“as schedule” vs. “as scheduled”

You say on schedule or behind schedule which consist of:

[preposition] + schedule (noun)

However, you say as scheduled and not as schedule. Why? What are the differences between these phrases?

Solution 1:

Phrases such as on schedule and behind schedule are idioms, so the form,

[preposition] + schedule (noun)

is not something that can be applied to every preposition and noun. However, as schedule is not commonly used. The phrase, as scheduled can be used in the following ways,

Everything is going as scheduled.

The food arrived as scheduled.

As scheduled is very similar to on schedule in that they are both used to describe an event that occurred on time or as expected. When one says something is behind schedule, then there is an indication that something failed to be done by a deadline. Some examples with on schedule and behind schedule,

He came right on schedule for the baby shower.

As long as every team member continues to his or her tasks on schedule, the project should be finished by February.

The boarding process for our flight is behind schedule, so we have to wait another half-hour.

The professor was behind schedule on his lectures and may ask the students to learn more material on their own.