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Uncommanded Dive


ac103010

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OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.
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OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.

 

Depending on the aircraft, it might be a mistake to click A/P Alt Hold. What you MAY be doing is clicking Alt Hold and that seeks to `hold` the alt at the moment you clicked the Hold. This will see you diving to the previously selected altitude...

 

If it's the elevator trim then you may have inadvertently checked a duplicate assignment.

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I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.

That's normal operation. The AP in FSX makes heavy use of the elevator trim, so that you effectively are fighting the AP if you try to adjust the trim and it WILL fight back.

 

I expect that Ray has the answer for the unexpected (not uncommanded) dive. You'll need to find what altitude the AP is set for or use CTRL-Z which should both engage the ALT HOLD and SET the current altitude hold value. Engaging ALT HOLD with the AP button(s) will try to maintain whatever altitude was already set in the AP.

 

So either use CTRL-Z or change the setting (perhaps an altimeter bug, or IAS hold window) of the AP.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Hi,

 

I've read all 3 replies.

 

First, let me say that this happens with a number of different aircraft: B747, A320, B737, A350.

 

Second, it's always been my understanding that when A/P Alt Hold are selected the aircraft would climb to the altitude set in the A/P window. I'm selecting the A/P passing, say, 3,000ft, and the A/P Alt Hold is set at 12,000ft, for example. And this is the way it always worked for me for about 3 years previously. This issue has only happened since I've installed FSX again after 5 years being away.

 

Then, when I say the elevator trim resets when taxiing, the A/P is not engaged at that point. I can view the trim operating when taxiing but without changing anything when I start my takeoff run it's reset.

 

Allan

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OK, so doesn't matter what aircraft or what airport, I get the same problem. I take off normally, rotate at a high enough speed select gear up, retract flaps, engage the autopilot and autothrottle, and heading. At this point everything's normal. But as soon as I select Altitude Hold the aircraft goes into a dive. I've tried selecting the elevator trim up max but, for some reason, whilst taxiing and/or whilst taking off, the elevator trim returns to neutral of it's own accord. It's driving me nuts.

 

Allan - Check all your AP settings on climb out. Make sure you haven't assigned a negative vertical speed assignment by mistake! You could have designated 12,000' for your desired altitude, but if you have a -2000' vertical speed assignment, and at 3,000' you engage the AP, you are going to go down! Oh, and make sure you are rolling the trim wheel in the right direction also! Like Larry says, the trim wheel will go wild on it's own responding to the needs of the autopilot!

 

It's happening on too many of your aircraft, something common you are doing wrong in all of them! :rolleyes:

 

Rick

Edited by Downwind66
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I don't have a clue after all of the above.

 

Alternatively I did read this on another site:

 

"Whenever I have seen this sort of thing, it was due to the elevator or ailerons being randomly assigned to some other joystick axis (in ADDITION to its normal assignment). I use the CH Products yoke & rudder peddles and when I make changes (which is rare), my computer has a bad habit of assigning random axis to the elevator or ailerons (the propeller pitch axes on the yoke gets assigned to the ailerons for example). Everything will function fine until you bump the pitch handle, and boom, it activates the control and the airplane flies out of control. And YES, you can have TWO axis assigned to the elevators for example (like the yoke and the prop pitch mixture lever). This means it may function normally for some time until you bump the other control also assigned to it - and then all #### breaks loose. You have to go to Settings/Controllers and check to see if they are assigned to more than one joystick axis. Not sure if this is your problem or if this will help."

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Update.

 

I deleted fsx.cfg and restarted. Went to joystick assignments and deleted every one except the 3 I knew were working previously. In settings I set everything as they were before deleting fsx.cfg

 

So no aircraft settings were changed. on the same B744.

 

Began my takeoff roll. At aroung 120 kts the aircraft tried to takeoff. I held it until I could engage the A/P but then the nose kept lifting until the aircraft stalled.

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Began my takeoff roll. At aroung 120 kts the aircraft tried to takeoff. I held it until I could engage the A/P but then the nose kept lifting until the aircraft stalled.

Am I understanding this correctly? Are you trying to engage the AP to do the takeoff? Did the nose lift before or after you engaged the AP? What were you doing with the trim at that point, if anything?

 

You say the aircraft tried to take off -- Was it too slow? Is that why you fought the take off?

 

As you might gather, I'm a bit puzzled about your procedure, both what and why.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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No, Inuss. I hadn't engaged the A/P and the speed was around 120 and accelerating when the nose started to lift. I held it down then raised the nose at about 150 kts with the joystick. Then gear up whilst holding the climb at around 10 deg, retracted the flaps at around 180, then, at around 3000 ft engaged the A/P and Alt Hold mode. Alt window was set to 12000 ft and A/T set at 230. It went into a dive.

 

 

Now, just to complicate matters, initially my B744, A350, A320 and B738 would takeoff normally until I engaged the A/P and Alt Hold, whereupon it would go into an immediate dive.

