turn the jackscrew will be ‘relatively’ low.
In the same way one does not require much ‘pull’ on a spanner to tighten a nut.
All to do with ‘mechanical advantage’.
The downside is you need many many turns of the jackscrew to move the nut (and thus the stabiliser) a short distance.
This is why the trim wheels (which I guess could better be named ‘stabiliser wheel’) has unlock pull-out winding handles on the side – both pilots can grab their own handle and start ‘cranking in unison’.
