Quadcopter motor smoked during transition


Today, I flew the VTBird 4+1 VTOL.

I autotuned my aircraft three times, and the quadcopter motor worked well in various flight modes.(QLOITER, QHOVER, QSTABILIZE)

However, when I attempted to land at the landing site from fixed-wing mode to multi-rotor mode, the multi-copter motor(number 4) suddenly caught fire and generated smoke.

I managed to land safely, but I can’t determine the reason for the issue.

According to the flight log, when I engaged the QHover mode, the motor burned after just 2 seconds.

Below is my flight log.