The radio link was lost (“on”) or has come back (“off”)

What ArduPilot says:

• Throttle failsafe …

“Throttle failsafe on” is how Plane says the radio control link is lost. For ten readings in a row (0.2 s) the throttle channel was below THR_FS_VALUE (950 µs by default), or no valid frame had come from the receiver for 1 s. From now on the sticks are ignored and the failsafe begins: first the short one, and after FS_LONG_TIMEOUT the long one. “Throttle failsafe off” means good frames are back. All of this needs THR_FAILSAFE to be 1.

What to do, most likely first

1. The plane flew out of range, or in a turn the airframe or the battery shadowed the receiver's aerial.

   CheckIn the log, look at the receiver's signal strength (RSSI or link quality) in the seconds before the message, and at the distance from home.

   FixFly closer. Mount the receiver's aerial clear of carbon, battery and wiring, and in the same orientation as the transmitter's.

2. The transmitter was switched off, or its battery ran out.

   FixSwitch it on; the failsafe clears as soon as frames arrive. Switching the transmitter off on the bench is the right way to test this failsafe: take the propeller off first.

3. THR_FS_VALUE is too close to the stick's lowest position, so closing the throttle trips it.

   CheckThe message comes exactly when you pull the throttle down. Compare THR_FS_VALUE with RC3_MIN.

   FixCalibrate the radio, then set THR_FS_VALUE well below RC3_MIN and above what the receiver sends when the link is lost.

4. The receiver lost power or a wire while the flight controller kept running.

   CheckThe signal strength was good until the moment it stopped. Check the receiver's connector at both ends.

   FixRepair the wiring before the next flight; the Board & Wiring Planner shows which pad the receiver belongs on.

The message is the same whether the receiver signals the loss by a low throttle, by stopping its frames or by its failsafe flag. With THR_FAILSAFE 2 the sticks are ignored while the link is lost, but no failsafe action follows and this message is not sent.

Parameters to look at

Where to do it:Radio Link PlannerBoard & Wiring Planner