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Guest Dianne_S

Does the real 737 autopilot use Stab Trim to maintain altitude?

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Guest Dianne_S

Just wondering, this question was brought up in the main FS2004 forum. Does the 737 autopilot use elevator input or rather stabilizer trim to maintain level flight?In the PMDG sim the autopilot seems to set stab trim as the aircraft will maintain a level course when the autopilot is disengaged.Thanks!

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There's definitely others from the beta team who can answer this better than I can, but I do seem to recall a discussion during testing to the effect that it actually uses both elevator and trim inputs to fly the plane on AP.I know of one instance for sure where it does use the trim - if you have the 800/900, perform a full autoland and watch at 400ft RA - you'll see/hear the trim bias itself up a bit to aid in the flare.Btw, are you real ATC or on VATSIM?


Ryan Maziarz
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Guest Loiosh

I am under the impression the real 737-NG uses both. You will certainly notice this on real life auto-land approaches where the autopilot will trim (clack-clack-clack-clack) the flare and then apply pressure to the elevator to hold the glide slope.Most the time (as read off another post) the airplane will adjust the elevator first, level, then trim to releave control pressure.What really made me laugh is the simulator report (it's on the AVSIM front page right now) from one of the con goers who was impressed with how -loud- the trim actually is the airplane. Without a doubt the NG is my favorite flight sim investment. I spent two months reviewing add-ons before I picked it and have -no- regrets. Congratulations to the team.

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Guest bobsk8

>I am under the impression the real 737-NG uses both. You will>certainly notice this on real life auto-land approaches where>the autopilot will trim (clack-clack-clack-clack) the flare>and then apply pressure to the elevator to hold the glide>slope.>>Most the time (as read off another post) the airplane will>adjust the elevator first, level, then trim to releave control>pressure.>>What really made me laugh is the simulator report (it's on>the AVSIM front page right now) from one of the con goers who>was impressed with how -loud- the trim actually is the>airplane. >>Without a doubt the NG is my favorite flight sim investment. I>spent two months reviewing add-ons before I picked it and>have -no- regrets. Congratulations to the team.I agree with you that the PMDG NG brings flying FS9 to a whole new level. I was overwhelmed with the program the first day or two that I owned it, since it seemed that I would never learn how to control the plane and the FMC, etc. I posted this on a newsgroup and someone on the newsgroup encouraged me to stick it out and sooner or later I would grow to really like the plane. He also mentioned that the NG was now the only thing he flew in FS9. Well I did stick it out, and now the NG is the only thing that "I" fly in FS9 :)

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Guest Dianne_S

Hi Ryan,I am real ATC :) I'm an ARTCC journeyman (woman actually) and I work at Salt Lake Center, Bryce Canyon LOW (SFC-FL230). Beats having a real job!Thanks for the info!

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Guest Dianne_S

Thanks for all the info guys!

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Hi Dianne,I hope Dr. Vaos, the almighty PMDG flight dynamics/auto pilot specialist doesn't see this post. That you can even remotely think that he has cheated when it comes to the modeling of the controls used by the AFDS. ;-) :(Seriously. As Ryan has stated, there are more knowledgeable persons who could answer this but to my recollection the PMDG NG uses elevator for pitch adjustments and stab trim for control pressure relief. As an interesting side note. If you try to trim manually while the A/P is engaged you will get a A/P disconnect. I think your NG rated pilot friend will concur that this is the correct behaviour.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Guest hlm65

Hi Dianne,from the 737 Flight Manual, I understand that the autopilot uses exclusively the stabilzer trim to control the pitch axis. In Chapter 6.9 "Pitch Control" it says: "The elevators are normally controlled by forward and aft movement of the control column. The stabilizer is normally controlled by either the stabilizer trim switches on the control wheel (my note: and if you operate any trim switch with A/P engaged, A/P will disengage, and this is one of the ways to get an A/P disconnection) or the autopilot". And further on, in the same chapter, under "Stabilizer Trim" and regarding the NG series: "The horizontal stabilizer may be positioned by a single electric trim motor, controlled by either the autopilot trim or by stabilizer trim switches on the control wheel or by manual cable control".Hope this helps clarify your question.Best regards.

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