On Airbus A320 The altitude alert signal works only in manual mode (without automatic pilot): in this mode when you cross the altitude set on the LVLCH ALT selector the alarm sounds ONLY if automatic pilot is OFF. For unknown reasons in some older version of Wilcopub Airbus Series soemtimes this alarm worked even in autopilot mode (but this NOT what happens in real Airbus models).On the Boing the Altitude Alert works always whatever mode (manual/autopilot) you are.