Radio link lost: the copter is acting on its own

What ArduPilot says:

• ERR FAILSAFE_RADIO-1 (Subsys 5, ECode 1)

The copter stopped hearing the transmitter and is doing what FS_THR_ENABLE says: 1 RTL (fly home and land), 3 Land, 4 SmartRTL or else RTL, 5 SmartRTL or else Land, 6 the mission's landing sequence or else RTL, 7 Brake or else Land. If RTL cannot start (no position estimate or no home), it lands where it is after a 4 s pause. Keep the transmitter on and move closer. When “Radio Failsafe Cleared” appears the copter still flies the failsafe mode: change the flight mode switch to take control back.

What to do, most likely first

1. The copter flew out of the radio's range, or something came between the antennas (trees, a building, your own body).

   CheckIn the log, see whether link quality and RSSI fell before the event. “ERR RADIO-2” right before “ERR FAILSAFE_RADIO-1” means frames stopped arriving for RC_FS_TIMEOUT (1 s by default); without it, the receiver kept sending frames with the throttle below FS_THR_VALUE (three in a row trigger the failsafe).

   FixMount the receiver's antennas clear of carbon, the battery and wires, do not point the tip of the transmitter's antenna at the copter, and do a range check on the ground before flying far.

2. The receiver lost power, or its connector worked loose in flight.

   CheckThe loss is instant, with no slow fall in link quality before it. On the bench, props off, wiggle the receiver's wires and watch the bars in the Radio Calibration screen.

   FixRe-seat or solder the connector, fix the wires so vibration cannot move them, and feed the receiver from a 5 V supply that does not sag when a video transmitter or servos draw current.

3. The transmitter's battery ran out, or the transmitter was switched off or to another model.

   FixCharge the transmitter before flying and set its low-battery alarm.

4. FS_THR_VALUE is above what the throttle stick gives at the bottom, so pulling the throttle down looks like a lost link.

   CheckThe failsafe comes exactly when you pull the throttle down, and the log has no “ERR RADIO-2”. Compare FS_THR_VALUE with RC3_MIN.

   FixCalibrate the radio, then set FS_THR_VALUE at least 10 µs below RC3_MIN and above what the receiver sends when the link is lost.

Rehearse it on the bench with the props off: arm in each mode you fly, switch the transmitter off and read what the copter says it does. Without a position estimate the result is not the return home that RTL promises.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning GuideSetup & Tuning Guide