 

 

So, I deleted my fsx.cfg file and restarted FSX, and changed all the settings back to those previoulsy. I also deleted the joystick assignments for everything except 3 buttons, none of which would change the flight dynamics. Then, my B744 acted as it did in the first paragraph. Now, with the B738, it was the same as the B744 except the nose didn't try to lift at first. However, once in the climb the A/P was engaged the A/C nose pitched up until the aircraft was almost vertical, and, of course, stalled.

 

I hope someone has a clue as to what's happening because I don't.

 

Going to run some more tests later.

 

Allan

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Decided to go back to first principles.

 

Without changing ANY settings from the post above I chose the default A321.

 

Set:

 

A/P Off

A/T Off

Speed set 210

Alt set 12,000

ROC 1200 ft/min

Elevator Trim 0.0 Deg

 

Flaps 2

 

Set 100% N1.

 

At about 150 kts the nose came up sharply of it's own accord and the aircraft pitched up to 30 deg plus. Unable to control it.

 

 

Repeated takeoff with 81% N1 and aircraft took off normally. Was abe to set the nose up angle and engage all aspects of the A/P without problems.

 

 

 

 

 

Repeated the exercise with a 3rd party B738.

 

All settings the same.

 

Selected 80% N1 but clear the aircraft wasn't going to reach V1 so increased to 100% N1.

Aircraft took off normally at about 150 kts although it did require a little more joystick input

 

In the climb cleaned the aircraft up and engaged the A/P.

Started to go into a dive although it was easy to control and once controlled the A/P took over normally.

 

 

 

I'll try the other aircraft later but I still can't understand why I never had these issues in the past.

 

Allan

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Fuel load? Flaps setting about 5 degrees? At 120 knots you should start feeling the nose rise and takeoff should be around 135-140 knots. Autothrottle should be set for 250 knots until reaching 10,000 feet. Flaps up by 180 knots and gear up after shortly after takeoff. V/S set for 1800. I recommend 92% N1 for takeoff and not 100%.

 

All this starting from a fresh new flight and not from a "saved" condition? What airport, weather conditions?

 

Keep in mind that max fuel load, airport altitude, and weather conditions will play a roll in how things go after wheels up.

Edited by mrzippy

Still thinking about a new flightsim only computer!  ✈️

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Hi Mrzippy,

 

I haven't yet got around to calculating fuel loads. Tried Flaps 5 and 10 deg. Under normal takeoffs I'd never experienced the nose starting to rise and T/O usually 140 kts plus. I set the A/T initially at something like 230 because I've sometimes found it's exceeded 250 before settling back, a result, obviously, of takeoff issues. I clean up by 180 kts.

 

Until these problems started I'd always used 100% N1.

 

These problems occur sometimes with fresh flights and sometimes with saved,

 

Weather is Fine and airport is Cardiff EGFF, 220 ft AMSL.

 

Will gather more information later today.

 

Allan

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OK.

 

Moved FSX from Program Files (x86) to C:\.

 

Default A321

Summer

No Weather

Cardiff (EGFF)

230ft AMSL

72m566 Kg Gross Weight

Flaps 2

Elevator Trim 0.0 deg

A/P Settings 230 kts and 9,000 ft at 1,200 fpm

 

N1 set at 90%

 

Nose did not attempt to pitch up during the T/O roll.

Rotate at 140 kts.

 

Smooth rotation, gear retracted, Aircraft Cleaned up.

 

Autopilot at A/T engaged.

 

No problems at all.

 

 

So now I've proven that the default aircraft is ok I can now go back to the other aircraft to see if they will perform.

 

Thanks for all the inputs. I'll come back sometime soon with results of my checks.

 

Allan

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OK, so I repeated the exercise today with the same default A321 and takeoff was perfect.

 

Changed to a 3rd party B738 - that I'd flown many times previously - with the same parameters. Takeoff was ok to start with but shortly afterwards the aircraft went into steep climb, then stalled. There was nothing I could do to control it.

 

So I went to the default B737-800. Again, same parameters. Takeoff seemed normal, engaged A/T and A/P and aircraft was steady in the climb. So I selected heading mode. The aircraft started a turn and the climb steepened. I could not get the angle of climb down, with the inevitable stall.

 

Haven't got a clue what's going on.

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Allan - Do you know how to provide this forum screen prints? Would be nice if you focus'd on just the default B737-800,

 

1) Take a screen shot of your panel, just before takeoff

 

2) When the unexpected happens, try to get a screen shot of the panel! Don't make a screen shot of the panel after you make any adjustments. Get it just as the event happens!

 

This way, we might be able to see what is going on in the cockpit, both pre-takeoff AND at the time the event happens! :rolleyes:

 

Rick :cool:

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ac10310, I am wondering if you accidentally set the V-Speed (Vertical Speed) too high. It will cause the aircraft to try to nose up and speed up to match your entry . The default 1800 is safest but I believe I have heard that most Boeing usually go at 2500. Rick's suggestion to take a screen shot of your AP panel is a great idea. I fly the default Boeings and Airbus all the time and do not have issues with using the AP shortly after manual takeoffs.
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Thanks Rich, and Allan, here's what I would like to see on your B737-800. I have two pics, one that will successfully climb to 6000' at 1800'/min, the other will manually climb until you engage the autopilot and then it is going down (to Mother Earth) at a descent rate of 1800'/min!!

