Please Help. Crash in Auto-Mode!

Hi Everyone

I need help understanding what happened during my flight yesterday.

My IRIS+ was in AUTO-Mode. Flow a circle at 20m high and 17m diameter.

At the middle of the circle the IRIS+ lost altitude and crash on the roof…

Why IRIS+ lost altitude in AUTO Mode???

Here is an Auto Analysis of your dataflash log:

Size (kb) 1402.41015625
No of lines 18979
Duration 0:01:17
Vehicletype ArduCopter
Firmware Version V3.2
Firmware Hash c8e0f3e1
Hardware Type
Free Mem 0
Skipped Lines 0

Test: Autotune = UNKNOWN - No ATUN log data
Test: Balance/Twist = GOOD -
Test: Brownout = FAIL - Truncated Log? Ends while armed at altitude 6.96m
Test: Compass = FAIL - Large change in mag_field (63.35%)

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = FAIL - ERRs found: CRASH FS_THR
Test: GPS = GOOD -
Test: IMU Mismatch = GOOD - (Mismatch: 0.44, WARN: 0.75, FAIL: 1.50)
Test: Parameters = GOOD -
Test: PM = GOOD -
Test: Pitch/Roll = FAIL - Roll (-178.07, line 18309) > maximum lean angle (30.00)
Test: Thrust = FAIL - Avg climb rate -40.35 cm/s for throttle avg 915
Test: VCC = GOOD -

Looks like you may have a had a brownout.

What means " a had a brownout." ??

And why i lost altitude bevor the Crash?

Auto-Mode say Circle at 20m Altitude. The IRIS+ lost altitude after a half circle.