Loiter crash

Today I tried the loiter mode on a new quad and had a sudden disaster. The althold was working very well and I had flown for about 3 minutes before switching to loiter and when I switched to loiter it was stable for a short time and then it suddenly got 2 or 3 power surges and significant altitude gains. At that time I switched back to althold mode and I panicked and may have cut the throttle because the motors quit and it dropped like a rock and broke an arm. Looking at the log shows the throttle surges but I can’t see why that would have happened because the altitude was pretty stable until the surges. Maybe someone can see something in the logs I am missing and I appreciate the help.

Too much vibration is really your only option for this kind of thing.

I had looked at the vibration level on accelz and the level seemed to be -5 to -15 for the most part, but is that considered too high or marginal? I had read that the acceptable level was -5 to -15.

It’s more complicated than just magnitude - it depends on the frequencies too.

Thanks, I will look at more isolation and I have replaced the HJ quad arms with DJI arms which appear to be considerably stiffer. It may help to eliminate some of the flex in the arms which may be allowing more resonated vibrations.

Yes this is not acceptable. You should keep your accelZ vibrations within -8 +12 to avoid bad behavior.And for a good loiter, keep it even lower.
Use kyosho zeal tape or moongel, these are the easiest isolation methods.