The lift motors started because the aircraft was too low

What ArduPilot says:

• Alt assist N.Nm

Assistance is the lift motors switching on by themselves in a fixed-wing mode. Here the height above ground stayed under Q_ASSIST_ALT for longer than Q_ASSIST_DELAY (0.5 s by default); the number is that height in metres. The height comes from a rangefinder when one may be used, from terrain data when following terrain, otherwise it is the height above home. Assistance stops once the aircraft has been back above for twice the delay. It needs Q_ASSIST_SPEED above 0, and never acts in a fixed-wing landing flare.

What to do, most likely first

1. The aircraft really sank: too slow, too little thrust, or a turn it could not hold height in.

   CheckIn the log, follow altitude, airspeed and throttle before the message.

   FixFind why it lost height before flying that profile again; the lift motors saved it this time, at a high cost in battery.

2. A low pass or a fixed-wing landing was flown on purpose through Q_ASSIST_ALT.

   FixGive a switch the QAssist function (RCn_OPTION 82) and put it low for the fixed-wing approach, back to the middle afterwards; or plan the pass above Q_ASSIST_ALT.

3. Without a rangefinder the height is taken above home, and the ground here is higher than home.

   CheckCompare the height in the message with what you saw.

   FixPlan heights with the terrain in mind, enable terrain following, or fit a rangefinder.

Parameters to look at