Background:  We've got a large scale powered glider  that has been flying pretty well, except for one thing that is baffling.  We've done Autotune, setup and checked each parameter carefully in the list and wondering if we potentially missed something critical with the throttle and pitch settings.  

Airframe: ASW20

Flight Controller: Pixhawk 1

Problems:

1) When in Auto mode, we get very strange spikes in desired pitch (which are NOT desired), and when switching from RTL to FBWA, the airplane immediately pitched down -35 degrees, lost 100m of altitude in 5 seconds and was unrecoverable.  New airframe on order, but electronics are fine.  When we rebuild, hopefully we can get these parameters correct...

Current params affecting pitch (please let me know if there are more):

KFF_THR2PTCH 0
PTCH2SRV_TCONST 0.4
PTCH2SRV_TCONST 0.4
PTCH2SRV_P 0.84426
PTCH2SRV_D 0.0675408
PTCH2SRV_I 0.06
PTCH2SRV_RMAX_UP 90
PTCH2SRV_RMAX_DN 90
PTCH2SRV_RLL 1
PTCH2SRV_IMAX 3000
PTCH2SRV_FF 0
TECS_PTCH_DAMP 0
TECS_PITCH_MAX 0
TECS_PITCH_MIN 0
STAB_PITCH_DOWN 2
LIM_PITCH_MAX 2000
LIM_PITCH_MIN -1500
TRIM_PITCH_CD 0

Initial reactions are we need to change TEC_PTCH_DAMP to 0.3 at least, TECS_PITCH_MAX should be set to 5m/s not disabled (0), and LIM_PITCH_MIN should probably be increased to -5000 (-5 degrees pitch down) such that throttle is mainly used to control the altitude.  Thoughts?? Experiences??

2) The throttle seems to wind up and down repeatedly throughout Auto waypoint folllowing.  When we watch the playback HD video you can hear how bad this sounds in the audio. Here's a graph of what the throttle looks like between FBWA and Auto:

Also, here's all the throttle params I could think of, I'm wondering if we set something up incorrectly or if we just fix it by adding some lowering the throttle gain (seems like a bandaid fix for an inherent issue):

TECS_THR_DAMP 0.5
THR_MIN 0
THR_MAX 100
THR_SLEWRATE 100
THR_SUPP_MAN 0
THR_PASS_STAB 0
TRIM_THROTTLE 45

THROTTLE_NUDGE 1

My initial reaction to reviewing these settings are we need to lower the TECS_THR_DAMP to 0.2, change THR_MIN to 30 and THR_MAX to 80, change THR_SLEWRATE to 10%/s, and turn THROTTLE_NUDGE off by setting it to 0.  Thoughts?  Experiences?

As it turns out for this particular project I cannot share the logs, but if you would like to see any particular plots etc... I can easily look myself and/or post them.