The throttle channel reads below the failsafe value

What ArduPilot says:

• PreArm: … below failsafe

The throttle channel's pulse is shorter than FS_THR_VALUE (975 µs by default). In flight that is how ArduPilot recognises a lost radio link, so on the ground it refuses to arm: it would be in radio failsafe at once.

What to do, most likely first

1. The transmitter is off: the receiver sends its failsafe throttle, which is set low on purpose.

   CheckSwitch the transmitter on and look at the throttle bar in the Radio Calibration screen: with the stick down it should read close to RC3_MIN.

   FixSwitch the transmitter on before arming.

2. FS_THR_VALUE is above what the throttle stick gives at the bottom.

   CheckCompare the two: FS_THR_VALUE must be at least 10 µs below the lowest throttle the stick gives (RC3_MIN after calibration) and above what the receiver sends when the link is lost.

   FixCalibrate the radio, then set FS_THR_VALUE between the receiver's failsafe throttle and RC3_MIN minus 10.

3. The throttle is on another channel than ArduPilot thinks, or reversed in the transmitter.

   CheckMove only the throttle stick and see which bar moves, and in which direction: it should be the channel RCMAP_THROTTLE names (3 by default), rising as the stick goes up.

   FixSet the channel order in the transmitter (AETR for the default mapping) or RCMAP_THROTTLE to the channel that moves, and reboot.

Parameters to look at

Where to do it:Setup & Tuning GuideFailsafe Explainer