On Tibet entry:

      If you have purchased a ticket whose destination is Tibet or which involves a connection in Tibet, please make sure that your travel documents, visa and the Letter of Authorized Entry into Tibet are all in compliance with applicable laws and policies and do confirm that you qualify to fly or enter Tibet. If your documents fail to comply with the applicable laws and policies, in which case you will be denied boarding or entry, you will be held responsible for all the consequences and losses arising therefrom, and Air China will not be held liable in that regard. Thank you for your understanding and cooperation.