My B738 Climbs and decends rapidly, why?

Question

My B738 is uncontrollably climbing and descending whenever the 'ALT HOLD' comes on the Autopilot. This has been happening for several flights now, and it has not stopped, I am able to take off ok, but whenever I hit my cruising altitude and try to level off, the plane starts climbing and descending rapidly all the way up to V.S. 9999+ and -9999. This issue has really started to annoy me and I really want this fixed.

VIDEO: https://www.youtube.com/watch?v=iPvnOAiXKac&feature=youtu.be

Answer 1

Hi again, your target speed is above max operating ias. Does it work better if you fly at 320 kts for instance?

Does the LVL CHG mode work better?

PS : you should stick to one thread per problem, so that other users can see the whole process to find a solution.

Confirm you have the latest X-plane 11.05r1 update?

Confirm deleting output/preferences folder did not improve anything?

Can you attach your log.txt?

Comments

Hi, I will try flying at 320 kts

LVL CHG mode doesnt work, it has the same response

I have the latest update.

I deleted the prefs folder and It did not improve it.

Log.txt:

0:32:28.838 D/ATC: p=0 (N816NN): Squawk 4702
0:32:43.255 D/ATC: p=1 (N01XP): Takeoff: 15
0:32:53.952 I/ATC: N816NN, Squawk 4 7 0 2.
0:32:53.952 D/ATC: p=0 (N816NN): Squawk 4702
0:33:02.409 D/ATC: p=0 (N816NN): Fly Heading: 310
0:33:02.409 I/ATC: N816NN, Fly heading 3 1 0.
0:33:07.816 I/ATC: Heading 3 1 0, N816NN.
0:33:17.188 I/ATC: N816NN, Squawk 4 7 0 2.
0:33:17.188 D/ATC: p=0 (N816NN): Squawk 4702
0:33:40.836 I/ATC: N816NN, Squawk 4 7 0 2.
0:33:40.836 D/ATC: p=0 (N816NN): Squawk 4702
0:33:47.488 I/ATC: Squawk 4 7 0 2, N816NN.
0:34:01.455 I/ATC: Removing p=1 (N01XP) from Active Queue
0:34:01.455 D/ATC: p=1 (N01XP): Fly Heading: 139
0:34:01.455 D/ATC: p=1 (N01XP): Contact controller on 127.80
0:34:01.455 D/ATC: p=1 (N01XP): Switching frequency to 127.80
0:34:01.455 I/ATC: Center, N01XP, Checking in.
0:34:01.455 E/ATC: Tried playing a wave file that doesn't exist: 'checking_in.wav'
0:34:07.696 I/ATC: N816NN, Squawk 4 7 0 2.
0:34:07.696 D/ATC: p=0 (N816NN): Squawk 4702
0:34:29.588 I/ATC: N816NN, Squawk 4 7 0 2.
0:34:29.588 D/ATC: p=0 (N816NN): Squawk 4702
0:34:46.623 D/ATC: p=0 (N816NN): Fly Heading: 312
0:34:46.623 I/ATC: N816NN, Fly heading 3 2 0.
0:34:50.952 D/STM: Transitioning from state state_Normal to state_ShuttingDown
0:34:50.952 I/JOY: Control response for axis joy_use_ptch: 0.500000
0:34:50.952 I/JOY: Control response for axis joy_use_roll: 0.500000
0:34:50.952 I/JOY: Control response for axis joy_use_hdng: 0.500000
0:34:50.952 I/JOY: Control response for axis joy_use_thro: 0.000000
0:34:50.952 I/JOY: Control response for axis joy_use_lbrk: 0.000000
0:34:50.952 I/JOY: Control response for axis joy_use_rbrk: 0.000000
0:34:50.952 I/JOY: Control response for axis joy_use_prop: 0.000000
0:34:50.952 I/JOY: Control response for axis joy_use_mixt: 0.000000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_ptch: 0.500000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_roll: 0.500000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_hdng: 0.500000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_thro: 0.000000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_lbrk: 0.000000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_rbrk: 0.000000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_prop: 0.000000
0:34:50.952 I/JOY: Stability augmentation for axis joy_use_mixt: 0.000000
0:34:50.952 I/JOY: UNREGISTER Joystick device: CH PRO PEDALS USB  - VID:1678PID:242
0:34:50.952 I/JOY: UNREGISTER Joystick device: CH FLIGHT SIM YOKE USB  - VID:1678PID:255

Thanks!

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It's better to include the whole log.txt file.

I could definitely not reproduce your bug :

Windows 8.1 64 bits I5 4690 3.5GHz/8Gb RAM/dedicated SSD 480Gb/MSI GTX970 4Gb xp11.05r1

Can you try and desengage your A/T, Autopilot and FD, and then activate them again in the following order?

FD on

A/T on

LVL CHG on

Autopilot engaged

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It has actually started to work! Thanks for the support!