def METHOD_NAME(self):
    """
    change mode to NAV.
    """
    if self.ap_mode is not None:
        msg = PprzMessage("ground", "DL_SETTING")
        msg['ac_id'] = self.ac_id
        msg['index'] = self.ap_mode.index
        try:
            msg['value'] = self.ap_mode.ValueFromName('Nav')  # AP_MODE_NAV
        except ValueError:
            try:
                msg['value'] = self.ap_mode.ValueFromName('NAV')  # AP_MODE_NAV
            except ValueError:
                msg['value'] = 13 # fallback to fixed index
        print("Setting mode to NAV: %s" % msg)
        self._interface.send(msg)