Short failsafe: the first seconds after the radio is lost

What ArduPilot says:

• RC Short Failsafe: switched to …
• RC Short Failsafe On
• Short Failsafe Cleared
• Flight mode … restored
• RC Short Failsafe Cleared

The short failsafe is what Plane does right after the radio is lost (“Throttle failsafe on”), while it waits for the link to return. FS_SHORT_ACTN decides. When you were flying by hand (MANUAL, FBWA, CRUISE and similar): CIRCLE for 0 or 1, FBWA with the motor stopped for 2, FBWB for 4. In AUTO, GUIDED or LOITER: nothing for 0, so the mission goes on, otherwise as above. “switched to …” names the new mode; “RC Short Failsafe On” means the mode was left alone. When the link returns: “Cleared”, then “Flight mode … restored” if the failsafe had changed the mode.

What to do, most likely first

1. The link dropped for a moment at the edge of range or in a turn.

   FixIf it cleared by itself and the mode was restored, nothing is broken: take it as a warning that you are at the limit of the radio and turn back. The causes are those of “Throttle failsafe on”.

2. The link stayed lost.

   FixAfter FS_LONG_TIMEOUT (5 s by default) the long failsafe takes over with FS_LONG_ACTN. For most planes 1 (RTL) is the action to have.

3. You switched the transmitter off on the ground to test it.

   FixThat is the test working. Do it with the propeller off: CIRCLE and RTL control the throttle themselves.

CIRCLE, TAKEOFF and RTL are never changed by the short failsafe, and it does nothing during an automatic landing or with FS_SHORT_ACTN 3. In 4.6 the long failsafe's time is counted from the start of the short one; in 4.7 from the last good frame.

Parameters to look at

Where to do it:Radio Link Planner