Runaway pitch trim v1.4.1 in the Saab 340A

[cwjohan]

When flying at lower speeds (e.g., during approach phase) with autopilot on I'm seeing the pitch trim go to an extreme high value (nose up).  The aircraft noses up and cannot be persuaded to descend.  I have to turn off the autopilot and set the pitch to something more reasonable, but doing that while struggling with an out of control aircraft can be quite difficult.  When adjusting the pitch trim, one cannot see where the aircraft is going.  I do have a couple joystick buttons assigned to pitch trim, so I can use those more conveniently.

However, my real question is what can cause the pitch trim to go extreme position like this while flying?  Center of gravity?  Just flying too slow?  I don't have this issue with other aircraft.  Also, I don't remember having this issue with an earlier version of the Saab 340A.  I have flown the Saab 340 v1.4.1 only twice so far, so I'll keep an eye out as to whether or not this is a consistent problem or intermittent.

[JGregory]

4 hours ago, cwjohan said:

When flying at lower speeds (e.g., during approach phase) with autopilot on I'm seeing the pitch trim go to an extreme high value (nose up).  The aircraft noses up and cannot be persuaded to descend.  I have to turn off the autopilot and set the pitch to something more reasonable, but doing that while struggling with an out of control aircraft can be quite difficult.  When adjusting the pitch trim, one cannot see where the aircraft is going.  I do have a couple joystick buttons assigned to pitch trim, so I can use those more conveniently.

However, my real question is what can cause the pitch trim to go extreme position like this while flying?  Center of gravity?  Just flying too slow?  I don't have this issue with other aircraft.  Also, I don't remember having this issue with an earlier version of the Saab 340A.  I have flown the Saab 340 v1.4.1 only twice so far, so I'll keep an eye out as to whether or not this is a consistent problem or intermittent.

My quick guess is yes, you were probably too slow.  However, in order to provide proper support, please post your log.txt file and GizmoLog.txt file from immediately after a sim session in which this problem occurred.

Edited July 8, 2017 by JGregory

[birdy.dma]

5 hours ago, cwjohan said:

When flying at lower speeds (e.g., during approach phase) with autopilot on I'm seeing the pitch trim go to an extreme high value (nose up).  The aircraft noses up and cannot be persuaded to descend.  I have to turn off the autopilot and set the pitch to something more reasonable, but doing that while struggling with an out of control aircraft can be quite difficult.  When adjusting the pitch trim, one cannot see where the aircraft is going.  I do have a couple joystick buttons assigned to pitch trim, so I can use those more conveniently.

However, my real question is what can cause the pitch trim to go extreme position like this while flying?  Center of gravity?  Just flying too slow?  I don't have this issue with other aircraft.  Also, I don't remember having this issue with an earlier version of the Saab 340A.  I have flown the Saab 340 v1.4.1 only twice so far, so I'll keep an eye out as to whether or not this is a consistent problem or intermittent.

May be a help pushing the trim reset button.

[cwjohan]

JGregory, logs here.  Don't see anything useful in them myself.  Loading KSAC had some issues, but was flying nowhere near there -- 143 km (89 miles) distant from KSAC.

GizmoLog.txt

Log.txt

[cwjohan]

6 hours ago, birdy.dma said:

May be a help pushing the trim reset button.

Thanks, birdy.dma.  I will try that.  I suspect that causes autopilot to disconnect, though.

[JGregory]

32 minutes ago, cwjohan said:

JGregory, logs here.  Don't see anything useful in them myself.  Loading KSAC had some issues, but was flying nowhere near there -- 143 km (89 miles) distant from KSAC.

GizmoLog.txt

Log.txt

Nothing obvious, but you are running a lot of scenery and plugins.  How are your frame rates?  You may try removing other plugins and see if that does anything. Other than that, and without any further info (ie. airspeed, weight, vvi, cg, etc.), I'm not sure I can offer any other help.  As of right now no one else is reporting this type of problem.  There are really no significant flight model changes in this update that would account for this type of behavior.

[cwjohan]

Did a couple of flights with no runaway pitch trim problem.  But on this flight, had the problem on ILS approach to runway 12 at KOAK.  Photo attached shows the elevator full up.  The rudder is completely gone  Managed land OK without adjusting the trim.  Outer flaps ended up full down, but I had them set for just 20°.  See video.  Approach speed was just under 120 kts.  I don't think the trim should have been that high at that speed.

GizmoLog.txt

Log.txt

LES_Saab_340A_1.avi

[JGregory]

According to the GizmoLog.txt an error occurred in one of the scripts, which could account for some of the odd behavior, but I can't guarantee it.  The cause of the script error will be researched and fixed asap.

[cwjohan]

6 hours ago, JGregory said:

According to the GizmoLog.txt an error occurred in one of the scripts, which could account for some of the odd behavior, but I can't guarantee it.  The cause of the script error will be researched and fixed asap.

Thanks, JGregory, for looking further into this issue.  The playing of pump sounds, as indicated in Gizmolog.txt, may not have a direct connection to the runaway pitch trim issue, but they may have a common cause.  My FPS are only in the 13 - 17 range, so simulator time and real time may be very different.  Also, my single-core CPU is maxed out.

SPECULATION: There may be algorithms controlling pitch trim when autopilot is on that are assuming simulator time is approximately equivalent to real time.  Timers based on real time may be popping too often with respect to simulator time because too much real time has passed.

On the other hand, at least some X-Plane aircraft are leaving all these kinds of autopilot calculations to X-Plane itself and not attempting to modify how it works with plugin code (or Gizmo code).  Which approach does the Q400 take -- does the Q400 code try to control the pitch trim when autopilot is on or does it leave it up to X-Plane default autopilot code?

Cheers,
Craig

[JGregory]

4 hours ago, cwjohan said:

Thanks, JGregory, for looking further into this issue.  The playing of pump sounds, as indicated in Gizmolog.txt, may not have a direct connection to the runaway pitch trim issue, but they may have a common cause.  My FPS are only in the 13 - 17 range, so simulator time and real time may be very different.  Also, my single-core CPU is maxed out.

SPECULATION: There may be algorithms controlling pitch trim when autopilot is on that are assuming simulator time is approximately equivalent to real time.  Timers based on real time may be popping too often with respect to simulator time because too much real time has passed.

On the other hand, at least some X-Plane aircraft are leaving all these kinds of autopilot calculations to X-Plane itself and not attempting to modify how it works with plugin code (or Gizmo code).  Which approach does the Q400 take -- does the Q400 code try to control the pitch trim when autopilot is on or does it leave it up to X-Plane default autopilot code?

Cheers,
Craig

Ummm... Q400?  I'll assume you just mistakenly wrote that.  Your frame rates are far too low to guarantee that anything will work right.  The autopilot input is modified in the Saab, but the output is mostly X-Plane.

[cwjohan]

Right.  Saab 340A, not Q400.  Ha ha.  Typing without thinking.  The Q400 is a twin turboprop that I fly a lot.