Greetings fellow aviation lovers! This is my first post so please excuse any noob mistakes/lack of etiquette.I'm using FSX SP2 in Win XP SP3 and recently re-installed FSX onto a new dedicated 640mb SataII drive with 64mb cache. Everything appears fine except for some odd flight characteristics. My main problem/key frustration:Flying the default 737 and posky 737-600/737-700 (both aliased to default 738 Virtual Cockpit), they fly very well with and without aircraft.cfg tweaks EXCEPT for landing. When on short finals using localizer/glidescope with autopilot in APP mode, the aircraft fly the glidepath as intended. At around 500 feet AGL I disconnect the autothrottle and at just above 200 feet AGL I disconnect the autopilot and this is where my problem is!Just after I disengage the AP i get a sudden and pronounced nose down attitude which can only be recovered from with a huge input of pull-back on the control-column! The result is either a very firm '3-point' touchdown or 'porpoising' either of which is very frustrating and takes away from the whole simming experience. I believe on disengaging the AP the aircraft should be trimmed correctly at the current attitude so this is really killing my experience.Before you ask, the aircraft is lightly loaded,well-balanced and take-off with correct trim/power-settings are pretty smooth!I read somewhere that the FSX default AP returns the control surface (or at least ailerons) to the last position BEFORE the AP is engaged. So I tried engaging AP at around 1000 feet after takeoff whilst commanding nose-up. I then centred the controls on finals before disengaging the AP...no joy! Nose pitched down on finals.So I tried centering the controls using Numpad-5 after take-off and then engaging AP. I again centred the controls on finals before disengaging the AP...no joy! Nose pitched down on finals.After googling for some time I found the following post where one poster states

sometimes certain Aircraft in FS have a bug which mean thats as you disengage the Autopilot, the Trim resets itself to 0.1 Degrees.

So I take off again on my testing flight as above with each of the above aircraft. After establishing on localizer and glidescope, aircraft configured with good speed I check the trim and it's within the green band. I center the controls and disengage autothrottle...no problem everything is looking good! I disengage the AP and the nose dives. I manage to recover and get on the ground.At the end of the runway I pause the sim and take a look at my throttle quadrant/pedestal.To my dismay ( and great horror) I see that my pitch has gone from within the green band to right up against the stop somewhere around 0.1!!!What I'd like to know:1) Has anyone experienced this and can you shed any light on this?2) Is there a solution to this problem (other than massive control inputs so close to the ground!)3) I'm using keyboard and mouse NOT a joystick...could this be the problem even though hand-flying is perfectly fine.I'm stumped on this one and could sure use some help. Many thanks.(I haven't tried any other AP controlled aircraft as yet but will try once I overcome my frustration with this :( )

I center the controls and disengage autothrottle...no problem everything is looking good! I disengage the AP and the nose dives.

Are you centering the controls (Num5) as part of your normal approach setup, or are you doing this as some kind of test because of this problem?... and, (bye the way) - Welcome to the Avsim forum!

Hi Paul, thanks for the reply and warm welcome! I'm centering the controls (Num5) to test whether it might have been some control position issue. It would appear this had no effect whatsoever. On disengaging the AP the nose takes a steep dive and it would appear that somehow the trim is going all the way to full nose-down on disengaging the AP.

=====================================QUICK UPDATE=====================================Forced FSX to reload fsx.cfg by renaming fsx.cfg to fsx.cfg.oldTried my test flight in default Airbus A321 and aircraft behaved normally...NO NOSEDIVE ON AUTOPILOT DISENGAGE and nice smooth landing. Tried again in default 737 and same problem persisting :( .Will retry Airbus tomorrow to make sure it wasn't a fluke (or self-delusion :( )...will also try Qualitywings 757-200 and -300 and see what happens.At this stage it would appear that this problem is only with default 738 or 737 models aliased to default 738 !?By the looks of things this might also turn out to be a one-man thread :( . Good night for now!Happy Flying!

I wouldn't rename it to that, try fsx.old

If you're at a proper landing weight, and the CoG is good.. there are two other possible problems:1) Approach speed for the weight2) Flaps for the weightIf you'v got landing flaps deployed, and have been established well out at a good approach speed, the default 737 is a joy to land.

