Crash after "EKF2 IMU0 is using GPS" and different AHR2 and ATT Attitude Estimationes

Unfortunately, my new Opterra 1.2 flew into the ground in FBWA-Mode - not responding to full pulled elevator stick:

After the start FBWA worked fine. Then i switched to autotune. As the rudders began to flutter, i switched back to FBWA (i would have better switched to manual). After “EKF2 IMU0 is using GPS” msg the Opterra shows no reaction to the full pulled stick and flew with increasing pitch down and speed (25 m/s) into the ground.

Firmware: ArduPlane V3.9.8 (6ea22c9c) ChibiOS d2030d88
Hardeware: Omnibusf4pro 004F0027 4B4B500C 20313548

Some excerpts from the logged AHR2 and ATT pitch datas:

Complete logfile and parameters:

In my opinion, the initial correct datas from ATT was used first. After the message “EKF2 IMU0 is using GPS” the AHR2 datas was used. However, from this moment on they falsely indicated a pitch upwards (!), while reality and ATT values showed (correctly) pitch downwards(!).

At estimated 99.9% of my other flights, AHR2 and ATT msg differ only minimally. I’m interested in why the two estimates deviate so much and in the end are still in the opposite direction

Regards Rolf

Hi Rolf,
The problem was that arming checks were disabled combined with not having a compass or an airspeed sensor. The EKF did not correctly handle the in-flight initialisation.
Flying without compass and airspeed will work if you wait for the EKF to initialise fully on the ground, but is unreliable if you bypass arming checks and let it initialise in flight.
I’ve opened an issue here:

note that if you really do want to bypass arming checks and fly with no compass or airspeed then you should disable the EKF completely using AHRS_EKF_TYPE=0

Hi Tridge,
Thanks for the clarifying words. I will act accordingly :smile: