SmartRTL could not start: falling back to RTL or Land

What ArduPilot says:

• SmartRTL Unavailable, Trying RTL Mode
• SmartRTL Unavailable, Using Land Mode

A failsafe asked for SmartRTL, which flies back along the path the copter came by, and SmartRTL refused to start. The copter falls back as the line says: to RTL, and to Land if RTL is refused too, or straight to Land (it descends after a 4 s pause). SmartRTL starts only while a path is being recorded: the position estimate was good when the copter armed and since, and the path memory (SRTL_POINTS points) has not been full for 15 s.

What to do, most likely first

1. No path was recorded: the position estimate was not good at arming, or was bad for 15 s in flight (“SmartRTL deactivated: bad position”).

   CheckSearch the log's messages for “SmartRTL deactivated”.

   FixWait for a good position before arming when a failsafe relies on SmartRTL, and find why the position was lost in flight (GPS, compass, EKF messages around that time).

2. The path memory filled up on a long or winding flight (“SmartRTL deactivated: buffer full”).

   CheckSRTL_POINTS is 300 by default, and points are kept no closer than SRTL_ACCURACY (2 m).

   FixRaise SRTL_POINTS (500 at most; 100 points take about 3 kB of memory) and reboot, or make SRTL_ACCURACY larger so points are stored further apart.

3. SRTL_POINTS is 0, which switches SmartRTL off altogether.

   FixSet SRTL_POINTS to 300 and reboot, or choose a failsafe action that does not use SmartRTL.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning Guide