The lift motors are at full power and the aircraft is still sinking

What ArduPilot says:

• Potential VTOL Thrust Loss (N)

For 1 s the lift motors were at 90% throttle or more (or at their upper limit) while the aircraft, asked to stay within 15° of level and holding its attitude to within 30°, kept descending. The number is the motor with the highest output, the likeliest to have failed. ArduPilot switches on thrust boost: it stops trying to balance on that motor so that the others can give all they have. The check exists from 4.7 on; Q_THRST_LOSS_OPT configures it.

What to do, most likely first

1. A lift motor, its ESC or its propeller failed in flight.

   CheckThe number in the message is the motor to look at first; in the log its output is at maximum while the others are lower.

   FixLand at once (QLAND). Before flying again inspect that motor, its propeller, its ESC and the wiring.

2. All lift motors are at their limit: the aircraft is too heavy for them, or the battery voltage has dropped.

   CheckIn the log: every lift motor near maximum, and the battery voltage at that time.

   FixLand at once. Reduce the weight or use a fresh battery: a healthy QuadPlane hovers well below full throttle.

3. It happened in a transition or during assistance in a fixed-wing mode, where the lift motors are asked for more than a hover needs.

   CheckDoes the message come only there, with the motors proven healthy in a hover?

   FixQ_THRST_LOSS_OPT bit 1 limits the check to VTOL modes; use it only after ruling out the first two causes.

Parameters to look at