737 Ng Sts

[airyana]

"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 ??

[Mad (Flt) Scientist]

It would make more sense to me if it said "disables" as that would imply that not only were you getting an automatic nose down trim (which both deters further decel through increasing the column force AND places the stab in a good nose-down position to assist recover if high thrust is applied) but also you're prevented from manually overriding that and trimming yourself into a stall.

Since trim is normally enabled, i can't see why you'd add another "enabling" above stick shaker.

Seems to me like a translation problem or a missing word - maybe it means "enabled above stick shaker SPEED" not "AOA"?

Total guesswork based on no specific B737 knowledge, will be interested to see the 'official' answer ...

The MAK report into the Aeroflot-Nord B737 accident states (page 76) that STS is only active below shaker AOA. So either they are wrong, the text quoted is wrong, or the term STS is being used in different senses by the two texts.

[airyana]

Statement copied and pasted from the 737 FCOM, and that's how it read through the last three updates ... I agree "disables" would make more sense

[airyana]

Is the post not clear enough ? Do you need me to post a larger section of the fcom text ? Just let me know ,,, any other thoughts anyone ...

[BOAC]

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

[STBYRUD]

I suppose what they are trying to say is that it works down to stickshaker AOA, then it stops trimming. Above that it will maintain speed stability, namely trimming the airplane nose down when speed tends to decrease from in-trim speed.

[airyana]

I hope so :) thanks

[Whippersnapper]

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.