The new flight mode refused to start

What ArduPilot says:

• Flight mode change failed

The plane was asked to change mode, the new mode refused to start, and the plane stays in the mode it was in; the buzzer plays its “failed” tune. In fixed-wing flight few modes can refuse: THERMAL when soaring is not active, AUTO on a QuadPlane that waits in GUIDED for a take-off while the mission has none, and from 4.7 AUTOLAND. When there is a reason, it is in the line right before this one.

What to do, most likely first

1. THERMAL is on the mode switch, and soaring is off.

   CheckSOAR_ENABLE is 0, or the soaring switch (RCn_OPTION 88) has it disabled.

   FixTHERMAL is entered by the soaring system itself when it finds rising air. Take it off the mode switch; set SOAR_ENABLE to 1 only if the plane is a glider and you want it to circle in thermals.

2. In 4.7: AUTOLAND was selected and could not start.

   FixRead the line before this one: “Must already be flying!”, “Takeoff initial direction not set” or “autoland not available”, each explained on its own.

3. On a QuadPlane: AUTO was selected before take-off and the mission does not begin with one.

   CheckThe line before reads “Takeoff waypoint required”.

   FixMake a take-off the first item of the mission and upload it.

Parameters to look at