towards the very end of the flight I switched to loiter mode. Initially, in loiter, it throttled up slightly, gaining altitude, then it decreased throttle, eventually cutting the motors off completely. I switched back out of loiter into stabilize and “landed” more quickly than I prefered on some soft grass.
So why did loiter mode cut power? Is the barometer saying there was an increase in altitude? Is the altitude PID parameters wrong? I don’t have enough experience to debug and was hoping someone could provide some suggestions.
There were no glaring problems in your log, mechanical or vibration-wise. The only thing that stood out to me is you were hovering at 75% throttle when you switched into Loiter and your THR_MID is only set to 510. For the weight you are lofting, your craft is very under powered. If you switch to Loiter from Stabilize with those values it will likely descend.
Remember the throttle acts very different in Loiter and AltHold modes versus Stabilize. In Loiter putting the throttle at the very middle of its travel should cause the craft to neither gain nor lose altitude…BUT only if the THR_MID is set to the throttle value actually used in hover. In your case, if you continued to fly in such an overloaded condition, your THR_MID should be set to around 730.
Baro readings looked fine as did your altitude PIDS. Simply an underpowered/overloaded craft not handling the transition between Stabilize and Loiter very well. Upping your THR_MID will make the best of the situation you have.