 

This aircraft will put a smile on your face!

2020-11-30_15-23-9-567.jpg

 

This aircraft will ruin you whole day!

2020-11-30_15-22-56-698.jpg

 

I know you might think of us not being confident in your flying abilities, but not so, we have no way of telling if your problems could be this simple to fix, or you have additional deep rooted problems with YOUR FSX program. We don't have a crystal ball and if we did, I wouldn't know how to use it, so we have to depend on you as the user and sometimes ask you to provide us with pics such as what I have just posted.

 

I hope you can soon resolve the issue as I know things like this are frustrating to say the least! :rolleyes:

 

Rick :cool:

Edited by Downwind66
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textrich and downwind. The rate of climb IS set as a +ve. I set it at 1,200 fpm but I have tried it as high as 2,000.

 

I would point out that I'd flown A320's and B737's - 200 and 800's for a number of years. As well as B747's 200 and 400's, B727's, DC10, MD11, and never had problems. Then I uninstalled FSX about 4 years ago and only reinstalled this year, and this is when the problems started.

 

And I would remind you what I said in an earlier message, that the same thing happens with the default B747 and with an added Camsim A350.

 

 

 

Rick, I don't know how to add pics but I understand what you're saying and I'll look into it later and forward pics.

 

 

Thanks for your inputs. Will get back with pics later.

 

Allan

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Allan - Just as a quick reference to check yours, here is the [autopilot] section of my B737-800 aircraft.cfg. Just compare it to yours to see if there is any significant differences between the two!

Looks like you and anyone trying to help you solve this, will be "shooting in the dark" as, I for one am at a loss as to what is going on? What really is confusing, you say you got a successful flight from your A321, okay, why not with the B737 then? This is why I suggested just focusing in on the B737 for now.

Good luck Allan!

 

[autopilot]

autopilot_available=1

flight_director_available=1

 

default_vertical_speed=800

 

autothrottle_available=1

autothrottle_arming_required=1

autothrottle_max_rpm = 90

autothrottle_takeoff_ga=1

pitch_takeoff_ga = 8;

max_pitch=10.0

max_pitch_acceleration=1.0

max_pitch_velocity_lo_alt=2.0

max_pitch_velocity_hi_alt=1.5

max_pitch_velocity_lo_alt_breakpoint=20000.0

max_pitch_velocity_hi_alt_breakpoint=28000.0

max_bank=25

max_bank_acceleration=1.8

max_bank_velocity=3.00

max_throttle_rate=0.10

nav_proportional_control=9.00

nav_integrator_control=0.25

nav_derivative_control=0.00

nav_integrator_boundary=2.50

nav_derivative_boundary=0.00

gs_proportional_control=9.52

gs_integrator_control=0.26

gs_derivative_control=0.00

gs_integrator_boundary=0.70

gs_derivative_boundary=0.00

yaw_damper_gain = 1.0

Edited by Downwind66
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Hi Downwind,

 

This is my Autopilot. The differences are highlighted.

 

The difference in the figure mean nothing to me.

 

[autopilot]

autopilot_available=1

flight_director_available=1

default_vertical_speed=1800

autothrottle_available=1

autothrottle_arming_required=1

autothrottle_max_rpm = 90

autothrottle_takeoff_ga=1

pitch_takeoff_ga = 8;

max_pitch=10.0

max_pitch_acceleration=1.0

max_pitch_velocity_lo_alt=2.0

max_pitch_velocity_hi_alt=1.5

max_pitch_velocity_lo_alt_breakpoint=20000.0

max_pitch_velocity_hi_alt_breakpoint=28000.0

max_bank=30,25,20,15,10

max_bank_acceleration=1.8

max_bank_velocity=3.00

max_throttle_rate=0.10

nav_proportional_control=12.00

nav_integrator_control=0.25

nav_derivative_control=0.00

nav_integrator_boundary=2.50

nav_derivative_boundary=0.00

gs_proportional_control=25.0

gs_integrator_control=0.53

gs_derivative_control=0.00

gs_integrator_boundary=0.70

gs_derivative_boundary=0.00

yaw_damper_gain = 1.0

 

 

Screen shots to follow (I hope)

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Your trim setting is likely out of whack. The autopilot can only retrim so fast when engaged. If your trim is too hi NU, the AP cannot retrim fast enough to prevent this pitchup at engagement.

 

Try a manual takeoff and initial climb without autopilot - that will tell you if your trim is out of line with your weight / cg location. If you have to fight to keep a typical transition attitude / acceleration to 250 KIAS (need to trim a lot nose down), that is the problem.

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