Sign in to follow this  
Guest OneTinSoldier

Question about disconnecting A/T

Recommended Posts

Hi All,I disconnected my A/T while in VNAV/LNAV A/P Mode and it didn't seem to want to maintain a proper amount of descent. I had my MCP ALT set at 4,000 and was out of about 15,000. As I gave it some throttle the nose seemed to come up even more, to pretty much level flight before I backed off and reengaged A/T. Can someone shed some light on this? I may be having many more questions coming up in the future. Hope that's ok. :)Jim Richards

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

"As I gave it some throttle the nose seemed to come up even more,.."I think most aircraft are designed to do this, Jim ;-) It's a safety feature. Look where the engines are located with respect to aircraft's center of gravity (i.e. below). Any increase in thrust is going to lift the nose due to "turning moments". Some have automatic trimming (even with the A/P disconnected), which may change this behaviour slightly depending on the aircraft speed and other factors. E.g. The 737NG has SPEED TRIM and MACH TRIM.737NG SPEED TRIM:Speed Trim is designed to keep the aircraft airspeed constant when the aircraft is at low speed and at high thrust settings. An increase in airspeed will make the stabilizer move to cause the aircraft nose to pitch up, slowing the aircraft. If the speed decreases, the aircraft will pitch down, increasing the speed of the aircraft.Depending on the aircaft type (-600/-700/-800/-900), the speed at which SPEED trim is active, varies. On the -700, according to my books it is below 202 kts (with the flaps extended).If the aircraft is close to stall speed, Speed Trim will push the nose downwards.This shouldn't apply in your case, but Speed Trimming is not active when the aircraft roll angle exceeds 40 degrees. 737NG MACH TRIM:On the 737NG, Mach trim actually moves the elevators rather than the stabilizer. The 737NG Mach Trim system moves the elevators to prevent a nose down manoeuvre at high speeds (Mach 0.6 to 0.84 on the -700). Strangely, it also moves the elevator to increase a nose up manoeuvre at takeoff.Hope this helps.Cheers.Ian.

Share this post


Link to post

Hi Ian,Yep, I expect the nose to rise if I give it thrust. But what I was expecting here was for it to continue on it's VNAV descent. All I did was turn off the A/T. Unless I'm mistaken, LNAV and VNAV were still lit. Thanks for the interesting lesson in trim dynamics tough. :)Regards,Jim Richards

Share this post


Link to post

Oooops..... misread your question, Jim... I thought you said you disconnected the A/P... You said A/T....Gotta keep off these caffeine drinks ;-)Cheers.Ian.

Share this post


Link to post

With a VNAV mode engaged, the FMC commands AFDS pitch and the A/T modes to fly the vertical profile. [h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]

Share this post


Link to post

Hey no problem. Just the blind leading the blind here. :-lolJust a little joke!Ciao,Jimmy Richards

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this