QuadPlane decrease the throttle in Qloiter mode and crash(3.5.3)

ArduPlane Plane 3.5
1

Hello,guys, hoping your help.
Not everytime,but frequently after the QuadPlane fly to a certain level(always some meters) in Qloiter mode, ardupilot will slowly decrease the output of RC5/6/7/8,and so plane fall on floor.

Cutdown throttle.jpg722×449 127 KB

2016-05-27 15-58-51.tlog (288.2 KB)
Download bin.rar

And actually I only fly to 5 meters.But the log show I fly to 18m, that’s strange. When i feel ardupilot cutdown my thottle,I change to QHover mode, and plane start to fall. So it shouldn’t fly to 18m.

ALT.jpg1280×363 112 KB

So i think is there any problem about my baro,I check EKF IPD,as below
IPD.jpg1328×338 91.8 KB

2

Dose any body have idea?

3

So nobody has ever encountered this big problem?

4

I think this is a serious problem and my friends and I both encountered it.
How can nobody else post similar problem.
Hoping your reply.

5

Nobody have any idea?