I have a quad copter that had an unexpected crash shortly after takeoff, running Arducopter version 4.0. This drone is fitted with dual GPS units, one regular GPS and one RTK GPS.
The craft was powered up and let sit for a while to allow RTK to get a lock. Very quickly after takeoff the craft lost altitude and crashed. The log shows a couple of EKF errors while in flight, and following the crash there were a ton of EKF_YAW_RESET errors repeating every 5 ms.
What could have caused this crash and why are there so many errors?
I’ve attached the binary log of this particular flight.
The EKF resets are all coming after the crash. I can’t say if that’s the chicken or the egg. But here’s my take on it:
While the drone was in a hover I see a torque imbalance, that the CW motors (3&4) are working harder than the CCW motors (1&2). This could mean that there is a twist in an arm, or some other mechanical issue with the airframe.
When everything goes wrong and it starts to pitch up the front motors (1&3) are slowing and the rear motors (2&4) are speeding up. To me this looks like it’s trying to correct the pitching up. Again, since the correction is not successful this is suggesting some kind of mechanical failure. The current monitoring isn’t working so it’s hard to say if you lost a motor/ESC, or if something in the airframe let go and the motor moved.
The problem is that it depends on the direction they are twisted. So as @amilcarlucas has said you can’t really tell from the log. It’s going to require a physical inspection to see. Measure the heights and locations of each motor. It won’t take much to cause a problem.
The motors with higher PWM are typically the ones that are twisted. BUT if two motors are twisted in a certain way their effect will cancel out. So long story short, you need to do as I told you above, but yes, you should start by the motors with higher PWM values, and you must inspect them all for the reasons stated above.
And please do not tell me that you are using ArduCopter 4.0.x. Update ASAP to ArduCopter 4.1.5