QLAND Mode attempts to bring the QuadPlane straight down at the position the vehicle is located when the mode is entered, descending to Q_LAND_FINAL_ALT at Q_WP_SPEED_DN until it reaches Q_LAND_FINAL_ALT, at which point it continues to descend at Q_LAND_SPEED until landing.
QuadPlane will recognize that it has landed if the motors are at minimum but its altitude does not change more than 0.2m for one second. It does not use the altitude itself to decide whether to shut off the motors except that the QuadPlane must also be below Q_LAND_FINAL_ALT above home(ie in the LAND FINAL phase). The altitude change for the decision can be increased, in case the altitude determination from the EKF is excessively noisy by increasing the Q_LAND_ALTCHG value from its default value of 0.2m.
If the QuadPlane appears to bounce or balloon back up a couple of times before settling down and turning the props off, try lowering the
Q_LAND_SPEEDparameter a bit.
If the vehicle has GPS lock the landing controller will attempt to control its horizontal position but the pilot can adjust the target horizontal position just as in QLOITER mode.
If the vehicle does not have GPS lock the horizontal control will be as in QSTABILIZE mode so the pilot can control the roll and pitch lean angle of the QuadPlane.
In any mode based on using the barometer: QLAND, QLOITER, QHOVER, QRTL, if your QuadPlanes operation becomes erratic when you are close to the ground or landing (and also if any auto landing procedure results in bouncing or failure to turn off motors properly after landing) you probably have the autopilot situated such that its barometer (altimeter) is being affected by the pressure created by the QuadPlanes prop-wash against the ground.
This is easily verified by looking at the altimeter (baro alt) reading in your logs and seeing if it spikes or oscillates when near the ground.
If this is a problem, move the autopilot out of prop wash effect or shield it with an appropriately ventilated enclosure.
Success can be verified by flight test and by log results.