Very interesting reading:
From
Adsto
15th Apr 2005: B737 Stall
http://www.pprune.org/1842832-post1.html
From
CaptainSandL #5 (permalink)
737-300 at 40T,
stick shaker triggers at approx 1.09Vs with flap 5 & gear up and approx 1.07Vs with flap 30 & gear down.
From
BOAC #6 (permalink)
Thanks for the Vs figures. 1.07 is not much when you
look at a Vs of around ?110kts? If my fading memory serves me right (going back to 1988) we went to the first proper 'symptom' of the stall (excluding stick shake), but Tempus Fugit may be playing tricks! IIRC We always found that at 1kt/sec you did not have enough back stick to keep the nose high enough to reach anything other than a gentle 'mush out', and never got to stall speed. I assumed the point of it was to try to ensure there were no 'nasty' asymmetries waiting to bite - but we never found out
But also:
http://www.pprune.org/tech-log/431167-737-ng-sts.html
airyana 19th Oct 2010, 22:17
"As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane."
Can anyone clarify what is meant by: "enables trim above stickshaker AOA"
And why ??
BOAC 22nd Oct 2010, 16:08
Never really understood it and ignored it - it is confusing because stab nose down is a/c nose up. I suspect they
missed out the word 'airplane'. I expect Cpt SandL or Checkboard will know the answer
Whippersnapper 24th Oct 2010, 18:02
It has been a while, but as I remember it, as the stall is approached, the elevator feel shift centre position moves and its hydraulic pressure increases. This is combined with the speed trim system trimming (aircraft) nose down. The combination is meant to emulate the old fashioned "stick pusher" on older aircraft with hydraulic controls.
My guess is that the speed trim is inhibited as the stick shaker is activated (perhaps as an assumption a trim system malfunction is the reason for the high AOA), but if AOA is further increased to the point that the stall is imminent, the speed trim is reactivated to trim nose down.
I was spared the suction of a good portion of dust.