Plane crash on auto takeoff due to battery failsafe autoland(?)

ArduPlane Plane 4.7
1

Hi all,

I had a plane=)
I was doing an auto takeoff. It seems I had weak battery, so it went below BATT_CRT_VOLT on a first second of a takeoff. I have BATT_FS_CRT_ACT=2 # Land so
plane have chosen direction to land, started to descend, rolling, but it was still in the auto takeoff mode and it even did not reduced motors RPM for landing. Speed were growing, rapid descend, done.

Messages:

2026-05-03 13:56:55.339 Armed AUTO, xaccel = -17.3 m/s/s, waiting 0.2 sec
2026-05-03 13:56:55.539 Triggered AUTO. GPS speed = 0.7
2026-05-03 13:56:59.079 Takeoff to 40m for 200.0m heading 294.7 deg
2026-05-03 13:56:59.399 Autoland direction= 310

Flight:

image
image1483Ă—839 159 KB

Log:
2026-05-03 13-56-45.bin (636 KB)

2

I never use battery failsafes at all for similar reasons. Once I was landing with very low battery and when the copter was 1 meter above ground it started climbing… Barely managed to get it down again before it woyld have tumbled down.

So… disable all battery failsafes and use common sense when managing battery

1 Like
3

Looks like your running custom firmware? Some arming checks are also skipped.

There is no battery failsafe, It is just a failed takeoff. The “Autoland direction= 310” thing is just to tell you that the auto land direction has been locked in from the take off heading. It does not mean that it is starting a auto land.

Has this vehicle flown before?

4

No, that was the first run. I had only GPS check disabled, I have seen “GPS glitch” in EKF helper too, but it was after arm. Also flag “takeoff” in EKF helper became “false” at the same time battery went below critical voltage.

Custom firmware adds only VTX power switch functionality for IRC tramp VTX, nothing more.

I have another idea why it has happen - maybe because of max_throttle on takeoff 80%, plane gained speed which was critical for this frame. But how did that lead to the right roll and to the pitch towards the ground? I disabled airspeed sensor that time to make its calibration.

Maybe you have other ideas of the cause why did takeoff fail?

Aircraft is swordfish, 2 engines, 7inch props, 4s battery

PS: regarding takeoff heading and land direction - it differs 15 degrees. Why so?

5

If its not flown before then it could just be that the tune is not correct, or the control surfaces are not setup correctly. There really is very little relationship between what the controllers are asking for and what the plane is doing.

1 Like
6

I’m going to go out on a limb here and suggest something physical, like your battery was loose and sliding around inside the fuselage, causing the un-commanded roll to the right. AP was trying to correct but at a max of 5 degrees due to LEVEL_ROLL_LIMIT = 5.