IRIS+ hovered in Loiter mode, FailSafeBatt-1 triggered, copter climbed and crashed

1) Describe the problem you are having. What is the expected verses the observed behavior?

I was simply loitering IRIS+ approximately 4 meters above ground. I expected failsafe battery to trigger LAND, because FS_BATT_ENABLE was set to 1:LAND. Instead, the copter climbed approximately to 20-30 meters and flew away from Home location, then crashed.

Even if the copter would specifically RTL, (as noted for the FS_BATT_VOLTAGE) then it still should have landed at the same location anyway, as it was ARMED their, instead, as noted above, it flew away from RTL location.

The moment it climbed up I did switch CH7 ON to force Land, but it didn’t help.

I was having issues with compass and then accelerometer calibration, but I re-calibrated everything from scratch and had no Pre-Arm Errors.

2) Provide hardware information such as the brand and version of your autopilot, GPS, radio, and compass, as well as any other pertinent details about your setup.
3DR Iris+, Pixhawk/PX4, 3DR UBlox GPS + Compass Module, 2.4Ghz FrSky D4R-II 4 Channel Telemetry Receiver and FrSky DJT Telemetry Transmitter, 433 Mhz 3DR Telemetry Radio for Tower (was connected when crashed)

3) What version of the APM:Copter firmware version are you running?
APM:Copter V3.3.3 Quad (acf2e10c) installed from Mission Planner 1.3.37 build 1.1.5917.13431
Frame Type: V, 3DR_Iris+param

4) For flight issues, provide your dataflash log.

Auto Analysis shows the following:

Log File C:/Users/
Size (kb) 5888.9365234375
No of lines 72399
Duration 0:07:56
Vehicletype ArduCopter
Firmware Version V3.3.3
Firmware Hash acf2e10c
Hardware Type
Free Mem 0
Skipped Lines 0

Test: Autotune = NA -
Test: Balance/Twist = NA -
Test: Brownout = GOOD -
Test: Compass = FAIL - Large change in mag_field (49.27%)
Max mag field length (590.26) > recommended (550.00)

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = FAIL - ERRs found: FS_BATT CRASH FS_THR
Test: GPS = GOOD -
Test: IMU Mismatch = FAIL - Check vibration or accelerometer calibration. (Mismatch: 3.45, WARN: 0.75, FAIL: 1.50)
Test: Parameters = GOOD -
Test: PM = NA -
Test: Pitch/Roll = NA -
Test: Thrust = NA -
Test: VCC = GOOD -

Looking forward to getting any clarifications that caused the crash.


That MAG field change is troubling. That would cause issues with any RTL action after a failsafe I would think. In a lot of cases, I’d rather set the failsafe to just Land. That way, it will descend straight down at a slow rate. I haven’t looked at the logs yet, but the autoanalysis specifically with that entry would be worrisome.

Ok, what did you do to your IRIS?

The logs show that a battery failsafe was reached and the copter entered Land mode to land. The problem is that the motors are maxed out and the vibrations in the Z axis were bad enough to send the throttle into full mode because it thought is was falling because of the vibrations.

IRIS copters typically have very low vibrations.


Thanks Mike…I hadn’t read it yet but the MAG field issue was the first thing I saw, then I saw the IMU failure which indicates vibrations. I hadn’t completely read it yet.

That is exactly what I did. My Iris+ was configured to LAND and not RTL.

I did nothing wrong.

Thanks for the explanation, but what could cause this Z fluctuations (I associate the word “vibration” with something purely mechanical)?

Now I’m afraid to use my X8+ :frowning:

I need to record a lesson learned, otherwise the risk of similar crashes happening would not be reduced.

I was just woundering if there was modification done to the copter. Or maybe it had a load attached to it that was heavy. These things can cause the motors to work hard causing some vibration issues. If this was a standard out of the box IRIS+ then there should not have been any issues with vibrations and the copter should have just landed like it was commanded to do.


OK, if you did nothing wrong, has this IRIS crashed before and if so did you have to do any re-assembly?

No modifications, but it was not a brand new copter. Just before the crash I struggled to fix the compass and accelerometer. I did eventually cause it did arm without errors. At least that’s what I thought.

Yes, it has crashed several times, but minor crashes.
Yes I did re-assemble it couple of times.

So it is the PHYSICAL VIBRATION that caused the accelerometer to malfunction?

It looks like it is. The problem here is that the IRIS+ does not have a Stabilize flight mode to use. Both Loiter and Altitude Hold and Position Hold relay on the accelerometer for flight. I think on the IRIS+ they label it STD mode which is actually Altitude Hold.

What would be nice is to do a test hover for a few minutes and check the vibration to see if there is still a problem. But this need to be done in Stabilize mode and not any other mode as a fly away is possible.

It maybe that something is binding or touching the flight controller in such a way that it is transferring vibrations to it.

In any case we need to see what going on here.


Thanks for sharing your knowledge! I have read a lot about vibration since then. Looks like I have got the answer and have regained my “pilot” confidence.

The IRIS+ arms are broken. I am likely to build it almost from scratch. Then I will go through the all the testing.

Thanks a lot!