Hi @Mael_Forestal ,
IIRC, terrain following is not available for fixedwing yet. I guess it should probably be implemented here: https://github.com/PX4/PX4-Autopilot/blob/835e3258ed941458c95c295c7746d9ad5a07ead0/src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp#L494
You can simply use the dist_bottom field from the vehicle_local_position topic to get the estimate of the distance below the drone (using a range finder or an optical flow sensor).
Good luck!