Long failsafe: the radio or the ground station stayed lost

What ArduPilot says:

• … Failsafe On: switched to …
• … Failsafe On: …
• RC Long Failsafe Cleared
• GCS Failsafe Off

The line begins with “RC Long” (the radio has been lost for FS_LONG_TIMEOUT, 5 s by default) or with “GCS” (no heartbeat from the ground station for the same time, with FS_GCS_ENABL on), and ends with the mode the plane is now in. FS_LONG_ACTN decides: 0 lets an AUTO or GUIDED flight continue and sends anything else to RTL, 1 is always RTL, 2 is FBWA (a glide with the motor stopped when the radio is lost), 3 releases the parachute, 4 switches to AUTO. Nothing changes during an automatic landing. “Cleared” or “Off” means the link is back; the plane stays in the failsafe's mode until you change it.

What to do, most likely first

1. The radio link stayed lost: range, aerial, transmitter battery.

   Check“Throttle failsafe on” and a short-failsafe line come right before it in the log.

   FixLet it fly home. When the link is back (“RC Long Failsafe Cleared”), move the mode switch to another position to take control: the plane does not leave RTL by itself.

2. The telemetry link to the ground station dropped while FS_GCS_ENABL is on.

   CheckThe line begins with “GCS”.

   FixFind why the telemetry dropped: range, the aerial, a pulled USB cable, a sleeping laptop. With FS_GCS_ENABL 1 it acts in every mode; 3 limits it to AUTO, where the ground station is the only link you rely on.

3. FS_LONG_ACTN is 0, so a plane in AUTO flew on with its mission without a link.

   FixSet FS_LONG_ACTN to 1 unless the mission is meant to continue out of radio range.

Check that RTL_ALTITUDE (100 m by default) clears everything between the plane and home. In TAKEOFF mode the long failsafe waits until the first climb is finished, unless the action is glide or parachute. In 4.7 the ground station's link coming back reads “GCS Failsafe Cleared”.

Parameters to look at

Where to do it:Radio Link PlannerStarting Parameters