No, because then it could not be overpowered by the pilot, and a runaway trim situation would be 100% fatal.
Peter wrote:
No, because then it could not be overpowered by the pilot, and a runaway trim situation would be 100% fatal.
Not sure of logic there.
Clearly there is a huge amount of gearing (mechanicl advantage) available to the pilots on that trimwheel.
Nevertheless, not sure how even a ‘captains boot’ could override that jackscrew motor.
Frankly I don’t know how that stabiliser / jackscrew / motor / trimwheel system is set-up.
The servo doesn’t drive the jackscrew directly. It is further upstream. You can always overpower the trim servo, in any aircraft, otherwise an electrical failure would be lethal.
Seems that (even before MAX8) if the stabiliser and the separate elevator (there are TWO moving devices back there + rudder of cause) are put is such a position to overcome trim-wheel runaway (jackscrew motor THEN turned off) the loads become so great that pilots working in unison would not be able to turn (crank) that trim-wheel backwards to stop the decent.
Seems the fix was.
Non intuitively push forward on the stick, loads on jackscrew get reduced, and crank trim-wheel back.
Them pull back on stick and repeat.
Push forward, resulting again in reduce jack-screw loads, crank trim-wheel back.
Repeat and repeat till stabiliser is moved sufficiently ‘down’ to arrest the decent and start a climb.
In the old day (737-200) this was known as the ‘roller coaster’ technique to regain control.
Seems a design problem, but got you out of jail.
Peter wrote:
The servo doesn’t drive the jackscrew directly. It is further upstream. You can always overpower the trim servo, in any aircraft, otherwise an electrical failure would be lethal.
Copy that Peter – but thought stablisier jackscrew was driven by this massive electric motor (no servos) and the main trim wheel mechanically turned the jackscrew in event of motor ‘off’ or failure.
I appologies, but really not sure of the desgin set-up on this one.
Trim wheel connected by cables to cable drum which is directly connected to jackscrew gearbox.
System works both ways
Electric motor turns jackscrew (and moves stabiliser) which also turns flightdesk trim wheel very fast.
Pilot turns trimwheel many time due gearing effect to manually slowly operate jackscrew.
See pic at next post – link not just correct
Yes; the pilot can always overpower the electric trim.
It sounds like there are unusual attitudes which combined with unusual speeds can generate large aerodynamic forces on the THS and make the manual trim impossible to turn, but that’s a different discussion.
Peter wrote:
It sounds like there are unusual attitudes which combined with unusual speeds can generate large aerodynamic forces on the THS and make the manual trim impossible to turn, but that’s a different discussion.
Indeed, but is that not the point.
Having turned off HCAS the were meant to rectifify the ‘nose down’ by winding / cranking back the trim wheel.
If stab, elevator, jackscrew, nut etc were in ‘lock’ due high loads – what then. Big panic.
Try turning STAB switches back on, and bloody hell more nose down kicks in.
Should have used tradititional ‘roller coaster’ method to get that stab. down and nose up.
We need input here from a real airline pilot. There are some on EuroGA.
