Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_58 (year_commissioned INTEGER, gross_head__metres_ INTEGER)
### Question:
What is the Year Commissioned of the power stationo with a Gross head of less than 18?
### Answer:
SELECT AVG(year_commissioned) FROM table_name_58 WHERE gross_head__metres_ < 18