Full throttle and still sinking: a motor may have failed

What ArduPilot says:

• Potential Thrust Loss (N)

For one second the copter was asked to stay within 15° of level, had the throttle above 90% or the motors at their upper limit, still held its attitude (less than 30° of error), and was going down. ArduPilot reads that as lost thrust. The number in brackets is the motor it is driving hardest, usually the one that delivers nothing. From then on it leaves that motor out when it shares out the thrust and lets the others run to their limit (thrust boost), giving up some yaw control to stay level. That can save a hexa or an octo; a quad cannot fly on three motors. Land at once.

What to do, most likely first

1. A prop came off or broke, or a motor or ESC stopped.

   CheckIn the log that motor's output (RCOU) sits at maximum while the others are lower; with ESC telemetry, its RPM or current is zero.

   FixLand where you are. Then inspect that arm (prop and its nut, motor screws and bearings, ESC, connectors, solder joints), replace what is damaged, and run the motor test with the props off before the next flight.

2. The copter is too heavy for its motors, or the battery sagged so far that full throttle no longer lifts it.

   CheckAll the motor outputs are near maximum together and the battery voltage is low. A healthy copter hovers at about half throttle: MOT_THST_HOVER well above 0.5 leaves little in reserve.

   FixTake weight off, or fit motors, props or a battery that give more thrust, and land before the battery is low.

3. One motor is weaker than the rest: worn bearings, a damaged winding, a tired ESC.

   CheckIn hover logs the same output runs clearly higher than the others, flight after flight.

   FixReplace the weak motor or ESC. The Motor & Prop Bench compares the motors at the same throttle and shows the one whose RPM or current is off.

4. The motors never get full power: MOT_SPIN_MAX is below its default of 0.95, or PWM ESCs are not calibrated to the output range.

   FixSet MOT_SPIN_MAX back to 0.95 and, with the props off, calibrate PWM ESCs.

The log records it as error THRUST_LOSS_CHECK. With bit 2 of FLIGHT_OPTIONS set, the gripper lets go of its load at this moment to lighten the copter.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning GuideMotor & Prop Bench