Do you have elevator trim assigned to a joystick axis/rotary?

I think FSUIPC has a Centre Elevator Trim On Autopilot Disengage option or something similar. That may help?

=====================================QUICK UPDATE=====================================Firstly thankyou for your kind replies.A temporary fix/solution: disconnect autopilot in level flight, correct the immediate nose-down pitch on AP disengage and re-engage AP whilst pulling nose-up. This seems to take care of the nose down pitching earlier, likewise, doing the same early on approach and hand flying down. However,it doesn't get to the root cause of why I'm getting this pitch down attitude on AP disconnect. Conversely, I get a nose-up pitch of some 3000 fpm on first ENGAGING the autopilot. This can be reduced by making sure there is a speed set in the V/S window on the MCP prior to engaging the AP (I currently have use_no_default_pitch = 1 in the aircraft.cfg but will also try setting a default pitch mode and see if that changes anything)@413X3

I wouldn't rename it to that, try fsx.old

Thankyou, I got all mixed up :( !@Brett_Henderson

If you're at a proper landing weight, and the CoG is good.. there are two other possible problems:1) Approach speed for the weight2) Flaps for the weightIf you'v got landing flaps deployed, and have been established well out at a good approach speed, the default 737 is a joy to land.

Thankyou you're absolutely right. Tightening up approach speed/flap settings certainly make for better approaches and landings but unfortunetly it doesn't stop this odd pitching behaviour in my case.@WarpD

Do you have elevator trim assigned to a joystick axis/rotary?

Not in this case. Using keyboard and mouse as yoke. Elevator trim down assigned to keypad (num7) + Elevator trim up assigned to keypad (num5)@rabid_david

I think FSUIPC has a Centre Elevator Trim On Autopilot Disengage option or something similar. That may help?

Thanks for the suggestion but I have an unregistered copy of FSUIPC so I don't think I can access that option.The quest continues. Just to reiterate the problem is a sudden nose down pitch on disengaging the AP and a sudden nose-up pitch on engaing the AP. Also it would appear that this only happens the first time AP is engaged and disengaged. But I would have to test that further before I could say for sure.Apologies if this is old hat or known issue but before re-installing I was mostly flying Carenado Cherokee and Captain Sim 757. It's been a long while since I flew default aircraft or Posky. Maybe this is why I stayed away from them? :( Happy Flying! Tomorrow's a new day...========================================CORRECTION=======================================:(WarpD

Do you have elevator trim assigned to a joystick axis/rotary?

Elevator trim down assigned to keypad (num7) + Elevator trim up assigned to keypad (num1)

==============================QUICK UPDATE and some more understanding==============================OK, after some more testing I have come to the following conclusions:1) The default 737 Autopilot or any aircraft aliased to it's VC appears to be sensitive to ELEVATOR POSITION at the time the AP is disengaged. This would explain my nose-down pitching on first disengage of the autopilot as I would have been pushing the nose down to begin acceleration for flap retraction before first engaging the autopilot. Not touching the yoke again till disengaging the autopilot meant that at autopilot disengage the aircraft appears to take this as a pitch-down command rather than leaving the controls/flight surfaces at the last position the AP had commanded. Why my centering the controls(by keying num5) didn't work!?It would appear num5 only centers the ailerons and NOT the elevators thus only centering the controls in the roll axis. I don't have rudder pedals so I can't test this for yaw :( ! So my efforts to center the controls before disengaging the AP were ok for rolling but USELESS for pitch!...hence that didn't help.2) The nose-up pitch on Engaging the AP would appear to be due to having the ALT HOLD button selected when engaging the AP. From what I've tried so far it's OK to select SPEED/HDG HOLD/NAVLOC before engaing the AP and then selecting ALT HOLD after the AP is engaged. I need to test this more.These seem to be some peculiarities in the default 737. Apologies again for those who fly these regularly if this is old news but I've got used to more realistic flight logic and this really threw me. At least I can continue and enjoy my Posky merge with this knowledge...Before I hit Google again, can anyone reccomend any utils for FSX that show control control surface position in flight?Many thanks and Happy Flying folks!