Originally Posted by
GarageYears
Ok, I have to confess I’m confused.
All this talk of manual trim forces, etc.
MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.
If MCAS runs again, again any pilot trim action defeats MCAS.
There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.
Am I wrong?
- GY
If the runaway MCAS is countered early enough, then nose up trim, and disabing the cutoff switches is sufficient. That was the official version from Boeing and the FAA, until shortly after the second MAX crash. If left too long, the situation enters uncharted territory, and nobody has come out alive (except those in the simulator).
Several sources indicate that electric trim was intentionally limited in scope, to avoid unintentional runaway nose up trim (whether by the pilot or by a wiring fault). Runaway nose up trim may be just as deadly as nose down trim, so there was logical justification for this restriction.
It has been suggested that pitted against runaway MCAS, the electric trim never wins enough authority to recover from severe nose down trim, where there is aerodynamic loading of the horizontal stabiliser. Whether electric trim would be sufficient against the upgraded MCAS is not clear, and that risk needs to be scrutinised.
Several tests, leaks, and EASA documentation have knocked a huge hole in the initial assertions. I don't have detailed references handy for all of those points, but they have been interspersed throughout the last few days posts. I expect to see more media articles and blogs on these topics.
Edit: The link posted by
ProPax gives the latest overview:
https://www.flightglobal.com/news/ar...ses-ne-457224/
The FAA is presumably not happy with all of this contradictory information, and being made to look sheepish by ongoing media revelations. The certification review should require detailed evidence, rather than the bland reassurances we had last November after the first MAX crash.