I've always made it a rule to allow my copters to acclimate for 3-5 minutes (minimum in very cold temps). I've never had an issue with GPS modes sticking to this procedure. The copter could have 15 satellites showing but not have a good position estimate. Just as with Arducopter, I think it is the same with Yuneec's FC that when it shows 'Ready' (green light blinking on Pixhawk) when GPS lock is attained, does not mean it has a good position estimate fix. Allowing it to sit for a few more minutes while it sorts out the position estimate before taking off should result in more stable flight, particularly when hovering. I brought this up because was reading at Ardupilot.org a post made by dev Randy McKay: Copter became hardly controllable and crashed; Log indicates a fail to enter the flight mode (AltHold) Daniel, Thanks for the testing and report. The issue is probably related to the fact that the vehicle was flying before the EKF had a good position estimate. This caused it to fail to enter Loiter mode but more importantly you may have uncovered an issue with AltHold before we get the position estimate. I'll check into this. For now, I think it would be good to wait for a good position estimate before taking off which you can do by switching into Loiter mode (before takeoff) and waiting for the LEDs to become green. While you're in stabilize mode (on the ground) the light will become flashing green but that actually doesn't mean that it has a good position estimate yet, it only means that it has a good GPS lock. I think maybe we should change this so that it only goes green once it has the good position estimate. That would make it consistent with the autonomous flight modes. I'll discuss with the other devs.