Fail to change to loiter from Alt Hld when airborne

My quad running Copter 4.1.2 (Pixhawk 2.8.4) failed to change to loiter mode after hovering in Alt Hld for a while. I then changed GPS_GNSS_mode from 67 (GPS+SBAS+GLONASS) to 3 (GPS+SBAS), and succeeded to change to loiter. Few days later, have not changed any param, it failed again to change to loiter. There is no problem changing to loiter on the bench everytime.

Pls suggest any possible reasons. Param and log attached. The RTL mode at the latter stage of flight was due to low battery, and I switched to loiter mode after landing (the last part of log).
S500__fail to loiter__6 July 2022.param (16.7 KB)

The log file is unavailable. “File is in Owner’s trash.”

Usually if you can’t switch to loiter then the GPS isn’t happy. Check HDOP, not just SAT count. Also if you have geofencing turned on then the quad won’t arm unless it has satisfactory GPS location.

And… as usual, update to ArduCopter 4.2.1

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