"按照预定计划" can be translated into English as "according to the predetermined plan" or "as scheduled".
"According to the predetermined plan" emphasizes following a plan that has been set in advance.
"As scheduled" is a more concise and commonly used expression, especially when referring to events or activities happening at their designated times.
For example:
"The project is progressing according to the predetermined plan."
"The meeting will start as scheduled."